secp256k1-0.2.0: contrib/lax_der_parsing.h

/**********************************************************************
 * Copyright (c) 2015 Pieter Wuille                                   *
 * Distributed under the MIT software license, see the accompanying   *
 * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
 **********************************************************************/

/* This file contains a code snippet that parses DER with various errors and
 * violations. This is not a part of the library itself, because the allowed
 * violations are chosen arbitrarily and do not follow or establish any
 * standard.
 *
 * In many places it matters that different implementations do not only accept
 * the same set of valid signatures, but also reject the same set of signatures.
 * The only means to accomplish that is by strictly obeying a standard, and not
 * accepting anything else.
 *
 * Nonetheless, sometimes there is a need for compatibility with systems that
 * use signatures which do not strictly obey DER. The snippet below shows how
 * certain violations are easily supported. You may need to adapt it.
 *
 * Do not use this for new systems. Use well-defined DER or compact signatures
 * instead if you have the choice (see secp256k1_ecdsa_signature_parse_der and
 * secp256k1_ecdsa_signature_parse_compact).
 *
 * The supported violations are:
 * - All numbers are parsed as nonnegative integers, even though X.609-0207
 *   section 8.3.3 specifies that integers are always encoded as two's
 *   complement.
 * - Integers can have length 0, even though section 8.3.1 says they can't.
 * - Integers with overly long padding are accepted, violation section
 *   8.3.2.
 * - 127-byte long length descriptors are accepted, even though section
 *   8.1.3.5.c says that they are not.
 * - Trailing garbage data inside or after the signature is ignored.
 * - The length descriptor of the sequence is ignored.
 *
 * Compared to for example OpenSSL, many violations are NOT supported:
 * - Using overly long tag descriptors for the sequence or integers inside,
 *   violating section 8.1.2.2.
 * - Encoding primitive integers as constructed values, violating section
 *   8.3.1.
 */

#ifndef _SECP256K1_CONTRIB_LAX_DER_PARSING_H_
#define _SECP256K1_CONTRIB_LAX_DER_PARSING_H_

#include <string.h>
#include <secp256k1.h>

static int secp256k1_ecdsa_signature_parse_der_lax(const secp256k1_context* ctx, secp256k1_ecdsa_signature* sig, const unsigned char *input, size_t inputlen);

static int secp256k1_ecdsa_signature_parse_der_lax(const secp256k1_context* ctx, secp256k1_ecdsa_signature* sig, const unsigned char *input, size_t inputlen) {
    size_t rpos, rlen, spos, slen;
    size_t pos = 0;
    size_t lenbyte;
    unsigned char tmpsig[64] = {0};
    int overflow = 0;

    /* Hack to initialize sig with a correctly-parsed but invalid signature. */
    secp256k1_ecdsa_signature_parse_compact(ctx, sig, tmpsig);

    /* Sequence tag byte */
    if (pos == inputlen || input[pos] != 0x30) {
        return 0;
    }
    pos++;

    /* Sequence length bytes */
    if (pos == inputlen) {
        return 0;
    }
    lenbyte = input[pos++];
    if (lenbyte & 0x80) {
        lenbyte -= 0x80;
        if (pos + lenbyte > inputlen) {
            return 0;
        }
        pos += lenbyte;
    }

    /* Integer tag byte for R */
    if (pos == inputlen || input[pos] != 0x02) {
        return 0;
    }
    pos++;

    /* Integer length for R */
    if (pos == inputlen) {
        return 0;
    }
    lenbyte = input[pos++];
    if (lenbyte & 0x80) {
        lenbyte -= 0x80;
        if (pos + lenbyte > inputlen) {
            return 0;
        }
        while (lenbyte > 0 && input[pos] == 0) {
            pos++;
            lenbyte--;
        }
        if (lenbyte >= sizeof(size_t)) {
            return 0;
        }
        rlen = 0;
        while (lenbyte > 0) {
            rlen = (rlen << 8) + input[pos];
            pos++;
            lenbyte--;
        }
    } else {
        rlen = lenbyte;
    }
    if (rlen > inputlen - pos) {
        return 0;
    }
    rpos = pos;
    pos += rlen;

    /* Integer tag byte for S */
    if (pos == inputlen || input[pos] != 0x02) {
        return 0;
    }
    pos++;

    /* Integer length for S */
    if (pos == inputlen) {
        return 0;
    }
    lenbyte = input[pos++];
    if (lenbyte & 0x80) {
        lenbyte -= 0x80;
        if (pos + lenbyte > inputlen) {
            return 0;
        }
        while (lenbyte > 0 && input[pos] == 0) {
            pos++;
            lenbyte--;
        }
        if (lenbyte >= sizeof(size_t)) {
            return 0;
        }
        slen = 0;
        while (lenbyte > 0) {
            slen = (slen << 8) + input[pos];
            pos++;
            lenbyte--;
        }
    } else {
        slen = lenbyte;
    }
    if (slen > inputlen - pos) {
        return 0;
    }
    spos = pos;
    pos += slen;

    /* Ignore leading zeroes in R */
    while (rlen > 0 && input[rpos] == 0) {
        rlen--;
        rpos++;
    }
    /* Copy R value */
    if (rlen > 32) {
        overflow = 1;
    } else {
        memcpy(tmpsig + 32 - rlen, input + rpos, rlen);
    }

    /* Ignore leading zeroes in S */
    while (slen > 0 && input[spos] == 0) {
        slen--;
        spos++;
    }
    /* Copy S value */
    if (slen > 32) {
        overflow = 1;
    } else {
        memcpy(tmpsig + 64 - slen, input + spos, slen);
    }

    if (!overflow) {
        overflow = !secp256k1_ecdsa_signature_parse_compact(ctx, sig, tmpsig);
    }
    if (overflow) {
        memset(tmpsig, 0, 64);
        secp256k1_ecdsa_signature_parse_compact(ctx, sig, tmpsig);
    }
    return 1;
}

#endif