Cryptography – Key management – Having particular key generator
Reexamination Certificate
1998-10-28
2003-05-06
Darrow, Justin T. (Department: 2132)
Cryptography
Key management
Having particular key generator
C380S047000, C380S262000, C380S264000, C380S277000, C711S216000
Reexamination Certificate
active
06560337
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to ciphering systems, methods and computer program products, and more particularly to systems, methods and computer program products for reducing effective key length of ciphers.
BACKGROUND OF THE INVENTION
Symmetric-key ciphers are widely used for encryption in both commercial and government applications to protect the privacy and integrity of a wide variety of information. The strength of a symmetric-key cipher is generally tied to its key length. Typically, as the number of independent key bits increases the cipher becomes stronger.
There are many symmetric-key block ciphers with different key lengths which can offer different levels of security, flexibility, and efficiency. These include DES, RC5, CAST, Blowfish, FEAL, SAFER, and IDEA. For example, DES uses a 56-bit key to encrypt a 64-bit input plaintext to produce a 64-bit output ciphertext. These ciphers are described for example, in Rivest, “The RC5 Encryption Algorithm”, Dr. Dobb's Journal, Vol. 20, No. 1, January 1995, pp. 146-148; Schneier, “
The Blowfish Encryption Algorithm
”, Dr. Dobb's Journal, Vol. 19, No. 4, April 1994, Pages 38-40; National Bureau of Standards, “
Data Encryption Standard
”, FIPS PUB 46, January 1977; Massey, “
SAFER K
-64
: A Byte-Oriented Block
-
Ciphering Algorithm
”, Fast Software Encryption, Cambridge Security Workshop Proceedings, Springer-Verlag, 1994, pp. 1-17; Adams, “
Constructing Symmetric Ciphers Using the CAST Design Procedure
”, Design, Codes, and Cryptography, Vol. 12, No. 3, November 1997, pp. 283-316; and Schneier, “
The IDEA Encryption Algorithm
”, Dr. Dobb's Journal, Vol. 18, No. 13, December 1993, pp. 50-56. The disclosures of all these publications are hereby incorporated herein by reference.
When used in commercial applications, data encryption is often subject to government regulations limiting the use, import and/or export of equipment supporting data privacy to certain key lengths. For example, under the United States Department of Commerce regulations, when the keys are limited to 40 bits, products supporting data encryption are generally exportable. Also, when work on key recovery is undertaken, export of DES with 56-bit key is generally allowed. Therefore, depending on the type of application or regulations, the key size may need to be varied.
Unfortunately, many ciphers are not designed to support variable key length. Moreover, even if a cipher supports variable key length, it is often desirable to reduce the “effective key length” (i.e., the number of independent key bits) rather than the actual key length in order to be able to use the same key management and key exchange protocols independent of the application or regulations. The effective key length of a cipher can be reduced by reducing the number of independent key bits in the key. This process is also referred to as “key shortening”.
Key shortening generates a “shortened key” form an initial (“long”) key. Thus, key shortening reduces the effective key length of the initial key without affecting its actual length. For example, denote the initial and shortened keys by K and K′, respectively. Let l be the actual length of K (or K′) in bits and let r be the number of bits by which K has to be shortened. Thus, the effective length of K′ is l−r bits.
One technique to derive K′ from K is to set r specific bits of K to some publicly known constant values. For example, the r left-most bits of K can be set to zero. Unfortunately, K′ becomes weaker than K by r bits since r bits of K′ are constant values which are known to the public. Thus, this key shortening technique which fixes some of the key bits to some publicly known constant values, is potentially susceptible to “short-cut” attacks. That is, the work factor required to break the shortened key K′ may be less than 2
l−r
, due to the way some ciphers work.
