• Introduction
• Version History
• Usage
• Function Reference
• Examples (Erlang)
• Examples (Elixir)
• Credits
• Licence

POT is an Erlang library for generating one time passwords. It supports both HMAC-based one time passwords (HOTP) and time based ones (TOTP). The generated passwords are based on RFC 4226 and RFC 6238, compatible with Google Authenticator.

POT is an almost direct translation of the Python OneTimePass library.

POT should work with any recent version of Erlang/OTP, Elixir, and other Erlang VM based languages.

In order to learn more about one time password generation, see the following Wikipedia articles:

• Google Authenticator
• HMAC-based One-time Password Algorithm (RFC 4226)
• Time-based One-time Password Algorithm (RFC 6238)

• Released version 1.0.2 with the following changes:

  • Fix type specs (Thanks to Krzysztof Jurewicz)
  • Added OTP 24.0 to CI (Thanks to Julius Beckmann)

• Released version 1.0.1 with the following changes:

  • Migrate from Travis to GitHub Actions (Thanks to Nicholas Lundgaard)
  • Update pot.erl to support sha256 and not use deprecated :crypto.hmac (Thanks to Francois Paul)

• Released version 1.0.0 with the following changes:

  • Move coveralls into project_plugins (Thanks to Bryan Paxton)

• Released version 0.11.0 with the following changes:

  • Improved types, README documentation (Thanks to Nicholas Lundgaard)
  • Add return_interval option to valid_hotp (Thanks to Nicholas Lundgaard)

• Released version 0.10.2 with the following change:

  • Fix valid_totp to support upper bound on check_candidate (Thanks to Nicholas Lundgaard)

• Released version 0.10.1 with the following change:

  • Added pot prefix to base32 module avoid name collision (Thanks to Girish Ramnani). This is a breaking change, base32 module was renamed to pot_base32.

• Released version 0.9.8 with the following bug fix:

  • Return boolean on pot:valid_hotp/2 and pot:valid_hotp/3 (Thanks to Zbigniew Pekala)

• pot:totp/2 supports setting the timestamp (Thanks to Julius Beckmann)

• Added options to support Android devices (Thanks to Pedro Vieira)

• Released version 0.9.5 with bug fixes (Thanks to Peter McLain)

• Embedded base32_erlang library

• Initial version

See the sections below on using pot in your Erlang and Elixir project.

We recommend using rebar3 for managing dependencies and building the library. POT is available on hex.pm, so you can just include the following in your rebar.config:

{deps, [pot]}.

See the Erlang examples

Include POT in your mix.exs as a dependency:

defp deps do
    [{:pot, "~> 1.0"}]
end

The functions below refer to the following common parameters:

• Interval is an integer that represents the counter value, the "moving factor" referenced in RFC 4226. It is an 8 byte unsigned integer; if a negative and/or too large integer is passed, it will be 2's complemented and truncated appropriately.
• Secret is a base-32-encoded secret key. Generally, it should be at least 128 bits, preferably 160 bits.
• Token is a HOTP/TOTP value represented as a string*. This is generally a 6-digit number, e.g., "123456", but its length may be modulated with the token_length option.

*Note: for Erlang uses of pot, all strings should be in binary() format.

Generate an RFC 4226 compatible HOTP token.

Erlang:

pot:hotp(Secret, Interval) -> Token
pot:hotp(Secret, Interval, Options) -> Token

Elixir:

:pot.hotp(Secret, Interval) -> Token
:pot.hotp(Secret, Interval, Options) -> Token

The following Options are allowed:

• digest_method controls the signing algorithm passed to the Erlang crypto module's hmac function. For RFC 4226 compliant tokens, it must be set to sha. For RFC 6238 compliant tokens, additional values such as sha256 or sha512 may be used.
• token_length controls the number of digits in output Token.

Generate an RFC 6238 compatible TOTP token.

Erlang:

pot:totp(Secret) -> Token
pot:totp(Secret, Options) -> Token

Elixir:

:pot.totp(Secret) -> Token
:pot.totp(Secret, Options) -> Token

The following Options are allowed:

• addwindow acts as an offset to the Interval extrapolated from dividing the timestamp by the interval_length per the algorithm described in RFC 6238.
• interval_length controls the number of seconds for the Interval computation.
• timestamp may be passed to specify a custom timestamp (in Erlang timestamp format) to use for computing the Interval used to generate a Token.

Validate that a given Token has the correct format (correct length, all digits).

Erlang:

pot:valid_token(Token) -> Boolean
pot:valid_token(Token, Options) -> Boolean

Elixir:

:pot.valid_token(Token) -> Boolean
:pot.valid_token(Token, Options) -> Boolean

The following Options are allowed:

Validate an RFC 4226 compatible HOTP token. Returns true if the Token is valid.

