From 6450224f3e3c78fdfa37eadbe6ada8301279f6c1 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Niels=20M=C3=B6ller?= <nisse@lysator.liu.se>
Date: Mon, 20 Jun 2016 20:04:56 +0200
Subject: Use mpz_powm_sec.
Subject: Check for invalid keys, with even p, in dsa_sign.
Subject: Reject invalid keys, with even moduli, in rsa_compute_root_tr.
Subject: Reject invalid RSA keys with even modulo.

diff --git a/bignum.h b/bignum.h
index 24158e0..0d30534 100644
--- a/bignum.h
+++ b/bignum.h
@@ -53,6 +53,8 @@
 # define mpz_combit mpz_combit
 # define mpz_import mpz_import
 # define mpz_export mpz_export
+/* Side-channel silent powm not available in mini-gmp. */
+# define mpz_powm_sec mpz_powm
 #else
 # include <gmp.h>
 #endif
diff --git a/configure.ac b/configure.ac
index e1ee64c..1e88477 100644
--- a/configure.ac
+++ b/configure.ac
@@ -236,9 +236,9 @@ fi
 # Checks for libraries
 if test "x$enable_public_key" = "xyes" ; then
   if test "x$enable_mini_gmp" = "xno" ; then
-    AC_CHECK_LIB(gmp, __gmpz_getlimbn,,
+    AC_CHECK_LIB(gmp, __gmpz_powm_sec,,
         [AC_MSG_WARN(
-    [GNU MP not found, or not 3.1 or up, see http://gmplib.org/.
+    [GNU MP not found, or too old. GMP-5.0 or later is needed, see http://gmplib.org/.
     Support for public key algorithms will be unavailable.])]
         enable_public_key=no)
 
diff --git a/dsa-sign.c b/dsa-sign.c
index 62c7d4a..b713743 100644
--- a/dsa-sign.c
+++ b/dsa-sign.c
@@ -56,6 +56,11 @@ dsa_sign(const struct dsa_params *params,
   mpz_t tmp;
   int res;
   
+  /* Check that p is odd, so that invalid keys don't result in a crash
+     inside mpz_powm_sec. */
+  if (mpz_even_p (params->p))
+    return 0;
+
   /* Select k, 0<k<q, randomly */
   mpz_init_set(tmp, params->q);
   mpz_sub_ui(tmp, tmp, 1);
@@ -65,7 +70,7 @@ dsa_sign(const struct dsa_params *params,
   mpz_add_ui(k, k, 1);
 
   /* Compute r = (g^k (mod p)) (mod q) */
-  mpz_powm(tmp, params->g, k, params->p);
+  mpz_powm_sec(tmp, params->g, k, params->p);
   mpz_fdiv_r(signature->r, tmp, params->q);
 
   /* Compute hash */
diff --git a/rsa-blind.c b/rsa-blind.c
index 7662f50..16b03d7 100644
--- a/rsa-blind.c
+++ b/rsa-blind.c
@@ -61,7 +61,7 @@ _rsa_blind (const struct rsa_public_key *pub,
   while (!mpz_invert (ri, r, pub->n));
 
   /* c = c*(r^e) mod n */
-  mpz_powm(r, r, pub->e, pub->n);
+  mpz_powm_sec(r, r, pub->e, pub->n);
   mpz_mul(c, c, r);
   mpz_fdiv_r(c, c, pub->n);
 
diff --git a/rsa-sign-tr.c b/rsa-sign-tr.c
index 3d80ed4..8542cae 100644
--- a/rsa-sign-tr.c
+++ b/rsa-sign-tr.c
@@ -60,7 +60,7 @@ rsa_blind (const struct rsa_public_key *pub,
   while (!mpz_invert (ri, r, pub->n));
 
   /* c = c*(r^e) mod n */
-  mpz_powm(r, r, pub->e, pub->n);
+  mpz_powm_sec(r, r, pub->e, pub->n);
   mpz_mul(c, m, r);
   mpz_fdiv_r(c, c, pub->n);
 
@@ -88,6 +88,14 @@ rsa_compute_root_tr(const struct rsa_public_key *pub,
   int res;
   mpz_t t, mb, xb, ri;
 
+  /* mpz_powm_sec handles only odd moduli. If p, q or n is even, the
+     key is invalid and rejected by rsa_private_key_prepare. However,
+     some applications, notably gnutls, don't use this function, and
+     we don't want an invalid key to lead to a crash down inside
+     mpz_powm_sec. So do an additional check here. */
+  if (mpz_even_p (pub->n) || mpz_even_p (key->p) || mpz_even_p (key->q))
+    return 0;
+
   mpz_init (mb);
   mpz_init (xb);
   mpz_init (ri);
@@ -97,7 +105,7 @@ rsa_compute_root_tr(const struct rsa_public_key *pub,
 
   rsa_compute_root (key, xb, mb);
 
-  mpz_powm(t, xb, pub->e, pub->n);
+  mpz_powm_sec(t, xb, pub->e, pub->n);
   res = (mpz_cmp(mb, t) == 0);
 
   if (res)
diff --git a/rsa-sign.c b/rsa-sign.c
index eba7388..4832352 100644
--- a/rsa-sign.c
+++ b/rsa-sign.c
@@ -96,11 +96,11 @@ rsa_compute_root(const struct rsa_private_key *key,
 
   /* Compute xq = m^d % q = (m%q)^b % q */
   mpz_fdiv_r(xq, m, key->q);
-  mpz_powm(xq, xq, key->b, key->q);
+  mpz_powm_sec(xq, xq, key->b, key->q);
 
   /* Compute xp = m^d % p = (m%p)^a % p */
   mpz_fdiv_r(xp, m, key->p);
-  mpz_powm(xp, xp, key->a, key->p);
+  mpz_powm_sec(xp, xp, key->a, key->p);
 
   /* Set xp' = (xp - xq) c % p. */
   mpz_sub(xp, xp, xq);
diff --git a/rsa.c b/rsa.c
index 19d93de..f594140 100644
--- a/rsa.c
+++ b/rsa.c
@@ -58,13 +58,18 @@ rsa_public_key_clear(struct rsa_public_key *key)
 }
 
 /* Computes the size, in octets, of a the modulo. Returns 0 if the
- * modulo is too small to be useful. */
-
+ * modulo is too small to be useful, or otherwise appears invalid. */
 size_t
 _rsa_check_size(mpz_t n)
 {
   /* Round upwards */
-  size_t size = (mpz_sizeinbase(n, 2) + 7) / 8;
+  size_t size;
+
+  /* Even moduli are invalid, and not supported by mpz_powm_sec. */
+  if (mpz_even_p (n))
+    return 0;
+
+  size = (mpz_sizeinbase(n, 2) + 7) / 8;
 
   if (size < RSA_MINIMUM_N_OCTETS)
     return 0;
diff --git a/testsuite/rsa-test.c b/testsuite/rsa-test.c
index e9b1c03..a429664 100644
--- a/testsuite/rsa-test.c
+++ b/testsuite/rsa-test.c
@@ -57,6 +57,13 @@ test_main(void)
 
   test_rsa_sha512(&pub, &key, expected);
 
+  /* Test detection of invalid keys with even modulo */
+  mpz_clrbit (pub.n, 0);
+  ASSERT (!rsa_public_key_prepare (&pub));
+
+  mpz_clrbit (key.p, 0);
+  ASSERT (!rsa_private_key_prepare (&key));
+
   /* 777-bit key, generated by
    *
    *   lsh-keygen -a rsa -l 777 -f advanced-hex
-- 
2.7.3