Patent Publication Number: US-2016247150-A1

Title: Format-preserving cryptographic systems

Description:
This application is a continuation of patent application Ser. No. 14/094,366, filed Dec. 2, 2013, which is a division of patent application Ser. No. 11/654,054, filed Jan. 16, 2007, each of which is hereby incorporated by reference herein in its entirety. This application claims the benefit of and claims priority to patent application Ser. No. 14/094,366, filed Dec. 2, 2013, and patent application Ser. No. 11/654,054, filed Jan. 16, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to cryptography and more particularly, to preserving data formats during encryption and decryption operations. 
     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. 
     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. 
     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. 
     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 unsensitive information in the table to retrieve the desired information without decrypting the entire table. 
     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. 
     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 
     In accordance with the present invention, a data processing system is provided in which a format-preserving cryptographic function may be used for format-preserving encryption operations and format-preserving decryption operations. The data processing system may include a key server. The key server may provide cryptographic keys to authorized key requesters. The key server may use policy rules to determine which key requesters are authorized to obtain a copy of a given key. If a key requester is authorized, the key server may generate the requested key and may provide the key to the key requester over a communications network. 
     Key requests may include identifiers. Identifiers help to identify key requesters and key requests. Suitable identifiers may include user names such as the name of an individual, the name of an organization, the name of a group, etc. Policy names and program names may also be used as identifiers. 
     If desired, key validity period information may be included in an identifier. With one suitable arrangement, data to be encrypted or decrypted using a key is credit card data and the validity period information is a credit card expiration date. 
     Using a format-preserving encryption function, plaintext may be encrypted to form ciphertext. Validity period information may be embedded in the ciphertext for use in requesting and generating an appropriate decryption key. The validity period information may be embedded by combining an index value that corresponds to a particular validity period with redundant information such as a checksum value in a credit card number. Upon receipt of the ciphertext containing the embedded validity period information, an application can extract the embedded validity period information. The extracted validity period information can be used in selecting an appropriate key to use in responding to the key request, so information such as the validity period information may sometimes be referred to as key selector information or a key selector. 
     In a data processing system including multiple applications that access a common database, testing can be facilitated by using a format-preserving encryption engine to encrypted sensitive data prior to testing. In a normal production environment for the data processing system, multiple applications access a production database that contains sensitive data. Proper testing of applications in a test environment requires that the format of the data be preserved. The format-preserving encryption engine is used to encrypt the sensitive items in the production database. The encrypted versions of the sensitive data items are then exported into a test version of the database. The applications can be tested using the encrypted data in the test database. 
     A plaintext string may include multiple plaintext parts. Each plaintext part may have a different sensitivity level. In this type of situation, it may be desirable to provide access to different parts of the plaintext to different applications or entities. By selectively encrypting each plaintext part, access can be controlled. Encryption keys for encrypting each part can be formed using the results of earlier encryption operations. In this way, a second plaintext part may be randomized relative to a first plaintext part during encryption, a third plaintext part may be randomized relative to the second plaintext part during encryption, etc. 
     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 
         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. 
         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. 
         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. 
         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. 
         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. 
         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. 
         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. 
         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. 
         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. 
         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. 
         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. 
         FIG. 11  is a diagram showing how validity period information can be embedded within a credit card number during format-preserving encryption operations in accordance with an embodiment of the present invention. 
         FIG. 12  is a diagram of an illustrative system in which format-preserving encryption and decryption operations are performed in accordance with an embodiment of the present invention. 
         FIG. 13  is a flow chart of illustrative steps involved with encrypting and decrypting a credit card number using format-preserving cryptographic techniques in which validity period information is embedded in the checksum digit of the credit card number in accordance with an embodiment of the present invention. 
         FIG. 14  is a diagram showing how a format-preserving encryption engine may be used to encrypt data before the data is exported from a production database in a production environment to a test database in a test environment in accordance with an embodiment of the present invention. 
         FIG. 15  is a diagram showing how different parts of a data item such as a credit card number can be divided into different plaintext parts for selective encryption in accordance with an embodiment of the present invention. 
         FIG. 16  is a diagram showing how three plaintext parts of a data string can be encrypted using four cryptographic keys in accordance with an embodiment of the present invention. 
         FIG. 17  is a diagram showing how three plaintext parts of a data string can be encrypted using three cryptographic keys in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 
     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. 
     The data that is handled by system  10  includes sensitive items such as individuals&#39; 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&#39; names may be considered sensitive. 
     In a typical scenario, a credit card company maintains a database  18  of account holders. The database lists each account holder&#39;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. 
     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. 
     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. 
     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. 
