Provides HMAC-based One-Time Password (HOTP) generating procedures. HOTP is defined in RFC 4226: RFC 4226.
K must be a bytevector represents shared secret.
C must be an exact integer represents counter.
digit must be an exact integer.
Generates an HMAC-based one-time password.
hotp is an alias of generate-hmac-based-one-time-password.