For example, DES includes 16 rounds of identical operations in which the data is mixed with the key. For each round, a key transformation algorithm derives a sub-key from the initial 56-bit key. In each round, a sub-key is mixed with the data. The way the DES key transformation algorithm works, it preserves the initial key bits. Thus, if some of the initial key bits are fixed, then these fixed bits will propagate into the sub-keys, reducing the entropy of the sub-keys. That is, some of the bits in the sub-keys will have known values. This may lead to short-cut attacks. To avoid such a problem, a key shortening scheme preferably should produce a shortened key from an initial key that is pseudo-random with no fixed bits.
Another potential issue with such a key shortening scheme is that exposure of the shortened key K′ may reveal information about the initial key K. That is, if K′ is exposed, an exhaustive search for K may take only 2
r
trials. This can be a problem since in some applications, it might be desirable to use both the keys for two different sessions, one for a “strong session” and one for a “weak session”.
The Commercial Data Masking Facility (CDMF) key shortening scheme can avoid these issues. CDMF is described in U.S. Pat. No. 5,323,464 to Elander et al., entitled “
Commercial Data Masking
”, assigned to the assignee of the present invention, the disclosure of which is incorporated by reference herein in its entirety. CDMF is also described in Johnson et al., “
The Commercial Data Masking facility
(
CDMF
)
Data Privacy Algorithm
”, IBM Journal of Research and Development, Vol. 38, No. 2, March 1994, pp. 217-226; and Johnson et al., “
Design of the Commercial Data Masking Facility Data Privacy Algorithm
”, 1
st
ACM Conference on Computer and Communications Security, ACM Press, 1993, pp. 93-96, the disclosures of which are incorporated by reference herein in their entirety. CDMF generates a pseudo-random 40-bit DES key with no fixed bits from a 56-bit key. Given a 64-bit DES key (including the 8 parity bits), the CDMF derives a 40-bit key using the following steps:
1. Set the parity bits (i.e., bits 8, 16, 24, 32, 40, 48, 56, and 64) in the key to 0.
2. Encrypt the output of Step 1 using DES and the key 0Xc408b0540ba1e0ae.
3. EXCLUSIVE-OR (XOR) the results of Steps 1 and 2.
4. Zero the parity bits (i.e., bits 8, 16, 24, 32, 40, 48, 56, and 64) and the following 16 bits in the output of Step 3: bits 1, 2, 3, 4, 17, 18, 19, 20, 33, 34, 35, 36, 49, 50, 51, and 52.
5. Encrypt the output of Step 4 using DES and the key 0Xef2c041ce6382fe6.
6. Set the parity bits (i.e., bits
8
,
16
,
24
,
32
,
40
,
48
,
56
, and
64
) in the result of Step 5 if desired.
The output of step 6 is the CDMF-derived shortened key which is used in a standard DES invocation.
Notwithstanding the improvement of CDMF, there continues to be a need for improved systems, methods and computer program products for reducing effective key length of ciphers. Key length reducing systems, methods and computer program products should preferably be generic in the sense that they can be used with any cipher and any size key, and can generate a shortened key which is of the same length as the initial long key.
SUMMARY OF THE INVENTION
The present invention includes systems, methods and/or computer program products that reduce effective key length of a symmetric key cipher by deriving an intermediate value from an initial key, using a one-way cryptographic function. Predetermined bit locations of the intermediate value are selected to obtain an intermediate key. An intermediate shortened key is derived from the intermediate key by setting predetermined bit locations of the intermediate key to predetermined values. A diffused intermediate shortened key is derived from the intermediate shortened key using the one-way cryptographic function. Predetermined bit locations of the diffused intermediate shortened key are then selected to obtain a shortened key.
Systems, methods and/or computer program products for reducing effective key length of a symmetric key cipher according to the invention can be used with any cipher
Matyas, Jr. Stephen Michael
Peyravian Mohammad
Zunic Nevenko
Darrow Justin T.
International Business Machines - Corporation
Myers Bigel & Sibley & Sajovec
Ray-Yarletts Jeanne S.
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