From f96b50d00568154aabff7f956b974ad295c38fd6 Mon Sep 17 00:00:00 2001
From: Francois Paul <francois@rails.co.za>
Date: Wed, 17 Mar 2021 00:44:52 +0100
Subject: [PATCH] Update pot.erl to support sha256 and not use deprecated
 :crypto.hmac (#30)

Update pot HMAC computation to support sha256 and be compatible with Erlang/OTP 23+
Resolves https://github.com/yuce/pot/issues/29
---
 src/pot.erl | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/src/pot.erl b/src/pot.erl
index 980b79c..591300e 100644
--- a/src/pot.erl
+++ b/src/pot.erl
@@ -90,8 +90,8 @@ hotp(Secret, IntervalsNo, Opts) ->
     TokenLength = proplists:get_value(token_length, Opts, 6),
     Key = pot_base32:decode(Secret),
     Msg = <<IntervalsNo:8/big-unsigned-integer-unit:8>>,
-    Digest = crypto:hmac(DigestMethod, Key, Msg),
-    <<_:19/binary, Ob:8>> = Digest,
+    Digest = hmac(DigestMethod, Key, Msg),
+    <<Ob:8>> = binary:part(Digest, {byte_size(Digest), -1}),
     O = Ob band 15,
     <<TokenBase0:4/integer-unit:8>> = binary:part(Digest, O, 4),
     TokenBase = TokenBase0 band 16#7fffffff,
@@ -205,3 +205,16 @@ valid_hotp_return(LastInterval, true = _ReturnInterval) ->
     {true, LastInterval};
 valid_hotp_return(_LastInterval, _ReturnInterval) ->
     true.
+
+-ifdef(OTP_RELEASE).
+-if(?OTP_RELEASE >= 23).
+hmac(DigestMethod, Key, Msg) ->
+    crypto:mac(hmac, DigestMethod, Key, Msg).
+-else.
+hmac(DigestMethod, K, S) ->
+    crypto:hmac(DigestMethod, K, S).
+-endif.
+-else.
+hmac(DigestMethod, K, S) ->
+    crypto:hmac(DigestMethod, K, S).
+-endif.