U.S. patent application number 13/517513 was filed with the patent office on 2017-05-25 for format-preserving cryptographic systems.
The applicant listed for this patent is Matthew J. Pauker, Terence Spies. Invention is credited to Matthew J. Pauker, Terence Spies.
Application Number | 20170149565 13/517513 |
Document ID | / |
Family ID | 49756817 |
Filed Date | 2017-05-25 |
United States Patent
Application |
20170149565 |
Kind Code |
A9 |
Pauker; Matthew J. ; et
al. |
May 25, 2017 |
FORMAT-PRESERVING CRYPTOGRAPHIC SYSTEMS
Abstract
Format-preserving encryption and decryption processes are
provided. The encryption and decryption processes may use a block
cipher. A string that is to be encrypted or decrypted may be
converted to a unique binary value. The block cipher may operate on
the binary value. If the output of the block cipher that is
produced is not representative of a string that is in the same
format as the original string, the block cipher may be applied
again. The block cipher may be repeatedly applied in this way
during format-preserving encryption operations and during
format-preserving decryption operations until a format-compliant
output is produced. Selective access may be provided to portions of
a string that have been encrypted using format-preserving
encryption.
Inventors: |
Pauker; Matthew J.; (San
Francisco, CA) ; Spies; Terence; (Mountain View,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Pauker; Matthew J.
Spies; Terence |
San Francisco
Mountain View |
CA
CA |
US
US |
|
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20130339252 A1 |
December 19, 2013 |
|
|
Family ID: |
49756817 |
Appl. No.: |
13/517513 |
Filed: |
June 13, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12432258 |
Apr 29, 2009 |
8208627 |
|
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13517513 |
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61050160 |
May 2, 2008 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04L 9/3213 20130101;
G06F 21/6209 20130101; H04L 9/0625 20130101; H04L 2209/24 20130101;
H04L 9/088 20130101; H04L 2209/56 20130101; H04L 9/3226 20130101;
H04L 2209/34 20130101; H04L 9/3239 20130101 |
International
Class: |
G06Q 20/40 20120101
G06Q020/40; H04L 9/28 20060101 H04L009/28 |
Claims
1. A method for performing encryption at computing equipment,
comprising: with an encryption engine on computing equipment,
obtaining an unencrypted string in a given format; with the
encryption engine on the computing equipment, encoding the
unencrypted string to produce an encoded value; with the encryption
engine on the computing equipment, applying a block cipher to the
encoded value to produce a block cipher output; after each
application of the block cipher, with the encryption engine on the
computing equipment, determining whether the block cipher output is
representative of a string in the given format; whenever it is
determined that the block cipher output is not representative of a
string in the given format, with the encryption engine on the
computing equipment, applying the block cipher an additional time
to update the block cipher output; and when it is determined that
the block cipher output is representative of a string in the given
format, with the encryption engine on the computing equipment,
processing the block cipher output to produce an encrypted version
of the unencrypted string.
2. The method defined in claim 1, wherein the given format
comprises a payment card number format and wherein obtaining the
unencrypted string comprises obtaining an unencrypted payment card
number.
3. The method defined in claim 2, further comprising: removing a
checksum value from the unencrypted string before the unencrypted
string is encoded; and computing a new checksum value for the
encrypted version of the string.
4. The method defined in claim 3, wherein computing the new
checksum value for the encrypted version of the string comprises
embedding information in the new checksum value.
5. The method defined in claim 1, wherein the given format
comprises a credit card number format and wherein obtaining the
unencrypted string comprises obtaining an unencrypted credit card
number.
6. The method defined in claim 1, wherein the given format
comprises a bank account number format and wherein obtaining the
unencrypted string comprises obtaining an unencrypted bank account
number.
7. The method defined in claim 1 wherein encoding the unencrypted
string comprises converting the string to numeric values and
multiplying the numeric values by coefficients.
8. The method defined in claim 7, wherein a given one of the
coefficients is associated with a given character in the string,
the method further comprising: with the encryption engine on the
computing equipment, computing the given one of the coefficients by
computing a product of a set of numbers each of which represents
how many possible character values are associated with a respective
character in the string prior to the given character.
9. A method for performing decryption at computing equipment,
comprising: with a decryption engine on computing equipment,
obtaining an encrypted string in a given format; with the
decryption engine on the computing equipment, encoding the
encrypted string to produce the encoded value; with the decryption
engine on the computing equipment, applying a block cipher to the
encoded value to produce a block cipher output; after each
application of the block cipher, with the decryption engine on the
computing equipment, determining whether the block cipher output is
representative of a string in the given format; whenever it is
determined that the block cipher output is not representative of a
string in the given format, with the decryption engine on the
computing equipment, applying the block cipher an additional time
to update the block cipher output; and when it is determined that
the block cipher output is representative of a string in the given
format, with the decryption engine on the computing equipment,
processing the block cipher output to produce a decrypted version
of the encrypted string.
10. The method defined in claim 9, wherein the given format
comprises a payment card number format and wherein obtaining the
encrypted string comprises obtaining an encrypted payment card
number.
11. The method defined in claim 1, wherein the given format
comprises a credit card number format and wherein obtaining the
encrypted string comprises obtaining an encrypted credit card
number.
12. The method defined in claim 1, wherein the given format
comprises a bank account number format and wherein obtaining the
encrypted string comprises obtaining an encrypted bank account
number.
13. The method defined in claim 9 wherein encoding the encrypted
string comprises converting the string to numeric values and
multiplying the numeric values by coefficients.
14. A method for using at least first and second cryptographic keys
to provide at least first and second users with selective access to
the contents of a string, comprising: with format-preserving
encryption, encrypting a first plaintext part of the string using
the first cryptographic key to produce first ciphertext that is in
the same format as the first plaintext part while leaving a second
plaintext part of the string unencrypted; and with
format-preserving encryption following encryption of the first
plaintext part of the string, encrypting both the second plaintext
part of the string and the first ciphertext to produce second
ciphertext, wherein the second ciphertext is in the same format as
the string.
15. The method defined in claim 14, further comprising obtaining
the string, wherein the string comprises a payment card number.
16. The method defined in claim 14, further comprising: providing
the first and second keys to the first user.
17. The method defined in claim 14, further comprising: providing
the second key to the second user without providing the first key
to the second user.
18. The method defined in claim 14 further comprising: at the
second user, decrypting the second ciphertext for the second user
to produce the first ciphertext part and the second plaintext
part.
19. The method defined in claim 14 further comprising: at the first
user, decrypting the first ciphertext part to produce the first
plaintext part.
20. The method defined in claim 19 wherein the decrypting the first
ciphertext part comprises decrypting the first ciphertext part
using a block cipher.
Description
[0001] This patent application claims the benefit of provisional
patent application No. 61/050,160, filed May 2, 2008, and patent
application Ser. No. 12/432,258, filed Apr. 29, 2009, which are
hereby incorporated by reference herein in their entireties.
BACKGROUND OF THE INVENTION
[0002] This invention relates to cryptography and more
particularly, to preserving data formats during encryption and
decryption operations.
[0003] Cryptographic systems are used to secure data in a variety
of contexts. For example, encryption algorithms are used to encrypt
sensitive information such as financial account numbers, social
security numbers, and other personal information. By encrypting
sensitive data prior to transmission over a communications network,
the sensitive data is secured, even if it passes over an unsecured
communications channel. Sensitive data is also sometimes encrypted
prior to storage in a database. This helps to prevent unauthorized
access to the sensitive data by an intruder.
[0004] Commonly used encryption algorithms include the Advanced
Encryption Standard (AES) encryption algorithm and the Data
Encryption Standard (DES) encryption algorithm. Using these types
of algorithms, an organization that desires to secure a large
quantity of sensitive information can place the sensitive
information in a data file. The data file can then be encrypted in
its entirety using the AES or DES algorithms.
[0005] Encrypting entire files of data can be an effective
technique for securing large quantities of data. However, bulk
encryption of files can be inefficient and cumbersome because it is
not possible to selectively access a portion of the encrypted data
in an encrypted file. Even if an application only needs to have
access to a portion of the data, the entire file must be decrypted.
Without the ability to selectively decrypt part of a file, it can
be difficult to design a data processing system that provides
different levels of data access for different application programs
and for different personnel.
[0006] To avoid the difficulties associated with encrypting entire
files of sensitive data, it would be desirable to be able to apply
cryptographic techniques such as the AES and DES encryption
algorithms with a finer degree of granularity. For example, it
might be desirable to individually encrypt social security numbers
in a database table, rather than encrypting the entire table. This
would allow software applications that need to access information
in the table that is not sensitive to retrieve the desired
information without decrypting the entire table.
[0007] Conventional encryption techniques can, however,
significantly alter the format of a data item. For example,
encryption of a numeric string such as a credit card number may
produce a string that contains non-numeric characters or a string
with a different number of characters. Because the format of the
string is altered by the encryption process, it may not be possible
to store the encrypted string in the same type of database table
that is used to store unencrypted versions of the string. The
altered format of the encrypted string may therefore disrupt
software applications that need to access the string from a
database. The altered format may also create problems when passing
the encrypted string between applications. Because of these
compatibility problems, organizations may be unable to incorporate
cryptographic capabilities into legacy data processing systems.
[0008] It would therefore be desirable to be able to provide
cryptographic tools that are capable of encrypting and decrypting
data without altering the format of the data.
SUMMARY OF THE INVENTION
[0009] In accordance with the present invention, format-preserving
encryption and decryption algorithms are provided. Using
format-preserving encryption, a plaintext string such as a string
of letters and digits can be encrypted to produce ciphertext
composed of letters and digits in the same format as the original
plaintext string. During format-preserving decryption, ciphertext
can be converted into plaintext in the same format as the
ciphertext.
[0010] The format-preserving encryption and decryption algorithms
may be implemented using a block cipher. During encryption, the
block cipher may be applied to a plaintext string. If the resulting
block cipher output does not correspond to a string that is in the
same format as the original string, the block cipher can be applied
one or more additional times. Once the block cipher output is
format-compliant, the block cipher output can be used to produce
the encrypted string. The block cipher may also be applied in this
fashion during format-preserving decryption operations.