     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&#39;s authentication credentials are valid. 
     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. 
     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. 
     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&#39;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. 
     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. 
     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. 
     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”. 
     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. 
     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 ). 
     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. 
     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. 
     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. 
     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. 
     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.). 
     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). 
     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. 
     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. 
     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 . 
     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. 
     In  FIG. 3 , the first half of the unencrypted string is labeled “L 1 ” and the second half of the unencrypted string is labeled “R 1 ”. During encryption operations with encryption engine  26 , the unencrypted string halves L 1  and R 1  are processed to form corresponding encrypted string halves L 3  and R 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 3  and R 2  are decrypted to produce unencrypted halves L 1  and R 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. 
     Although shown as involving three rounds in the example of  FIG. 3 , the operations of  FIG. 3  may, if desired, be implemented using four or more rounds. The use of a three-round block cipher is described as an example. 
     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. The block cipher uses a subkey generation algorithm  38 . The subkey generation algorithm  38  has three inputs: a key K, a constant C (C 1  for round  40 , C 2  for round  42 , and C 3  for round  44 ), and a string S (S 1 =R 1  for round  40 , S 2 =L 2  for round  42 , and S 3 =R 2  for round  44 ). 
     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. 
     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 1  that is used in round  40  is equal to 1, the constant C 2  that is used in round  42  is 2, and the constant C 3  that is used in round  44  is 3. The value of S varies in each round. In round  40 , S 1  is equal to the first half of the unencrypted string R 1 . In round  42 , S 2  is equal to the L 2 . In round  44 , S 3  is equal to R 2 . 
     In round  40 , the output of the subkey generation algorithm is subkey SK 1 , as shown in equation 2. 
         SK 1= H ( S   1   |C   1   |K )  (2)
 
     In round  42 , the output of the subkey generation algorithm is subkey SK 2 , as shown in equation 3. 
         SK 2= H ( S   2   |C   2   |K )  (3)
 
     In round  44 , the output of the subkey generation algorithm is subkey SK 3 , as shown in equation 4. 
         SK 3= H ( S   3   |C   3   |K )  (4)
 
     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). 
     A format-preserving combining operation (labeled “+” in  FIG. 3 ) is used to combine the subkeys SK 1 , SK 2 , and SK 3  with respective string portions. 
     During encryption operations, format-preserving combining operation  46  combines SK 1  with string L 1  to produce string L 2 . During decryption operations, format-preserving combining operation  46  combines SK 1  with string L 2  to produce string L 1 . Format-preserving combining operation  48  combines SK 2  with string R 1  to produce string R 2  during encryption operations and combines SK 2  with string R 2  to produce string R 1  during decryption operations. Format-preserving combining operation  50  is used to process subkey SK 3 . During encryption, format-preserving combining operation  50  combines SK 3  with string L 2  to produce string L 3 . During decryption, format-preserving combining operation  50  combines SK 3  with string L 3  to produce string L 2 . 
     The format-preserving combining operation + preserves the format of the strings L 1 , L 2 , L 3 , R 1 , and R 2  as they are combined with the subkeys SK 1 , SK 2 , and SK 3 . For example, the string L 2  that is produced by combining string L 1  and subkey SK 1  has the same format as the string L 1 . 
     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 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 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 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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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 . 
     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. 
     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. 
     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. 
     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. 
     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 ). 
     The flow chart of  FIG. 5  shows steps involved in encrypting a data string. 
     As shown in  FIG. 5 , the data string is preprocessed at step  64 , encrypted at step  72 , and postprocessed at step  74 . 
     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 . 
     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. 
     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). 
     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. 
     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 . 
     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. 
     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. 
     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. 
     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&#39;s format be used. 
     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. 
     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. 
     As shown in  FIG. 6 , the encrypted data string is preprocessed at step  82 , is decrypted at step  90 , and postprocessed at step  92 . 
     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 . 
     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 . 
     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. 
     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 . 
     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. 
     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. 
     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. 
     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. 
     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 ). 
     At step  98 , the decrypted string is provided to an application  16  or database  18 . 
     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. 
     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. 
     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  ( FIG. 1 ). As an example, key K may be computed by key server  20  using equation 5. 
         K=f ( R SECRET,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. 
     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. 
     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. 
     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). 
     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.” 
     Illustrative steps involved in forming a key K using equation 5 are shown in  FIG. 7A . 
     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.). 
     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. 
     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. 
     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 . 
     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. 
     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. 
     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.). 
     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 . 
     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 . 
     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. 
     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). 
     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. 
     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. 
     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. 
     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&#39;s membership in a directory group, etc. 
     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 . 
