From 2018d1063ee7ff7f92c588318bb9cba8add7a221 Mon Sep 17 00:00:00 2001
From: jsing <jsing@openbsd.org>
Date: Wed, 17 Apr 2024 14:47:17 +0000
Subject: [PATCH] Rewrite BN_mpi2bn() using CBS and bn_bin2bn_cbs().

ok tb@
---
 lib/libcrypto/bn/bn_convert.c | 61 +++++++++++++++++------------------
 1 file changed, 29 insertions(+), 32 deletions(-)

diff --git a/lib/libcrypto/bn/bn_convert.c b/lib/libcrypto/bn/bn_convert.c
index 0bfb00e9589..7c5ba853052 100644
--- a/lib/libcrypto/bn/bn_convert.c
+++ b/lib/libcrypto/bn/bn_convert.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_convert.c,v 1.19 2024/04/17 14:45:46 jsing Exp $ */
+/* $OpenBSD: bn_convert.c,v 1.20 2024/04/17 14:47:17 jsing Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -720,45 +720,42 @@ BN_bn2mpi(const BIGNUM *a, unsigned char *d)
 LCRYPTO_ALIAS(BN_bn2mpi);
 
 BIGNUM *
-BN_mpi2bn(const unsigned char *d, int n, BIGNUM *ain)
+BN_mpi2bn(const unsigned char *d, int n, BIGNUM *bn_in)
 {
-	BIGNUM *a = ain;
-	long len;
+	BIGNUM *bn = bn_in;
+	uint32_t mpi_len;
+	uint8_t v;
 	int neg = 0;
+	CBS cbs;
+
+	if (n < 0)
+		return NULL;
+
+	CBS_init(&cbs, d, n);
 
-	if (n < 4) {
+	if (!CBS_get_u32(&cbs, &mpi_len)) {
 		BNerror(BN_R_INVALID_LENGTH);
-		return (NULL);
+		return NULL;
+
 	}
-	len = ((long)d[0] << 24) | ((long)d[1] << 16) | ((int)d[2] << 8) |
-	    (int)d[3];
-	if ((len + 4) != n) {
+	if (CBS_len(&cbs) != mpi_len) {
 		BNerror(BN_R_ENCODING_ERROR);
-		return (NULL);
+		return NULL;
+	}
+	if (CBS_len(&cbs) > 0) {
+		if (!CBS_peek_u8(&cbs, &v))
+			return NULL;
+		neg = (v >> 7) & 1;
 	}
 
-	if (a == NULL)
-		a = BN_new();
-	if (a == NULL)
-		return (NULL);
+	if (!bn_bin2bn_cbs(&bn, &cbs, 0))
+		return NULL;
 
-	if (len == 0) {
-		a->neg = 0;
-		a->top = 0;
-		return (a);
-	}
-	d += 4;
-	if ((*d) & 0x80)
-		neg = 1;
-	if (BN_bin2bn(d, (int)len, a) == NULL) {
-		if (ain == NULL)
-			BN_free(a);
-		return (NULL);
-	}
-	BN_set_negative(a, neg);
-	if (neg) {
-		BN_clear_bit(a, BN_num_bits(a) - 1);
-	}
-	return (a);
+	if (neg)
+		BN_clear_bit(bn, BN_num_bits(bn) - 1);
+
+	BN_set_negative(bn, neg);
+
+	return bn;
 }
 LCRYPTO_ALIAS(BN_mpi2bn);
-- 
2.20.1