[0011] Strings may be made up of characters. During
format-preserving encryption and decryption operations, the strings
may be converted to numeric values using an index. During these
encoding operations, unique binary values may be produced for each
string using a binary encoding function. The binary values that are
produced may be operated upon using the block cipher.
[0012] Different users (applications) may be provided with
selective access to different portions of a string that has been
encrypted using format-preserving encryption. For example, a string
may have first and second plaintext parts. The first plaintext part
may be encrypted using a first cryptographic key to produce first
ciphertext. By using format-preserving encryption, the first
ciphertext may have the same format as the first plaintext part.
Both the second plaintext part and the first ciphertext may be
encrypted together using format-preserving encryption and a second
cryptographic key to produce second ciphertext that is in the same
format as first ciphertext and the second plaintext part. With this
type of arrangement, the ciphertext has the same format as the
original string that is composed of the first and second plaintext
parts.
[0013] To provide selective access to the different portions of
plaintext, a first user may be provided with the first and second
keys and a second user may be provided with the second key but not
the first key. This allows the second user to access the second
plaintext part but not the first plaintext part and allows the
first user to access both the first and second plaintext parts.
[0014] Further features of the invention, its nature and various
advantages will be more apparent from the accompanying drawings and
the following detailed description of the preferred
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a diagram of an illustrative system environment in
which cryptographic tools with format-preserving encryption and
decryption features may be used in accordance with an embodiment of
the present invention.
[0016] FIG. 2 is a diagram showing how encryption and decryption
engines preserve the format of a string in accordance with an
embodiment of the present invention.
[0017] FIG. 3 is a diagram of an illustrative format-preserving
block cipher that may be used during data encryption and decryption
in accordance with an embodiment of the present invention.
[0018] FIG. 4 is a flow chart of illustrative steps that may be
used in setting up format-preserving encryption and decryption
engines for use in a data processing system of the type shown in
FIG. 1 in accordance with an embodiment of the present
invention.
[0019] FIG. 5 is a flow chart of illustrative steps involved in
using a format-preserving encryption engine to encrypt a data
string in accordance with an embodiment of the present
invention.
[0020] FIG. 6 is a flow chart of illustrative steps involved in
using a format-preserving decryption engine to decrypt a data
string in accordance with an embodiment of the present
invention.
[0021] FIG. 7A is a flow chart of illustrative steps involved in
generating a key that is based on an identifier in accordance with
an embodiment of the present invention.
[0022] FIG. 7B is a flow chart of illustrative steps involved in
generating a key and storing the generated key with an association
between the stored key and an identifier in accordance with an
embodiment of the present invention.
[0023] FIG. 8 is a flow chart of illustrative steps involved in
requesting and obtaining a key from a key server in accordance with
an embodiment of the present invention.
[0024] FIG. 9 is a flow chart of illustrative steps involved in
requesting and obtaining a key from a key server in accordance with
another embodiment of the present invention.
[0025] FIG. 10 is a flow chart of illustrative steps involved in
requesting and obtaining a key from a key server in accordance with
yet another embodiment of the present invention.
[0026] FIG. 11 is a diagram showing how different parts of a data
item can be divided into different plaintext parts and selectively
encrypted in accordance with an embodiment of the present
invention.
[0027] FIG. 12 is a diagram showing how multiple keys may be used
to provide selective access to different parts of a data item in
accordance with an embodiment of the present invention.
[0028] FIG. 13 is a flow chart of illustrative steps involved in
using multiple keys to provide selective access to different part
of a data item in accordance with an embodiment of the present
invention.
[0029] FIG. 14 is a diagram showing how a string may be represented
as a unique binary value in accordance with an embodiment of the
present invention.
[0030] FIG. 15 is a diagram showing how a string format may be
preserved when converting a string to a binary value in accordance
with an embodiment of the present invention.
[0031] FIG. 16 a flow chart of illustrative steps involved in using
a format-preserving encryption engine to encrypt a data string
represented using a unique binary value in accordance with an
embodiment of the present invention.
[0032] FIG. 17 is a flow chart of illustrative steps involved in
using a format-preserving decryption engine to decrypt a data
string represented using a unique binary value in accordance with
an embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0033] An illustrative cryptographic system 10 in accordance with
the present invention is shown in FIG. 1. System 10 includes
computing equipment 12 and communications network 14. The computing
equipment 12 may include one or more personal computers,
workstations, computers configured as servers, mainframe computers,
portable computers, etc. The communications network 14 may be a
local area network or a wide area network such as the internet.
System 10 may be used in processing data for one or more
organizations.
[0034] Computing equipment 12 may be used to support applications
16 and databases 18. In computing equipment 12 in which multiple
applications run on the same computer platform, applications and
databases may communicate with each other directly. If desired,
applications 16 can communicate with each other and with databases
18 remotely using communications network 14. For example, an
application 16 that is run on a computer in one country may access
a database 18 that is located in another country or an application
16 running on one computer may use network 14 to transmit data to
an application 16 that is running on another computer. Applications
16 may be any suitable applications, such as financial services
applications, governmental record management applications, etc.
[0035] The data that is handled by system 10 includes sensitive
items such as individuals' addresses, social security numbers and
other identification numbers, license plate numbers, passport
numbers, financial account numbers such as credit card and bank
account numbers, telephone numbers, email addresses, etc. In some
contexts, information such as individuals' names may be considered
sensitive.
[0036] In a typical scenario, a credit card company maintains a
database 18 of account holders. The database lists each account
holder's name, address, credit card number, and other account
information. Representatives of the credit card company may be
located in many different geographic locations. The representatives
may use various applications 16 to access the database. For
example, a sales associate may retrieve telephone numbers of
account holders to make sales calls using one application, whereas
a customer service representative may retrieve account balance
information using another application. Automated applications such
as error-checking housekeeping applications may also require access
to the database.
[0037] To prevent unauthorized access to sensitive data and to
comply with data privacy regulations and other restrictions,
sensitive data may need to be encrypted. Encryption operations may
be performed before data is passed between applications 16 or
before data is stored in a database 18. Because various
applications may need to access different types of data, the system
10 preferably allows data to be selectively encrypted. As an
example, each of the telephone numbers and each of the credit card
numbers can be individually encrypted using separate cryptographic
keys. With this type of selective encryption arrangement,
applications that require access to telephone numbers need not be
provided with access to credit card numbers and vice versa.
[0038] To support encryption and decryption operations in system 10
applications 16 may be provided with encryption and decryption
engines. For example, an application 16 that accesses a database 18
over a communications network 14 may have an encryption engine for
encrypting sensitive data before it is provided to the database 18
and stored and may have a decryption engine for use in decrypting
encrypted data that has been retrieved from database 18 over
communications network 14. As another example, a first application
may have an encryption engine for encrypting sensitive data before
passing the encrypted data to a second application. The second
application may have a decryption engine for decrypting the
encrypted data that has been received from the first
application.
[0039] Any suitable technique may be used to provide applications
16 with encryption and decryption capabilities. For example, the
encryption and decryption engines may be incorporated into the
software code of the applications 16, may be provided as
stand-alone applications that are invoked from within a calling
application, or may be implemented using a distributed arrangement
in which engine components are distributed across multiple
applications and/or locations.
[0040] Key server 20 may be used to generate and store
cryptographic keys that are used by the encryption and decryption
engines. Key server 20 may include policy information 22 that key
server 20 uses in determining whether to fulfill key requests. As
an example, policy information 22 may include a set of policy rules
that dictate that keys should only be released if they have not
expired and if the key requester's authentication credentials are
valid.
[0041] In a typical scenario, an application requests a key from
key server 22. When requesting the key, the application provides
authentication credentials to the key server 20. The key server 20
provides the authentication credentials to authentication server
24. Authentication server 24 verifies the authentication
credentials and provides the results of the verification operation
to the key server over communications network 14. If the key
requester is successfully authenticated and if the key server
determines that the expiration period has not yet expired, the key
server can satisfy the key request by providing the requested key
to the application over a secure path in network 14 (e.g., over a
secure sockets layer link). Other authentication techniques and key
request arrangements may be used if desired.
[0042] The data handled by the applications 16 and databases 18 of
system 10 is represented digitally. The data includes strings of
characters (i.e., names, addresses, account numbers, etc.). As
shown in FIG. 2, during encryption operations, an encryption engine
26 encrypts unencrypted strings of characters (sometimes referred
to as plaintext) into encrypted strings of characters (sometimes
referred to as ciphertext). During decryption operations, a
decryption engine 28 decrypts encrypted strings of characters to
form unencrypted strings of characters.
[0043] The data strings that are handled in a typical data
processing system have defined formats. For example, an
identification number may be made up of three letters followed by
ten digits. The encryption and decryption engines of the present
invention are able to encrypt and decrypt strings without changing
a string's format (i.e., so that a plaintext identification number
made up of three letters followed by ten digits would be encrypted
to form corresponding ciphertext make up of three letters and ten
digits). The ability to preserve the format of a data string
greatly simplifies system operations and allows systems with legacy
applications to be provided with cryptographic capabilities that
would not be possible using conventional techniques.
[0044] Conventional encryption algorithms can alter the format of a
string during encryption, so that it becomes difficult or
impossible to use the encrypted version of the string. For example,
it may be impossible to store a conventionally-encrypted credit
card number in a database table that has been designed to handle
strings that contain only digits.
[0045] In accordance with the present invention, data stings can be
encrypted and decrypted while preserving the format of the strings.
Consider, as an example, the encryption and decryption of credit
card numbers. Credit card numbers generally have between 13 and 18
digits. The format for a particular valid credit card number might
require that the credit card number have 16 digits. This type of
credit card number will be described as an example.
[0046] In a 16-digit credit card number, the digits are typically
organized in four groups of four each, separated by three spaces.
