Side-channel-safe rewrite of the Miller-Rabin test.

Thanks to Mark Wooding for explaining the method of doing this. At
first glance it seemed _obviously_ impossible to run an algorithm that
needs an iteration per factor of 2 in p-1, without a timing leak
giving away the number of factors of 2 in p-1. But it's not, because
you can do the M-R checks interleaved with each step of your whole
modular exponentiation, and they're cheap enough that you can do them
in _every_ step, even the ones where the exponent is too small for M-R
to be interested in yet, and then do bitwise masking to exclude the
spurious results from the final output.

keygen/millerrabin.c

@@ -95,10 +95,8 @@
 struct MillerRabin {
     MontyContext *mc;
 
-    size_t k;
-    mp_int *q;
-
-    mp_int *two, *pm1, *m_pm1;
+    mp_int *pm1, *m_pm1;
+    mp_int *lowbit, *two;
 };
 
 MillerRabin *miller_rabin_new(mp_int *p)
@@ -108,16 +106,19 @@ MillerRabin *miller_rabin_new(mp_int *p)
     assert(mp_hs_integer(p, 2));
     assert(mp_get_bit(p, 0) == 1);
 
-    mr->k = 1;
-    while (!mp_get_bit(p, mr->k))
-        mr->k++;
-    mr->q = mp_rshift_safe(p, mr->k);
+    mr->pm1 = mp_copy(p);
+    mp_sub_integer_into(mr->pm1, mr->pm1, 1);
+
+    /*
+     * Standard bit-twiddling trick for isolating the lowest set bit
+     * of a number: x & (-x)
+     */
+    mr->lowbit = mp_new(mp_max_bits(mr->pm1));
+    mp_sub_into(mr->lowbit, mr->lowbit, mr->pm1);
+    mp_and_into(mr->lowbit, mr->lowbit, mr->pm1);
 
     mr->two = mp_from_integer(2);
 
-    mr->pm1 = mp_unsafe_copy(p);
-    mp_sub_integer_into(mr->pm1, mr->pm1, 1);
-
     mr->mc = monty_new(p);
     mr->m_pm1 = monty_import(mr->mc, mr->pm1);
 
@@ -126,10 +127,10 @@ MillerRabin *miller_rabin_new(mp_int *p)
 
 void miller_rabin_free(MillerRabin *mr)
 {
-    mp_free(mr->q);
-    mp_free(mr->two);
     mp_free(mr->pm1);
     mp_free(mr->m_pm1);
+    mp_free(mr->lowbit);
+    mp_free(mr->two);
     monty_free(mr->mc);
     smemclr(mr, sizeof(*mr));
     sfree(mr);
@@ -144,35 +145,93 @@ void miller_rabin_free(MillerRabin *mr)
  */
 static struct mr_result miller_rabin_test_inner(MillerRabin *mr, mp_int *mw)
 {
-    /*
-     * Compute w^q mod p.
-     */
-    mp_int *wqp = monty_pow(mr->mc, mw, mr->q);
+    mp_int *acc = mp_copy(monty_identity(mr->mc));
+    mp_int *spare = mp_new(mp_max_bits(mr->pm1));
+    size_t bit = mp_max_bits(mr->pm1);
 
     /*
-     * See if this is 1, or if it is -1, or if it becomes -1
-     * when squared at most k-1 times.
+     * The obvious approach to Miller-Rabin would be to start by
+     * calling monty_pow to raise w to the power q, and then square it
+     * k times ourselves. But that introduces a timing leak that gives
+     * away the value of k, i.e., how many factors of 2 there are in
+     * p-1.
+     *
+     * Instead, we don't call monty_pow at all. We do a modular
+     * exponentiation ourselves to compute w^((p-1)/2), using the
+     * technique that works from the top bit of the exponent
+     * downwards. That is, in each iteration we compute
+     * w^floor(exponent/2^i) for i one less than the previous
+     * iteration, by squaring the value we previously had and then
+     * optionally multiplying in w if the next exponent bit is 1.
+     *
+     * At the end of that process, once i <= k, the division
+     * (exponent/2^i) yields an integer, so the values we're computing
+     * are not just w^(floor of that), but w^(exactly that). In other
+     * words, the last k intermediate values of this modexp are
+     * precisely the values M-R wants to check against +1 or -1.
+     *
+     * So we interleave those checks with the modexp loop itself, and
+     * to avoid a timing leak, we check _every_ intermediate result
+     * against (the Montgomery representations of) both +1 and -1. And
+     * then we do bitwise masking to arrange that only the sensible
+     * ones of those checks find their way into our final answer.
      */
+
+    unsigned active = 0;
+
     struct mr_result result;
-    result.passed = false;
-    result.potential_primitive_root = false;
+    result.passed = result.potential_primitive_root = 0;
 
