ellswift: introduce ElligatorSwift encoding and decoding funcs

[Crypt-iQ]

The BIP324 ElligatorSwift test vectors are also included. They can be found here and here. This code could be more optimized, possibly by not using so many new(FieldVal) invocations among some other things.

[coveralls]

Pull Request Test Coverage Report for Build 11147308122

Details

• 162 of 275 (58.91%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.01%) to 57.244%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
btcec/ellswift.go	162	275	58.91%

Files with Coverage Reduction	New Missed Lines	%
connmgr/connmanager.go	2	87.32%

Totals
Change from base Build 10970025081:	0.01%
Covered Lines:	30023
Relevant Lines:	52447

---

💛 - Coveralls

[lnliz]

This is great! fwiw, I had great success using the csv files from the BIP324 directly for the tests and it gave me great test coverage too, see esp the big test running through all the "packet_encoding_test_vectors.csv" rows here

[saubyk]

cc: @ellemouton @ProofOfKeags for review

[ProofOfKeags]

Looks to be a correct implementation of the BIP324 gear.

[Crypt-iQ]

This is great! fwiw, I had great success using the csv files from the BIP324 directly for the tests and it gave me great test coverage too, see esp the big test running through all the "packet_encoding_test_vectors.csv" rows here

I don't have a strong opinion on whether we should add the csv files directly. In other parts of the codebase (i.e. other BIP test vectors), we just have the test vectors in the format that I have in this PR

[saubyk]

cc @ellemouton for review comments on this pr. Thanks

[saubyk]

cc: @kcalvinalvin for review

[lnliz]

nit: instead of relying on the init function you should also be able to do this in the var section

_ = c.SetByteSlice(cBytes[:])

[lnliz]

nit: I think would it make sense to only have the ellswiftOurs / ellswiftTheirs order toggle in the if to make clear that everything else stays the same?

Something like this:

	var msg []byte

	if initiating {
		// Initiating, place our public key encoding first.
		var msg []byte
		msg = append(msg, ellswiftOurs[:]...)
		msg = append(msg, ellswiftTheirs[:]...)
	} else {
		msg = append(msg, ellswiftTheirs[:]...)
		msg = append(msg, ellswiftOurs[:]...)
	}

	msg = append(msg, ecdhPoint[:]...)
	return chainhash.TaggedHash(ellswiftTag, msg), nil