Constant-time utilities

Pull constant-time methods out to a separate header, add tests.

Reviewed-by: Bodo Moeller <bodo@openssl.org>

1 files changed, 169 insertions, 0 deletions

diff --git a/crypto/constant_time_locl.h b/crypto/constant_time_locl.h
new file mode 100644
index 0000000000..782da6c8b2
--- /dev/null
+++ b/crypto/constant_time_locl.h
@@ -0,0 +1,169 @@
+/* crypto/constant_time_locl.h */
+/*
+ * 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.]
+ */
+
+#ifndef HEADER_CONSTANT_TIME_LOCL_H
+#define HEADER_CONSTANT_TIME_LOCL_H
+
+#include "e_os.h"  /* For 'inline' */
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*
+ * The following methods return a bitmask of all ones (0xff...f) for true
+ * and 0 for false. This is useful for choosing a value based on the result
+ * of a conditional in constant time. For example,
+ *
+ * if (a < b) {
+ *   c = a;
+ * } else {
+ *   c = b;
+ * }
+ *
+ * can be written as
+ *
+ * unsigned int lt = constant_time_lt(a, b);
+ * c = a & lt | b & ~lt;
+ */
+
+/*
+ * 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.
+ */
+static inline unsigned int constant_time_msb(unsigned int a);
+
+/*
+ * Returns 0xff..f if a < b and 0 otherwise.
+ */
+inline unsigned int constant_time_lt(unsigned int a, unsigned int b);
+/* Convenience method for getting an 8-bit mask. */
+inline unsigned char constant_time_lt_8(unsigned int a, unsigned int b);
+
+/*
+ * Returns 0xff..f if a >= b and 0 otherwise.
+ */
+inline unsigned int constant_time_ge(unsigned int a, unsigned int b);
+/* Convenience method for getting an 8-bit mask. */
+inline unsigned char constant_time_ge_8(unsigned int a, unsigned int b);
+
+/*
+ * Returns 0xff..f if a == 0 and 0 otherwise.
+ */
+inline unsigned int constant_time_is_zero(unsigned int a);
+/* Convenience method for getting an 8-bit mask. */
+inline unsigned char constant_time_is_zero_8(unsigned int a);
+
+
+/*
+ * Returns 0xff..f if a == b and 0 otherwise.
+ */
+inline unsigned int constant_time_eq(unsigned int a, unsigned int b);
+/* Convenience method for getting an 8-bit mask. */
+inline unsigned char constant_time_eq_8(unsigned int a, unsigned int b);
+
+static inline unsigned int constant_time_msb(unsigned int a)
+	{
+	return (unsigned int)((int)(a) >> (sizeof(int) * 8 - 1));
+	}
+
+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);
+	}
+
+inline unsigned char constant_time_lt_8(unsigned int a, unsigned int b)
+	{
+	return (unsigned char)(constant_time_lt(a, b));
+	}
+
+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);
+	}
+
+inline unsigned char constant_time_ge_8(unsigned int a, unsigned int b)
+	{
+	return (unsigned char)(constant_time_ge(a, b));
+	}
+
+inline unsigned int constant_time_is_zero(unsigned int a)
+	{
+	return constant_time_msb(~a & (a - 1));
+	}
+
+inline unsigned char constant_time_is_zero_8(unsigned int a)
+	{
+	return (unsigned char)(constant_time_is_zero(a));
+	}
+
+inline unsigned int constant_time_eq(unsigned int a, unsigned int b)
+	{
+	return constant_time_is_zero(a ^ b);
+	}
+
+inline unsigned char constant_time_eq_8(unsigned int a, unsigned int b)
+	{
+	return (unsigned char)(constant_time_eq(a, b));
+	}
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  /* HEADER_CONSTANT_TIME_LOCL_H */