Remove a secret-dependent branch in Montgomery multiplication

ChangeLog.d/montmul-cmp-branch.txt

@@ -0,0 +1,6 @@
+Security
+   * Fix a side channel vulnerability in modular exponentiation that could
+     reveal an RSA private key used in a secure enclave. Noticed by Sangho Lee,
+     Ming-Wei Shih, Prasun Gera, Taesoo Kim and Hyesoon Kim (Georgia Institute
+     of Technology); and Marcus Peinado (Microsoft Research). Reported by Raoul
+     Strackx (Fortanix) in #3394.

library/bignum.c

@@ -243,6 +243,22 @@ void mbedtls_mpi_swap( mbedtls_mpi *X, mbedtls_mpi *Y )
     memcpy(  Y, &T, sizeof( mbedtls_mpi ) );
 }
 
+/*
+ * Conditionally assign dest = src, without leaking information
+ * about whether the assignment was made or not.
+ * dest and src must be arrays of limbs of size n.
+ * assign must be 0 or 1.
+ */
+static void mpi_safe_cond_assign( size_t n,
+                                  mbedtls_mpi_uint *dest,
+                                  const mbedtls_mpi_uint *src,
+                                  unsigned char assign )
+{
+    size_t i;
+    for( i = 0; i < n; i++ )
+        dest[i] = dest[i] * ( 1 - assign ) + src[i] * assign;
+}
+
 /*
  * Conditionally assign X = Y, without leaking information
  * about whether the assignment was made or not.
@@ -262,10 +278,9 @@ int mbedtls_mpi_safe_cond_assign( mbedtls_mpi *X, const mbedtls_mpi *Y, unsigned
 
     X->s = X->s * ( 1 - assign ) + Y->s * assign;
 
-    for( i = 0; i < Y->n; i++ )
-        X->p[i] = X->p[i] * ( 1 - assign ) + Y->p[i] * assign;
+    mpi_safe_cond_assign( Y->n, X->p, Y->p, assign );
 
-    for( ; i < X->n; i++ )
+    for( i = Y->n; i < X->n; i++ )
         X->p[i] *= ( 1 - assign );
 
 cleanup:
@@ -1328,9 +1343,12 @@ int mbedtls_mpi_add_abs( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
 }
 
 /*
- * Helper for mbedtls_mpi subtraction
+ * Helper for mbedtls_mpi subtraction:
+ * d -= s where d and s have the same size and d >= s.
  */
-static void mpi_sub_hlp( size_t n, mbedtls_mpi_uint *s, mbedtls_mpi_uint *d )
+static void mpi_sub_hlp( size_t n,
+                         const mbedtls_mpi_uint *s,
+                         mbedtls_mpi_uint *d )
 {
     size_t i;
     mbedtls_mpi_uint c, z;
@@ -1975,18 +1993,34 @@ static void mpi_montg_init( mbedtls_mpi_uint *mm, const mbedtls_mpi *N )
     *mm = ~x + 1;
 }
 
-/*
- * Montgomery multiplication: A = A * B * R^-1 mod N  (HAC 14.36)
- */
-static int mpi_montmul( mbedtls_mpi *A, const mbedtls_mpi *B, const mbedtls_mpi *N, mbedtls_mpi_uint mm,
+/** Montgomery multiplication: A = A * B * R^-1 mod N  (HAC 14.36)
+ *
+ * \param[in,out]   A   One of the numbers to multiply.
+ *                      It must have at least one more limb than N
+ *                      (A->n >= N->n + 1).
+ *                      On successful completion, A contains the result of
+ *                      the multiplication A * B * R^-1 mod N where
+ *                      R = (2^ciL)^n.
+ * \param[in]       B   One of the numbers to multiply.
+ *                      It must be nonzero and must not have more limbs than N
+ *                      (B->n <= N->n).
+ * \param[in]       N   The modulo. N must be odd.
+ * \param           mm  The value calculated by `mpi_montg_init(&mm, N)`.
+ *                      This is -N^-1 mod 2^ciL.
+ * \param[in,out]   T   A bignum for temporary storage.
+ *                      It must be at least twice the limb size of N plus 2
+ *                      (T->n >= 2 * (N->n + 1)).
+ *                      Its initial content is unused and
+ *                      its final content is indeterminate.
+ *                      Note that unlike the usual convention in the library
+ *                      for `const mbedtls_mpi*`, the content of T can change.
+ */
+static void mpi_montmul( mbedtls_mpi *A, const mbedtls_mpi *B, const mbedtls_mpi *N, mbedtls_mpi_uint mm,
                          const mbedtls_mpi *T )
 {
     size_t i, n, m;
     mbedtls_mpi_uint u0, u1, *d;
 
-    if( T->n < N->n + 1 || T->p == NULL )
-        return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
-
     memset( T->p, 0, T->n * ciL );
 
     d = T->p;
@@ -2009,28 +2043,33 @@ static int mpi_montmul( mbedtls_mpi *A, const mbedtls_mpi *B, const mbedtls_mpi
 
     memcpy( A->p, d, ( n + 1 ) * ciL );
 
