From 587eb12f2939f2de7ec3b59d80ce14dfa6c9e436 Mon Sep 17 00:00:00 2001
From: tb <>
Date: Tue, 4 Feb 2025 12:47:58 +0000
Subject: Inline BN_reciprocal() in its only caller

This is simpler, doesn't need an auxiliary function of dubious value,
avouds an auxiliary variable and gets rid of a bunch of comments that
are hard to make sense of.

This doesn't bother to invalidate recp->shift since on error you should
not be reusing the RECP_CTX without reinitializing it.

ok jsing
---
 src/lib/libcrypto/bn/bn_recp.c | 46 +++++++++---------------------------------
 1 file changed, 10 insertions(+), 36 deletions(-)

diff --git a/src/lib/libcrypto/bn/bn_recp.c b/src/lib/libcrypto/bn/bn_recp.c
index 757ed0c3d2..d5070bc003 100644
--- a/src/lib/libcrypto/bn/bn_recp.c
+++ b/src/lib/libcrypto/bn/bn_recp.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_recp.c,v 1.31 2025/02/04 05:09:53 tb Exp $ */
+/* $OpenBSD: bn_recp.c,v 1.32 2025/02/04 12:47:58 tb Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -104,34 +104,6 @@ BN_RECP_CTX_free(BN_RECP_CTX *recp)
 	freezero(recp, sizeof(*recp));
 }
 
-/* len is the expected size of the result
- * We actually calculate with an extra word of precision, so
- * we can do faster division if the remainder is not required.
- */
-/* r := 2^len / m */
-static int
-BN_reciprocal(BIGNUM *r, const BIGNUM *m, int len, BN_CTX *ctx)
-{
-	int ret = -1;
-	BIGNUM *t;
-
-	BN_CTX_start(ctx);
-	if ((t = BN_CTX_get(ctx)) == NULL)
-		goto err;
-
-	if (!BN_set_bit(t, len))
-		goto err;
-
-	if (!BN_div_ct(r, NULL, t, m, ctx))
-		goto err;
-
-	ret = len;
-
-err:
-	BN_CTX_end(ctx);
-	return ret;
-}
-
 int
 BN_div_reciprocal(BIGNUM *dv, BIGNUM *rem, const BIGNUM *m, BN_RECP_CTX *recp,
     BN_CTX *ctx)
@@ -174,13 +146,15 @@ BN_div_reciprocal(BIGNUM *dv, BIGNUM *rem, const BIGNUM *m, BN_RECP_CTX *recp,
 	if (j > i)
 		i = j;
 
-	/* Nr := round(2^i / N) */
-	if (i != recp->shift)
-		recp->shift = BN_reciprocal(recp->Nr, recp->N, i, ctx);
-
-	/* BN_reciprocal returns i, or -1 for an error */
-	if (recp->shift == -1)
-		goto err;
+	/* Compute Nr := (1 << i) / N if necessary. */
+	if (i != recp->shift) {
+		BN_zero(recp->Nr);
+		if (!BN_set_bit(recp->Nr, i))
+			goto err;
+		if (!BN_div_ct(recp->Nr, NULL, recp->Nr, recp->N, ctx))
+			goto err;
+		recp->shift = i;
+	}
 
 	/* d := |round(round(m / 2^BN_num_bits(N)) * recp->Nr / 2^(i - BN_num_bits(N)))|
 	 *    = |round(round(m / 2^BN_num_bits(N)) * round(2^i / N) / 2^(i - BN_num_bits(N)))|
-- 
cgit v1.2.3-55-g6feb