One Time Passwords
========================

One time password schemes are a user authentication method that relies on a
fixed secret key which is used to derive a sequence of short passwords, each of
which is accepted only once. Commonly this is used to implement two-factor
authentication (2FA), where the user authenticates using both a conventional
password (or a public key signature) and an OTP generated by a small device such
as a mobile phone.

Botan implements the HOTP and TOTP schemes from RFC 4226 and 6238.

Since the range of possible OTPs is quite small, applications must rate limit
OTP authentication attempts to some small number per second. Otherwise an attacker
could quickly try all 1000000 6-digit OTPs in a brief amount of time.

HOTP
^^^^^^

HOTP generates OTPs that are a short numeric sequence, between 6 and 8 digits
(most applications use 6 digits), created using the HMAC of a 64-bit counter
value. If the counter ever repeats the OTP will also repeat, thus both parties
must assure the counter only increments and is never repeated or
decremented. Thus both client and server must keep track of the next counter
expected.

Anyone with access to the client-specific secret key can authenticate as that
client, so it should be treated with the same security consideration as would be
given to any other symmetric key or plaintext password.

.. cpp:class:: HOTP

    Implement counter-based OTP

    .. cpp:function:: HOTP(const SymmetricKey& key, const std::string& hash_algo = "SHA-1", size_t digits = 6)

       Initialize an HOTP instance with a secret key (specific to each client),
       a hash algorithm (must be SHA-1, SHA-256, or SHA-512), and the number of
       digits with each OTP (must be 6, 7, or 8).

       In RFC 4226, HOTP is only defined with SHA-1, but many HOTP
       implementations support SHA-256 as an extension. The collision attacks
       on SHA-1 do not have any known effect on HOTP's security.

    .. cpp:function:: uint32_t generate_hotp(uint64_t counter)

       Return the OTP assosciated with a specific counter value.

    .. cpp:function:: std::pair<bool,uint64_t> verify_hotp(uint32_t otp, \
                      uint64_t starting_counter, size_t resync_range = 0)

       Check if a provided OTP matches the one that should be generated for
       the specified counter.

       The *starting_counter* should be the counter of the last successful
       authentication plus 1. If *resync_resync* is greater than 0, some number
       of counter values above *starting_counter* will also be checked if
       necessary. This is useful for instance when a client mistypes an OTP on
       entry; the authentication will fail so the server will not update its
       counter, but the client device will subsequently show the OTP for the
       next counter. Depending on the environment a *resync_range* of 3 to 10
       might be appropriate.

       Returns a pair of (is_valid,next_counter_to_use). If the OTP is invalid
       then always returns (false,starting_counter), since the last successful
       authentication counter has not changed.