add hash function Tip5

[jan-ferdinand]

This PR is trying to address #187.

Unfortunately, the tests are not passing. I suspect this has to do with me raising the upper bound for the number of bytes in a Digest (see here). However, the feedback provided by the failing tests doesn't let me pinpoint the root cause very easily. Since I don't know the codebase very well, I'd be happy if someone in the know could take a look.

Additionally, I'm not sure if my handling of bytes is correct in all places. For example (but not limited to): my current implementation of the Hasher's trait method fn hash(bytes: &[u8]) interprets 8 bytes as a field element of the field with $2^{64} - 2^{32} + 1$ elements¹. This can lead to hash collisions, depending on the type of objects being fed into the method. I'd appreciate some guidance here.

Footnotes

1. This is the only field the hash function Tip5 is defined over, so there is no need to be generic with respect to the field. ↩

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[ameya-deshmukh]

Hi! I'll try hacking on this and will let you know if tests pass for me. In addition to this, happy to reproduce and review this PR

[Nashtare]

I don't think it makes sense to increase the size of Digest bytes for all existing instantiations, as this need comes solely from the specific version of Tip5 you want to implement. Instead I'd suggest having the output length defined by an associated constant.

[jan-ferdinand]

@Nashtare, I agree. However, taken straight from the description of the Digest trait:

Ideally, the length of the returned array should be defined by an associated constant, but using associated constants in const generics is not supported by Rust yet. Thus, we put an upper limit on the possible digest size. For digests which are smaller than 32 [now 40] bytes, the unused bytes should be set to 0.

Refactoring the trait was beyond what I was willing to invest into this PR for the time being.