During a format-preserving encryption operation, an unencrypted
credit card number such as "4408 0412 3456 7890" may be transformed
into credit-card-formatted ciphertext such as "4417 1234 5678 9114"
and during decryption, the ciphertext "4417 1234 5678 9114" may be
transformed back into the unencrypted credit card number "4408 0412
3456 7890".
[0047] The value of a valid sixteenth digit in a credit card number
is formed by performing a checksum operation on the first 15 digits
using the so-called Luhn algorithm. Any single-digit error in the
credit card number and most adjacent digit transpositions in the
credit card number will alter the checksum value, so that data
entry errors can be identified.
[0048] During encryption operations, the encryption engine 26 can
compute a new checksum value using the first 15 digits of the
ciphertext. The new checksum digit can be used in the ciphertext
or, if desired, policy information such as a validity period may be
embedded within the checksum digit by adding an appropriate
validity period index value to the new checksum value. When a
validity period is embedded within a checksum digit, the resulting
modified checksum value will generally no longer represent a valid
checksum for the string. However, applications in system 10 will be
able to retrieve the validity period information from the checksum
digit and will be able to use the extracted validity period
information in obtaining a decryption key from key server 20 (FIG.
1).
[0049] This type of embedding operation may be used to store any
suitable information within encrypted data. The use of credit card
numbers and, more particularly, the use of validity period
information that has been embedded within the checksum digits of
credit card numbers are described herein as examples.
[0050] Because encryption and decryption engines 26 and 28 of FIG.
2 can preserve a desired format for a string during encryption and
decryption operations, sensitive data can be secured without
requiring entire files to be encrypted.
[0051] The encryption and decryption engines 26 and 28 preferably
use index mappings to relate possible character values in a given
string position to corresponding index values in an index. By
mapping string characters to and from a corresponding index, the
encryption and decryption engines 26 and 28 are able to perform
encryption and decryption while preserving string formatting.
[0052] In a typical scenario, an index mapping may be formed using
a table having two columns and a number of rows. The first column
of the mapping corresponds to the potential character values in a
given string position (i.e., the range of legal values for
characters in that position). The second column of the mapping
corresponds to an associated index. Each row in the mapping defines
an association between a character value and a corresponding index
value.
[0053] Consider, as an example, a situation in which the string
being encrypted has first, fifth, sixth, and seventh string
characters that are digits and second, third, and fourth characters
that are uppercase letters. In this situation, the possible
character values in the first, fifth, sixth, and seventh character
positions within the plaintext version of the string might range
from 0 to 9 (i.e., the first character in the string may be any
digit from 0 through 9, the fifth character in the string may be
any digit from 0 to 9, etc.). The possible character values in the
second, third, and fourth positions in the string range from A to Z
(i.e., the second character in the unencrypted version of the
string may be any uppercase letter in the alphabet from A to Z, the
third character in the unencrypted version of the string may be any
uppercase letter from A through Z, etc.).
[0054] The index mapping in this type of situation may map the ten
possible digit values for the first, fifth, sixth, and seventh
string characters into ten corresponding index values (0 . . . 9).
For the second, third, and fourth character positions, 26 possible
uppercase letter values (A . . . Z) may be mapped to 26
corresponding index values (0 . . . 25).
[0055] In a typical string, not all characters have the same range
of potential character values. If there are two ranges of potential
character values, two index mappings may be used, each of which
maps a different set of possible character values to a different
set of index values. If there are three ranges of potential
character values within the string, three index mappings may be
used. For example, a first index mapping may relate a digit
character to a first index, a second index mapping may relate a
uppercase letter character to a second index, and a third index
mapping may relate an alphanumeric character to a third index. In
strings that contain a larger number of different character types,
more index mappings may be used.
[0056] In general, a string contains a number of characters N. The
potential character values in the string are related to
corresponding index values using index mappings. An index mapping
is created for each character. The indexes used to represent each
character may have any suitable size. For example, an index
containing 52 index values may be associated with string characters
with character values that span both the uppercase and lowercase
letters. Because not all of the characters typically have the same
range of potential character values, there are generally at least
two different index mappings used to map character values in the
string to corresponding index values. In a string with N
characters, N index mappings are used, up to N of which may be
different index mappings.
[0057] Any suitable cryptographic formulation may be used for the
format-preserving encryption and decryption engines 26 and 28,
provided that the cryptographic strength of the encryption
algorithm is sufficiently strong. With one suitable approach,
encryption engine 26 and decryption engine 28 use a cryptographic
algorithm based on the well known Luby-Rackoff construction. The
Luby-Rackoff construction is a method of using pseudo-random
functions to produce a pseudo-random permutation (also sometimes
referred to as a block cipher). A diagram showing how encryption
engine 26 and decryption engine 28 may be implemented using the
Luby-Rackoff construction is shown in FIG. 3.
[0058] During encryption operations, an unencrypted string is
divided into two portions. The unencrypted string may be divided
into two portions using any suitable scheme. For example, the
string may be divided into odd and even portions by selecting
alternating characters from the string for the odd portion and for
the even portion. With another suitable approach, the unencrypted
string is divided into two portions by splitting the string into
left and right halves.
[0059] In FIG. 3, the first half of the unencrypted string is
labeled "L.sub.1" and the second half of the unencrypted string is
labeled "R.sub.1". During encryption operations with encryption
engine 26, the unencrypted string halves L.sub.1 and R.sub.1 are
processed to form corresponding encrypted string halves L.sub.3 and
R.sub.2. During decryption operations with decryption engine 28,
processing flows from the bottom of FIG. 3 towards the top, so that
encrypted string halves L.sub.3 and R.sub.2 are decrypted to
produce unencrypted halves L.sub.1 and R.sub.1. Processing occurs
in three rounds 40, 42, and 44. During encryption, the operations
of round 40 are performed first, the operations of round 42 are
performed second, and the operations of round 44 are performed
third. During decryption, the operations of round 44 are performed
first, the operations of round 42 are performed second, and the
operations of round 40 are performed third.
[0060] As indicated by dots 51 in FIG. 3, the operations of FIG. 3
may, if desired, be implemented using four or more rounds. For
example, eight rounds of a block cipher may be performed.
[0061] The block cipher structure of FIG. 3 encrypts (or decrypts)
a string of a particular known size to produce an output string of
the same size. During encryption, plaintext is converted to
ciphertext (i.e., the block cipher of FIG. 3 is operated from top
to bottom). During decryption, ciphertext is converted to plaintext
(i.e., the block cipher of FIG. 3 is operated from bottom to
top).
[0062] The block cipher uses a subkey generation algorithm 38. The
subkey generation algorithm 38 has three inputs: a key K, a
constant C (C.sub.1 for round 40, C.sub.2 for round 42, and C.sub.3
for round 44), and a string S (S.sub.1=R.sub.1 for round 40,
S.sub.2=L.sub.2 for round 42, and S.sub.3=R.sub.2 for round
44).
[0063] The subkey generation algorithm 38 may be a function H' that
is based on a cryptographic hash function H and that takes as an
input S, C, and K. With one suitable approach, the subkey
generation algorithm H' is given by equation 1.
H'=H(S|C|K) (1)
In equation 1, the symbol "|" represents the concatenation
function. The cryptographic hash function H is preferably chosen so
that the subkey generation algorithm has a suitable cryptographic
strength. Illustrative cryptographic hash functions that can be
used for hash function H include the SHA1 hash function and the AES
algorithm used as a hash function.
[0064] The value of the key K is the same for rounds 40, 42, and
44. The value of the constant C is different for each round. With
one suitable arrangement, the constant C.sub.1 that is used in
round 40 is equal to 1, the constant C.sub.2 that is used in round
42 is 2, and the constant C.sub.3 that is used in round 44 is 3.
The value of S varies in each round. In round 40, S.sub.1 is equal
to the first half of the unencrypted string R.sub.1. In round 42,
S.sub.2 is equal to the L.sub.2. In round 44, S.sub.3 is equal to
R.sub.2.
[0065] In round 40, the output of the subkey generation algorithm
is subkey SK1, as shown in equation 2.
SK1=H(S.sub.1|C.sub.1|K) (2)
In round 42, the output of the subkey generation algorithm is
subkey SK2, as shown in equation 3.
SK2=H(S.sub.2|C.sub.2|K) (3)
In round 44, the output of the subkey generation algorithm is
subkey SK3, as shown in equation 4.
SK3=H(S.sub.3|C.sub.3|K) (4)
[0066] Equations 1-4 involve the use of a cryptographic hash
function for the subkey generation algorithm. If desired, the
subkey generation algorithm may be implemented using a
cryptographic message authentication code (MAC) function. A
cryptographic message authentication code function is a keyed hash
function. Using a cryptographic message authentication code
function, equation 1 would become H'=MACF(S|C,K), where MACF is the
message authentication code function. An example of a message
authentication code function is CMAC (cipher-based MAC), which is a
block-cipher-based message authentication code function. The
cryptographic message authentication code function AES-CMAC is a
CMAC function based on the 128-bit advanced encryption standard
(AES).
[0067] A format-preserving combining operation (labeled "+" in FIG.
3) is used to combine the subkeys SK1, SK2, and SK3 with respective
string portions.
[0068] During encryption operations, format-preserving combining
operation 46 combines SK1 with string L.sub.1 to produce string
L.sub.2. During decryption operations, format-preserving combining
operation 46 combines SK1 with string L.sub.2 to produce string
L.sub.1. Format-preserving combining operation 48 combines SK2 with
string R.sub.1 to produce string R.sub.2 during encryption
operations and combines SK2 with string R.sub.2 to produce string
R.sub.1 during decryption operations. Format-preserving combining
operation 50 is used to process subkey SK3. During encryption,
format-preserving combining operation 50 combines SK3 with string
L.sub.2 to produce string L.sub.3. During decryption,
format-preserving combining operation 50 combines SK3 with string
L.sub.3 to produce string L.sub.2.
[0069] The format-preserving combining operation + preserves the
format of the strings L.sub.1, L.sub.2, L.sub.3, R.sub.1, and
R.sub.2 as they are combined with the subkeys SK1, SK2, and SK3.
