Update constant-time operations.
(Based on upstream's 42af669ff2754dfbe1dd55a0ab56664f82284dc4)
Change-Id: I4d3954fea7471e274c626483a0dfb9d7b3250b74
diff --git a/crypto/constant_time_test.c b/crypto/constant_time_test.c
new file mode 100644
index 0000000..bc127c1
--- /dev/null
+++ b/crypto/constant_time_test.c
@@ -0,0 +1,307 @@
+/*
+ * Utilities for constant-time cryptography.
+ *
+ * Author: Emilia Kasper (emilia@openssl.org)
+ * Based on previous work by Bodo Moeller, Emilia Kasper, Adam Langley
+ * (Google).
+ * ====================================================================
+ * Copyright (c) 2014 The OpenSSL Project. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the copyright
+ * notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ * 3. All advertising materials mentioning features or use of this software
+ * must display the following acknowledgement:
+ * "This product includes cryptographic software written by
+ * Eric Young (eay@cryptsoft.com)"
+ * The word 'cryptographic' can be left out if the rouines from the library
+ * being used are not cryptographic related :-).
+ * 4. If you include any Windows specific code (or a derivative thereof) from
+ * the apps directory (application code) you must include an acknowledgement:
+ * "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"
+ *
+ * THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ * The licence and distribution terms for any publically available version or
+ * derivative of this code cannot be changed. i.e. this code cannot simply be
+ * copied and put under another distribution licence
+ * [including the GNU Public Licence.]
+ */
+
+#include "internal.h"
+
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+
+static const unsigned int CONSTTIME_TRUE = (unsigned)(~0);
+static const unsigned int CONSTTIME_FALSE = 0;
+static const uint8_t CONSTTIME_TRUE_8 = 0xff;
+static const uint8_t CONSTTIME_FALSE_8 = 0;
+
+static int test_binary_op(unsigned int (*op)(unsigned int a, unsigned int b),
+ const char* op_name, unsigned int a, unsigned int b,
+ int is_true) {
+ unsigned c = op(a, b);
+ if (is_true && c != CONSTTIME_TRUE) {
+ fprintf(stderr,
+ "Test failed for %s(%du, %du): expected %du (TRUE), got %du\n",
+ op_name, a, b, CONSTTIME_TRUE, c);
+ return 1;
+ } else if (!is_true && c != CONSTTIME_FALSE) {
+ fprintf(stderr,
+ "Test failed for %s(%du, %du): expected %du (FALSE), got %du\n",
+ op_name, a, b, CONSTTIME_FALSE, c);
+ return 1;
+ }
+ return 0;
+}
+
+static int test_binary_op_8(uint8_t (*op)(unsigned int a, unsigned int b),
+ const char* op_name, unsigned int a, unsigned int b,
+ int is_true) {
+ uint8_t c = op(a, b);
+ if (is_true && c != CONSTTIME_TRUE_8) {
+ fprintf(stderr,
+ "Test failed for %s(%du, %du): expected %u (TRUE), got %u\n",
+ op_name, a, b, CONSTTIME_TRUE_8, c);
+ return 1;
+ } else if (!is_true && c != CONSTTIME_FALSE_8) {
+ fprintf(stderr,
+ "Test failed for %s(%du, %du): expected %u (FALSE), got %u\n",
+ op_name, a, b, CONSTTIME_FALSE_8, c);
+ return 1;
+ }
+ return 0;
+}
+
+static int test_is_zero(unsigned int a) {
+ unsigned int c = constant_time_is_zero(a);
+ if (a == 0 && c != CONSTTIME_TRUE) {
+ fprintf(stderr,
+ "Test failed for constant_time_is_zero(%du): expected %du (TRUE), "
+ "got %du\n",
