# Tag Info

11

Do all ciphers suffer from the problem of multiple equivalent decryption keys? No. The number of non-equivalent keys is bounded by the number of permutations. Since the number of permutations is very high there is a very big chance that ciphers do not have equivalent keys. This is especially true for ciphers with a high block size (AES with 128 bits). Even ...

4

Schneier is talking about distinguishing a block cipher from an ideal cipher - or in other words, about formal definitions for security. Think of a game, where the attacker is given a ciphertext encrypted either with a block cipher or with an ideal cipher (with equal probability), and has to guess which cipher encrypted the message. Let's say this attacker ...

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The size we speak of with regard to elliptic curves is the size of the field over which the elliptic curve is defined. This is not necessarily exactly the size of the private key. For example: Curve25519 is a 255-bit elliptic curve and has, effectively, 252-bit private keys, though they are usually encoded as 256-bit values with four fixed bits. Public keys ...

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I was going to make this a comment; however you asked for hint, and these are hints. Suppose you had an Oracle that solved the second game for you; how could you use that Oracle to solve the first game? Does that imply that a MAC where the first game is unsolvable imply that the second game is also unsolvable?

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An exhaustive search of half the key space requires $2^{n-1}$ work and provides the right answer 75% of the time. I haven't read that book, and so they may give a cavaet about the larger picture. However, as specified, I don't believe that's correct, but not for the reason you think. If you present a distinguisher with a copy of the cipher, it will give ...

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