crypto, crypto/secp256k1: add CompressPubkey (#15626)

This adds the inverse to DecompressPubkey and improves a few minor
details in crypto/secp256k1.

1 files changed, 23 insertions, 17 deletions

diff --git a/crypto/secp256k1/ext.h b/crypto/secp256k1/ext.h
index b0f30b73c..9b043c724 100644
--- a/crypto/secp256k1/ext.h
+++ b/crypto/secp256k1/ext.h
@@ -19,7 +19,7 @@ static secp256k1_context* secp256k1_context_create_sign_verify() {
 	return secp256k1_context_create(SECP256K1_CONTEXT_SIGN | SECP256K1_CONTEXT_VERIFY);
 }
 
-// secp256k1_ecdsa_recover_pubkey recovers the public key of an encoded compact signature.
+// secp256k1_ext_ecdsa_recover recovers the public key of an encoded compact signature.
 //
 // Returns: 1: recovery was successful
 //          0: recovery was not successful
@@ -27,7 +27,7 @@ static secp256k1_context* secp256k1_context_create_sign_verify() {
 //  Out:    pubkey_out: the serialized 65-byte public key of the signer (cannot be NULL)
 //  In:     sigdata:    pointer to a 65-byte signature with the recovery id at the end (cannot be NULL)
 //          msgdata:    pointer to a 32-byte message (cannot be NULL)
-static int secp256k1_ecdsa_recover_pubkey(
+static int secp256k1_ext_ecdsa_recover(
 	const secp256k1_context* ctx,
 	unsigned char *pubkey_out,
 	const unsigned char *sigdata,
@@ -46,7 +46,7 @@ static int secp256k1_ecdsa_recover_pubkey(
 	return secp256k1_ec_pubkey_serialize(ctx, pubkey_out, &outputlen, &pubkey, SECP256K1_EC_UNCOMPRESSED);
 }
 
-// secp256k1_ecdsa_verify_enc verifies an encoded compact signature.
+// secp256k1_ext_ecdsa_verify verifies an encoded compact signature.
 //
 // Returns: 1: signature is valid
 //          0: signature is invalid
@@ -55,7 +55,7 @@ static int secp256k1_ecdsa_recover_pubkey(
 //          msgdata:    pointer to a 32-byte message (cannot be NULL)
 //          pubkeydata: pointer to public key data (cannot be NULL)
 //          pubkeylen:  length of pubkeydata
-static int secp256k1_ecdsa_verify_enc(
+static int secp256k1_ext_ecdsa_verify(
 	const secp256k1_context* ctx,
 	const unsigned char *sigdata,
 	const unsigned char *msgdata,
@@ -74,28 +74,34 @@ static int secp256k1_ecdsa_verify_enc(
 	return secp256k1_ecdsa_verify(ctx, &sig, msgdata, &pubkey);
 }
 
-// secp256k1_decompress_pubkey decompresses a public key.
+// secp256k1_ext_reencode_pubkey decodes then encodes a public key. It can be used to
+// convert between public key formats. The input/output formats are chosen depending on the
+// length of the input/output buffers.
 //
-// Returns: 1: public key is valid
-//          0: public key is invalid
+// Returns: 1: conversion successful
+//          0: conversion unsuccessful
 // Args:    ctx:        pointer to a context object (cannot be NULL)
-//  Out:    pubkey_out: the serialized 65-byte public key (cannot be NULL)
-//  In:     pubkeydata: pointer to 33 bytes of compressed public key data (cannot be NULL)
-static int secp256k1_decompress_pubkey(
+//  Out:    out:        output buffer that will contain the reencoded key (cannot be NULL)
+//  In:     outlen:     length of out (33 for compressed keys, 65 for uncompressed keys)
+//          pubkeydata: the input public key (cannot be NULL)
+//          pubkeylen:  length of pubkeydata
+static int secp256k1_ext_reencode_pubkey(
 	const secp256k1_context* ctx,
-	unsigned char *pubkey_out,
-	const unsigned char *pubkeydata
+	unsigned char *out,
+	size_t outlen,
+	const unsigned char *pubkeydata,
+	size_t pubkeylen
 ) {
 	secp256k1_pubkey pubkey;
 
-	if (!secp256k1_ec_pubkey_parse(ctx, &pubkey, pubkeydata, 33)) {
+	if (!secp256k1_ec_pubkey_parse(ctx, &pubkey, pubkeydata, pubkeylen)) {
 		return 0;
 	}
-	size_t outputlen = 65;
-	return secp256k1_ec_pubkey_serialize(ctx, pubkey_out, &outputlen, &pubkey, SECP256K1_EC_UNCOMPRESSED);
+	unsigned int flag = (outlen == 33) ? SECP256K1_EC_COMPRESSED : SECP256K1_EC_UNCOMPRESSED;
+	return secp256k1_ec_pubkey_serialize(ctx, out, &outlen, &pubkey, flag);
 }
 
-// secp256k1_pubkey_scalar_mul multiplies a point by a scalar in constant time.
+// secp256k1_ext_scalar_mul multiplies a point by a scalar in constant time.
 //
 // Returns: 1: multiplication was successful
 //          0: scalar was invalid (zero or overflow)
@@ -104,7 +110,7 @@ static int secp256k1_decompress_pubkey(
 //  In:     point:    pointer to a 64-byte public point,
 //                    encoded as two 256bit big-endian numbers.
 //          scalar:   a 32-byte scalar with which to multiply the point
-int secp256k1_pubkey_scalar_mul(const secp256k1_context* ctx, unsigned char *point, const unsigned char *scalar) {
+int secp256k1_ext_scalar_mul(const secp256k1_context* ctx, unsigned char *point, const unsigned char *scalar) {
 	int ret = 0;
 	int overflow = 0;
 	secp256k1_fe feX, feY;