For example, the string L.sub.2 that is produced by combining
string L.sub.1 and subkey SK1 has the same format as the string
L.sub.1.
[0070] The format-preserving combining operation + may be based on
any suitable mathematical combining operation. For example, the
function + may be addition mod x or the function + may be
multiplication mod x, where x is an integer of an appropriate size
(i.e., x=y.sup.z, where z is equal to the length of the string S,
and where y is equal to the number of possible character values for
each character in the string S). If, as an example, the string S
contains 16 digits (each digit having one of 10 possible values
from 0 to 9), x would be 10.sup.16. If the string S contains three
uppercase letters (each uppercase letter having one of 26 possible
values from A to Z), x would be 26.sup.3. These are merely
illustrative examples. The format-preserving combining function +
may be any reversible logical or arithmetic operation that
preserves the format of its string input when combined with the
subkey.
[0071] Illustrative steps involved in setting up the encryption
engine 26 and decryption engine 28 are shown in FIG. 4. At step 52,
the desired formatting to be used for the encrypted and decrypted
strings is defined.
[0072] For example, unencrypted strings may be social security
numbers that follow the format ddd-dd-dddd, where d is a digit from
0 to 9. The encryption engine 26 may produce corresponding
encrypted strings with the identical format.
[0073] As another example, the string format may be dddd dddd dddd
dddc, where d is a digit from 0 to 9 and where c is a checksum
digit (a digit from 0 to 9). The block cipher may be applied to the
leading 15 digits of the credit card number and a checksum value
may be recomputed from the encrypted version of the leading 15
digits using the Luhn algorithm. Validity period information may be
embedded into the checksum digit by adding a validity period index
to the recomputed checksum value. The index may, as an example,
specify that an index value of 1 corresponds to the year 2006, an
index value of 2 corresponds to the year 2007, an index value of 3
corresponds to the year 2008, etc. If the recomputed checksum is 3
(as an example), and the validity period for the encryption
operation is 2006, the index value of 1 (corresponding to year
2006) may be added to the checksum value of 3 to produce a checksum
digit of 4 for the ciphertext. In this situation, the final version
of the encrypted string has the form dddd dddd dddd dddc, where the
value of c is 4. The overall encryption process implemented by the
encryption engine 26 maintains the digit format of the string,
because both the unencrypted and encrypted versions of the string
contain 16 digits.
[0074] The inclusion of additional constraints on the format of the
encrypted string may be necessary to ensure that the encrypted
strings are fully compliant with legacy applications. During step
52, a user decides which of these ancillary constraints are to be
included in the definition of the required format for the
string.
[0075] At step 54, for each character in the string, an index
mapping is created by defining a set of legal character values and
a corresponding index of sequential values that is associated with
the legal characters values. For example, if the legal characters
for a particular character position in a string include the 10
digits (0.9) and the 26 lowercase letters (a . . . z), a suitable
indexing scheme associates digits 0 through 9 with index values 1
through 10 and associates letters a through z with index values
11-36. In this index mapping, the index values that are created are
all adjacent. Because there are no gaps in the indices, index value
10 is adjacent to index value 11 (in the present example). If the
string contains more than one type of character, there will be more
than one index mapping associated with the characters in the
string.
[0076] At step 56, a value for key K is obtained. The value of K
may be obtained, for example, by generating K from a root secret
and other information using a key generation algorithm in key
server 20.
[0077] At step 58, the format-preserving combining operation "+" is
defined. As described in connection with FIG. 3, the
format-preserving combining operation may be addition modulo x,
multiplication modulo x, or any other suitable logical or
arithmetic operation that preserves the format of the string when
combining the string with a subkey and that is reversible.
[0078] At step 60, a block cipher structure is selected for the
encryption engine 26 and decryption engine 28. The block cipher
structure may, for example, by a Luby-Rackoff construction of the
type described in connection with FIG. 3. Other suitable block
cipher structures may be used if desired.
[0079] At step 62, a subkey generation algorithm is selected.
Suitable subkey generation algorithms include those based on
cryptographic hash functions such the SHA1 hash function and AES
algorithm used as a hash function. Suitable subkey generation
algorithms also include those built on cryptographic message
authentication code functions such as AES-CMAC.
[0080] After performing the setup steps of FIG. 4, the encryption
engine 26 and decryption engine 28 can be implemented in system 10
and sensitive data can be secured.
[0081] Illustrative steps involved in using the encryption engine
26 and decryption engine 28 when processing strings of data in
system 10 are shown in FIGS. 5 and 6. As described in connection
with FIGS. 1 and 2, the encryption engine 26 and decryption engine
28 may be called by an application or may be part of an application
16 that is running on data processing system 10. The data strings
that are encrypted and decrypted may be strings that are retrieved
from and stored in fields in a database 18 or may be strings that
are passed between applications 16 (e.g., applications 16 that are
running on the same computing equipment 12 or that are
communicating remotely over a communications network 14).
[0082] The flow chart of FIG. 5 shows steps involved in encrypting
a data string.
[0083] As shown in FIG. 5, the data string is preprocessed at step
64, encrypted at step 72, and postprocessed at step 74.
[0084] At step 66, the encryption engine obtains the unencrypted
string. The string may be retrieved from a database 18 or received
from an application 16.
[0085] At step 68, the string is processed to identify relevant
characters. During step 68, dashes spaces, checksums, and other
undesired characters can be removed from the string and the
relevant characters in the string can be retained.
[0086] For example, if the string is a social security number that
contains nine digits separated by two dashes, the string can be
processed to remove the dashes. Although the dashes could be left
in the string, there is no purpose in encrypting a dash character
in the unencrypted string to produce a corresponding dash character
in the encrypted string (as would be required to preserve the
format of the entire string).
[0087] As another example, if the string being processed is a
credit card number containing 16 digits and three spaces, the
spaces can be removed. The checksum portion of the 16 digit credit
card can be ignored by extracting the 15 leading digits of the
credit card number as the relevant characters to be processed
further.
[0088] At step 70, the encryption engine 26 uses the index mappings
that were created during step 54 of FIG. 4 to convert the processed
string (i.e., the string from which the irrelevant characters have
been removed) into an encoded unencrypted string. For example,
consider a license plate number in which the first, fifth, sixth,
and seventh character positions contain digits (i.e., numbers from
0 through 9) and the second, third, and fourth character positions
contain uppercase letters. An index mapping may be used to convert
the character values in the first, fifth, sixth, and seventh
character positions into corresponding index values ranging from 0
through 9. Another index mapping may be used to convert the
character values in the second, third, and fourth character
positions into corresponding index values ranging from 0 through
25. The index values used in each index mapping may be sequential.
Once the characters have been encoded using the sequential index
values, processing can continue at step 72.
[0089] At step 72, the encryption engine 26 encrypts the encoded
string using the format-preserving block cipher that was
established during the operations of FIG. 4. For example, the
encryption engine 26 can perform the Luby-Rackoff encryption
operations described in connection with FIG. 3. During step 72, the
subkey generation algorithm that was selected at step 62 of FIG. 4
and the format-preserving combining algorithm + that was defined at
step 58 of FIG. 4 are used to transform the unencrypted encoded
string into an encrypted encoded string.
[0090] At step 76, the same index mappings that were used during
the encoding operations of step 70 are used to convert the index
values of the encrypted string back into characters (i.e.,
characters in the legal set of character values that were defined
for each character position at step 54). Decoding the encoded
version of the string using the index mappings returns the string
to its original character set.
[0091] At step 78, the decoded encrypted string is processed to
restore elements such as dashes and spaces that were removed at
step 68. When replacing a checksum value, a new valid checksum
value can be computed from the encrypted version of the string and
validity period information or other suitable information can be
embedded within the checksum digit (e.g., by adding a validity
period index to the new valid checksum value to produce a checksum
digit for the decoded encrypted string). The decoded encrypted
string is ciphertext that corresponds to the plaintext unencrypted
string that was obtained at step 66. If desired, the entire string
can be encrypted. With this type of arrangement, the checksum
removal operation of step 68 and the checksum digit computation
operation of step 78 can be omitted.
[0092] By processing the string at step 78, the extraneous elements
of the string that were removed at step 68 are inserted back into
the string. Because the extraneous elements are reinserted into the
string and because a format-preserving block cipher was used in
step 72, the encrypted string that is produced will have the same
format as the original unencrypted string. This allows the
encrypted string to be used by applications 16 and databases 18
that require that the original string's format be used.
[0093] At step 80, the encrypted string is provided to an
application 16 or database 18. Legacy applications and databases
that require a specific string format may be able to accept the
encrypted string.
[0094] Illustrative steps involved in using decryption engine 28 to
decrypt a string that has been encrypted using the process of FIG.
5 are shown in FIG. 6. The decryption engine 28 may be invoked by
an application 16 or may be part of an application 16 that is
running on data processing system 10. The data string that is being
decrypted in the process of FIG. 6 may be an encrypted string that
has been retrieved from a database 18 or may be a string that has
been retrieved from an application.
[0095] As shown in FIG. 6, the encrypted data string is
preprocessed at step 82, is decrypted at step 90, and postprocessed
at step 92.
[0096] At step 84, the decryption engine obtains the encrypted
string. The encrypted string may be retrieved from a database 18 or
received from an application 16.
[0097] At step 86, the encrypted string is processed to identify
relevant characters. During step 86, dashes spaces, checksums, and
other extraneous elements can be removed from the string. The
relevant characters in the string are retained. The process of
removing extraneous characters during step 86 is the same as that
used during the processing of the unencrypted string that was
performed during step 68 of FIG. 5.
[0098] If the string being decrypted is a social security number
that contains nine digits separated by two dashes, the encrypted
string can be processed to remove the dashes.
[0099] As another example, if the string being processed during
step 86 is a credit card number containing 16 digits and three
spaces, the spaces can be removed prior to decryption. The checksum
digit of the 16 digit credit card can be ignored by extracting the
15 leading digits of the encrypted credit card number as the
relevant characters to be decrypted. If information is embedded in
the checksum digit (e.g., validity period information), the
checksum digit may be processed to extract this information during
step 86.
