From 75d26b134f5d4f956095253608cd702f63d43bd2 Mon Sep 17 00:00:00 2001
From: The MMGen Project <mmgen@tuta.io>
Date: Sat, 15 Aug 2026 14:13:08 +0000
Subject: [PATCH] secp256k1 extmod: new `pubkey_decompress()` function

---
 extmod/secp256k1mod.c    | 28 ++++++++++++++++++++++++++++
 mmgen/bip_hd/__init__.py | 10 +++-------
 test/modtest_d/ecc.py    |  8 +++++++-
 3 files changed, 38 insertions(+), 8 deletions(-)

diff --git a/extmod/secp256k1mod.c b/extmod/secp256k1mod.c
index bd1a7dc1..013e0104 100755
--- a/extmod/secp256k1mod.c
+++ b/extmod/secp256k1mod.c
@@ -197,6 +197,28 @@ static PyObject * pubkey_check(PyObject *self, PyObject *args) {
 	return Py_BuildValue("I", 1);
 }
 
+static PyObject * pubkey_decompress(PyObject *self, PyObject *args) {
+	const unsigned char * in_pubkey_bytes;
+	Py_ssize_t in_pubkey_bytes_len;
+	if (!PyArg_ParseTuple(args, "y#", &in_pubkey_bytes, &in_pubkey_bytes_len)) {
+		PyErr_SetString(PyExc_ValueError, "Unable to parse extension mod arguments");
+		return NULL;
+	}
+	secp256k1_context *ctx = create_context(1);
+	secp256k1_pubkey pubkey;
+	if (!pubkey_parse_with_check(ctx, &pubkey, in_pubkey_bytes, in_pubkey_bytes_len)) {
+		return NULL;
+	}
+	size_t pubkey_bytes_len = 65;
+	unsigned char pubkey_bytes[pubkey_bytes_len];
+	if (secp256k1_ec_pubkey_serialize(ctx, pubkey_bytes, &pubkey_bytes_len, &pubkey,
+			SECP256K1_EC_UNCOMPRESSED) != 1) {
+		PyErr_SetString(PyExc_RuntimeError, "Public key serialization failed");
+		return NULL;
+	}
+	return Py_BuildValue("y#", pubkey_bytes, pubkey_bytes_len);
+}
+
 /*
  * returns 64-byte serialized signature (r + s) plus integer recovery ID in range 0-3
  */
@@ -396,6 +418,12 @@ static PyMethodDef secp256k1_methods[] = {
 		METH_VARARGS,
 		"Recover a serialized pubkey from a recoverable signature plus signed message"
 	},
+	{
+		"pubkey_decompress",
+		pubkey_decompress,
+		METH_VARARGS,
+		"Convert a compressed or uncompressed serialized pubkey into an uncompressed serialized pubkey"
+	},
 	{NULL, NULL}
 };
 
diff --git a/mmgen/bip_hd/__init__.py b/mmgen/bip_hd/__init__.py
index 3f954e12..63dc7218 100755
--- a/mmgen/bip_hd/__init__.py
+++ b/mmgen/bip_hd/__init__.py
@@ -26,7 +26,7 @@ from ..addr import MMGenAddrType
 from ..key import PrivKey
 from ..protocol import CoinProtocol, init_proto
 from ..proto.btc.common import hash160, b58chk_encode, b58chk_decode
-from ..proto.secp256k1.secp256k1 import pubkey_tweak_add, pubkey_check
+from ..proto.secp256k1.secp256k1 import pubkey_tweak_add, pubkey_check, pubkey_decompress
 
 from . import chainparams
 chainparams_data = chainparams.parse_data()
@@ -56,10 +56,6 @@ def compress_pubkey(pubkey_bytes):
 	# see: proto.secp256k1.keygen.pubkey_format()
 	return (b'\x02', b'\x03')[pubkey_bytes[-1] & 1] + pubkey_bytes[1:33]
 
-def decompress_pubkey(pubkey_bytes):
-	import ecdsa
-	return b'\x04' + ecdsa.VerifyingKey.from_string(pubkey_bytes, curve=ecdsa.curves.SECP256k1).to_string()
-
 class Bip32ExtendedKey(Lockable):
 
 	def __init__(self, key_b58):
@@ -73,7 +69,7 @@ class Bip32ExtendedKey(Lockable):
 
 		# Serialization:
 		#   ver_bytes | depth | par_print | idx      | chaincode  | serialized_key
-		#   0:4 (4)   | 4 (1) | 5:9 (4)   | 9:13 (4) | 13:45 (32) | 45(46): 33(32)
+		#   0:4 (4)   | 4 (1) | 5:9 (4)   | 9:13 (4) | 13:45 (32) | 45: (33), or 46: (32)
 		ver_hex = key[:4].hex()
 		bipnum, cp_entry = parse_version_bytes(ver_hex)
 
@@ -273,7 +269,7 @@ class BipHDNode(Lockable):
 	def address(self):
 		return self.cfg.ag.to_addr(
 			keygen_public_data(
-					pubkey        = self.key if self.cfg.addr_type.compressed else decompress_pubkey(self.key),
+					pubkey        = self.key if self.cfg.addr_type.compressed else pubkey_decompress(self.key),
 					viewkey_bytes = None,
 					pubkey_type   = self.cfg.addr_type.pubkey_type,
 					compressed    = self.cfg.addr_type.compressed)
diff --git a/test/modtest_d/ecc.py b/test/modtest_d/ecc.py
index 2f93a289..46498cb8 100755
--- a/test/modtest_d/ecc.py
+++ b/test/modtest_d/ecc.py
@@ -8,6 +8,7 @@ from mmgen.proto.secp256k1.secp256k1 import (
 	pubkey_gen,
 	pubkey_tweak_add,
 	pubkey_check,
+	pubkey_decompress,
 	sign_msghash,
 	pubkey_recover,
 	verify_sig)
@@ -101,7 +102,7 @@ class unit_tests:
 		return True
 
 	def pubkey_ops(self, name, ut):
-		vmsg('  Generating pubkey, adding scalar 123456789 to pubkey:')
+		vmsg('  Generating, checking, decompressing, and adding scalar to pubkey:')
 		pk_addend_bytes = int.to_bytes(123456789, length=32, byteorder='big')
 
 		for privkey in (
@@ -126,6 +127,11 @@ class unit_tests:
 				assert len(res1) == length
 				assert res1 == res2
 
+				res3 = pubkey_decompress(res2)
+				pubkey_check(res3)
+				assert res3[0] == 4
+				assert len(res3) == 65
+
 		return True
 
 	def pubkey_errors(self, name, ut):