Does the security of Skein as a hash imply the security of Threefish as a block cipher?

Question

The Skein hash proposed for SHA-3 boasts some impressive speed results, which I suspect would be applicable for the Threefish block cipher at its heart - but, if Skein is approved for SHA-3, would this imply that Threefish is considered secure as well? That is, would any vulnerability in Threefish imply a vulnerability in SHA-3? (and thus, a lack of known issues and a general trust in SHA-3 imply the same for Threefish)

Answer 1

Nope. The security of Skein does not imply the security of Threefish. Putting it positively, if someone finds a weakness in Threefish then this does not imply that Skein is also insecure.

The question however, is quite intersting an applies to other hash functions too. Skein uses a Davis-Meyer construction with some modification. MD5, SHA1 and many other hash functions are also using this Davis-Meyer construction and hence they are in principle based on a block cipher. Just in case of MD5 or SHA1 that block cipher does not have a name and I'm not aware of much research on how suitable these constructs are.

The requirements for a good block cipher and for a good hash function are different. Somewhat simplified, if E is a block cipher and it is not feasible to find two keys K, K' and two messages M, M' such that E_K(M) xor M = E_K'(M') xor M' then E is suitable for constructing a hash function using Davis-Meyer. But to be secure as a block cipher E would need other properties. E would have to resist chosen-ciphertext attacks, chosen-plaintext attacks etc.

Furthermore, if E is a good block cipher then that does also not mean it gives a good hash function. Microsoft had to learn this the hard way with the hash they used in the XBOX. This hash was based on the block cipher TEA that had a weakness that was insignificant for a block cipher, but proved fatal when used for a hash function.

To be fair, there are some relations between being a good block cipher and being suitable for a hash function. E.g., in both cases differential attacks need to be avoided. Hence some design methods used for construction good block ciphers can be used to construct good hash functions.

Let me also add that some of the proposals for SHA-3 are based on AES. So far, I haven't seen much support for favoring AES based hash functions, just because AES is already a standard. These hash functions are analyzed just like any other SHA-3 proposal.

Answer 2

Disregard my previous answer. I misunderstood the relationship between Skein and Threefish. I still don't think Skein being approved absolutely proves Threefish is generally secure (it's possible Threefish is only secure when used in a particular manner), but it would be an indication.