lax_der_parsing.c File Reference

#include <string.h>
#include "lax_der_parsing.h"

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Functions
int	ecdsa_signature_parse_der_lax (const secp256k1_context *ctx, secp256k1_ecdsa_signature *sig, const unsigned char *input, size_t inputlen)

Function Documentation

ecdsa_signature_parse_der_lax()

int ecdsa_signature_parse_der_lax	(	const secp256k1_context *	ctx,
		secp256k1_ecdsa_signature *	sig,
		const unsigned char *	input,
		size_t	inputlen )

Parse a signature in "lax DER" format

Returns: 1 when the signature could be parsed, 0 otherwise. Args: ctx: a secp256k1 context object Out: sig: a pointer to a signature object In: input: a pointer to the signature to be parsed inputlen: the length of the array pointed to be input

This function will accept any valid DER encoded signature, even if the encoded numbers are out of range. In addition, it will accept signatures which violate the DER spec in various ways. Its purpose is to allow validation of the Bitcoin blockchain, which includes non-DER signatures from before the network rules were updated to enforce DER. Note that the set of supported violations is a strict subset of what OpenSSL will accept.

After the call, sig will always be initialized. If parsing failed or the encoded numbers are out of range, signature validation with it is guaranteed to fail for every message and public key.

Definition at line 11 of file lax_der_parsing.c.

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

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