-    if (mp_cmp_eq(wqp, monty_identity(mr->mc))) {
-        result.passed = true;
-    } else {
-        for (size_t i = 0; i < mr->k; i++) {
-            if (mp_cmp_eq(wqp, mr->m_pm1)) {
-                result.passed = true;
-                result.potential_primitive_root = (i == mr->k - 1);
-                break;
-            }
-            if (i == mr->k - 1)
-                break;
-            monty_mul_into(mr->mc, wqp, wqp, wqp);
-        }
+    while (bit-- > 1) {
+        /*
+         * In this iteration, we're computing w^(2e) or w^(2e+1),
+         * where we have w^e from the previous iteration. So we square
+         * the value we had already, and then optionally multiply in
+         * another copy of w depending on the next bit of the exponent.
+         */
+        monty_mul_into(mr->mc, acc, acc, acc);
+        monty_mul_into(mr->mc, spare, acc, mw);
+        mp_select_into(acc, acc, spare, mp_get_bit(mr->pm1, bit));
+
+        /*
+         * mr->lowbit is a number with only one bit set, corresponding
+         * to the lowest set bit in p-1. So when that's the bit of the
+         * exponent we've just processed, we'll detect it by setting
+         * first_iter to true. That's our indication that we're now
+         * generating intermediate results useful to M-R, so we also
+         * set 'active', which stays set from then on.
+         */
+        unsigned first_iter = mp_get_bit(mr->lowbit, bit);
+        active |= first_iter;
+
+        /*
+         * Check the intermediate result against both +1 and -1.
+         */
+        unsigned is_plus_1 = mp_cmp_eq(acc, monty_identity(mr->mc));
+        unsigned is_minus_1 = mp_cmp_eq(acc, mr->m_pm1);
+
+        /*
+         * M-R must report success iff either: the first of the useful
+         * intermediate results (which is w^q) is 1, or _any_ of them
+         * (from w^q all the way up to w^((p-1)/2)) is -1.
+         *
+         * So we want to pass the test if is_plus_1 is set on the
+         * first iteration, or if is_minus_1 is set on any iteration.
+         */
+        result.passed |= (first_iter & is_plus_1);
+        result.passed |= (active & is_minus_1);
+
+        /*
+         * In the final iteration, is_minus_1 is also used to set the
+         * 'potential primitive root' flag, because we haven't found
+         * any exponent smaller than p-1 for which w^(that) == 1.
+         */
+        if (bit == 1)
+            result.potential_primitive_root = is_minus_1;
     }
 
-    mp_free(wqp);
+    mp_free(acc);
+    mp_free(spare);
 
     return result;
 }

sshkeygen.h

@@ -97,8 +97,8 @@ void miller_rabin_free(MillerRabin *mr);
 /* Perform a single Miller-Rabin test, using a specified witness value.
  * Used in the test suite. */
 struct mr_result {
-    bool passed;
-    bool potential_primitive_root;
+    unsigned passed;
+    unsigned potential_primitive_root;
 };
 struct mr_result miller_rabin_test(MillerRabin *mr, mp_int *w);
 

test/cryptsuite.py

@@ -1255,6 +1255,25 @@ class keygen(MyTestBase):
         assert(pow(2, n-1, n) == 1) # Fermat test would pass, but ...
         self.assertEqual(miller_rabin_test(mr, 2), "failed") # ... this fails
 
+        # A white-box test for the side-channel-safe M-R
+        # implementation, which has to check a^e against +-1 for every
+        # exponent e of the form floor((n-1) / power of 2), so as to
+        # avoid giving away exactly how many of the trailing values of
+        # that sequence are significant to the test.
+        #
+        # When the power of 2 is large enough that the division was
+        # not exact, the results of these comparisons are _not_
+        # significant to the test, and we're required to ignore them!
+        #
+        # This pair of values has the property that none of the values
+        # legitimately computed by M-R is either +1 _or_ -1, but if
+        # you shift n-1 right by one too many bits (losing the lowest
+        # set bit of 0x6d00 to get 0x36), then _that_ power of the
+        # witness integer is -1. This should not cause a spurious pass.
+        n = 0x6d01
+        mr = miller_rabin_new(n)
+        self.assertEqual(miller_rabin_test(mr, 0x251), "failed")
+
 class crypt(MyTestBase):
     def testSSH1Fingerprint(self):
         # Example key and reference fingerprint value generated by