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/*
* asn1.c
* ------
* Minimal ASN.1 implementation in support of Cryptech libhal.
*
* The functions in this module are not intended to be part of the
* public API. Rather, these are utility functions used by more than
* one module within the library, which would otherwise have to be
* duplicated. The main reason for keeping these private is to avoid
* having the public API depend on any details of the underlying
* bignum implementation (currently libtfm, but that might change).
*
* As of this writing, the ASN.1 support we need is quite minimal, so,
* rather than attempting to clean all the unecessary cruft out of a
* general purpose ASN.1 implementation, we hand code the very small
* number of data types we need. At some point this will probably
* become impractical, at which point we might want to look into using
* something like the asn1c compiler.
*
* Authors: Rob Austein
* Copyright (c) 2015, SUNET
*
* 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 above 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.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "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
* COPYRIGHT OWNER 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.
*/
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <stddef.h>
#include <string.h>
#include <assert.h>
#include "hal.h"
#include "asn1_internal.h"
/*
* Encode tag and length fields of an ASN.1 object.
*
* Sets *der_len to the size of of the ASN.1 header (tag and length
* fields); caller supplied length of value field, so presumably
* already knows it.
*
* If der is NULL, just return the size of the header that would be
* encoded and returns HAL_OK.
*
* If der isn't NULL, returns HAL_ERROR_RESULT_TOO_LONG unless full
* header plus value will fit; this is a bit weird, but is useful when
* using this to construct encoders for complte ASN.1 objects.
*/
hal_error_t hal_asn1_encode_header(const uint8_t tag,
const size_t value_len,
uint8_t *der, size_t *der_len, const size_t der_max)
{
size_t header_len = 2; /* Shortest encoding is one octet each for tag and length */
if (value_len >= 128) /* Add octets for longer length encoding as needed */
for (size_t n = value_len; n > 0; n >>= 8)
++header_len;
if (der_len != NULL)
*der_len = header_len;
if (der == NULL) /* If caller just wanted the length, we're done */
return HAL_OK;
/*
* Make sure there's enough room for header + value, then encode.
*/
if (value_len + header_len > der_max)
return HAL_ERROR_RESULT_TOO_LONG;
*der++ = tag;
if (value_len < 128) {
*der = (uint8_t) value_len;
}
else {
*der = 0x80 | (uint8_t) (header_len -= 2);
for (size_t n = value_len; n > 0 && header_len > 0; n >>= 8)
der[header_len--] = (uint8_t) (n & 0xFF);
}
return HAL_OK;
}
/*
* Encode an unsigned ASN.1 INTEGER from a libtfm bignum. If der is
* NULL, just return the length of what we would have encoded.
*/
hal_error_t hal_asn1_encode_integer(const fp_int * const bn,
uint8_t *der, size_t *der_len, const size_t der_max)
{
if (bn == NULL)
return HAL_ERROR_BAD_ARGUMENTS;
/*
* We only handle unsigned INTEGERs, so we need to pad data with a
* leading zero if the most significant bit is set, to avoid
* flipping the ASN.1 sign bit. Conveniently, this also handles the
* difference between libtfm's and ASN.1's encoding of zero.
*/
if (fp_cmp_d(unconst_fp_int(bn), 0) == FP_LT)
return HAL_ERROR_BAD_ARGUMENTS;
const int leading_zero = fp_iszero(bn) || (fp_count_bits(unconst_fp_int(bn)) & 7) == 0;
const size_t vlen = fp_unsigned_bin_size(unconst_fp_int(bn)) + leading_zero;
hal_error_t err;
size_t hlen;
err = hal_asn1_encode_header(ASN1_INTEGER, vlen, der, &hlen, der_max);
if (der_len != NULL)
*der_len = hlen + vlen;
if (der == NULL || err != HAL_OK)
return err;
assert(hlen + vlen <= der_max);
der += hlen;
if (leading_zero)
*der++ = 0x00;
fp_to_unsigned_bin(unconst_fp_int(bn), der);
return HAL_OK;
}
/*
* Parse tag and length of an ASN.1 object. Tag must match value
* specified by the caller. On success, sets hlen and vlen to lengths
* of header and value, respectively.
*/
hal_error_t hal_asn1_decode_header(const uint8_t tag,
const uint8_t * const der, size_t der_max,
size_t *hlen, size_t *vlen)
{
assert(der != NULL && hlen != NULL && vlen != NULL);
if (der_max < 2 || der[0] != tag)
return HAL_ERROR_ASN1_PARSE_FAILED;
if ((der[1] & 0x80) == 0) {
*hlen = 2;
*vlen = der[1];
}
else {
*hlen = 2 + (der[1] & 0x7F);
*vlen = 0;
if (*hlen > der_max)
return HAL_ERROR_ASN1_PARSE_FAILED;
for (size_t i = 2; i < *hlen; i++)
*vlen = (*vlen << 8) + der[i];
}
if (*hlen + *vlen > der_max)
return HAL_ERROR_ASN1_PARSE_FAILED;
return HAL_OK;
}
/*
* Decode an ASN.1 INTEGER into a libtfm bignum. Since we only
* support (or need to support, or expect to see) unsigned integers,
* we return failure if the sign bit is set in the ASN.1 INTEGER.
*/
hal_error_t hal_asn1_decode_integer(fp_int *bn,
const uint8_t * const der, size_t *der_len, const size_t der_max)
{
if (bn == NULL || der == NULL)
return HAL_ERROR_BAD_ARGUMENTS;
hal_error_t err;
size_t hlen, vlen;
if ((err = hal_asn1_decode_header(ASN1_INTEGER, der, der_max, &hlen, &vlen)) != HAL_OK)
return err;
if (der_len != NULL)
*der_len = hlen + vlen;
if (vlen < 1 || (der[hlen] & 0x80) != 0x00)
return HAL_ERROR_ASN1_PARSE_FAILED;
fp_init(bn);
fp_read_unsigned_bin(bn, (uint8_t *) der + hlen, vlen);
return HAL_OK;
}
/*
* Local variables:
* indent-tabs-mode: nil
* End:
*/
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