+ a, CONSTTIME_TRUE, c);
+ return 1;
+ } else if (a != 0 && c != CONSTTIME_FALSE) {
+ fprintf(stderr,
+ "Test failed for constant_time_is_zero(%du): expected %du (FALSE), "
+ "got %du\n",
+ a, CONSTTIME_FALSE, c);
+ return 1;
+ }
+ return 0;
+}
+
+static int test_is_zero_8(unsigned int a) {
+ uint8_t c = constant_time_is_zero_8(a);
+ if (a == 0 && c != CONSTTIME_TRUE_8) {
+ fprintf(stderr,
+ "Test failed for constant_time_is_zero(%du): expected %u (TRUE), "
+ "got %u\n",
+ a, CONSTTIME_TRUE_8, c);
+ return 1;
+ } else if (a != 0 && c != CONSTTIME_FALSE) {
+ fprintf(stderr,
+ "Test failed for constant_time_is_zero(%du): expected %u (FALSE), "
+ "got %u\n",
+ a, CONSTTIME_FALSE_8, c);
+ return 1;
+ }
+ return 0;
+}
+
+static int test_select(unsigned int a, unsigned int b) {
+ unsigned int selected = constant_time_select(CONSTTIME_TRUE, a, b);
+ if (selected != a) {
+ fprintf(stderr,
+ "Test failed for constant_time_select(%du, %du,"
+ "%du): expected %du(first value), got %du\n",
+ CONSTTIME_TRUE, a, b, a, selected);
+ return 1;
+ }
+ selected = constant_time_select(CONSTTIME_FALSE, a, b);
+ if (selected != b) {
+ fprintf(stderr,
+ "Test failed for constant_time_select(%du, %du,"
+ "%du): expected %du(second value), got %du\n",
+ CONSTTIME_FALSE, a, b, b, selected);
+ return 1;
+ }
+ return 0;
+}
+
+static int test_select_8(uint8_t a, uint8_t b) {
+ uint8_t selected = constant_time_select_8(CONSTTIME_TRUE_8, a, b);
+ if (selected != a) {
+ fprintf(stderr,
+ "Test failed for constant_time_select(%u, %u,"
+ "%u): expected %u(first value), got %u\n",
+ CONSTTIME_TRUE, a, b, a, selected);
+ return 1;
+ }
+ selected = constant_time_select_8(CONSTTIME_FALSE_8, a, b);
+ if (selected != b) {
+ fprintf(stderr,
+ "Test failed for constant_time_select(%u, %u,"
+ "%u): expected %u(second value), got %u\n",
+ CONSTTIME_FALSE, a, b, b, selected);
+ return 1;
+ }
+ return 0;
+}
+
+static int test_select_int(int a, int b) {
+ int selected = constant_time_select_int(CONSTTIME_TRUE, a, b);
+ if (selected != a) {
+ fprintf(stderr,
+ "Test failed for constant_time_select(%du, %d,"
+ "%d): expected %d(first value), got %d\n",
+ CONSTTIME_TRUE, a, b, a, selected);
+ return 1;
+ }
+ selected = constant_time_select_int(CONSTTIME_FALSE, a, b);
+ if (selected != b) {
+ fprintf(stderr,
+ "Test failed for constant_time_select(%du, %d,"
+ "%d): expected %d(second value), got %d\n",
+ CONSTTIME_FALSE, a, b, b, selected);
+ return 1;
+ }
+ return 0;
+}
+
+static int test_eq_int(int a, int b) {
+ unsigned int equal = constant_time_eq_int(a, b);
+ if (a == b && equal != CONSTTIME_TRUE) {
+ fprintf(stderr,
+ "Test failed for constant_time_eq_int(%d, %d): expected %du(TRUE), "
+ "got %du\n",
+ a, b, CONSTTIME_TRUE, equal);
+ return 1;
+ } else if (a != b && equal != CONSTTIME_FALSE) {
+ fprintf(stderr,
+ "Test failed for constant_time_eq_int(%d, %d): expected "
+ "%du(FALSE), got %du\n",
+ a, b, CONSTTIME_FALSE, equal);
+ return 1;
+ }
+ return 0;
+}
+
+static int test_eq_int_8(int a, int b) {
+ uint8_t equal = constant_time_eq_int_8(a, b);
+ if (a == b && equal != CONSTTIME_TRUE_8) {
+ fprintf(stderr,
+ "Test failed for constant_time_eq_int_8(%d, %d): expected "
+ "%u(TRUE), got %u\n",
+ a, b, CONSTTIME_TRUE_8, equal);
+ return 1;
