Malleable field elements #45
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This PR introduces a concept of malleable field elements - i.e., elements which might have redundant internal representation for the same canonical representation. For example, this is the case for field elements where internally elements are stored in the range
[0, 2M)(rather than[0, M)) which we use for ourf62field (thef128field does not have this issue because we don't use Montgomery arithmetic for it).This also allows us to hash elements consistently by first normalizing their internal representation, and only then hashing them. We do this for malleable elements only, since non-malleable elements don't need to be normalized, and thus, we can hash them directly.
We also make sure that we always write elements into the proof in canonical representation. This avoids issues when trying to deserialize elements where canonical and internal representations differ.
The above fixes should resolve the problems mentioned in #13 (comment).