Rather than using keys such as huge as a digit disposable, is used more often algorithms that have a secret key relatively small (80 to 128 bits, but using this key to a seemingly complex it is impossible for an enemy to find the value.
Therefore, since the key is so small, it is not easy to try all possibilities to reach the decryption? In fact, no computer could be achieved in a realistic timeframe. Indeed, if the key was 128 bits, ie a sequence of 128 "0" and "1", there would be 2128 or 100000000000000000000000000000000000000 possible keys!
The objective is to: develop from a random message M numbers, or at least that seems random, only the detention of the key K to decrypt.
Specifically, it involves constructing a bijective function "pseudo-random":
- It must be a bijection, since it must correspond to each digit of the plaintext message a number of coded message, and because it must allow power from a number C, trace uniquely corresponds to the message plain
- It must be random or appear: in cryptography, perfection is random, the coded message is appearing to arise directly from the chance to reduce the risk of an attack by analyzing the ciphertext, its redundancy, etc..
The development of a function involving these two conditions posed a problem for cryptographers until the 1950s when Feistel showed a pseudo-random function is transformed by a simple method, bijection. Currently, the method of secret-key encryption used more frequently.
Explanation:
- Let a function f that takes as argument a word of n bits.
- The encryption algorithm will proceed by encrypting blocks of 2n bits, which divides into 2 parts of n bits each: the parties (L) and right (D).
- The image of the block (G, D) is the block (L, R) with L and R D = G = XOR f (D).
- This transformation is bijective, because if we have a couple (L, R), there are many (G, D) with D = L and G = R XOR F (L).
- The right side has not been processed (sent just to the left). It must therefore repeat the Feistel scheme a number of times (known as rounds).
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