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Length extension attack The reason why $H(k || m)$ is insecure with most older hashes is that they use the Merkle–Damgård construction which suffers from length extensions. When length extensions are available it's possible to compute $H(k || m || m^\prime)$ knowing only $H(k || m)$ but not $k$. This violates the security requirements of a MAC. Like all ...

3

Internally, Skein-1024 uses the Threefish-1024 compression function and is extended to 1024 bits of internal state whereas Skein-512 uses the Threefish-512 compression function. There are a few additional changes but they are very similar (just like SHA-256 and SHA-512 are, for instance). The tweak handling is, however, the same since all Threefish variants ...

2

The main reason for designing a cryptographic hash function using an output transformation is flexibility. In the following, I try to motivate the need for an output transformation. For example, SHA-256 has no output transformation and a fixed output size of 256-bit. What happens if you truncate a SHA-256 hash value? In this case, there are no security ...

2

On multi-threading: Read the code. As Paulo writes, that's up to the implementation. Read the source code of your library and see for yourself what it is doing; it shouldn't be too hard to figure it out.

2

There are two parts to this proposal: the use of a code book and a scheme to send short confidential and authenticated messages utilising an existing shared symmetric key. A code book can be used alone to provide a degree of confidentiality, or can be used to ascribe specific pre-agreed meanings to short messages, in combination with any scheme for sending ...

2

Is there any advantage – other than potential memory or speed performance reasons – of picking a state size different from 512? If, what would the advantage(s) be? Yes. With 256-bit state, the main advantages are memory use and hardware implementation area. With 1024-bit state, a hardware implementation can be faster, but there are also security ...

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The paper describing the Skein hash function does contain a short description in section 4.9 on how to use it as a PRNG (as otus and SEJPM pointed out). If you use that mode then Skein can be used as PRNG. If the security of Skein holds then you should be able to use it for very large amounts of output data, not just 16 KiB of output. Note that Skein is "...

1

Yes, I don't see why this scheme would not be secure. It uses a MAC over known data - protected against replay. If that data is received or calculated locally shouldn't matter. But as you already showed yourself, you bring down the security of the tag with the amount of bits required to calculate the options. So in the end you could as well just CTR-encrypt ...

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