-    if( mbedtls_mpi_cmp_abs( A, N ) >= 0 )
-        mpi_sub_hlp( n, N->p, A->p );
-    else
-        /* prevent timing attacks */
-        mpi_sub_hlp( n, A->p, T->p );
-
-    return( 0 );
+    /* If A >= N then A -= N. Do the subtraction unconditionally to prevent
+     * timing attacks. */
+    /* Set d to A + (2^biL)^n - N. */
+    d[n] += 1;
+    mpi_sub_hlp( n, N->p, d );
+    /* Now d - (2^biL)^n = A - N so d >= (2^biL)^n iff A >= N.
+     * So we want to copy the result of the subtraction iff d->p[n] != 0.
+     * Note that d->p[n] is either 0 or 1 since A - N <= N <= (2^biL)^n. */
+    mpi_safe_cond_assign( n + 1, A->p, d, (unsigned char) d[n] );
+    A->p[n] = 0;
 }
 
 /*
  * Montgomery reduction: A = A * R^-1 mod N
+ *
+ * See mpi_montmul() regarding constraints and guarantees on the parameters.
  */
-static int mpi_montred( mbedtls_mpi *A, const mbedtls_mpi *N,
-                        mbedtls_mpi_uint mm, const mbedtls_mpi *T )
+static void mpi_montred( mbedtls_mpi *A, const mbedtls_mpi *N,
+                         mbedtls_mpi_uint mm, const mbedtls_mpi *T )
 {
     mbedtls_mpi_uint z = 1;
     mbedtls_mpi U;
 
     U.n = U.s = (int) z;
     U.p = &z;
 
-    return( mpi_montmul( A, &U, N, mm, T ) );
+    mpi_montmul( A, &U, N, mm, T );
 }
 
 /*
@@ -2116,13 +2155,13 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
     else
         MBEDTLS_MPI_CHK( mbedtls_mpi_copy( &W[1], A ) );
 
-    MBEDTLS_MPI_CHK( mpi_montmul( &W[1], &RR, N, mm, &T ) );
+    mpi_montmul( &W[1], &RR, N, mm, &T );
 
     /*
      * X = R^2 * R^-1 mod N = R mod N
      */
     MBEDTLS_MPI_CHK( mbedtls_mpi_copy( X, &RR ) );
-    MBEDTLS_MPI_CHK( mpi_montred( X, N, mm, &T ) );
+    mpi_montred( X, N, mm, &T );
 
     if( wsize > 1 )
     {
@@ -2135,7 +2174,7 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
         MBEDTLS_MPI_CHK( mbedtls_mpi_copy( &W[j], &W[1]    ) );
 
         for( i = 0; i < wsize - 1; i++ )
-            MBEDTLS_MPI_CHK( mpi_montmul( &W[j], &W[j], N, mm, &T ) );
+            mpi_montmul( &W[j], &W[j], N, mm, &T );
 
         /*
          * W[i] = W[i - 1] * W[1]
@@ -2145,7 +2184,7 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
             MBEDTLS_MPI_CHK( mbedtls_mpi_grow( &W[i], N->n + 1 ) );
             MBEDTLS_MPI_CHK( mbedtls_mpi_copy( &W[i], &W[i - 1] ) );
 
-            MBEDTLS_MPI_CHK( mpi_montmul( &W[i], &W[1], N, mm, &T ) );
+            mpi_montmul( &W[i], &W[1], N, mm, &T );
         }
     }
 
@@ -2182,7 +2221,7 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
             /*
              * out of window, square X
              */
-            MBEDTLS_MPI_CHK( mpi_montmul( X, X, N, mm, &T ) );
+            mpi_montmul( X, X, N, mm, &T );
             continue;
         }
 
@@ -2200,12 +2239,12 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
              * X = X^wsize R^-1 mod N
              */
             for( i = 0; i < wsize; i++ )
-                MBEDTLS_MPI_CHK( mpi_montmul( X, X, N, mm, &T ) );
+                mpi_montmul( X, X, N, mm, &T );
 
             /*
              * X = X * W[wbits] R^-1 mod N
              */
-            MBEDTLS_MPI_CHK( mpi_montmul( X, &W[wbits], N, mm, &T ) );
+            mpi_montmul( X, &W[wbits], N, mm, &T );
 
             state--;
             nbits = 0;
@@ -2218,18 +2257,18 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
      */
     for( i = 0; i < nbits; i++ )
     {
-        MBEDTLS_MPI_CHK( mpi_montmul( X, X, N, mm, &T ) );
+        mpi_montmul( X, X, N, mm, &T );
 
         wbits <<= 1;
 
         if( ( wbits & ( one << wsize ) ) != 0 )
-            MBEDTLS_MPI_CHK( mpi_montmul( X, &W[1], N, mm, &T ) );
+            mpi_montmul( X, &W[1], N, mm, &T );
     }
 
     /*
      * X = A^E * R * R^-1 mod N = A^E mod N
      */
-    MBEDTLS_MPI_CHK( mpi_montred( X, N, mm, &T ) );
+    mpi_montred( X, N, mm, &T );
 
     if( neg && E->n != 0 && ( E->p[0] & 1 ) != 0 )
     {