From 0b199a883e9170cdfe8e61c150bbaf8d8951f3e7 Mon Sep 17 00:00:00 2001
From: Samuel Weiser <samuel.weiser@iaik.tugraz.at>
Date: Tue, 5 Dec 2017 15:55:17 +0100
Subject: [PATCH] Replaced variable-time GCD with consttime inversion to avoid
 side-channel attacks on RSA key generation

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5170)

(cherry picked from commit 9db724cfede4ba7a3668bff533973ee70145ec07)
---
 crypto/rsa/rsa_gen.c | 30 ++++++++++++++++++++++++------
 1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/Cryptlib/OpenSSL/crypto/rsa/rsa_gen.c b/Cryptlib/OpenSSL/crypto/rsa/rsa_gen.c
index a85493d6097..8553772f062 100644
--- a/Cryptlib/OpenSSL/crypto/rsa/rsa_gen.c
+++ b/Cryptlib/OpenSSL/crypto/rsa/rsa_gen.c
@@ -109,6 +109,7 @@ static int rsa_builtin_keygen(RSA *rsa, int bits, BIGNUM *e_value,
     BIGNUM *pr0, *d, *p;
     int bitsp, bitsq, ok = -1, n = 0;
     BN_CTX *ctx = NULL;
+    unsigned long error = 0;
 
     /*
      * When generating ridiculously small keys, we can get stuck
@@ -155,16 +156,25 @@ static int rsa_builtin_keygen(RSA *rsa, int bits, BIGNUM *e_value,
     if (BN_copy(rsa->e, e_value) == NULL)
         goto err;
 
+    BN_set_flags(rsa->e, BN_FLG_CONSTTIME);
     /* generate p and q */
     for (;;) {
         if (!BN_generate_prime_ex(rsa->p, bitsp, 0, NULL, NULL, cb))
             goto err;
         if (!BN_sub(r2, rsa->p, BN_value_one()))
             goto err;
-        if (!BN_gcd(r1, r2, rsa->e, ctx))
-            goto err;
-        if (BN_is_one(r1))
+        if (BN_mod_inverse(r1, r2, rsa->e, ctx) != NULL) {
+            /* GCD == 1 since inverse exists */
             break;
+        }
+        error = ERR_peek_last_error();
+        if (ERR_GET_LIB(error) == ERR_LIB_BN
+            && ERR_GET_REASON(error) == BN_R_NO_INVERSE) {
+            /* GCD != 1 */
+            ERR_clear_error();
+        } else {
+            goto err;
+        }
         if (!BN_GENCB_call(cb, 2, n++))
             goto err;
     }
@@ -177,10 +187,18 @@ static int rsa_builtin_keygen(RSA *rsa, int bits, BIGNUM *e_value,
         } while (BN_cmp(rsa->p, rsa->q) == 0);
         if (!BN_sub(r2, rsa->q, BN_value_one()))
             goto err;
-        if (!BN_gcd(r1, r2, rsa->e, ctx))
-            goto err;
-        if (BN_is_one(r1))
+        if (BN_mod_inverse(r1, r2, rsa->e, ctx) != NULL) {
+            /* GCD == 1 since inverse exists */
             break;
+        }
+        error = ERR_peek_last_error();
+        if (ERR_GET_LIB(error) == ERR_LIB_BN
+            && ERR_GET_REASON(error) == BN_R_NO_INVERSE) {
+            /* GCD != 1 */
+            ERR_clear_error();
+        } else {
+            goto err;
+        }
         if (!BN_GENCB_call(cb, 2, n++))
             goto err;
     }
