1 files changed, 29 insertions, 32 deletions

diff --git a/src/lib/libcrypto/bn/bn_convert.c b/src/lib/libcrypto/bn/bn_convert.c
index 0bfb00e958..7c5ba85305 100644
--- a/src/lib/libcrypto/bn/bn_convert.c
+++ b/src/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);