btcec+chaincfg: use pre-computed tag hash values

In this commit, we optimize our signature implementation slightly, by
defining pre-computed sha256(tag) variables for the commonly used
values.  If a tag matches this, then we'll use that hash value to avoid
an extra round of hashing.

btcec/schnorr/signature.go

@@ -22,14 +22,6 @@ const (
 	scalarSize = 32
 )
 
-var (
-	tagHashAux = []byte("BIP0340/aux")
-
-	tagHashNonce = []byte("BIP0340/nonce")
-
-	tagHashChallenge = []byte("BIP0340/challenge")
-)
-
 var (
 	// rfc6979ExtraDataV0 is the extra data to feed to RFC6979 when
 	// generating the deterministic nonce for the BIP-340 scheme.  This
@@ -183,7 +175,7 @@ func schnorrVerify(sig *Signature, hash []byte, pubKeyBytes []byte) error {
 	pBytes := SerializePubKey(pubKey)
 
 	commitment := chainhash.TaggedHash(
-		tagHashChallenge, rBytes[:], pBytes, hash,
+		chainhash.TagBIP0340Challenge, rBytes[:], pBytes, hash,
 	)
 
 	var e btcec.ModNScalar
@@ -325,7 +317,7 @@ func schnorrSign(privKey, nonce *btcec.ModNScalar, pubKey *btcec.PublicKey, hash
 	pBytes := SerializePubKey(pubKey)
 
 	commitment := chainhash.TaggedHash(
-		tagHashChallenge, rBytes[:], pBytes, hash,
+		chainhash.TagBIP0340Challenge, rBytes[:], pBytes, hash,
 	)
 
 	var e btcec.ModNScalar
@@ -492,7 +484,9 @@ func Sign(privKey *btcec.PrivateKey, hash []byte,
 		//
 		// t = bytes(d) xor tagged_hash("BIP0340/aux", a)
 		privBytes := privKeyScalar.Bytes()
-		t := chainhash.TaggedHash(tagHashAux, (*opts.authNonce)[:])
+		t := chainhash.TaggedHash(
+			chainhash.TagBIP0340Aux, (*opts.authNonce)[:],
+		)
 		for i := 0; i < len(t); i++ {
 			t[i] ^= privBytes[i]
 		}
@@ -504,7 +498,7 @@ func Sign(privKey *btcec.PrivateKey, hash []byte,
 		// We snip off the first byte of the serialized pubkey, as we
 		// only need the x coordinate and not the market byte.
 		rand := chainhash.TaggedHash(
-			tagHashNonce, t[:], pubKeyBytes[1:], hash,
+			chainhash.TagBIP0340Nonce, t[:], pubKeyBytes[1:], hash,
 		)
 
 		// Step 8.

chaincfg/chainhash/hash.go

@@ -17,6 +17,25 @@ const HashSize = 32
 // MaxHashStringSize is the maximum length of a Hash hash string.
 const MaxHashStringSize = HashSize * 2
 
+var (
+	// TagBIP0340Challenge is the BIP-0340 tag for challenges.
+	TagBIP0340Challenge = []byte("BIP0340/challenge")
+
+	// TagBIP0340Aux is the BIP-0340 tag for aux data.
+	TagBIP0340Aux = []byte("BIP0340/aux")
+
+	// TagBIP0340Nonce is the BIP-0340 tag for nonces.
+	TagBIP0340Nonce = []byte("BIP0340/nonce")
+
+	// precomputedTags is a map containing the SHA-256 hash of the BIP-0340
+	// tags.
+	precomputedTags = map[string]Hash{
+		string(TagBIP0340Challenge): sha256.Sum256(TagBIP0340Challenge),
+		string(TagBIP0340Aux):       sha256.Sum256(TagBIP0340Aux),
+		string(TagBIP0340Nonce):     sha256.Sum256(TagBIP0340Nonce),
+	}
+)
+
 // ErrHashStrSize describes an error that indicates the caller specified a hash
 // string that has too many characters.
 var ErrHashStrSize = fmt.Errorf("max hash string length is %v bytes", MaxHashStringSize)
@@ -84,10 +103,13 @@ func NewHash(newHash []byte) (*Hash, error) {
 // TaggedHash implements the tagged hash scheme described in BIP-340. We use
 // sha-256 to bind a message hash to a specific context using a tag:
 // sha256(sha256(tag) || sha256(tag) || msg).
-//
-// TODO(roasbeef): add fast paths for common known tags
 func TaggedHash(tag []byte, msgs ...[]byte) *Hash {
-	shaTag := sha256.Sum256(tag)
+	// Check to see if we've already pre-computed the hash of the tag. If
+	// so then this'll save us an extra sha256 hash.
+	shaTag, ok := precomputedTags[string(tag)]
+	if !ok {
+		shaTag = sha256.Sum256(tag)
+	}
 
 	// h = sha256(sha256(tag) || sha256(tag) || msg)
 	h := sha256.New()