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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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My question is - are there any known methods (e.g. ever used in history) for selecting specified positions in point 2)? Your construction is quite similar to the Running Key Cipher, which is soemtimes considered a variant of the Vigenere cipher. There they just start at one position and use all subsequent symbols as keystream. ... He told me that he ...

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Thanks to everyone for the hints and comments. I would like to propose you my proof by contradiction and kindly ask for a further comment. Being $\pi=(KG,TAG,VRFY)$ an uuf MAC according to the first game, we suppose that $\pi$ isn't uuf according to the second one. On the basis of this hypothesis, an attacker A can guess the secret key $k$. More formally, $... 0 It isn't secure at all unless the GUIDs are completely inaccessible to everything except the encryption code. I doubt that is the case in a real application; I assume every DB query and API call passes those GUIDs around. Any anyone who can access the database can easily hash the GUIDs as well and decrypt any of the data. Your scheme as described is ... 2 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? 1 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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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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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 ...

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