     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 . 
     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). 
     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 . 
     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. 
     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.). 
     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. 
     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). 
     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. 
     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 ). 
     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 . 
     With the approach of  FIG. 10 , authentication operations are performed by key server  20 , so authentication server  24  need not be used. 
     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). 
     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. 
     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 . 
     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 ). 
     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. 
     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. 
     To ensure that applications are properly informed of which validity period to use when processing a given data item, the validity period can be embedded in the data item. When an application needs to determine what validity period applies to a particular data item, the validity period can be extracted from the data item by the application. 
     Consider, as an example, credit card numbers. The last digit of a credit card number is a checksum digit. In a normal valid credit card number, the value of the checksum digit represents a valid checksum that is computed based on the preceding numbers of the credit card (i.e., the sixteenth digit in a credit card number is a checksum digit computed from the first fifteen digits of the credit card number). The checksum digit can be used to determine whether a given credit card number is valid. 
     Validity period information can be embedded in the credit card number by adding a validity period index to the checksum. With one suitable arrangement, the validity period index matches index values 1 through 9 with years 2006, 2007, . . . 2014, respectively. The validity period index value for 2006 is 1, the validity period index value of 2 represents a validity period of 2007, etc. By combining an appropriate validity period index with a checksum number, the validity period can be embedded into the checksum digit and therefore into the credit card number. 
     Validity period embedding is illustrated in  FIG. 11 . In the example of  FIG. 11 , the valid checksum digit for an unencrypted credit card (plaintext) is 0. Following application of a format-preserving encryption function, the first 15 digits of the credit card number are transformed into encrypted digits. A new valid checksum can be computed based on these encrypted digits. In the example of  FIG. 11 , the recomputed valid checksum is 3. The validity period that is to be embedded into the checksum digit is 2006 (in this example). The index value for validity period 2006 is 1. 
     As shown in  FIG. 11 , the validity period can be embedded into the checksum digit by adding 1 (the index value for 2006) to 3 (the checksum). The resulting modified checksum digit will be 4. When this modified checksum digit is used in the ciphertext version of the credit card number, it will not represent a valid checksum for the ciphertext version of the credit card number. However, applications will be able to extract the validity period from the ciphertext, obviating the need to keep track of the validity period separately. 
       FIG. 12  is a diagram showing how a credit card numbering scheme with embedded validity period information may be implemented in a system such as system  10  of  FIG. 1 . As shown by line  142 , a first application  16 - 1  may receive plaintext such as a credit card number. The plaintext may be manually input into application  16 - 1  by an operator, may be received from another application, etc. 
     Application  16 - 1  encrypts the plaintext to form ciphertext. As indicated by line  144 , application  16 - 1  may request a copy of a key K from key server  20 . If application  16 - 1  is authorized, key server  20  will provide the requested key K to application  16 - 1  (line  146 ). Application  16 - 1  encrypts the plaintext using encryption engine  26  and key K (line  148 ) to produce ciphertext. As part of the encryption operation, application  16 - 1  can embed validity period information into the ciphertext. 
     Application  16 - 1  can store the ciphertext in database  18  for subsequent retrieval by application  16 - 2  (lines  150  and  152 ). Alternatively, application  16 - 1  can provide the ciphertext to application  16 - 2  directly (line  154 ). Application  16 - 2  extracts the validity period information from the ciphertext and uses this validity period information in requesting an appropriate key K for decrypting the ciphertext (line  156 ). If authorized, key server  20  provides the requested key to application  16 - 2  (line  158 ). The key is used in decryption engine  28  by application  16 - 2  to decrypt the ciphertext, producing plaintext (line  160 ). The plaintext may be used by application  16 - 2  or other applications in system  10  to which application  16 - 2  provides the plaintext. 
     Illustrative steps involved in encrypting and decrypting credit card numbers in an arrangement of the type shown in  FIG. 12  in which validity period information is embedded in credit card checksum digits are shown in  FIG. 13 . 
     At step  162  of  FIG. 13 , application  16 - 1  obtains an unencrypted (plaintext) credit card number. The credit card number has a valid checksum digit (e.g., the 16th digit out of 16 digits in the credit card number). 
     An encryption engine  26  associated with application  16 - 1  requires a key K to encrypt the credit card number. The application  16 - 1  therefore obtains a key K (step  164 ). Suitable techniques for obtaining key K are described in connection with  FIGS. 8, 9, and 10 . With one suitable arrangement, application  16 - 1  provides key server  20  with a key request that contains an identity IDEN containing a validity period. Key server  20  may generate key K using equation 5. 