+ } else if (a != b && equal != CONSTTIME_FALSE_8) {
+ fprintf(stderr,
+ "Test failed for constant_time_eq_int_8(%d, %d): expected "
+ "%u(FALSE), got %u\n",
+ a, b, CONSTTIME_FALSE_8, equal);
+ return 1;
+ }
+ return 0;
+}
+
+static unsigned int test_values[] = {0, 1, 1024, 12345, 32000, UINT_MAX / 2 - 1,
+ UINT_MAX / 2, UINT_MAX / 2 + 1,
+ UINT_MAX - 1, UINT_MAX};
+
+static uint8_t test_values_8[] = {0, 1, 2, 20, 32, 127, 128, 129, 255};
+
+static int signed_test_values[] = {
+ 0, 1, -1, 1024, -1024, 12345, -12345,
+ 32000, -32000, INT_MAX, INT_MIN, INT_MAX - 1, INT_MIN + 1};
+
+int main(int argc, char* argv[]) {
+ unsigned int a, b, i, j;
+ int c, d;
+ uint8_t e, f;
+ int num_failed = 0, num_all = 0;
+ fprintf(stdout, "Testing constant time operations...\n");
+
+ for (i = 0; i < sizeof(test_values) / sizeof(int); ++i) {
+ a = test_values[i];
+ num_failed += test_is_zero(a);
+ num_failed += test_is_zero_8(a);
+ num_all += 2;
+ for (j = 0; j < sizeof(test_values) / sizeof(int); ++j) {
+ b = test_values[j];
+ num_failed +=
+ test_binary_op(&constant_time_lt, "constant_time_lt", a, b, a < b);
+ num_failed += test_binary_op_8(&constant_time_lt_8, "constant_time_lt_8",
+ a, b, a < b);
+ num_failed +=
+ test_binary_op(&constant_time_lt, "constant_time_lt_8", b, a, b < a);
+ num_failed += test_binary_op_8(&constant_time_lt_8, "constant_time_lt_8",
+ b, a, b < a);
+ num_failed +=
+ test_binary_op(&constant_time_ge, "constant_time_ge", a, b, a >= b);
+ num_failed += test_binary_op_8(&constant_time_ge_8, "constant_time_ge_8",
+ a, b, a >= b);
+ num_failed +=
+ test_binary_op(&constant_time_ge, "constant_time_ge", b, a, b >= a);
+ num_failed += test_binary_op_8(&constant_time_ge_8, "constant_time_ge_8",
+ b, a, b >= a);
+ num_failed +=
+ test_binary_op(&constant_time_eq, "constant_time_eq", a, b, a == b);
+ num_failed += test_binary_op_8(&constant_time_eq_8, "constant_time_eq_8",
+ a, b, a == b);
+ num_failed +=
+ test_binary_op(&constant_time_eq, "constant_time_eq", b, a, b == a);
+ num_failed += test_binary_op_8(&constant_time_eq_8, "constant_time_eq_8",
+ b, a, b == a);
+ num_failed += test_select(a, b);
+ num_all += 13;
+ }
+ }
+
+ for (i = 0; i < sizeof(signed_test_values) / sizeof(int); ++i) {
+ c = signed_test_values[i];
+ for (j = 0; j < sizeof(signed_test_values) / sizeof(int); ++j) {
+ d = signed_test_values[j];
+ num_failed += test_select_int(c, d);
+ num_failed += test_eq_int(c, d);
+ num_failed += test_eq_int_8(c, d);
+ num_all += 3;
+ }
+ }
+
+ for (i = 0; i < sizeof(test_values_8); ++i) {
+ e = test_values_8[i];
+ for (j = 0; j < sizeof(test_values_8); ++j) {
+ f = test_values_8[j];
+ num_failed += test_select_8(e, f);
+ num_all += 1;
+ }
+ }
+
+ if (!num_failed) {
+ fprintf(stdout, "ok (ran %d tests)\n", num_all);
+ fprintf(stdout, "PASS\n");
+ return EXIT_SUCCESS;
+ } else {
+ fprintf(stdout, "%d of %d tests failed!\n", num_failed, num_all);
+ return EXIT_FAILURE;
+ }
+}
diff --git a/crypto/internal.h b/crypto/internal.h
index e32f460..5b980d8 100644
--- a/crypto/internal.h
+++ b/crypto/internal.h
@@ -172,21 +172,45 @@
* c = constant_time_select(lt, a, b); */
/* constant_time_msb returns the given value with the MSB copied to all the
- * other bits. Uses the fact that arithmetic shift shifts-in the sign bit.
