crypto/ed25519: add edge case tests

[FiloSottile]

We'll want to add tests for the edge cases that led to #40475.

Ideally, they would go into Wycheproof and run from there, making sure they run against all architectures.

/cc @hdevalence @katiehockman

[hdevalence]

https://github.com/hdevalence/ed25519consensus is a fork of crypto/ed25519 with a VerifyConsensus method implementing the ZIP215 validation rules described in that repo. (Please forgive my Go, I don't write it regularly).

Relevant to this issue, the zip215_test.go file in that repo has a list of 196 = 14^2 test vectors consisting of purported (public key, signature) pairs on the message b"Zcash". These are constructed by taking pairs from a list of 14 encodings of low-order points, 8 of which are canonically encoded and 6 of which are non-canonically encoded.

The difference between (my interpretation of) RFC 8032 and the current Go behaviour can be visualized as follows:

RFC 8032 (as interpreted)	Go crypto/ed25519

On the left, the dark shade indicates MUST-reject (according to RFC 8032, which was written after this implementation and is incompatible with it), medium indicates MAY-accept, light indicates MUST-accept. (RFC 8032 does not require implementations to agree on whether signatures are valid). On the right, dark and light indicate rejected and accepted, respectively.

Note that all of these test cases require carefully crafted public keys. Therefore, there's no security issue with the signatures themselves, because an attacker who can control the public key can sign any message whatsoever. The problem is just the inconsistency, which could allow, e.g., a malicious party to inject faults into a consensus algorithm and cause consensus divergence.

[FiloSottile]

There's more context in @hdevalence's article here: https://hdevalence.ca/blog/2020-10-04-its-25519am

Indeed, since there is no security risk we probably should not change the behavior now and risk forking some blockchains. We'll add test cases and a paragraph to the docs.

[odeke-em]

Thanks for the report @FiloSottile and for the details and work @hdevalence! We didn't get much action for the edge cases and Go1.16 is due, thus I shall kindly punt this to Go1.17. @hdevalence, it would be awesome to have you as a contributor to the Go project, your work on ed25519 is much appreciated too. I kindly nudge you @hdevalence to feel free to send some changes for Go1.17 and we'll review.

[gopherbot]

Change https://golang.org/cl/304349 mentions this issue: crypto/ed25519: add comprehensive edge-case test vectors