What is the codebook attack? Why this attack is feasible when 16 or 32-bit blocks have been used; but not feasible when 64-bit blocks have been used?
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A codebook attack is a type of cryptographic attack where an attacker tries to break a block cipher by discovering the secret key used to encrypt or decrypt data. In this attack, the attacker creates a "codebook" that contains all possible key-value pairs, mapping every possible plaintext to its corresponding ciphertext.
When 16 or 32-bit blocks are used, the number of possible key-value pairs in the codebook is manageable for an attacker. With 16-bit blocks, for example, there are only 2^16 (65536) possible plaintext-ciphertext pairs. An attacker can precompute this codebook, storing all possible plaintexts encrypted with all possible keys.
However, when 64-bit blocks are used, the number of possible key-value pairs becomes unmanageable. With 64-bit blocks, there are 2^64 (18,446,744,073,709,551,616) possible plaintext-ciphertext pairs, which makes precomputing a codebook infeasible due to the vast number of combinations.
Therefore, a codebook attack is feasible when 16 or 32-bit blocks have been used due to the manageable number of pairs, but it becomes infeasible when 64-bit blocks are used due to the exponentially larger number of pairs.
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