If he gets back a cipher that decrypts to his challenge message using d to decrypt it, he will know that it was in all probability created by someone knowing e and hence that the other communicant is probably A. This is a publicly known function that maps any message into a smaller message—called a digest—in which each bit of the digest is dependent on every bit of the message in such a way that changing even one bit in the message is apt to change, in a cryptosecure way, half of the bits in the digest.
By cryptosecure is meant that it is computationally infeasible for anyone to find a message that will produce a preassigned digest and equally hard to find another message with the same digest as a known one. To sign a message—which may not even need to be kept secret— A encrypts the digest with the secret e , which he appends to the message.
Anyone can then decrypt the message using the public key d to recover the digest, which he can also compute independently from the message. If the two agree, he must conclude that A originated the cipher, since only A knew e and hence could have encrypted the message. Thus far, all proposed two-key cryptosystems exact a very high price for the separation of the privacy or secrecy channel from the authentication or signature channel. As a result, the main application of two-key cryptography is in hybrid systems. In such a system a two-key algorithm is used for authentication and digital signatures or to exchange a randomly generated session key to be used with a single-key algorithm at high speed for the main communication.
At the end of the session this key is discarded. In general, cipher systems transform fixed-size pieces of plaintext into ciphertext. In older manual systems these pieces were usually single letters or characters—or occasionally, as in the Playfair cipher , digraphs, since this was as large a unit as could feasibly be encrypted and decrypted by hand.
Systems that operated on trigrams or larger groups of letters were proposed and understood to be potentially more secure, but they were never implemented because of the difficulty in manual encryption and decryption.
Contemporary cryptology—The science of information integrity
A block cipher breaks the plaintext into blocks of the same size for encryption using a common key: the block size for a Playfair cipher is two letters, and for the DES described in the section History of cryptology: The Data Encryption Standard and the Advanced Encryption Standard used in electronic codebook mode it is 64 bits of binary-encoded plaintext. Although a block could consist of a single symbol, normally it is larger. A stream cipher also breaks the plaintext into units, normally of a single character, and then encrypts the i t h unit of the plaintext with the i t h unit of a key stream.
Vernam encryption with a onetime key is an example of such a system, as are rotor cipher machines and the DES used in the output feedback mode in which the ciphertext from one encryption is fed back in as the plaintext for the next encryption to generate a key stream. This means that they must be synchronized initially and stay in sync thereafter, or else the cipher will be decrypted into a garbled form until synchrony can be reestablished.
- Bounds on the Effective Theory of Gravity in Models of Particle Physics and Cosmology.
- Index for C-star-subalgebras.
- Bibliographic Information.
This latter property of self-synchronizing cipher systems results in what is known as error propagation , an important parameter in any stream-cipher system. Article Media.
Classical and Contemporary Cryptology | Open Library
Info Print Print. Table Of Contents.
- ISBN 13: 9780780353527.
- Context of Cryptography!
- Characteristics of Modern Cryptography?
- Dark King and Lightning Tower (The Horus Heresy, Book 8).
- Saint (Paradise Series, Book 2) (The Books of History Chronicles)?
- Cryptography - Wikibooks, open books for an open world;
- Reiki: A Comprehensive Guide.
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