Electrical computers and digital processing systems: support – Multiple computer communication using cryptography – Protection at a particular protocol layer
Reexamination Certificate
2000-11-29
2003-06-10
Peeso, Thomas R. (Department: 2132)
Electrical computers and digital processing systems: support
Multiple computer communication using cryptography
Protection at a particular protocol layer
C713S152000, C380S037000, C380S255000
Reexamination Certificate
active
06578150
ABSTRACT:
FIELD OF INVENTION
This invention relates to block cipher secret-key cryptographic systems and methods. More particularly, the invention relates to improvements in a secret-key cryptographic system and method which uses data-dependent and variable rotations of data in block cipher rounds which are dependent, directly or indirectly, on plain text data being enciphered.
BACKGROUND OF THE INVENTION
Cryptography is the science of securing communications and information. In recent years, the importance of cryptographic systems has been magnified by the explosive growth and deployment of telecommunications technology. Increasing volumes of confidential data are being transmitted across telecommunications channels and are being stored in file servers, where such data ranges from financial information to electronic votes. It is desired that systems provide security from unsanctioned or illicit interception or modification of such confidential information.
There are two basic operations used in secret-key or symmetric block cipher cryptography. Encryption or encipherment is the process of disguising a communication to hide its content. During encryption, the communication which is known as plaintext is encrypted into what is known as ciphertext. Decryption or decipherment is the inverse process of using the same secret-key values to recover the plaintext from the ciphertext output. While the two basic operations of encryption and decryption may be distinguished in practice, there is in general no necessary mathematical difference between the two operations, other than that they are inverse transformations of each other.
Ciphertext output of a secure block cipher has little or no statistical relation to its corresponding plaintext input. The output (or input) is uncorrelated to the input (or output). Every bit of ciphertext output reflects every bit of the plaintext input and every bit of the key in a complex uncorrelated manner, just as every bit of recovered plaintext input reflects every bit of the ciphertext output and every bit of the key in a complex uncorrelated manner.
Block ciphers, generally, are binary ciphers receiving inputs consisting of a fixed number of bits (a block of n-bits), and have outputs of the same fixed number of bits (an equal sized block of n-bits). The input and output of such ciphers are one-to-one mappings: each ordered n-bit input is transformed by the block cipher into only one ordered n-bit output; and further, when the transformation is computed in reverse each ordered n-bit output may be transformed back into only one ordered n-bit input.
Secret key values are the values which influence the mapping of input to output provided by the block cipher. It is useful to divide secret keys into two categories: secret input keys and secret keys. Secret input keys may be based on varied input from a user or the encryption system which may be of fixed or variable length, and a secret key is often a transformed secret key input. A secret key is usually of fixed length. A block cipher usually operates on a secret key, but in some cases may operate on an secret input key. If a block cipher first operates on a secret input key, potentially it may use some algorithm to transform the secret input key into a secret key in a standard format. Then, a block cipher expands the secret key to form subkeys whose length or number of bits exceeds that of the secret key.
Block ciphers and have many rounds calculated in series where each round depends on plaintext through the output of the immediately prior round where generally in each round the same operations are performed iteratively in the same manner. The n-bit input into the block cipher may be called n-bit cipher input. After encryption, the result may be called n-bit cipher output. In each of these rounds, the ordered binary input may be called n-bit cipher round input, and the n-bit ordered binary output may be called n-bit cipher round output. An n-bit cipher input or n-bit cipher output refers to the variable n-bit binary input or variable n-bit binary output of a binary block cipher. Such n-bit cipher input and n-bit cipher output are typically plaintext input and ciphertext output. By contrast, key inputs or subkey values used by a binary block cipher are not variable binary inputs, but are generally fixed or predetermined values for a given use of the block cipher. An n-bit cipher round input or n-bit cipher round output refers to the variable n-bit binary input or variable n-bit binary output of one (and typically of one operative round) round of a binary block cipher.
An operative round of a binary block cipher is an iterative round which calculates new values for each of x primary segments in the round, where x may vary in different operative rounds, where there are a total of n-bits in the x primary segments, and where the new values of the x primary segments determine the n-bit round output. Operative rounds of a binary block cipher refer to iterative rounds which calculate new values for each of x primary segments in a given round, where x may vary in different rounds, where the n-bit cipher round output consists of these x segments of new values, and where the total of all bits of the x segments equals n bits. Binary block ciphers are ciphers receiving inputs consisting of n ordered bits of input and have outputs of the same number of ordered bits (n bits). A mapping of block cipher inputs to outputs reveals that every possible combination of n input bits from 2{circumflex over ( )}n possible combinations has only one corresponding combination of n output bits, and likewise every combination of n output bits from 2{circumflex over ( )}n possible combinations has only one corresponding combination of n input bits. In other words, binary block ciphers transform input values to output values in a manner such that the mapping of this transformation relates the members of the set of all possible ordered input values of n-bits in a one-to-one manner with the members of the set of all possible ordered output values of n-bits.
While a segment is defined simply as a plurality of ordered bits, it is also possible to classify types of segments. There are also round segments and one-to-one round segments.
A round segment is a segment within a round (and typically an operative round) of a binary block cipher which is part of n-bit cipher input or n-bit cipher output, or is calculated within a round or operative round the operative round and is intermediate between input and output; is affected by n-bit cipher round input; and affects n-bit cipher round output. For example, a first value in a calculation is said to affect a second value if, after taking into account the specifics of the particular calculation, a random change in all bits of the first value is likely to change at least one bit of the second value with a chance of at least one in three.
A one-to-one round segment is defined as a member of a one-to-one round segment set. A one-to-one round segment set is defined as a set of ordered round segments in an operative round of a binary block cipher where it is true that each n-bit round input corresponds with only one possible result or group of particular values of the ordered segments of that set, and that any group of particular values of the ordered segments of that set correspond with only one possible n-bit round input. For example, the set of segments in the n-bit cipher output are a one-to-one round segment set. The set of segments in any of the n-bit round input or the n-bit round output of each operative round are also one-to-one round segment sets. Where one-to-one round segment sets are calculated in a binary block cipher which operates on n-bits of input or output, it obviously follows that all such one-to-one round segment sets consist of exactly n-bits.
Note that in general there are usually more one-to-one round segment sets than the examples just mentioned. For example, in most binary block ciphers it is possible to form one-to-one round segment sets by combining particular round segments which ar
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