btcec/schnorr: fix signing and verifying to match BIP340 and libsecp256k1

behavior

Sign, and Verify: accept variable length messages, as required by
BIP340 and implemented by libsecp256k1

Sign: don't use RFC6979 when auxRand bytes are not passed by the
caller, instead, match BIP340 spec and libsecp256k1 implementation by
making not passing CustomNonce equivalent to passing a zeroed 32 byte
array.

Deprecated CustomNonce functional option, replaced by AuxRand due to misleading
name.

Author: allocz

btcec/schnorr/bench_test.go

@@ -136,16 +136,17 @@ func BenchmarkSign(b *testing.B) {
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
 		sig, err = Sign(
-			privKey, msgHash, CustomNonce(auxBytes), FastSign(),
+			privKey, msgHash, AuxRand(auxBytes), FastSign(),
 		)
 	}
 
 	testSig = sig
 	testErr = err
 }
 
-// BenchmarkSignRfc6979 benchmarks how long it takes to sign a message.
-func BenchmarkSignRfc6979(b *testing.B) {
+// BenchmarkSignDefault benchmarks how long it takes to sign a message when
+// AuxRand is not passed in
+func BenchmarkSignDefault(b *testing.B) {
 	// Randomly generated keypair.
 	d := hexToModNScalar("9e0699c91ca1e3b7e3c9ba71eb71c89890872be97576010fe593fbf3fd57e66d")
 	privKey := secp256k1.NewPrivateKey(d)