[0100] At step 88, the decryption engine 26 uses the index mappings
that were defined at step 54 of FIG. 4 and that were used during
the encryption operations of FIG. 5 to convert each of the
characters of the processed encrypted string (i.e., the encrypted
string from which the extraneous characters have been removed) into
an encoded encrypted string. If, as an example, the legal set of
characters associated with the first character of the encrypted
string is defined as the set of 10 digits, a 10 digit index may be
used to encode the first character of the encrypted string. If the
legal set of characters associated with the second character of the
encrypted string is defined as the set of 26 uppercase letters, a
26-digit index may be used to encode the second character of the
encrypted string. During step 88, each character of the string is
converted to a corresponding index value using an appropriate index
mapping.
[0101] At step 90, the encoded version of the encrypted string is
decrypted. The decryption engine 28 decrypts the string using the
format-preserving block cipher that was established during the
operations of FIG. 4. For example, the decryption engine 26 can
perform the Luby-Rackoff decryption operations described in
connection with FIG. 3. During step 90, the subkey generation
algorithm that was selected at step 62 of FIG. 4 and the
format-preserving combining algorithm + that was defined at step 58
of FIG. 4 are used to transform the encrypted encoded string into a
decrypted encoded string.
[0102] At step 94, the index mappings that were used during the
encoding operations of step 88 are used to convert the index values
of the decrypted string back into their associated characters
(i.e., characters in the legal set of character values that were
defined for each character position at step 54). This returns the
decrypted string to its original character set. In strings that
contain more than one different type of character, multiple
different index mappings are used.
[0103] At step 96, the decoded decrypted string is processed to
restore elements such as dashes, spaces, and checksum values that
were removed at step 88. When replacing a checksum value, a new
valid checksum value may be computed from the decrypted version of
the string. This ensures that the decrypted version of the string
will be returned to its original valid state.
[0104] During the string processing operations of step 96, the
extraneous elements of the string that were removed at step 88 are
inserted back into the string. This restores the string to its
original unencrypted state (i.e., the state of the string when
obtained at step 66 of FIG. 5).
[0105] At step 98, the decrypted string is provided to an
application 16 or database 18.
[0106] By incorporating format-preserving encryption and decryption
engines 26 and 28 into data processing system 10, legacy
applications and databases and other applications and databases can
be provided with cryptographic capabilities without disrupting
their normal operation.
[0107] The key K that is used by encryption and decryption engines
26 and 28 may be produced using any suitable technique. For
example, key K may be supplied to key server 20 manually and may be
distributed to encryption and decryption engines 26 and 28 in
satisfaction of valid key requests. With one particularly suitable
arrangement, key K is derived mathematically from a secret. The
secret, which is sometimes referred to as a root secret, may be
maintained at key server 20. The root secret may be supplied to key
server 20 manually or may be produced using a pseudo-random number
generator.
[0108] To ensure that keys are only distributed to authorized
applications 16, it may be advantageous to mathematically compute
each key K from policy information 22
[0109] (FIG. 1). As an example, key K may be computed by key server
20 using equation 5.
K=f(RSECRET,IDEN) (5)
In equation 5, the parameter IDEN is an identifier, the parameter
RSECRET is a root secret, and the function f is a one-way function
such as a hash function. An example of a hash function that may be
used for function f is the SHA1 hash function. If desired, other
hash functions and one-way functions may be used for function
f.
[0110] The identifier IDEN may include information that identifies
an individual, a group, a policy, or an application. As an example,
the identifier may be based on the name of an individual, the name
of an organization, the name of a group, or any other suitable user
name. The identifier may also be based on the name of a policy
(e.g., "PCI" indicating that cryptographic operations should be
performed in accordance with payment card industry standards) or
may be based on the name of an application. When an application
requests key K from key server 20, the key server 20 may use all or
part of the value of IDEN in determining whether the key requester
is authorized to receive K. If the key requester is authorized, the
function of equation 5 may be used to generate K.
[0111] To support version-based functions in system 10, it may be
desirable to allow identities and their associated keys K to
expire. Identity and key expiration may be implemented by requiring
that a validity period be included in each identity IDEN. The
validity period indicates the dates on which the key K is valid.
Validity periods can be expressed in terms of absolute dates,
abbreviated dates, version numbers that relate to valid date ranges
or key versions, etc.
[0112] One suitable format for the validity period is an expiration
date. For example, a validity period for IDEN may be made up of a
year of expiration (e.g., 2007), may be made up of a week of
expiration (e.g., week number 45), may be made up of a month and
year of expiration (e.g., 03/2007 or 03/07), etc. Validity periods
may also be constructed using a date range (e.g., 2006-2007) during
which key K is valid. With one suitable arrangement for use when
encrypting and decrypting credit cards, the validity period in an
identity IDEN may be a credit card expiration date (e.g.,
05/08).
[0113] The credit card expiration date or other such information
(e.g., a record locator, cardholder name, etc.) may be combined
with information that labels the identity IDEN as being associated
with credit cards and the payment card industry (PCI). The value of
IDEN might be formed, for example, by combining the strings "Joe
Smith" (the name of a holder of a credit card), "PCI" (indicating
the payment card industry), and a credit card expiration date to
form (as an example) a value for IDEN of "JOE_SMITH_PCI_05/08."
[0114] Illustrative steps involved in forming a key K using
equation 5 are shown in FIG. 7A.
[0115] At step 100, key server 20 obtains the parameter RSECRET
(e.g., using a pseudorandom number generator operating at key
server 20, by retrieving RSECRET from a cache at key server 20,
etc.).
[0116] At step 102, the key server 20 obtains the parameter IDEN.
The parameter IDEN may be provided to key server 20 as part of a
key request (e.g., in a single transmission requesting a key or in
a series of related transmissions requesting a key). Information
such as a user identity (e.g., a username or part of a username, a
group identity, etc.), validity period (e.g., an expiration date, a
valid date range, a version number, or a combination of such
validity period information), and industry/key type (e.g., "PCI"
for the payment card industry) may be included in the value of the
IDEN string. If desired, components of the IDEN string may be
represented using multiple strings or additional information may be
included in the IDEN string.
[0117] At step 104, key server 20 may use function f of equation 5
(e.g., a SHA1 hash function or other one-way function) to compute K
from the known values of the root secret RSECRET and the identifier
IDEN.
[0118] Keys may be generated using the operations of FIG. 7A at any
suitable time. For example, key server 20 may generate a key K
whenever a valid key request is received. If desired, key server 20
may maintain a key cache in which previously generated keys are
stored. Use of a key cache may reduce the processing burden on key
server 20.
[0119] A flow chart of illustrative steps involved in generating
key K using an approach in which generated key K is persistently
stored is shown in FIG. 7B. With the approach of FIG. 7B, key K is
generated randomly at step 105. For example, key K may be generated
using a pseudorandom number generator at key server 20 when a key
is requested in a key request containing an identifier IDEN.
[0120] At step 107, the key K is stored in persistent storage
(e.g., a key cache maintained at key server 20). Key server 20 also
stores an association between the key K that has been generated and
the value of identifier IDEN from the key request. The association
may be provided by making an entry in a database that contains the
key and the related identifier IDEN (as an example). At a later
time, when key K is requested, key server 20 can retrieve the
correct key K from storage to satisfy the request using the value
of the identifier IDEN that is provided in the key (step 109). The
approach of FIG. 7B therefore allows the key generator 20 to obtain
the key K by generating the key K randomly (if no key value has
been cached) or by retrieving a previously stored version of key K
using the identity value IDEN.
[0121] Key server 20 also preferably maintains policy information
22 (FIG. 1). Policy information 22 includes policy rules that may
be used in determining which key requests should be granted and
which key requests should be denied. An example of a policy rule is
a rule that requires that a key requester authenticate successfully
as part of a PCI LDAP (Lightweight Directory Access Protocol) group
whenever the parameter IDEN includes the industry type "PCI." As
another example, a policy rule might specify that key requests
should only be satisfied if made at date that falls within the
validity period specified in the IDEN parameter. Key server 20 may
maintain a clock or may otherwise obtain trustworthy external
information on the current date. External information such as this
may be used by key server 20 in evaluating whether the policy rules
have been satisfied for a particular key request. In a typical
scenario, the policy rules at key server 20 will specify multiple
criteria that must be satisfied (e.g., proper authentication of a
given type must be performed, a validity period restriction must be
satisfied, etc.).
[0122] In some situations, authentication server 24 is used in
authenticating key requesters. In other situations, key server 20
may perform authentication. Key requests may be made by encryption
engine 26 when a copy of a key K is needed to perform an encryption
operation or by decryption engine 28 when a copy of key K is needed
to perform a decryption operation. In general, any suitable
technique may be used to process key requests. Flow charts
presenting three illustrative ways in which key requests for key K
may be handled in system 10 are shown in FIGS. 8, 9, and 10.
[0123] In the example of FIG. 8, an encryption engine or decryption
engine associated with an application 16 makes a key request to key
server 20 at step 106. Key requests such as the key request of step
106 may be made in a single transmission over network 14 between
the computing equipment 12 on which the requesting application
resides or may be made in multiple associated transmissions. The
key request may include authentication credentials and an
identifier such as the identifier parameter IDEN described in
connection with FIGS. 7A and 7B. The identifier that is associated
with the key request may include information such as a validity
period (e.g., a credit card expiration date), user name, etc.
Different types of keys may require different levels of
authentication. The authentication credentials that are provided as
part of the key request are preferably provided in a form that is
suitable for the type of key being requested. One example of
authentication credentials is a userID and password. Biometric
authentication credentials may also be used (as an example). At
step 108, key server 20 forwards the authentication credentials
that have been received from the key requester to authentication
server 24 over communications network 14.
[0124] At step 110, authentication server 24 verifies the
authentication credentials. For example, if the authentication
credentials include a userID and password, authentication server 24
may compare the userID and password to a list of stored valid
userIDs and passwords.
