Programming language: Crystal
License: MIT License
Tags: Algorithms And Data Structures    
Latest version: v0.5.0

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The Crystal One Time Password library. Use this to generate HOTP or TOTP codes for two factor authentication.

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Add this to your application's shard.yml:

    github: philnash/crotp



require "crotp"

hotp = CrOTP::HOTP.new("secret")
counter = 1

# Generate a token
token = hotp.generate(counter)
# => "533881"

# Verify code
result = hotp.verify(token, counter)
# => true


require "crotp"

totp = CrOTP::TOTP.new("secret")

# Generate a code at a specific time stamp (by default, #generate will make a
# code using Time.now)
token = totp.generate(at: 1484007247)
# => "020567"

# Verify code at a specific time stamp
result = totp.verify(token, at: 1484007247)
# => true

# Verify code at different time stamp, with allowed drift
result = totp.verify(token, at: 1484007299, allowed_drift: 1)
# => true

# Verify code at different time stamp, outside allowed drift
result = totp.verify(token, at: 1484007300, allowed_drift: 1)
# => false

TOTP hashing algorithms

According to RFC 6238 section 1.2:

TOTP implementations MAY use HMAC-SHA-256 or HMAC-SHA-512 functions, based on SHA-256 or SHA-512 hash functions, instead of the HMAC-SHA-1 function that has been specified for the HOTP computation in RFC4226.

To use either SHA-256 or SHA-512 as the hashing function, initialise your CrOTP::TOTP object with the algorithm you want:

require "crotp"

totp = CrOTP::TOTP.new("secret", algorithm: OpenSSL::Algorithm::SHA512)

Note: authenticator applications may ignore the algorithm parameter when you encode your secret in a URL/QR code as below.

Authenticator applications

To share secrets with an authenticator application, like Authy or Google Authenticator you need a URI that you can share as a QR code. The implementation details for the URI are in the Google Authenticator wiki.

Here is how you can get the URI and, in case your user can't scan the code, the base 32 representation of the secret.


# For HOTP you need the initial counter and an issuer
puts hotp.authenticator_uri(initial_counter: 0, issuer: "Test app")
# => otpauth://hotp/Test%20app?secret=ONSWG4TFOQ&algorithm=SHA1&counter=0&digits=6&issuer=Test%20app

# You can add a user account detail too, normally an email address or username, that shows up in the authenticator app
puts hotp.authenticator_uri(initial_counter: 0, issuer: "Test app", user: "[email protected]")
# => otpauth://hotp/Test%20app:philnash%40example.com?secret=ONSWG4TFOQ&algorithm=SHA1&counter=0&digits=6&issuer=Test%20app


# For TOTP you only need an issuer
puts totp.authenticator_uri(issuer: "Test app")
# => otpauth://totp/Test%20app?secret=ONSWG4TFOQ&algorithm=SHA1&period=30&digits=6&issuer=Test%20app

# You can add a user detail here too
puts totp.authenticator_uri(issuer: "Test app", user: "[email protected]")
# => otpauth://totp/Test%20app:philnash%40example.com?secret=ONSWG4TFOQ&algorithm=SHA1&period=30&digits=6&issuer=Test%20app

Base 32 secret

puts hotp.base32_secret

puts totp.base32_secret

You can see and run these examples and more in example/crotp.cr.


  • [x] Basic HOTP and TOTP generation and verification
  • [x] Rewrite int_to_bytes and extract from CrOTP::OTP
  • [x] Verifying a token over a window of counters/time
  • [x] Google Authenticator otpauth URI generation
  • [x] Ability to choose algorithm (currently only sha1)
  • [ ] Ability to choose size of period in TOTP
  • [ ] Example application using Kemal
  • [ ] Much more documentation

Running the project locally

First clone the project:

git clone https://github.com/philnash/crotp.git
cd crotp

Run the tests with:

crystal spec


  1. Fork it ( https://github.com/philnash/crotp/fork )
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create a new Pull Request


This code is available as open source under the terms of the MIT License.


*Note that all licence references and agreements mentioned in the CrOTP README section above are relevant to that project's source code only.