sha256 token creation

[franc]

I'm trying to generate and verify totp tokens.

this works well when using [digest_method: :sha], but I need to use rfc6238 SHA256 tokens.

getting failures when trying digest_method: :sha256

opts = [window: 2, interval_length: 3600, digest_method: :sha256]
token = secret |> :pot.totp(opts)
** (MatchError) no match of right hand side value: <<221, 169, 235, 199, 207, 194, 226, 210, 209, 123, 203, 138, 78, 57, 209, 132, 19, 111, 104, 107, 2, 237, 8, 212, 246, 217, 124, 97, 13, 245, 28, 211>>
    (pot 1.0.0) /home/ubuntu/code/maya/deps/pot/src/pot.erl:94: :pot.hotp/3

which points at this:

   Digest = crypto:hmac(DigestMethod, Key, Msg),
    <<_:19/binary, Ob:8>> = Digest,`

iex(17)> :crypto.hmac(:sha, key, "3600")
<<241, 110, 101, 95, 110, 107, 142, 144, 152, 77, 206, 54, 241, 74, 120, 64, 0,
  182, 49, 236>>
iex(18)> :crypto.hmac(:sha256, key, "3600")
<<19, 219, 50, 5, 1, 39, 84, 50, 141, 123, 3, 12, 60, 31, 81, 114, 171, 189, 89,
  13, 111, 243, 53, 50, 228, 50, 173, 173, 161, 193, 179, 4>>`

SHA256 returns 32 bytes, not 20. hotp seems to assume SHA-1 - which is fine, except if it is used for totp it should also support SHA256 and SHA512

[franc]

this seems to work:

    Digest = crypto:hmac(DigestMethod, Key, Msg),
    <<Ob:8>> = binary:part(Digest, {byte_size(Digest), -1}),
    O = Ob band 15,

[franc]

:crypto.hmac is deprecated though

[franc]

but there is this drop-in replacement: Digest = crypto:mac(hmac, DigestMethod, Key, Msg),

I'll see if i can get a pr up