hex string to byte array, C

gistfile1.c

unsigned char* hexstr_to_char(const char* hexstr)
{
    size_t len = strlen(hexstr);
    IF_ASSERT(len % 2 != 0)
        return NULL;
    size_t final_len = len / 2;
    unsigned char* chrs = (unsigned char*)malloc((final_len+1) * sizeof(*chrs));
    for (size_t i=0, j=0; j<final_len; i+=2, j++)
        chrs[j] = (hexstr[i] % 32 + 9) % 25 * 16 + (hexstr[i+1] % 32 + 9) % 25;
    chrs[final_len] = '\0';
    return chrs;
}

@guigarma This is pretty helpful. When checking if the string is valid hex it only checks for capital letters A-F (ASCII 65 - 70). If you use something like sprintf to produce the hex string then the output will use lowercase letters and the function doesn't work. I added a small check to fix that.

int HexStringToBytes(const char *hexStr,
                     unsigned char *output,
                     unsigned int *outputLen)
{
    size_t len = strlen(hexStr);
    if (len % 2 != 0) {
        return -1;
    }
    size_t finalLen = len / 2;
    *outputLen = finalLen;
    for (size_t inIdx = 0, outIdx = 0; outIdx < finalLen; inIdx += 2, outIdx++) {
        if ((hexStr[inIdx] - 48) <= 9 && (hexStr[inIdx + 1] - 48) <= 9) {
            goto convert;
        } else {
            if (((hexStr[inIdx] - 65) <= 5 && (hexStr[inIdx + 1] - 65) <= 5) || ((hexStr[inIdx] - 97) <= 5 && (hexStr[inIdx + 1] - 97) <= 5)) {
                goto convert;
            } else {
                *outputLen = 0;
                return -1;
            }
        }
    convert:
        output[outIdx] =
            (hexStr[inIdx] % 32 + 9) % 25 * 16 + (hexStr[inIdx + 1] % 32 + 9) % 25;
    }
    output[finalLen] = '\0';
    return 0;
}