Convert bn_exp to BN_MONT_CTX_create()

This simplifies the handling of the BN_MONT_CTX passed in and unifies the
exit paths. Also zap some particularly insightful comments by our favorite
captain.

ok jsing

1 files changed, 38 insertions, 53 deletions

diff --git a/src/lib/libcrypto/bn/bn_exp.c b/src/lib/libcrypto/bn/bn_exp.c
index 85c192b30a..e925d325d2 100644
--- a/src/lib/libcrypto/bn/bn_exp.c
+++ b/src/lib/libcrypto/bn/bn_exp.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_exp.c,v 1.57 2025/02/12 21:21:34 tb Exp $ */
+/* $OpenBSD: bn_exp.c,v 1.58 2025/02/13 11:15:09 tb Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -425,18 +425,10 @@ BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
 
         BN_CTX_start(ctx);
 
-        /*
-         * Allocate a Montgomery context if it was not supplied by the caller.
-         * If this is not done, things will break in the montgomery part.
-         */
-        if (in_mont != NULL)
-                mont = in_mont;
-        else {
-                if ((mont = BN_MONT_CTX_new()) == NULL)
-                        goto err;
-                if (!BN_MONT_CTX_set(mont, m, ctx))
-                        goto err;
-        }
+        if ((mont = in_mont) == NULL)
+                mont = BN_MONT_CTX_create(m, ctx);
+        if (mont == NULL)
+                goto err;
 
         /* Get the window size to use with size of p. */
         window = BN_window_bits_for_ctime_exponent_size(bits);
@@ -636,14 +628,16 @@ BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
         /* Convert the final result from montgomery to standard format */
         if (!BN_from_montgomery(rr, &tmp, mont, ctx))
                 goto err;
+
         ret = 1;
 
-err:
-        if ((in_mont == NULL) && (mont != NULL))
+ err:
+        if (mont != in_mont)
                 BN_MONT_CTX_free(mont);
-        freezero(powerbufFree, powerbufLen + MOD_EXP_CTIME_MIN_CACHE_LINE_WIDTH);
         BN_CTX_end(ctx);
-        return (ret);
+        freezero(powerbufFree, powerbufLen + MOD_EXP_CTIME_MIN_CACHE_LINE_WIDTH);
+
+        return ret;
 }
 LCRYPTO_ALIAS(BN_mod_exp_mont_consttime);
 
@@ -688,17 +682,10 @@ BN_mod_exp_mont_internal(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, const BIG
         if ((val[0] = BN_CTX_get(ctx)) == NULL)
                 goto err;
 
-        /* If this is not done, things will break in the montgomery
-         * part */
-
-        if (in_mont != NULL)
-                mont = in_mont;
-        else {
-                if ((mont = BN_MONT_CTX_new()) == NULL)
-                        goto err;
-                if (!BN_MONT_CTX_set(mont, m, ctx))
-                        goto err;
-        }
+        if ((mont = in_mont) == NULL)
+                mont = BN_MONT_CTX_create(m, ctx);
+        if (mont == NULL)
+                goto err;
 
         if (!BN_nnmod(val[0], a,m, ctx))
                 goto err;
@@ -783,13 +770,15 @@ BN_mod_exp_mont_internal(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, const BIG
         }
         if (!BN_from_montgomery(rr, r,mont, ctx))
                 goto err;
+
         ret = 1;
 
-err:
-        if ((in_mont == NULL) && (mont != NULL))
+ err:
+        if (mont != in_mont)
                 BN_MONT_CTX_free(mont);
         BN_CTX_end(ctx);
-        return (ret);
+
+        return ret;
 }
 
 int
@@ -879,14 +868,10 @@ BN_mod_exp_mont_word(BIGNUM *rr, BN_ULONG a, const BIGNUM *p, const BIGNUM *m,
         if ((t = BN_CTX_get(ctx)) == NULL)
                 goto err;
 
-        if (in_mont != NULL)
-                mont = in_mont;
-        else {
-                if ((mont = BN_MONT_CTX_new()) == NULL)
-                        goto err;
-                if (!BN_MONT_CTX_set(mont, m, ctx))
-                        goto err;
-        }
+        if ((mont = in_mont) == NULL)
+                mont = BN_MONT_CTX_create(m, ctx);
+        if (mont == NULL)
+                goto err;
 
         r_is_one = 1; /* except for Montgomery factor */
 
@@ -954,13 +939,15 @@ BN_mod_exp_mont_word(BIGNUM *rr, BN_ULONG a, const BIGNUM *p, const BIGNUM *m,
                 if (!BN_from_montgomery(rr, r, mont, ctx))
                         goto err;
         }
+
         ret = 1;
 
-err:
-        if ((in_mont == NULL) && (mont != NULL))
+ err:
+        if (mont != in_mont)
                 BN_MONT_CTX_free(mont);
         BN_CTX_end(ctx);
-        return (ret);
+
+        return ret;
 }
 
 int
@@ -1203,14 +1190,10 @@ BN_mod_exp2_mont(BIGNUM *rr, const BIGNUM *a1, const BIGNUM *p1,
         if ((val2[0] = BN_CTX_get(ctx)) == NULL)
                 goto err;
 
-        if (in_mont != NULL)
-                mont = in_mont;
-        else {
-                if ((mont = BN_MONT_CTX_new()) == NULL)
-                        goto err;
-                if (!BN_MONT_CTX_set(mont, m, ctx))
-                        goto err;
-        }
+        if ((mont = in_mont) == NULL)
+                mont = BN_MONT_CTX_create(m, ctx);
+        if (mont == NULL)
+                goto err;
 
         window1 = BN_window_bits_for_exponent_size(bits1);
         window2 = BN_window_bits_for_exponent_size(bits2);
@@ -1335,11 +1318,13 @@ BN_mod_exp2_mont(BIGNUM *rr, const BIGNUM *a1, const BIGNUM *p1,
         }
         if (!BN_from_montgomery(rr, r,mont, ctx))
                 goto err;
+
         ret = 1;
 
-err:
-        if ((in_mont == NULL) && (mont != NULL))
+ err:
+        if (mont != in_mont)
                 BN_MONT_CTX_free(mont);
         BN_CTX_end(ctx);
-        return (ret);
+
+        return ret;
 }