Erlang:

pot:valid_hotp(Token, Secret) -> Boolean
pot:valid_hotp(Token, Secret, Options) -> Boolean | {true, interval()}

Elixir:

:pot.valid_hotp(Token, Secret) -> Boolean
:pot.valid_hotp(Token, Secret, Options) -> Boolean | {true, interval()}

The following Options are allowed:

• last is the Interval value of the previous valid Token; the next Interval after last is used as the first candidate for validating the Token.
• trials controls the number of incremental Interval values after last to try when validating the Token. If a matching candidate is not found within trials attempts, the Token is considered invalid.
• return_interval controls whether the matching Interval of a valid Token is returned with the result. if set to true, then valid_hotp/2 will return {true, Interval} (e.g., {true, 123}) when a valid Token is provided.

Validate an RFC 6238 compatible TOTP token. Returns true if the Token is valid.

Erlang:

pot:valid_totp(Token, Secret) -> Boolean
pot:valid_totp(Token, Secret, Options) -> Boolean

Elixir:

:pot.valid_totp(Token, Secret) -> Boolean
:pot.valid_totp(Token, Secret, Options) -> Boolean

The following Options are allowed:

• window is a range used for expanding Interval value derived from the timestamp. This is done by considering the window Intervals before and after the one derived from the timestamp. This allows validation to be relaxed to allow for successful validation of TOTP Tokens generated by clients with some degree of unknown clock drift from the server, as well as some client entry delay.

POT works with binary tokens and secrets.

Secret = <<"MFRGGZDFMZTWQ2LK">>,
Token = pot:totp(Secret),
% Do something with the token

Secret = <<"MFRGGZDFMZTWQ2LK">>,
CurrentTrial = 3,
Token = pot:hotp(Secret, CurrentTrial),
% Do something with the token

Secret = <<"MFRGGZDFMZTWQ2LK">>,
Token = <<"123456">>,
IsValid = pot:valid_totp(Token, Secret),
% Do something

Secret = <<"MFRGGZDFMZTWQ2LK">>,
Token = <<"123456">>,
LastUsed = 5,  % last successful trial
IsValid = pot:valid_hotp(Token, Secret, [{last, LastUsed}]),
% Do something

Alternatively, to get the last interval from a validated token:

Secret = <<"MFRGGZDFMZTWQ2LK">>,
Token = <<"123456">>,
LastUsed = 5,  % last successful trial
Options = [{last, LastUsed}, {return_interval, true}],
NewLastUsed = case pot:valid_hotp(Token, Secret, Options) of
                  {true, LastInterval} -> LastInterval;
                  false -> LastUsed
              end,
% Do something

Secret = <<"MFRGGZDFMZTWQ2LK">>,
Token = pot:totp(Secret, [{addwindow, 1}]),
% Do something

Secret = <<"MFRGGZDFMZTWQ2LK">>,
Token = <<"123456">>,
IsValid = pot:valid_totp(Token, Secret, [{window, 1}, {addwindow, 1}]),
% Do something

Time format is {MegaSecs, Secs, MicroSecs} received by os:timestamp()

Secret = <<"MFRGGZDFMZTWQ2LK">>,
Token = pot:totp(Secret, [{timestamp, {1518, 179058, 919315}}]),
% Token will be <<"151469">>

secret = "MFRGGZDFMZTWQ2LK"
token = :pot.totp(secret)
# Do something with the token

secret = "MFRGGZDFMZTWQ2LK"
current_trial = 3
token = :pot.hotp(secret, current_trial)
# Do something with the token

secret = "MFRGGZDFMZTWQ2LK"
token = "123456"
is_valid = :pot.valid_totp(token, secret)
# Do something

secret = "MFRGGZDFMZTWQ2LK"
token = "123456"
last_used = 5  # last successful trial
is_valid = :pot.valid_hotp(token, secret, [{:last, last_used}])
# Do something

Alternatively, to get the last interval from a validated token:

secret = "MFRGGZDFMZTWQ2LK"
token = "123456"
last_used = 5  # last successful trial
options = [{:last, last_used}, {:return_token, true}]
new_last_used =
    case :pot.valid_hotp(token, secret, options) do
        {true, last_interval} -> last_interval
        false -> last_used
    end
# Do something

secret = "MFRGGZDFMZTWQ2LK"
token = :pot.totp(secret, [addwindow: 1])
# Do something

secret = "MFRGGZDFMZTWQ2LK"
token = "123456"
is_valid = :pot.valid_totp(token, secret, [window: 1, addwindow: 1])
# Do something

Time format is {MegaSecs, Secs, MicroSecs} received by :os.timestamp()

secret = "MFRGGZDFMZTWQ2LK"
token = :pot.totp(secret, [timestamp: {1518, 179058, 919315}])
# Token will be <<"151469">>

• Yüce Tekol
• Tomasz Jaskowski: OneTimePass Python library
• Andrew Tunnell-Jones: base32_erlang library

Thanks to contributors.

• 2020 - ... : Nicholas Lundgaard
• 2014 - 2020 : Yüce Tekol

Copyright (c) 2014-2021 POT Contributors

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.