Make explicit _ct and _nonct versions of bn_mod_exp funcitons that

matter for constant time, and make the public interface only used
external to the library.

This moves us to a model where the important things are constant time
versions unless you ask for them not to be, rather than the opposite.
I'll continue with this method by method.

Add regress tests for same.
ok jsing@

1 files changed, 55 insertions, 11 deletions

diff --git a/src/lib/libcrypto/bn/bn_exp.c b/src/lib/libcrypto/bn/bn_exp.c
index 83c62be25a..ed4bc666bf 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 1.27 2017/01/21 04:34:16 beck Exp $ */
+/* $OpenBSD: bn_exp.c,v 1.28 2017/01/21 09:38:58 beck Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -172,9 +172,9 @@ err:
         return (ret);
 }
 
-int
-BN_mod_exp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
-    BN_CTX *ctx)
+static int
+BN_mod_exp_internal(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
+    BN_CTX *ctx, int ct)
 {
         int ret;
 
@@ -213,12 +213,11 @@ BN_mod_exp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
          */
 
         if (BN_is_odd(m)) {
-                if (a->top == 1 && !a->neg &&
-                    (BN_get_flags(p, BN_FLG_CONSTTIME) == 0)) {
+                if (a->top == 1 && !a->neg && !ct) {
                         BN_ULONG A = a->d[0];
                         ret = BN_mod_exp_mont_word(r, A,p, m,ctx, NULL);
                 } else
-                        ret = BN_mod_exp_mont(r, a,p, m,ctx, NULL);
+                        ret = BN_mod_exp_mont_ct(r, a,p, m,ctx, NULL);
         } else  {
                 ret = BN_mod_exp_recp(r, a,p, m, ctx);
         }
@@ -228,6 +227,30 @@ BN_mod_exp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
 }
 
 int
+BN_mod_exp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
+    BN_CTX *ctx)
+{
+        return BN_mod_exp_internal(r, a, p, m, ctx,
+            (BN_get_flags(p, BN_FLG_CONSTTIME) != 0));
+}
+
+int
+BN_mod_exp_ct(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
+    BN_CTX *ctx)
+{
+        return BN_mod_exp_internal(r, a, p, m, ctx, 1);
+}
+
+
+int
+BN_mod_exp_nonct(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
+    BN_CTX *ctx)
+{
+        return BN_mod_exp_internal(r, a, p, m, ctx, 0);
+}
+
+
+int
 BN_mod_exp_recp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
     BN_CTX *ctx)
 {
@@ -361,9 +384,9 @@ err:
         return (ret);
 }
 
-int
-BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
-    BN_CTX *ctx, BN_MONT_CTX *in_mont)
+static int
+BN_mod_exp_mont_internal(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
+    BN_CTX *ctx, BN_MONT_CTX *in_mont, int ct)
 {
         int i, j, bits, ret = 0, wstart, wend, window, wvalue;
         int start = 1;
@@ -373,7 +396,7 @@ BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
         BIGNUM *val[TABLE_SIZE];
         BN_MONT_CTX *mont = NULL;
 
-        if (BN_get_flags(p, BN_FLG_CONSTTIME) != 0) {
+        if (ct) {
                 return BN_mod_exp_mont_consttime(rr, a, p, m, ctx, in_mont);
         }
 
@@ -513,6 +536,27 @@ err:
         return (ret);
 }
 
+int
+BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
+    BN_CTX *ctx, BN_MONT_CTX *in_mont)
+{
+        return BN_mod_exp_mont_internal(rr, a, p, m, ctx, in_mont,
+            (BN_get_flags(p, BN_FLG_CONSTTIME) != 0));
+}
+
+int
+BN_mod_exp_mont_ct(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
+    BN_CTX *ctx, BN_MONT_CTX *in_mont)
+{
+        return BN_mod_exp_mont_internal(rr, a, p, m, ctx, in_mont, 1);
+}
+
+int
+BN_mod_exp_mont_nonct(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
+    BN_CTX *ctx, BN_MONT_CTX *in_mont)
+{
+        return BN_mod_exp_mont_internal(rr, a, p, m, ctx, in_mont, 0);
+}
 
 /* BN_mod_exp_mont_consttime() stores the precomputed powers in a specific layout
  * so that accessing any of these table values shows the same access pattern as far