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Added in 3.3

$hotp(<key>, <count>, [hash], [digits], [encoding])

Returns an HOTP (HMAC-based One-Time Password) based on the specified parameters.

HOTP is designed for hashes no shorter than 160 bits, so using md5 hash is not secure enough and should never be used with $hotp.

See also $totp, $hmac.

Parameters

key - The key to hash. (Auto-detected between text/hex/base32 as described below)
count - a unique (sequential) number. valid range 0-2^64-1
hash - Hash method to hash the key with. (sha1, sha256, sha384, sha512, md5, sha1 is default)
digits - Number of digits to return. (3 - 10, default is 6) (AdiIRC only: digits=0 returns entire internal HMAC string)
encoding - Sets encoding method for 'key'. (t = UTF8 plain text, x = hex, a = base32) (AdiIRC only)

Default when 'encoding' is not used, attempts to support 'key' in several formats as follows:

if (key length excluding spaces is any of lengths 40|64|128 and $remove(key,$chr(32)) is hex ) encoding = x
if (key length excluding spaces is any of lengths 16|26|32 and $remove(key,$chr(32)) is base32) encoding = a
These assume hex keys of 160|256|512 bits, or base32 keys of 80|128|160 bits. all other cases: encoding = t
encoding 'x' or 'a' ignore all spaces padding, but 't' does not. All encoding formats reject key being $null or entirely consisting of spaces.

Note: definition of base32 is case-insensitive [a-zA-Z2-7] after removing spaces, and '=' padding is NOT allowed

Note: It's recommended that the secret 'key' contain entropy no less than the bit length of the 'hash' used. Also, hash blocklength is 64 except for sha512|sha384 having 128. Key is shortened to hash(key) if key is longer than 'blocklength'. i.e. Using key longer than 512 bits with hash=sha1 shortens key to 160 bits:

//var -s %key $regsubex(foo,$str(x,65),/x/g,$rand(a,z)) | echo -a $hotp(%key,123) same as $hotp($sha1(%key),123)
(Equivalence is due to the length 40 string seen as encoding=x)

Updated by Paul Janson 9 months ago · 7 revisions

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