     At step  166 , application  16 - 1  (e.g., encryption engine  26  at application  16 - 1 ) removes the original checksum from the plaintext (e.g., to produce a 15 digit string) and encrypts the string from which the checksum digit has been removed using a format-preserving encryption function of the type described in connection with  FIG. 3 . The format-preserving encryption operation uses the key K and the 15-digit string (number) as inputs and produces an encrypted 15-digit string (number) as an output (in this example). 
     At step  168 , the application  16 - 1  (e.g., encryption engine  26 ) computes a new valid checksum from the 15-digit encrypted string. The application  16 - 1  then embeds validity period information in the checksum digit. Any suitable technique may be used to mathematically combine the validity period information and the valid checksum. With one suitable approach, a validity period index is created (e.g., an index value of 1 corresponding to a validity period of 2006, etc.) and this validity period index value is added to the checksum to produce a checksum digit into which the validity period information has been embedded. If desired, other suitable mathematical functions may be used to embed the validity period information into the checksum (or into other redundant information in a string). The ciphertext that results from the processing of step  168  includes a leading 15 digits of encrypted credit card data followed by a single checksum digit into which the validity period information has been embedded. 
     At step  170 , the ciphertext is provided from application  16 - 1  to application  16 - 2  through a database  18  or direct transfer. 
     At step  172 , application  16 - 2  receives the ciphertext version of the credit card number. Application  16 - 2  separates the portion of the ciphertext that does not include the embedded validity period from the ciphertext (e.g., application  16 - 2  separates the leading 15 digits of the ciphertext from the checksum digit). Application  16 - 2  computes a valid checksum for the leading 15 digits. Application  16 - 2  uses the newly computed valid checksum to extract the validity period from the checksum digit. The mathematical function that is used to extract the validity period reverses the embedding process used at step  168 . For example, if the validity period was embedded into the checksum digit by adding the validity period to the checksum at step  168 , application  16 - 2  subtracts the newly computed valid checksum from the checksum digit at step  172  to reveal the embedded validity period index value. 
     At step  174 , application  16 - 2  uses the validity period that has been extracted from the checksum digit in obtaining a copy of key K. In particular, application  16 - 2  may formulate a key request for key server  20  that includes the extracted validity period. If application  16 - 2  is authorized, key server  20  may use the validity period in generating the key K (see, e.g., equation 5) and may provide the requested key K to application  16 - 2 . 
     At step  176 , application  16 - 2  may use decryption engine  28  to decrypt the ciphertext. Decryption engine  28  uses the key K and the ciphertext as inputs and produces the original plaintext version of the credit card number as an output. During decryption, decryption engine  28  may apply a format-preserving decryption algorithm of the type described in connection with  FIG. 3  to decrypt the first 15 digits of the ciphertext version of the credit card to produce 15 corresponding plaintext credit card digits. The decryption engine  28  may also compute a valid checksum for the 15 decrypted digits and may append the valid checksum to the 15 decrypted digits to produce a complete 16-digit plaintext credit card number having a valid checksum. 
     The format-preserving cryptographic functions of encryption engine  26  and decryption engine  28  may be used to facilitate software testing. 
     As shown in  FIG. 14 , a typical production environment  178  has multiple applications  16  that access a common database  18 . During normal use of applications  16  in a production system, applications  16  access data  182 . Data  182  may be provided in the form of one or more tables, some of which may contain sensitive data items (e.g., credit card numbers, etc.). 
     Before applications such as applications  16  are released into general use in production environment  178 , testing is performed in a test environment  180 . As shown in  FIG. 14 , the system of test environment  180  has applications  16  and a database  18  that are similar to those in production environment  178 . Data  184  (e.g., tables of data of the type stored in database  18  in production environment  178 ) is stored in database  18  of test environment  180 . However, the data  184  in test environment  180  is generally less secure than the data  182  in production environment  178 . This is because test environments typically lack the sophisticated security measures (strong firewalls, up-to-date antivirus software, etc.) that are found in production environments. 
     Although test environments such as test environment  180  are often less secure than normal production environments such as environment  178 , it is generally desired to test the applications  16  in test environment  180  using realistic data (e.g., credit card numbers and other data items that have appropriate string lengths and character values characteristic of valid real data). With conventional testing arrangements, a test environment database is created by copying a production environment database. This may expose sensitive information such as credit card numbers to attacks in the test environment. 
     To ensure that sensitive data is not exposed to attacks, format preserving encryption engine  26  is used to encrypt the data in table  182 . The encrypted data  184  may be exported to a database in test environment  180 . Any suitable amount of data may be encrypted and exported in this way. For example, the entire contents of database  18  in production environment  18  may be encrypted prior to exporting the encrypted data to database  18  in test environment  180 . If desired, only sensitive data may be encrypted (e.g., credit card numbers and social security number), while less sensitive data is not encrypted (or is encrypted or obscured using less sophisticated techniques). 
