Raise maxStandardSigScriptSize to 1650.

This commit raises the maximum allowed size for a standard signature
script to cover a 15-of-15 multi-signature pay-to-script-hash with
compressed pubkeys.

This mirrors a recent change to remain compatible with Bitcoin Core.

ok @owinga who also helped verify and correct the script math.

Closes #128.

mempool.go

@@ -53,12 +53,22 @@ const (
 
 	// maxStandardSigScriptSize is the maximum size allowed for a
 	// transaction input signature script to be considered standard.  This
-	// value allows for a CHECKMULTISIG pay-to-sript-hash with 3 signatures
-	// since each signature is about 80-bytes, the 3 corresponding public
-	// keys are 65-bytes each if uncompressed, and the script opcodes take
-	// a few extra bytes.  This value also adds a few extra bytes for
-	// prosperity.  3*80 + 3*65 + 65 = 500
-	maxStandardSigScriptSize = 500
+	// value allows for a 15-of-15 CHECKMULTISIG pay-to-script-hash with
+	// compressed keys.
+	//
+	// The form of the overall script is: OP_0 <15 signatures> OP_PUSHDATA2
+	// <2 bytes len> [OP_15 <15 pubkeys> OP_15 OP_CHECKMULTISIG]
+	//
+	// For the p2sh script portion, each of the 15 compressed pubkeys are
+	// 33 bytes (plus one for the OP_DATA_33 opcode), and the thus it totals
+	// to (15*34)+3 = 513 bytes.  Next, each of the 15 signatures is a max
+	// of 73 bytes (plus one for the OP_DATA_73 opcode).  Also, there is one
+	// extra byte for the initial extra OP_0 push and 3 bytes for the
+	// OP_PUSHDATA2 needed to specify the 513 bytes for the script push.
+	// That brings the total to 1+(15*74)+3+513 = 1627.  This value also
+	// adds a few extra bytes to provide a little buffer.
+	// (1 + 15*74 + 3) + (15*34 + 3) + 23 = 1650
+	maxStandardSigScriptSize = 1650
 
 	// maxStandardMultiSigKeys is the maximum number of public keys allowed
 	// in a multi-signature transaction output script for it to be