1 files changed, 35 insertions, 24 deletions

diff --git a/src/lib/libcrypto/bn/bn_convert.c b/src/lib/libcrypto/bn/bn_convert.c
index d509a86c08..d38747e017 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 1.21 2024/04/17 21:55:43 tb Exp $ */
+/* $OpenBSD: bn_convert.c,v 1.22 2024/06/22 16:33:00 jsing Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -690,32 +690,43 @@ BN_hex2bn(BIGNUM **bnp, const char *s)
 LCRYPTO_ALIAS(BN_hex2bn);
 
 int
-BN_bn2mpi(const BIGNUM *a, unsigned char *d)
+BN_bn2mpi(const BIGNUM *bn, unsigned char *d)
 {
-        int bits;
-        int num = 0;
-        int ext = 0;
-        long l;
-
-        bits = BN_num_bits(a);
-        num = (bits + 7) / 8;
-        if (bits > 0) {
-                ext = ((bits & 0x07) == 0);
-        }
+        uint8_t *out_bin;
+        size_t out_len, out_bin_len;
+        int bits, bytes;
+        int extend;
+        CBB cbb, cbb_bin;
+
+        bits = BN_num_bits(bn);
+        bytes = (bits + 7) / 8;
+        extend = (bits != 0) && (bits % 8 == 0);
+        out_bin_len = extend + bytes;
+        out_len = 4 + out_bin_len;
+
         if (d == NULL)
-                return (num + 4 + ext);
-
-        l = num + ext;
-        d[0] = (unsigned char)(l >> 24) & 0xff;
-        d[1] = (unsigned char)(l >> 16) & 0xff;
-        d[2] = (unsigned char)(l >> 8) & 0xff;
-        d[3] = (unsigned char)(l) & 0xff;
-        if (ext)
-                d[4] = 0;
-        num = BN_bn2bin(a, &(d[4 + ext]));
-        if (a->neg)
+                return out_len;
+
+        if (!CBB_init_fixed(&cbb, d, out_len))
+                goto err;
+        if (!CBB_add_u32_length_prefixed(&cbb, &cbb_bin))
+                goto err;
+        if (!CBB_add_space(&cbb_bin, &out_bin, out_bin_len))
+                goto err;
+        if (BN_bn2binpad(bn, out_bin, out_bin_len) != out_bin_len)
+                goto err;
+        if (!CBB_finish(&cbb, NULL, NULL))
+                goto err;
+
+        if (bn->neg)
                 d[4] |= 0x80;
-        return (num + 4 + ext);
+
+        return out_len;
+
+ err:
+        CBB_cleanup(&cbb);
+
+        return -1;
 }
 LCRYPTO_ALIAS(BN_bn2mpi);