Function DescriptorChecksum

src/script/descriptor.cpp:122–167

}

std::string DescriptorChecksum(const std::span<const char>& span)
{
    /** A character set designed such that:
     *  - The most common 'unprotected' descriptor characters (hex, keypaths) are in the first group of 32.
     *  - Case errors cause an offset that's a multiple of 32.
     *  - As many alphabetic characters are in the same group (while following the above restrictions).
     *
     * If p(x) gives the position of a character c in this character set, every group of 3 characters
     * (a,b,c) is encoded as the 4 symbols (p(a) & 31, p(b) & 31, p(c) & 31, (p(a) / 32) + 3 * (p(b) / 32) + 9 * (p(c) / 32).
     * This means that changes that only affect the lower 5 bits of the position, or only the higher 2 bits, will just
     * affect a single symbol.
     *
     * As a result, within-group-of-32 errors count as 1 symbol, as do cross-group errors that don't affect
     * the position within the groups.
     */
    static const std::string INPUT_CHARSET =
        "0123456789()[],'/*abcdefgh@:$%{}"
        "IJKLMNOPQRSTUVWXYZ&+-.;<=>?!^_|~"
        "ijklmnopqrstuvwxyzABCDEFGH`#\"\\ ";

    /** The character set for the checksum itself (same as bech32). */
    static const std::string CHECKSUM_CHARSET = "qpzry9x8gf2tvdw0s3jn54khce6mua7l";

    uint64_t c = 1;
    int cls = 0;
    int clscount = 0;
    for (auto ch : span) {
        auto pos = INPUT_CHARSET.find(ch);
        if (pos == std::string::npos) return "";
        c = PolyMod(c, pos & 31); // Emit a symbol for the position inside the group, for every character.
        cls = cls * 3 + (pos >> 5); // Accumulate the group numbers
        if (++clscount == 3) {
            // Emit an extra symbol representing the group numbers, for every 3 characters.
            c = PolyMod(c, cls);
            cls = 0;
            clscount = 0;
        }
    }
    if (clscount > 0) c = PolyMod(c, cls);
    for (int j = 0; j < 8; ++j) c = PolyMod(c, 0); // Shift further to determine the checksum.
    c ^= 1; // Prevent appending zeroes from not affecting the checksum.

    std::string ret(8, ' ');
    for (int j = 0; j < 8; ++j) ret[j] = CHECKSUM_CHARSET[(c >> (5 * (7 - j))) & 31];
    return ret;
}

std::string AddChecksum(const std::string& str) { return str + "#" + DescriptorChecksum(str); }