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/*
* Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License").
* You may not use this file except in compliance with the License.
* A copy of the License is located at
*
* http://aws.amazon.com/apache2.0
*
* or in the "license" file accompanying this file. This file is distributed
* on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
* express or implied. See the License for the specific language governing
* permissions and limitations under the License.
*/
#include "utils/s2n_safety.h"
#include <stdint.h>
#include <stdio.h>
#include "utils/s2n_annotations.h"
/**
* Given arrays "a" and "b" of length "len", determine whether they
* hold equal contents.
*
* The execution time of this function is independent of the values
* stored in the arrays.
*
* Timing may depend on the length of the arrays, and on the location
* of the arrays in memory (e.g. if a buffer has been paged out, this
* will affect the timing of this function).
*
* Returns:
* Whether all bytes in arrays "a" and "b" are identical
*/
bool s2n_constant_time_equals(const uint8_t *a, const uint8_t *b, const uint32_t len)
{
S2N_PUBLIC_INPUT(a);
S2N_PUBLIC_INPUT(b);
S2N_PUBLIC_INPUT(len);
/* if len is 0, they're always going to be equal */
if (len == 0) {
return true;
}
/* check if a and b are readable - if so, allow them to increment their pointer */
uint8_t a_inc = S2N_MEM_IS_READABLE(a, len) ? 1 : 0;
uint8_t b_inc = S2N_MEM_IS_READABLE(b, len) ? 1 : 0;
/* reserve a stand-in pointer to replace NULL pointers */
static uint8_t standin = 0;
/* if the pointers can increment their values, then use the
* original value; otherwise use the stand-in */
const uint8_t *a_ptr = a_inc ? a : &standin;
const uint8_t *b_ptr = b_inc ? b : &standin;
/* start by assuming they are equal only if both increment their pointer */
uint8_t xor = !((a_inc == 1) & (b_inc == 1));
/* iterate over each byte in the slices */
for (size_t i = 0; i < len; i++) {
/* Invariants must hold for each execution of the loop
* and at loop exit, hence the <= */
S2N_INVARIANT(i <= len);
/* mix the current cursor values in to the result */
xor |= *a_ptr ^ *b_ptr;
/* increment the pointers by their "inc" values */
a_ptr += a_inc;
b_ptr += b_inc;
}
/* finally check to make sure xor is still 0 */
return (xor == 0);
}
/**
* Given arrays "dest" and "src" of length "len", conditionally copy "src" to "dest"
* The execution time of this function is independent of the values
* stored in the arrays, and of whether the copy occurs.
*
* Timing may depend on the length of the arrays, and on the location
* of the arrays in memory (e.g. if a buffer has been paged out, this
* will affect the timing of this function).
*
*/
int s2n_constant_time_copy_or_dont(uint8_t *dest, const uint8_t *src, uint32_t len, uint8_t dont)
{
S2N_PUBLIC_INPUT(dest);
S2N_PUBLIC_INPUT(src);
S2N_PUBLIC_INPUT(len);
uint8_t mask = (((0xFFFF & dont) - 1) >> 8) & 0xFF;
/* dont = 0 : mask = 0xff */
/* dont > 0 : mask = 0x00 */
for (size_t i = 0; i < len; i++) {
uint8_t old = dest[i];
uint8_t diff = (old ^ src[i]) & mask;
dest[i] = old ^ diff;
}
return 0;
}
/* If src contains valid PKCS#1 v1.5 padding of exactly expectlen bytes, decode
* it into dst, otherwise leave dst alone. Execution time is independent of the
* content of src, but may depend on srclen/expectlen.
*
* Normally, one would fill dst with random bytes before calling this function.
*/
int s2n_constant_time_pkcs1_unpad_or_dont(uint8_t *dst, const uint8_t *src, uint32_t srclen, uint32_t expectlen)
{
S2N_PUBLIC_INPUT(dst);
S2N_PUBLIC_INPUT(src);
S2N_PUBLIC_INPUT(srclen);
S2N_PUBLIC_INPUT(expectlen);
/* Before doing anything else, some basic sanity checks on input lengths */
if (srclen < expectlen + 3) {
/* Not enough room for PKCS#1v1.5 padding, so treat it as bad padding */
return 0;
}
/* First, determine (in constant time) whether the padding is valid.
* If the padding is valid we expect that:
* Bytes 0 and 1 will equal 0x00 and 0x02
* Bytes (srclen-expectlen-1) will be zero
* Bytes 2 through (srclen-expectlen-1) will be nonzero
*/
uint8_t dont_copy = 0;
const uint8_t *start_of_data = src + srclen - expectlen;
dont_copy |= src[0] ^ 0x00;
dont_copy |= src[1] ^ 0x02;
dont_copy |= *(start_of_data - 1) ^ 0x00;
for (size_t i = 2; i < srclen - expectlen - 1; i++) {
/* Note! We avoid using logical NOT (!) here; while in practice
* many compilers will use constant-time sequences for this operator,
* at least on x86 (e.g. cmp -> setcc, or vectorized pcmpeq), this is
* not guaranteed to hold, and some architectures might not have a
* convenient mechanism for generating a branchless logical not. */
uint8_t mask = (((0xFFFF & src[i]) - 1) >> 8) & 0xFF;
/* src[i] = 0 : mask = 0xff */
/* src[i] > 0 : mask = 0x00 */
dont_copy |= mask;
}
s2n_constant_time_copy_or_dont(dst, start_of_data, expectlen, dont_copy);
return 0;
}
static bool s_s2n_in_unit_test = false;
bool s2n_in_unit_test()
{
return s_s2n_in_unit_test;
}
int s2n_in_unit_test_set(bool newval)
{
s_s2n_in_unit_test = newval;
return S2N_SUCCESS;
}
int s2n_align_to(uint32_t initial, uint32_t alignment, uint32_t *out)
{
POSIX_ENSURE_REF(out);
POSIX_ENSURE(alignment != 0, S2N_ERR_SAFETY);
if (initial == 0) {
*out = 0;
return S2N_SUCCESS;
}
const uint64_t i = initial;
const uint64_t a = alignment;
const uint64_t result = a * (((i - 1) / a) + 1);
POSIX_ENSURE(result <= UINT32_MAX, S2N_ERR_INTEGER_OVERFLOW);
*out = (uint32_t) result;
return S2N_SUCCESS;
}
int s2n_mul_overflow(uint32_t a, uint32_t b, uint32_t *out)
{
POSIX_ENSURE_REF(out);
const uint64_t result = ((uint64_t) a) * ((uint64_t) b);
POSIX_ENSURE(result <= UINT32_MAX, S2N_ERR_INTEGER_OVERFLOW);
*out = (uint32_t) result;
return S2N_SUCCESS;
}
int s2n_add_overflow(uint32_t a, uint32_t b, uint32_t *out)
{
POSIX_ENSURE_REF(out);
uint64_t result = ((uint64_t) a) + ((uint64_t) b);
POSIX_ENSURE(result <= UINT32_MAX, S2N_ERR_INTEGER_OVERFLOW);
*out = (uint32_t) result;
return S2N_SUCCESS;
}
int s2n_sub_overflow(uint32_t a, uint32_t b, uint32_t *out)
{
POSIX_ENSURE_REF(out);
POSIX_ENSURE(a >= b, S2N_ERR_INTEGER_OVERFLOW);
*out = a - b;
return S2N_SUCCESS;
}
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