Refactor BN_uadd() and BN_usub().

Unlike bn_add_words()/bn_sub_words(), the s2n-bignum bignum_add() and
bignum_sub() functions correctly handle inputs with differing word
lengths. This means that they can be called directly, without needing to
fix up any remaining words manually.

Split BN_uadd() in two - the default bn_add() implementation calls
bn_add_words(), before handling the carry for any remaining words.
Likewise split BN_usub() in two - the default bn_sub() implementation
calls bn_sub_words(), before handling the borrow for any remaining words.

On amd64, provide an implementation of bn_add() that calls s2n-bignum's
bignum_add() directly, similarly with an implementation of bn_sub() that
calls s2n-bignum's bignum_sub() directly.

ok tb@

2 files changed, 22 insertions, 2 deletions

diff --git a/src/lib/libcrypto/bn/arch/amd64/bn_arch.c b/src/lib/libcrypto/bn/arch/amd64/bn_arch.c
index aedefc76e2..dc3000fe8b 100644
--- a/src/lib/libcrypto/bn/arch/amd64/bn_arch.c
+++ b/src/lib/libcrypto/bn/arch/amd64/bn_arch.c
@@ -1,4 +1,4 @@
-/*      $OpenBSD: bn_arch.c,v 1.2 2023/01/29 14:00:41 jsing Exp $ */
+/*      $OpenBSD: bn_arch.c,v 1.3 2023/02/02 18:39:26 jsing Exp $ */
 /*
  * Copyright (c) 2023 Joel Sing <jsing@openbsd.org>
  *
@@ -21,6 +21,15 @@
 #include "bn_local.h"
 #include "s2n_bignum.h"
 
+#ifdef HAVE_BN_ADD
+BN_ULONG
+bn_add(BIGNUM *r, int rn, const BIGNUM *a, const BIGNUM *b)
+{
+        return bignum_add(rn, (uint64_t *)r->d, a->top, (uint64_t *)a->d,
+            b->top, (uint64_t *)b->d);
+}
+#endif
+
 #ifdef HAVE_BN_ADD_WORDS
 BN_ULONG
 bn_add_words(BN_ULONG *rd, const BN_ULONG *ad, const BN_ULONG *bd, int n)
@@ -30,6 +39,15 @@ bn_add_words(BN_ULONG *rd, const BN_ULONG *ad, const BN_ULONG *bd, int n)
 }
 #endif
 
+#ifdef HAVE_BN_SUB
+BN_ULONG
+bn_sub(BIGNUM *r, int rn, const BIGNUM *a, const BIGNUM *b)
+{
+        return bignum_sub(rn, (uint64_t *)r->d, a->top, (uint64_t *)a->d,
+            b->top, (uint64_t *)b->d);
+}
+#endif
+
 #ifdef HAVE_BN_SUB_WORDS
 BN_ULONG
 bn_sub_words(BN_ULONG *rd, const BN_ULONG *ad, const BN_ULONG *bd, int n)
diff --git a/src/lib/libcrypto/bn/arch/amd64/bn_arch.h b/src/lib/libcrypto/bn/arch/amd64/bn_arch.h
index 9e4b6b9442..c41a84409b 100644
--- a/src/lib/libcrypto/bn/arch/amd64/bn_arch.h
+++ b/src/lib/libcrypto/bn/arch/amd64/bn_arch.h
@@ -1,4 +1,4 @@
-/*      $OpenBSD: bn_arch.h,v 1.9 2023/01/31 05:53:49 jsing Exp $ */
+/*      $OpenBSD: bn_arch.h,v 1.10 2023/02/02 18:39:26 jsing Exp $ */
 /*
  * Copyright (c) 2023 Joel Sing <jsing@openbsd.org>
  *
@@ -22,6 +22,7 @@
 
 #ifndef OPENSSL_NO_ASM
 
+#define HAVE_BN_ADD
 #define HAVE_BN_ADD_WORDS
 
 #define HAVE_BN_DIV_WORDS
@@ -36,6 +37,7 @@
 #define HAVE_BN_SQR_COMBA8
 #define HAVE_BN_SQR_WORDS
 
+#define HAVE_BN_SUB
 #define HAVE_BN_SUB_WORDS
 
 #if defined(__GNUC__)