Showing papers on "Ciphertext published in 1982"

Cryptographic protocols

Abstract: A cryptographic transformation is a mapping f from a set of cleartext messages, M, to a set of ciphertext messages. Since for m e M, f(m) should hide the contents of m from an enemy, f-1 should, in a certain technical sense, be difficult to infer from f(m) and public knowledge about f. A cryptosystem is a model of computation and communication which permits the manipulation of messages by cryptographic transformations.

Why and how to establish a private code on a public network

Abstract: The Diffie and Hellman model of a Public Key Cryptosystem has received much attention as a way to provide secure network communication. In this paper, we show that the original Diffie and Hellman model does not guarantee security against other users in the system. It is shown how users, which are more powerful adversarys than the traditionally considered passive eavesdroppers, can decrypt other users messages, in implementations of Public Key Cryptosystem using the RSA function, the Rabin function and the Goldwasser&Micali scheme. This weakness depends on the bit security of the encryption function. For the RSA (Rabin) function we show that computing, from the cyphertext, specific bits of the cleartext, is polynomially equivalent to inverting the function (factoring). As for many message spaces, this bit can be easily found out by communicating, the system is insecure. We present a modification of the Diffie and Hellman model of a Public-Key Cryptosystem, and one concrete implementation of the modified model. For this implementation, the difficulty of extracting partial information about clear text messages from their encoding, by eavesdroppers, users or by Chosen Cyphertext Attacks is proved equivalent to the computational difficulty of factoring. Such equivalence proof holds in a very strong probabilistic sense and for any message space. No additional assumptions, such as the existence of a perfect signature scheme, or a trusted authentication center, are made.

Cryptanalysis of the data encryption standard by the method of formal coding

Abstract: The "Method of Formal Coding" consists in representing each bit of a DES ciphertext block as an XOR-sum-of-products of the plaintext bits and the key bits. Subsequent introduction of the "MFC-complexity measure" yields interesting results on the security of the DES and the influecce of various parameters.

Multiplexed sequences: some properties of the minimum polynomial

Abstract: In recent years considerable interest has been shown in the generation of binary sequences which have good randomness properties. Such sequences play an important role in cipher systems. In many situations the enciphering process begins with the conversion of the plaintext into string of bits by means of a binary “alphabet”. The sequence is then added to the plaintext bit by bit, using modulo 2 arithmetic and the resulting ciphertext is then transmitted. Decipherment is accomplished simply by adding the sequence to the ciphertext it a similar manner.

Forward Search as a Cryptanalytic Tool Against a Public Key

Abstract: In symmetric cryptosystems that depend on a single (secret) key for both encryption and decryption, a cryptanalyst -- since the key is unknown to him -- must either work backward from the cipher or else from the cipher and some known pairs of plain-text messages and matching ciphers in attempting to recover the plaintext. In an asymmetric (two key) cryptosystem used in the public key, i.e., privacy channel, mode where the encryption key is publicly expcxsedso that anyone who wishes can encrypt messages that can mly be decrypted by the person having the (secret) decryption key, a cryptanalytic weakness may occur that has no counterpart in symmetric systems. If the entropy of the input messages is too small (roughly speaking if it is computationally feasible to search through the most likely messages) either because the total number of messages is small or because a small number of the messages occur with high probabilities, the cryptanalyst can pre-encrypt these messages to form a cipher file that can then be matched against observed ciphers to accomplish a simple substitution decryption. This forward search cryptanalytic weakness of a public key privacy channel is purely a function of the entropy of the plaintext messages and does not depend on the existence of any cryptanalytic weakness in the concealment of the secret decryption key from a knowledge of the public encryption key in the underlying public key algorithm.

Forward search as a cryptanalytic tool against a public key privacy channel

Abstract: In symmetric cryptosystems that depend on a single (secret) key for both encryption and decryption, a cryptanalyst - since the key is unknown to him - must either work backward from the cipher or else from the cipher and some known pairs of plaintext messages and matching ciphers in attempting to recover the plaintext. In an asymmetric (two key) cryptosystem used in the public key, i.e., privacy channel, mode where the encryption key is publicly exposed so that anyone who wishes can encrypt messages that can only be decrypted by the person having the (secret) decryption key, a cryptanalytic weakness may occur that has no counterpart in symmetric systems. If the entropy of the input messages is too small (roughly speaking if it is computationally feasible to search through the most likely messages) either because the total number of messages is small or because a small number of the messages occur with high probabilities, the cryptanalyst can pre-encrypt these messages to form a cipher file that can then be matched against observed ciphers to accomplish a simple substitution decryption. This forward search cryptanalytic weakness of a public key privacy channel is purely a function of the entropy of the plaintextmore » messages and does not depend on the existence of any cryptanalytic weakness in the concealment of the secret decryption key from a knowledge of the public encryption key in the underlying public key algorithm.« less