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As per title, other than brute force, are there any attacks on Threefish-512 using only a single plaintext block? Are there any attacks like this in any other cipher?

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A single plaintext/ciphertext pair is much too small a sample to do anything on modern ciphers. On older, or significantly broken ciphers, it may be enough for successful cryptanalysis though. For Threefish-512, there are none, to my knowledge. –  Thomas Jul 18 '12 at 0:34
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According to Schneier, there is an attack on 42 rounds of ThreeFish-512, but if you check out the analysis, it relies on multiple plaintext values.

Wikipedia lists no singular known plaintext attack, only a boomerang attack, which also depends on multiple known plaintexts.

So given these sources, it would seem there is not a well known and effective attack on three fish 512 with only one plaintext block.

Generally speaking, given only an encryption of a single plaintext block it would be difficult to use cryptanalysis to break a cipher. Even block cipher modes of operation which are deprecated, such as ECB, will operate securely for single block-key encryptions.

The Known-Plaintext-Attack seems to utilize single blocks of plaintext in some situations. That wiki site lists its usage on a cipher used during WWII, so technically Yes, there do exist ciphers which have been susceptible to a single known plaintext block at some points in history. But for modern ciphers, No, it does not seem to be an effective means of attack

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Both attacks only work on reduced round ThreeFish, and not on the full Threefish. The first one only works that well on older versions of Threefish. –  CodesInChaos Jul 18 '12 at 15:49
    
yep all of that it correct. I'll edit to make it more clear –  crawfish Jul 18 '12 at 15:51
    
Thank you for your answer. Marked as such. –  Erkling Jul 21 '12 at 14:38
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