12-06-2012, 02:57 PM
CLASSICAL ENCRYPTION TECHNIQUE
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SYMMETRIC CIPHER MODEL
There are 2 requirements for secure use of conventional encryption:
1. We need a strong encryption algorithm – the opponent should be unable to decrypt ciphertext or to discover the key even if s/he is in the possession of a number of ciphertexts together with the plaintext that produced each ciphertext
2. Sender and receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure. If someone can discover the key and knows the algorithm, all communication using this key is readable
We assume that it is impractical to decrypt a message on the basis of the ciphertext plus knowledge of the encryption/decryption algorithm, i.e. we do not need to keep the algorithm secret; we need to keep only the key secret.
CRYPTOGRAPHY
Cryptographic systems are characterized by
1. The type of operations used for transforming plaintext to ciphertext (substitution, transposition). Fundamental requirement – no information be lost
2. The number of keys used (1 key – symmetric, single-key, secret-key; 2 keys – asymmetric, two-key, public-key)
3. The way in which the plaintext is processed (block cipher, stream cipher). Stream cipher may be viewed as a block cipher with block size equal to 1 element.
CRYPTANALYSIS
There are two general approaches to attacking a conventional encryption scheme:
1. Cryptanalysis: attempts to use characteristics of the plaintext or even some plaintext-ciphertext pairs to deduce a specific plaintext or key being used
2. Brute-force attack: every possible key is tried until an intelligible translation into plaintext is obtained. On average, half of all possible keys should be tried to achieve success.
SUBSTITUTION TECHNIQUE
A substitution technique is one in which the letters of plaintext are replaced by other letters or by numbers. If the plaintext is viewed as a sequence of bits, then substitution involves replacing plaintext bit patterns with ciphertext bit patterns.
[attachment=24212]
SYMMETRIC CIPHER MODEL
There are 2 requirements for secure use of conventional encryption:
1. We need a strong encryption algorithm – the opponent should be unable to decrypt ciphertext or to discover the key even if s/he is in the possession of a number of ciphertexts together with the plaintext that produced each ciphertext
2. Sender and receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure. If someone can discover the key and knows the algorithm, all communication using this key is readable
We assume that it is impractical to decrypt a message on the basis of the ciphertext plus knowledge of the encryption/decryption algorithm, i.e. we do not need to keep the algorithm secret; we need to keep only the key secret.
CRYPTOGRAPHY
Cryptographic systems are characterized by
1. The type of operations used for transforming plaintext to ciphertext (substitution, transposition). Fundamental requirement – no information be lost
2. The number of keys used (1 key – symmetric, single-key, secret-key; 2 keys – asymmetric, two-key, public-key)
3. The way in which the plaintext is processed (block cipher, stream cipher). Stream cipher may be viewed as a block cipher with block size equal to 1 element.
CRYPTANALYSIS
There are two general approaches to attacking a conventional encryption scheme:
1. Cryptanalysis: attempts to use characteristics of the plaintext or even some plaintext-ciphertext pairs to deduce a specific plaintext or key being used
2. Brute-force attack: every possible key is tried until an intelligible translation into plaintext is obtained. On average, half of all possible keys should be tried to achieve success.
SUBSTITUTION TECHNIQUE
A substitution technique is one in which the letters of plaintext are replaced by other letters or by numbers. If the plaintext is viewed as a sequence of bits, then substitution involves replacing plaintext bit patterns with ciphertext bit patterns.