     The encrypted data may be encrypted on a field-by-field basis. For example, credit card fields in the production database may be individually encrypted using format-preserving encryption engine  26  to produce encrypted credit card numbers in the test database. Because the encrypted credit card numbers have the same format as the unencrypted credit card numbers, accurate testing of applications  16  and database  18  can be performed in test environment  180 . Because sensitive information is encrypted before it is exported to the test environment, the risk of exposing sensitive data to attackers in the test environment is significantly reduced. 
     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. 15 , a sixteen digit credit card number may include three parts. The leading six digits of the credit card number (plaintext part P 1 ) are sometimes referred to as the bank identification number (BIN). The next six digits of the credit card number (plaintext part P 2 ) are sometimes referred to as the account number core and form part of the credit card holder&#39;s account number. The last four digits of the credit card number (plaintext part P 3 ) are sometimes referred to as user account number information and are used with the account number core to identify a credit card holder&#39;s account. 
     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. 
     As shown in  FIG. 16 , selective access may be accomplished by using encryption engine  26  to encrypt part P 1  with key K 1 , producing encrypted part P 1  (ciphertext C 1 ). Encryption engine  26  may encrypt part P 2  with key K 2  to produce encrypted part P 2  (ciphertext C 2 ). Part P 3  may be encrypted with the encryption engine using key K 3 , producing encrypted part P 3  (ciphertext C 3 ). Encryption engine  26  may then be used to encrypt C 1 , C 2 , and C 3  together using key K 4  to produce credit card ciphertext (i.e., ciphertext for all of parts P 1 , P 2 , and P 3  together). Keys K 1 , K 2 , K 3 , and K 4  may be four independent cryptographic keys. Encryption with key K 4  helps to prevent matching attacks (e.g., attacks in which an attacker attempts to gather information on the unencrypted credit card numbers by noting when values of C 2  are identical for two different credit card numbers). Selective decryption may be performed by decrypting the ciphertext with key K 4  and then decrypting a selected one of C 1 , C 2 , and C 3  using an appropriate one of K 1 , K 2 , and K 3 . 
     In some situations, it may be desirable to grant different parties or applications with access to portions of a data string (e.g., P 1 , P 2 , and P 3  in the present example) according to their sensitivity. In a credit card number, part P 1  is considered less sensitive than part P 2  and part P 2  is considered less sensitive than part P 3 . Parts P 1 , P 2 , and P 3  can therefore be ranked according to their sensitivity, with P 1  being the least sensitive and with P 3  being the most sensitive. 
     With one suitable encryption scheme, which is shown in  FIG. 17 , plaintext part P 1  is encrypted first. With this scheme, encryption engine  26  uses a key K 1  to produce encrypted P 1  (ciphertext C 1 ). Engine  26  uses a one-way function H such as a hash function (e.g., SHA1) to compute H(C 1 ), which is combined with a key BASE K 2  to produce a key K 2 . The function + in  FIG. 17  represents any suitable combining function such as concatenation or addition. The key K 2  is used to encrypt plaintext part P 2 , producing encrypted part P 2  (ciphertext C 2 ). This randomizes P 2  relative to P 1 . Once C 2  has been computed, engine  26  may compute H(C 2 ) and may combine H(C 2 ) with key BASE K 3  to produce key K 3 . Engine  26  may then use key K 3  to encrypt plaintext part P 3 . This produces ciphertext C 3  and randomizes P 3  relative to parts P 1  and P 2 . 
     When it is desired to provide access to part P 1  without providing access to part P 2 , an application may be provided with key K 1 , but not keys K 2  and K 3 . Selective access to part P 2  can be granted by providing an application with key K 2 . Key K 3  can be provided to an application that desires selective access to part P 3 . Encryption can be provided using only three encryption operations, rather than the four encryption operations used in the approach of  FIG. 16 . Moreover, decryption of any given part can be performed in a single step, rather than using two steps. To produce P 1 , it is only necessary to decrypt C 1  with K 1 . Similarly, decryption engine  28  can produce P 2  by decrypting C 2  with K 2  and can produce P 3  by decrypting C 3  with K 3 . 
     With the technique of  FIG. 17 , matching attacks on part P 1  are accepted, which is possible because the BIN number portion of a credit card number is not generally considered highly sensitive. Parts P 2  and P 3 , which are more sensitive than part P 1 , are secure against matching attacks. 
     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.