2 files changed, 45 insertions, 30 deletions

diff --git a/src/lib/libcrypto/bn/bn_internal.h b/src/lib/libcrypto/bn/bn_internal.h
index b6e903553f..a1f1515b57 100644
--- a/src/lib/libcrypto/bn/bn_internal.h
+++ b/src/lib/libcrypto/bn/bn_internal.h
@@ -1,4 +1,4 @@
-/*      $OpenBSD: bn_internal.h,v 1.18 2025/05/25 04:58:32 jsing Exp $ */
+/*      $OpenBSD: bn_internal.h,v 1.19 2025/05/25 05:12:05 jsing Exp $ */
 /*
  * Copyright (c) 2023 Joel Sing <jsing@openbsd.org>
  *
@@ -45,6 +45,8 @@ void bn_mod_mul_words(BN_ULONG *r, const BN_ULONG *a, const BN_ULONG *b,
 void bn_montgomery_multiply_words(BN_ULONG *rp, const BN_ULONG *ap,
     const BN_ULONG *bp, const BN_ULONG *np, BN_ULONG *tp, BN_ULONG n0,
     int n_len);
+void bn_montgomery_reduce_words(BN_ULONG *r, BN_ULONG *a, const BN_ULONG *n,
+    BN_ULONG n0, int n_len);
 
 #ifndef HAVE_BN_CT_NE_ZERO
 static inline int
diff --git a/src/lib/libcrypto/bn/bn_mont.c b/src/lib/libcrypto/bn/bn_mont.c
index ce88b23ca9..950846fa5b 100644
--- a/src/lib/libcrypto/bn/bn_mont.c
+++ b/src/lib/libcrypto/bn/bn_mont.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_mont.c,v 1.67 2025/05/25 04:58:32 jsing Exp $ */
+/* $OpenBSD: bn_mont.c,v 1.68 2025/05/25 05:12:05 jsing Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -316,6 +316,44 @@ BN_MONT_CTX_set_locked(BN_MONT_CTX **pmctx, int lock, const BIGNUM *mod,
 LCRYPTO_ALIAS(BN_MONT_CTX_set_locked);
 
 /*
+ * bn_montgomery_reduce_words() performs Montgomery reduction, reducing the input
+ * from its Montgomery form aR to a, returning the result in r. a must be twice
+ * the length of the modulus. Note that the input is mutated in the process of
+ * performing the reduction.
+ */
+void
+bn_montgomery_reduce_words(BN_ULONG *r, BN_ULONG *a, const BN_ULONG *n,
+    BN_ULONG n0, int n_len)
+{
+        BN_ULONG v, mask;
+        BN_ULONG carry = 0;
+        int i;
+
+        /* Add multiples of the modulus, so that it becomes divisible by R. */
+        for (i = 0; i < n_len; i++) {
+                v = bn_mul_add_words(&a[i], n, n_len, a[i] * n0);
+                bn_addw_addw(v, a[i + n_len], carry, &carry, &a[i + n_len]);
+        }
+
+        /* Divide by R (this is the equivalent of right shifting by n_len). */
+        a = &a[n_len];
+
+        /*
+         * The output is now in the range of [0, 2N). Attempt to reduce once by
+         * subtracting the modulus. If the reduction was necessary then the
+         * result is already in r, otherwise copy the value prior to reduction
+         * from the top half of a.
+         */
+        mask = carry - bn_sub_words(r, a, n, n_len);
+
+        for (i = 0; i < n_len; i++) {
+                *r = (*r & ~mask) | (*a & mask);
+                r++;
+                a++;
+        }
+}
+
+/*
  * bn_montgomery_reduce() performs Montgomery reduction, reducing the input
  * from its Montgomery form aR to a, returning the result in r. Note that the
  * input is mutated in the process of performing the reduction, destroying its
@@ -325,7 +363,6 @@ static int
 bn_montgomery_reduce(BIGNUM *r, BIGNUM *a, BN_MONT_CTX *mctx)
 {
         BIGNUM *n;
-        BN_ULONG *ap, *rp, n0, v, carry, mask;
         int i, max, n_len;
 
         n = &mctx->N;
@@ -341,7 +378,8 @@ bn_montgomery_reduce(BIGNUM *r, BIGNUM *a, BN_MONT_CTX *mctx)
 
         /*
          * Expand a to twice the length of the modulus, zero if necessary.
-         * XXX - make this a requirement of the caller.
+         * XXX - make this a requirement of the caller or use a temporary
+         * allocation.
          */
         if ((max = 2 * n_len) < n_len)
                 return 0;
@@ -350,33 +388,8 @@ bn_montgomery_reduce(BIGNUM *r, BIGNUM *a, BN_MONT_CTX *mctx)
         for (i = a->top; i < max; i++)
                 a->d[i] = 0;
 
-        carry = 0;
-        n0 = mctx->n0[0];
-
-        /* Add multiples of the modulus, so that it becomes divisible by R. */
-        for (i = 0; i < n_len; i++) {
-                v = bn_mul_add_words(&a->d[i], n->d, n_len, a->d[i] * n0);
-                bn_addw_addw(v, a->d[i + n_len], carry, &carry,
-                    &a->d[i + n_len]);
-        }
-
-        /* Divide by R (this is the equivalent of right shifting by n_len). */
-        ap = &a->d[n_len];
-
-        /*
-         * The output is now in the range of [0, 2N). Attempt to reduce once by
-         * subtracting the modulus. If the reduction was necessary then the
-         * result is already in r, otherwise copy the value prior to reduction
-         * from the top half of a.
-         */
-        mask = carry - bn_sub_words(r->d, ap, n->d, n_len);
+        bn_montgomery_reduce_words(r->d, a->d, n->d, mctx->n0[0], n_len);
 
-        rp = r->d;
-        for (i = 0; i < n_len; i++) {
-                *rp = (*rp & ~mask) | (*ap & mask);
-                rp++;
-                ap++;
-        }
         r->top = n_len;
 
         bn_correct_top(r);