[0125] If the authentication server 24 determines that the
authentication credentials are not valid, the authentication
process fails. A suitable response to this failure may be generated
at step 112. For example, authentication server 24 can notify key
server 20 that the authentication credentials are not valid and can
generate suitable alert messages for entities in system 10. Other
suitable actions include generating an error message that prompts
key server 20 and/or the key requester to resubmit the credentials
(e.g., to avoid the possibility that the authentication failure was
due to mistyped authentication credentials).
[0126] If the authentication server 24 determines that the
authentication credentials are valid, the authentication server 24
notifies the key server 20 accordingly. In a typical scenario, the
authentication server provides the key server 20 with an
"assertion" indicating that the credentials are valid. The
assertion may include information on group membership and roles and
rights for the authenticated party.
[0127] At step 114, key server 20 applies policy rules 22 to the
key request. Information such as the identity information IDEN, the
authentication results from authentication server 24 (e.g., the
assertion), and external information such as the current date may
be used by the key server 20 in enforcing the policy rules.
[0128] As an example, identity information, authentication results,
and external information may be used in determining which policy
rules should be applied. Certain policy rules may be applied when
IDEN indicates that the key requester is making a "PCI" key
request. Such rules may, as an example, require a particular level
of authentication. Certain policy rules may also be applied when a
key request is made on particular times and dates (e.g., more
stringent authentication may be required for evening and weekend
key requests). Certain policy rules may apply to particular groups
of users, etc.
[0129] In addition to determining which policy rules should be
applied, key server 20 may also use identity information,
authentication results, and external information in determining
whether the applicable policy rules have been satisfied. For
example, during step 114, key server 20 may determine whether the
key request includes valid validity period information (e.g.,
whether an expiration period has expired). Key server 20 may also
check to make sure that appropriate valid authentication results
have been received from authentication server 24, may check the key
requester's membership in a directory group, etc.
[0130] If the criteria set forth in the applicable policy rules are
not satisfied, the key request fails and appropriate error
notifications may be generated or other actions may be taken at
step 116.
[0131] If the applicable policy rules are satisfied, key server 20
may generate a key K to satisfy the key request at step 118. The
key K may be generated using operations of the type shown in FIG.
7A or may be generated or retrieved using operations of the type
shown in FIG. 7B. The key K may then be supplied to the key
requester over a secure path in communications network 14.
[0132] In this example, key server 20 applies the applicable policy
rules to the key request following successful verification of the
authentication credentials by authentication server 24. If desired,
the policy rules can be applied between steps 106 and 108. In this
type of scenario, the key server need not submit the authentication
credentials to the authentication server if the policy rules are
not satisfied (e.g., if validity period information indicates that
an expiration date has passed).
[0133] Another illustrative technique that may be used by an
encryption engine or decryption engine associated with an
application to obtain key K is shown in FIG. 9. With this
technique, authentication is performed using authentication server
24 before the key request is made to key server 20.
[0134] At step 120, an application 16 that desires a key K provides
authentication credentials to authentication server 24 for
verification. If desired, the application may also provide an
identifier (e.g., parameter IDEN) to authentication server 24,
which may use this information to determine what type of assertion
to provide to the application following successful verification of
the authentication credentials.
[0135] At step 122, authentication server 24 verifies the
authentication credentials. If the authentication credentials are
not valid, an appropriate response may be made at step 124 (e.g.,
by providing the application with another chance to provide valid
credentials, by issuing an alert, etc.).
[0136] If the authentication credentials are determined to be
valid, the authentication server provides the application with an
assertion over communications network 14. The assertion may be, for
example, a Kerberos ticket.
[0137] At step 126, the application uses the assertion that has
been received from the authentication server in making a key
request to key server 20. The key request may include the assertion
from authentication server 24 and an identifier (e.g., parameter
IDEN).
[0138] At step 128, the key server applies policy rules 22 to the
key request to determine whether the key request should be
satisfied. Key server 20 may use identity information (e.g.,
parameter IDEN, which may include a validity period),
authentication results (e.g., the assertion), and external
information (e.g., the current date) in determining which policy
rules should be applied to the key request. The key server may also
use this information in determining whether the applicable policy
rules have been satisfied. As an example, key server 20 may
determine whether the key request includes valid validity period
information during step 128 and may check to determine whether the
assertion is valid and sufficient to satisfy the policy rules.
[0139] If the applicable policy rules are not satisfied, the key
server 20 may request that the application issue a new request or
may take other suitable actions in response to the failure (step
130).
[0140] If the key server determines that the applicable key access
policy rules have been satisfied, the key server may retrieve key K
from cache or may generate an appropriate key K, as discussed in
connection with FIGS. 7A and 7B. At step 132, the key K may be
provided from key server 20 to the requesting application over
communications network 14.
[0141] With the approach of FIG. 10, authentication operations are
performed by key server 20, so authentication server 24 need not be
used.
[0142] At step 134, an application that needs key K makes a key
request to key server 20. The key request may include an identifier
(e.g., parameter IDEN) and shared secret information. The shared
secret information may be, for example, a shared secret (i.e., a
secret known by the application and by the key server) or shared
secret information that is derived from the shared secret (e.g., by
hashing the shared secret with an identifier such as parameter
IDEN).
[0143] At step 136, the key server verifies the shared secret
information. The key server may, as an example, compare the shared
secret information from the key request to previously generated and
stored shared secret information or to shared secret information
that is generated in real time based on the received identity
(e.g., IDEN). If the shared secret information is valid, the key
server can determine which key access policy rules are to be
applied to the key request (e.g., using external information such
as the current date, using identity information IDEN, etc.). After
determining which policy rules to use, key server 20 applies the
appropriate policy rules to the key request.
[0144] If the criteria set forth in the policy rules are not
satisfied, the key request fails and appropriate actions can be
taken at step 138.
[0145] If the policy rules are satisfied, the key server can
retrieve key K from cache or may generate key K in real time (e.g.,
using the operations of FIGS. 7A and 7B). The requested key may
then be provided to the key requester over communications network
12 (step 140).
[0146] One of the potential advantages of using key server 20 is
that it helps to avoid problems that might otherwise arise when
storing keys in local cache on computing equipment 12. If keys are
only maintained in local storage, it may be difficult to recreate a
key when needed to resurrect a server that has crashed. By using
key server 20, keys can be regenerated as needed at the key
server.
[0147] Systems such as system 10 of FIG. 1 may use validity periods
to control when keys are valid. A first application may encrypt
plaintext using a cryptographic key that is based on a given
validity period. The resulting ciphertext may then be stored in a
database and retrieved by a second application or may be provided
directly to the second application over network 14. The second
application must obtain a copy of key K to decrypt the ciphertext.
The key K must be generated using the given validity period. If an
incorrect validity period is used in generating K, the value of K
will be incorrect and the second application will not be able to
use that value of K to decrypt the ciphertext.
[0148] It may be desirable to selectively grant applications access
to different parts of a data string. Consider the example of a
credit card number. As shown in FIG. 11, a sixteen digit credit
card number may include three parts. The leading six digits of the
credit card number are sometimes referred to as the bank
identification number (BIN). The next six digits of the credit card
number are sometimes referred to as the account number core and
form part of the credit card holder's account number. The last four
digits of the credit card number are sometimes referred to as user
account number information and are used with the account number
core to identify a credit card holder's account.
[0149] Different parties may be entitled to access different parts
of the credit card number. Some parties may only need access to the
BIN. Other parties may require access to the entire credit card
number. As a result, it may be desirable to selectively grant
access to different portions of the credit card number to different
parties.
[0150] Selective access may be provided to different portions of a
data item using format-preserving cryptography. Users (individuals,
organizations, applications, etc.) that require access to one part
of a data item, may be provided with appropriate cryptographic
key(s) to access that part of the data item, whereas users that
require access to another portion of the data item may be provided
with appropriate key(s) to access that portion. Using this type of
arrangement, a first user might, for example, be provided with
access to an entire social security number, whereas a second user
might only be provided with access to the last four digits of a
social security number. As another example, a first user might be
provided with access to the first or last four digits of a credit
card number, whereas a second user might be provided with access to
the entire credit card number. In scenarios with three or more
users, each user may likewise be provided with access to different
portions of a social security number, credit card number, or other
data item.
[0151] Consider, as an example, a plaintext data item P. As shown
in FIG. 12, plaintext P may be made up of two parts: plaintext P1
and plaintext P2. As an example, plaintext P might be a credit card
number and plaintext portion P2 might be the last four digits of
the credit card number. In this type of example, plaintext P1 would
represent the leading twelve digits of the credit card number.
[0152] Using a first format-preserving cryptographic algorithm
(e.g., a block-cipher-based algorithm of the type described in
connection with FIG. 3) and a first cryptographic key K1, plaintext
P1 can be converted into ciphertext C1 and ciphertext C1 can be
converted into plaintext P1, as shown in FIG. 12. A second
format-preserving cryptographic algorithm (e.g., a
block-cipher-based algorithm of the type described in connection
with FIG. 3) and a second cryptographic key K2 may be used to
convert ciphertext C1 and plaintext portion P2 into ciphertext C
and may be used to convert ciphertext C into ciphertext portion C1
and plaintext portion P2.
[0153] During a first set of encryption operations,
format-preserving encryption algorithm E.sub.1 and cryptographic
key K1 are used to encrypt plaintext P1 to produce corresponding
ciphertext C1, whereas plaintext portion P2 is left unencrypted.
During a second set of encryption operations, format-preserving
encryption algorithm E.sub.2 and cryptographic key K2 are used to
encrypt ciphertext C1 and plaintext P2 to produce ciphertext C.
Encryption algorithm E.sub.1 is a format-preserving encryption
algorithm that maps plaintext P1 to ciphertext C1 that is in the
same format as plaintext P1. Encryption algorithm E.sub.2 is a
format-preserving encryption algorithm that maps input (C1, P2) to
ciphertext C that is in the same format as (C1, P2). In this
context, input (C1, P2) serves as "plaintext" corresponding to
output ciphertext C.
