Function hotp_with 

pub fn hotp_with(
    secret: &[u8],
    counter: u64,
    algorithm: Algorithm,
    digits: u32,
) -> String

HOTP (RFC 4226) for one counter value, with the algorithm and digit count the issuer chose.

digits is clamped to MIN_DIGITS..=MAX_DIGITS rather than rejected: this is the innermost function and it is reached only through validated constructors, so a panic here would be a crash in a card renderer for a number a user typed. Params::validate is where a bad value is refused.