- * However, this is not ensured by the C standard so you may need to replace
- * this with something else on odd CPUs. */
+ * other bits. */
static inline unsigned int constant_time_msb(unsigned int a) {
return (unsigned int)((int)(a) >> (sizeof(int) * 8 - 1));
}
/* constant_time_lt returns 0xff..f if a < b and 0 otherwise. */
static inline unsigned int constant_time_lt(unsigned int a, unsigned int b) {
- unsigned int lt;
- /* Case 1: msb(a) == msb(b). a < b iff the MSB of a - b is set.*/
- lt = ~(a ^ b) & (a - b);
- /* Case 2: msb(a) != msb(b). a < b iff the MSB of b is set. */
- lt |= ~a & b;
- return constant_time_msb(lt);
+ /* Consider the two cases of the problem:
+ * msb(a) == msb(b): a < b iff the MSB of a - b is set.
+ * msb(a) != msb(b): a < b iff the MSB of b is set.
+ *
+ * If msb(a) == msb(b) then the following evaluates as:
+ * msb(a^((a^b)|((a-b)^a))) ==
+ * msb(a^((a-b) ^ a)) == (because msb(a^b) == 0)
+ * msb(a^a^(a-b)) == (rearranging)
+ * msb(a-b) (because ∀x. x^x == 0)
+ *
+ * Else, if msb(a) != msb(b) then the following evaluates as:
+ * msb(a^((a^b)|((a-b)^a))) ==
+ * msb(a^(𝟙 | ((a-b)^a))) == (because msb(a^b) == 1 and 𝟙
+ * represents a value s.t. msb(𝟙) = 1)
+ * msb(a^𝟙) == (because ORing with 1 results in 1)
+ * msb(b)
+ *
+ *
+ * Here is an SMT-LIB verification of this formula:
+ *
+ * (define-fun lt ((a (_ BitVec 32)) (b (_ BitVec 32))) (_ BitVec 32)
+ * (bvxor a (bvor (bvxor a b) (bvxor (bvsub a b) a)))
+ * )
+ *
+ * (declare-fun a () (_ BitVec 32))
+ * (declare-fun b () (_ BitVec 32))
+ *
+ * (assert (not (= (= #x00000001 (bvlshr (lt a b) #x0000001f)) (bvult a b))))
+ * (check-sat)
+ * (get-model)
+ */
+ return constant_time_msb(a^((a^b)|((a-b)^a)));
}
/* constant_time_lt_8 acts like |constant_time_lt| but returns an 8-bit mask. */
@@ -196,12 +220,7 @@
/* constant_time_gt returns 0xff..f if a >= b and 0 otherwise. */
static inline unsigned int constant_time_ge(unsigned int a, unsigned int b) {
- unsigned int ge;
- /* Case 1: msb(a) == msb(b). a >= b iff the MSB of a - b is not set.*/
- ge = ~((a ^ b) | (a - b));
- /* Case 2: msb(a) != msb(b). a >= b iff the MSB of a is set. */
- ge |= a & ~b;
- return constant_time_msb(ge);
+ return ~constant_time_lt(a, b);
}
/* constant_time_ge_8 acts like |constant_time_ge| but returns an 8-bit mask. */
@@ -211,6 +230,18 @@
/* constant_time_is_zero returns 0xff..f if a == 0 and 0 otherwise. */
static inline unsigned int constant_time_is_zero(unsigned int a) {
+ /* Here is an SMT-LIB verification of this formula:
+ *
+ * (define-fun is_zero ((a (_ BitVec 32))) (_ BitVec 32)
+ * (bvand (bvnot a) (bvsub a #x00000001))
+ * )
+ *
+ * (declare-fun a () (_ BitVec 32))
+ *
+ * (assert (not (= (= #x00000001 (bvlshr (is_zero a) #x0000001f)) (= a #x00000000))))
+ * (check-sat)
+ * (get-model)
+ */
return constant_time_msb(~a & (a - 1));
}