[0154] When decrypting ciphertext C, a decryption algorithm D.sub.2
(corresponding to encryption algorithm E.sub.2) may perform
decryption operations with key K2 to produce ciphertext C1 and
plaintext part P2. Decryption algorithm D.sub.2 is a
format-preserving decryption algorithm, so output (C1, P2) is in
the same format as input C. Following decryption operations with
decryption algorithm D.sub.2, decryption algorithm D.sub.1
(corresponding to encryption algorithm E.sub.1) may perform
decryption operations on C1 using key K1 to produce plaintext P1.
Decryption algorithm D.sub.1 is a format-preserving decryption
algorithm, so output P1 is in the same format as input C1.
[0155] A first user (USER1) may be provided with key K1 and key K2.
A second user (USER2) may be provided with only key K2. Because
USER1 has both keys, USER1 can decrypt C with decryption function
D.sub.2 and key K2 to produce C1 and P2 and can decrypt C1 with
decryption function D.sub.1 and K1 to produce P1. Plaintext P1 and
plaintext P2 can be assembled to produce plaintext P, so USER1 can
access all of P. Because USER2 has only key K2, USER2 can only use
decryption function D.sub.2 to decrypt C and produce ciphertext C1
and plaintext P2. USER2 is therefore able to access only plaintext
P2, not plaintext P1.
[0156] Illustrative steps involved in using format-preserving
encryption and decryption algorithms to selectively provide access
to different parts of a data item are shown in FIG. 13. At step
142, key server 20 or other suitable entity may be used to
selectively distribute keys to users (i.e., to software
applications or parts of applications that are associated with
individuals, organizations, etc.). Different keys may be
distributed to different users depending on the level of access
that it is desired to grant each user. For example, at step 144,
first and second keys such as keys K1 and K2 of FIG. 12 may be
provided to a first user USER1, as described in connection with
FIG. 12. At step 146, second key K2 may be provided to a second
user USER2, as described in connection with FIG. 12.
[0157] At step 148, keys K1 and K2 are used in encrypting different
portions of the data item P. For example, key K1 may be used to
encrypt plaintext part P1 of P to produce corresponding ciphertext
C1, whereas key K2 may be used in encrypting both C1 and plaintext
part P2 of P to produce corresponding ciphertext C. The selective
encryption operations of step 148 may be performed at any suitable
time (e.g., before, after, or during the key distribution
operations of step 142). Each encryption step may use a unique
encryption algorithm such as encryption algorithms E.sub.1 and
E.sub.2 of FIG. 12. Encryption algorithm E.sub.1, which may be used
to encrypt plaintext P1, may be a format-preserving encryption
algorithm that is suitable for encrypting the last four digits of a
credit card number (as an example). Encryption algorithm E.sub.2
may be a format-preserving encryption algorithm that is suitable
for encrypting a 16 digit credit card number (as an example).
[0158] During step 150, format-preserving decryption algorithms
(e.g., decryption algorithm D.sub.2 corresponding to encryption
algorithm E.sub.2 and decryption algorithm D.sub.1 corresponding to
encryption algorithm E.sub.1) may be used to reverse the encryption
operations of step 142. In particular, decryption algorithm D.sub.2
and key K2 may be used by USER2 to decrypt C to produce ciphertext
C1 and plaintext P2, thereby providing USER2 with access to
plaintext part P2 of plaintext P, but not access to plaintext part
P1 of plaintext P. Because USER1 obtained both keys K1 and K2
during step 142, USER1 is able to decrypt C with D.sub.2 using key
K2 to produce C1 and plaintext P2 and is able to decrypt C1 with
D.sub.1 and key K1 to produce P1. This allows USER1 to access all
parts of P (i.e., both P1 and P2 in this example).
[0159] If desired, more than two levels of encryption may be
provided. For example, plaintext P may be divided into three or
more portions Pi, three or more format-preserving encryption
algorithms Ei and three or more corresponding keys Ki may be used
to encrypt each portion in a successive operation as described in
connection with FIGS. 12 and 13. Following selective key
distribution to three or more users, a corresponding number (i.e.,
three or more) matching decryption algorithms Di may be used to
provide these users with selective access to the different
plaintext portions Pi. The arrangement of FIGS. 12 and 13 in which
a string P was divided into two portions and in which two users
were provided with selective access to different portions of the
string is merely illustrative.
[0160] During string preprocessing operations (e.g., the encoding
operations of step 70 of FIG. 5 and step 88 of FIG. 6), it may be
desirable to convert characters into unique binary values. These
unique binary values may then be encrypted (as described in
connection with the format-preserving encryption operations of step
72 of FIG. 5) or decrypted (as described in connection with the
format-preserving decryption operations of step 90 of FIG. 6).
[0161] One type of technique that may be used when encoding strings
as unique binary values is illustrated in FIG. 14. In the example
of FIG. 14, an unencoded (plaintext) string P is converted into a
unique binary value UBV. String P (in this example) is made up of
characters P4, P3, P2, P1, and P0. Characters P4, P2, and P1 are
digits and may therefore have any value between 0 and 9 (i.e., 0 1,
2, 3, . . . 9). Characters P3 and P0 are letters and may have any
letter value (i.e., A, B, C, D, . . . Z). Each digit character may
have one of ten values and each letter character may have one of 26
values. To uniquely convert string P to binary, a formula such as
formula 152 may be used.
[0162] In formula 152, P0 represents a numeric value for letter P0.
In a typical indexing scheme for letters, "A" corresponds to 0, "B"
corresponds to 1, . . . and "Z" corresponds to 25, so if P0 is the
letter "B," the numeric value for P0 in equation 152 will be 1. P1
is a digit. In a typical encoding scheme for digits, 0 corresponds
to 0, 1 corresponds to 1, . . . and 9 corresponds to 9, so if P1 is
the digit "3," the value of P1 will be 3. Using an encoding scheme
of this type, character P2 will have a numeric value of 0 to 9, P3
will have a numeric value of 0 to 25, and P4 will have a numeric
value of 0 to 9.
[0163] As shown in formula 152, the numeric values of the
characters P0, P1, P2, P3, and P4 are multiplied by respective
coefficients 154, 156, 158, and 160 (the coefficient of P0 is "1").
The value of each multiplicative coefficient in formula 152
represents the number of possible values of the previous character.
For example, in formula 152, the numeric value of character P1 is
multiplied by coefficient 154. The value of coefficient 154 is 26,
because P0 (the character that is just prior to character P1 in
string P) may have any one of 26 possible values. Coefficient 156
of numeric value P2 has a value of 260, because the combination of
preceding characters P0 and P1 could have any of 260 possible
values. Likewise, the value of coefficient 158 of P3 is 2600
because there are 2600 possible combinations of numeric values for
preceding characters P0, P1, and P2 and the value of coefficient
160 of P4 is 67600 because there are 67600 possible combinations of
numeric values for preceding characters P0, P1, P2, and P3.
Although the numbers in the coefficients in FIG. 14 are represented
in base 10 for clarity, when computing the unique binary value UBV,
the base 10 numeric values of the coefficients and the numeric
values of the encoded characters (e.g., the numeric values of the
digits and letters of string P) are represented in binary (i.e., in
base 2, as "1s" and "0s"). The mapping provided by unique binary
value encoding function 152 is unique in that no two character
strings PA and PB will map to the same binary value UBV, when PA is
not the same as PB.
[0164] To ensure that the operation of the format-preserving
encryption and decryption functions of FIGS. 5 and 6 are successful
at preserving the format of a binary-encoded string, care should be
taken that the binary values that are produced during encryption
and decryption operations are format compliant. The encrypted
version of the binary-encoded string that is produced at the output
of step 72 in FIG. 5 should be format compliant to ensure that the
postprocessed string that is produced at the output of step 74 of
FIG. 5 is in the same format as the string originally obtained at
step 66. Similarly, the decrypted version of the binary-encoded
string that is produced at the output of step 90 of FIG. 6 should
be format compliant to ensure that the postprocessed string
produced following the operations of step 92 of FIG. 6 is in the
same format as the string originally obtained at step 84.
[0165] Encoded binary values are considered to be format compliant
when their unencoded form lies within the same range of values as
their original form. Consider, as an example, the character "9".
This character is a digit and can be numerically represented by the
base 10 number "9." In binary, the number 9 is 1001. Using a block
cipher of the type described in connection with FIG. 3, encryption
operations may be performed that transform the unencrypted binary
value 1001 to an encrypted value of 1111 (as an example). This
binary value is not format compliant, because it corresponds to a
base 10 value of "15," which is not within the permissible range
for a single digit (i.e., "15" does not lie within the range of
"0," "1", "2," . . . . "9"). The same type of problem can occur
during decryption if the decrypted version of a binary-encoded
value at the output of step 90 is not format compliant.
[0166] To ensure that encrypted and decrypted values are format
compliant, the block cipher operations of step 72 (FIG. 5) and step
90 (FIG. 6) can be repeated while checking the output of the cipher
for format compliance. Each iteration of the block cipher will
modify the binary output. Eventually, the binary output of the
block cipher will be format compliant, at which point no further
iterations of the block cipher are performed.
[0167] This type of arrangement is shown in FIG. 15. In the example
of FIG. 15, an unencrypted string P is being encrypted. The string
P in the FIG. 15 example is the digit "9." Using an index in which
digits are mapped to corresponding numeric values (i.e., "0" is
mapped to "0," "1" is mapped to "1," . . . and "9" is mapped to
"9), the string P is converted to a numeric value of 9. In
binary-encoded format, the unencrypted encoded version of string P
is 1001. As shown in FIG. 15, the left half of the binary-encoded
version of P ("10") is used as the input L1 to the block cipher of
FIG. 3 and the right half of the binary-encoded version of P ("01")
is used as the input R1 to the block cipher of FIG. 3.
