HMAC-Based and Time-Based One-Time Passwords (HOTP & TOTP)
Implements the HMAC-Based One-Time Password Algorithm (HOTP) as defined in RFC 4226 and the Time-Based One-Time Password Algorithm (TOTP) as defined in RFC 6238.
The TOTP and HOTP algorithms are commonly used to implement two-step verification (2FA) (e.g. by Google Authenticator ).
See module Data.OTP for API documentation.
OTP: HMAC-Based and Time-Based One-Time Passwords (HOTP & TOTP) 
Please refer to the package description for an overview of OTP.
Usage examples
Generating one-time passwords
If you need to generate HOTP password described in RFC4226, then use
>>> hotp SHA1 "1234" 100 6
317569
>>> hotp SHA512 "1234" 100 6
134131
Or
>>> totp SHA1 "1234" (read "2010-10-10 00:01:00 UTC") 30 8
43388892
to generate TOTP password described in RFC6238.
Checking one-time passwords
hotpCheck :: HashAlgorithm -- ^ Hashing algorithm
-> Secret -- ^ Shared secret
-> (Word8, Word8) -- ^ how much counters to take lower and higher than ideal
-> Word64 -- ^ ideal (expected) counter value
-> Word8 -- ^ Number of digits in password
-> Word32 -- ^ Password entered by user
-> Bool -- ^ True if password acceptable
>>> hotpCheck SHA1 "1234" (0,0) 10 6 50897
True
>>> hotpCheck SHA1 "1234" (0,0) 9 6 50897
False
>>> hotpCheck SHA1 "1234" (0,1) 9 6 50897
True
Here almost the same aguments as for hotp function, but there is also (0, 0) tuple. This tuple describes range of counters to check in case of desynchronisation of counters between client and server. I.e. if you specify (1, 1) and ideal counter will be 10 then function will check passwords for [9, 10, 11] list of counters.
Here is the same for TOTP:
>>> totpCheck SHA1 "1234" (0, 0) (read "2010-10-10 00:00:00 UTC") 30 6 778374
True
>>> totpCheck SHA1 "1234" (0, 0) (read "2010-10-10 00:00:30 UTC") 30 6 778374
False
>>> totpCheck SHA1 "1234" (1, 0) (read "2010-10-10 00:00:30 UTC") 30 6 778374
True