Currently, the multisig descriptor constructions are limited to 16 keys only possibly because of template parsing reasons. Under p2sh context, such scripts would be invalid anyway because of 520-byte rule, but we can support up to 20 keys for CHECKMULTISIG in witness script context. 


{
    uint8_t c0 = c >> 35;
    c = ((c & 0x7ffffffff) << 5) ^ val;
    if (c0 & 1) c ^= 0xf5dee51989;
    if (c0 & 2) c ^= 0xa9fdca3312;
    if (c0 & 4) c ^= 0x1bab10e32d;
    if (c0 & 8) c ^= 0x3706b1677a;
    if (c0 & 16) c ^= 0x644d626ffd;
    return c;
}
std::string DescriptorChecksum(const 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.