[0168] As described in connection with FIG. 3, a certain number of
rounds of the block cipher (e.g., eight rounds) may be used to
perform an initial encryption operation on the binary-encoded value
1001. In the FIG. 15 example, this initial encoding operation
results in the binary value of 1111. If this value were format
compliant, the encryption process would be complete. However, in
the present example, the binary value of 1111 is not format
compliant, because it corresponds to a base 10 numeric value of 15,
which is not within the permissible range numeric values for a
digit (i.e., 15 does not lie within the range of 0-9, so the string
"15" is not in the same format as the original string "9"). As a
result, additional rounds of the block cipher are performed (e.g.,
an additional eight rounds). This produces the encrypted binary
value 1000 (in the FIG. 15 example). Because the binary value 1000
corresponds to a base 10 numeric value of 8, which lies within the
permissible digit numeric values of 0-9, the binary value 1000
forms a proper format-compliant encrypted binary-encoded version of
string P. No further rounds of the block cipher need be
performed.
[0169] Decryption operations may be performed in the same way. If,
following an initial application of a given number of rounds of the
block cipher, the initial binary value of a string that is produced
is not format compliant, additional decryption operations can be
performed. As soon as a binary output is produced that is format
compliant, the block cipher operations may be terminated and the
binary value may be converted into an appropriately formatted
string of characters (e.g., letters and digits), as described in
connection with step 92.
[0170] Illustrative steps involved in performing format-preserving
encryption operations on a string using one or more repeated
iterations of a block cipher to ensure format compliance are shown
in FIG. 16.
[0171] As shown in FIG. 16, a data string may be preprocessed at
step 64, encrypted at step 162, and postprocessed at step 74.
[0172] As described in connection with steps 66, 68, and 70 of FIG.
5, during step 64, encryption engine 26 may obtain the unencrypted
string from a database 18 or an application 16 and may process the
string to identify relevant characters. Dashes spaces, checksums,
and other undesired characters can be removed from the string and
the relevant characters in the string can be retained. Encryption
engine 26 may then use the index mappings that were created during
step 54 of FIG. 4 to convert the processed string (i.e., the string
from which the irrelevant characters have been removed) into an
encoded unencrypted string. For example, each digit in the string
may be converted into a corresponding numeric value from 0-9, each
letter in the string may be converted into a corresponding numeric
value from 0-25, etc. During these encoding operations, encryption
engine 26 may use a formula such as formula 152 of FIG. 14 to
encode the string to a unique binary value. The actual formula that
is used during encoding depends on the nature of the string's
format. The values used for coefficients 154, 156, 158, and 160 in
FIG. 14 are applicable to a situation in which the string has the
format "digit, letter, digit, digit, and letter," as shown at the
top of FIG. 14. For strings with other formats, different
coefficients may be used in the unique binary value encoding
function represented by formula 152.
[0173] After preprocessing the string so that the string is
represented as a unique encoded binary value, block cipher
format-preserving encryption operations may be performed at step
162. During the operations of step 162, the binary-encoded string
may be processed using a block cipher such as a block cipher of the
type shown in FIG. 3. As shown in FIG. 16, the block cipher may be
applied to the binary-encoded string at step 164. The operations of
step 164 may involve any suitable number of rounds of the block
cipher. For example, during step 164, eight rounds of the block
cipher may be performed. The use of eight rounds of the block
cipher is, however, merely illustrative. Any suitable number of
rounds of the block cipher may be performed at step 164 if
desired.
[0174] As a result of the block cipher operations of step 164, the
binary value obtained from step 64 is converted to an encrypted
binary value. At step 166, the encrypted binary value that is
produced at step 164 is analyzed to determine whether it is format
compliant. As described in connection with FIG. 15, when a binary
value is operated on by the block cipher, the resulting binary
value at the output of the block cipher may not match the format of
the original input string. If it is determined at step 166 that the
format of the string no longer matches the format of the original
string (e.g., if a digit has been transformed into a value that no
longer falls within its allowed range of 0-9 as described in
connection with FIG. 15), processing can loop back to step 164, as
indicated by line 165. In this situation, the current block cipher
can again be applied to the binary value. By applying the block
cipher to the current block cipher output, the block cipher can
update the current block cipher output. This loop can continue
until it is determined during step 166 that the format of the
encrypted binary value (the current block cipher output) matches
the format of the original string obtained at step 64. When it is
determined during step 166 that the encrypted binary value output
by the block cipher is format compliant, processing may proceed to
step 74.
[0175] During the operations of step 74, the same unique binary
value encoding function that was used during the encoding
operations of step 64 and the same index mappings that were used
during the encoding operations of step 64 are used to convert the
encrypted string back into characters (i.e., characters in the
legal set of character values that were defined for each character
position). Decoding the encoded version of the string using the
unique binary value encoding function and index mappings returns
the string to its original character set.
[0176] The decoded encrypted string may then be processed to
restore elements such as dashes and spaces that were removed during
the preprocessing operations of step 64. When replacing a checksum
value, a new valid checksum value can be computed from the
encrypted version of the string and validity period information or
other suitable information can be embedded within the checksum
digit (e.g., by adding a validity period index to the new valid
checksum value to produce a checksum digit for the decoded
encrypted string). The decoded encrypted string is ciphertext that
corresponds to the plaintext unencrypted string that was obtained
at step 64. If desired, the entire string can be encrypted. With
this type of arrangement, the checksum removal operation and the
checksum digit computation operation of can be omitted.
[0177] By processing the string during step 74, the extraneous
elements of the string that were removed during step 64 may be
inserted back into the string. Because the extraneous elements are
reinserted into the string and because a format-preserving block
cipher encryption process was used during step 162, the encrypted
string that is produced will have the same format as the original
unencrypted string. This allows the encrypted string to be used by
applications 16 and databases 18 that require that the original
string's format be used. When the encrypted string is provided to
an application 16 or database 18, legacy applications and databases
that require a specific string format may be able to accept the
encrypted string.
[0178] Illustrative steps involved in performing format-preserving
decryption operations on a string using one or more repeated
iterations of a block cipher to ensure format compliance are shown
in FIG. 17.
[0179] As shown in FIG. 17, a data string may be preprocessed at
step 82, decrypted at step 168, and postprocessed at step 92.
[0180] During step 82, the decryption engine obtains the encrypted
string. The encrypted string may be retrieved from a database 18 or
received from an application 16. The encrypted string is processed
to identify relevant characters. For example, dashes spaces,
checksums, and other extraneous elements can be removed from the
string, whereas relevant characters in the string can be retained.
The process of removing extraneous characters during step 82 is the
same as that used during the processing of the unencrypted string
that was performed during step 68 of FIG. 5 (step 64 of FIG. 16).
After extraneous characters have been removed, decryption engine 28
may use the index mappings that were defined at step 54 of FIG. 4
to convert the processed string (i.e., the string from which the
irrelevant characters have been removed) into an encoded encrypted
string. For example, each digit in the string may be converted into
a corresponding numeric value from 0-9, each letter in the string
may be converted into a corresponding numeric value from 0-25, etc.
During these encoding operations, decryption engine 28 may use a
formula such as formula 152 of FIG. 14 to encode the string to a
unique binary value. As with the encoding operations performed
during preprocessing step 64 of FIG. 16, the formula that is used
during the encoding of step 82 depends on the nature of the
string's format. The values used for coefficients 154, 156, 158,
and 160 in the FIG. 14 example are merely illustrative.
[0181] After preprocessing the string so that the string is
represented as a unique encoded binary value, format-preserving
decryption operations may be performed at step 168. During the
operations of step 168, the binary-encoded string may be processed
using a block cipher such as a block cipher of the type shown in
FIG. 3. The block cipher may be applied to the binary-encoded
string at step 170. The operations of step 170 may involve any
suitable number of rounds of the block cipher. For example, eight
rounds of the block cipher may be performed during step 170.
[0182] As a result of the block cipher operations of step 170, the
binary value obtained from step 82 is converted to another binary
value. If only one pass through loop 165 of FIG. 16 was used during
encryption, a single pass through decryption step 170 will be
sufficient to convert the binary value from step 82 into a
decrypted binary value. If more passes through loop 165 were used
to produce the format-compliant encrypted string, a correspondingly
increased number of passes through loop 171 will be required during
decryption operations.
[0183] At each step 172, the binary value that was produced at step
170 by application of the decrypting block cipher is analyzed to
determine whether the binary value is format compliant. When the
block cipher is applied to a binary value, the resulting updated
binary value at the output of the block cipher may not match the
format of the original input string. If it is determined at step
172 that the format of the data string represented by the current
binary value (i.e., the current version of the block cipher output)
does not match its original format (e.g., if a digit has been
transformed into a value that no longer falls within its allowed
range of 0-9), processing can loop back to step 170, as indicated
by line 171. In this situation, the decrypting block cipher can
again be applied to the binary value at step 170 to update the
block cipher output. This loop can continue until it is determined
during step 172 that the format of the current binary value
produced at the output of the decrypting block cipher matches the
format of the original encrypted string obtained at step 82. When
it is determined during step 172 that the binary value output by
the block cipher is format compliant, the encrypted string has been
successfully decrypted and processing may proceed to step 92.
[0184] During step 92, the index mappings and unique binary
encoding scheme that were used during the encoding operations of
step 82 may be used to convert the index values of the decrypted
string back into their associated characters (i.e., characters in
the legal set of character values that were defined for each
character position). This returns the decrypted string to its
original character set. In strings that contain more than one
different type of character, multiple different index mappings may
be used. The decoded decrypted string may then be processed to
restore elements such as dashes, spaces, and checksum values that
were removed. When replacing a checksum value, a new valid checksum
value may be computed from the decrypted version of the string.
This ensures that the decrypted version of the string will be
returned to its original valid state.
[0185] During the string processing operations of step 92, the
extraneous elements of the string that were removed at step 82 are
inserted back into the string. This restores the string to its
original unencrypted state, so that the decrypted string may be
provided to an application 16 or database 18.
[0186] The foregoing is merely illustrative of the principles of
this invention and various modifications can be made by those
skilled in the art without departing from the scope and spirit of
the invention.
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