Improve pad character handling in base64 MIME decoding functions

Implement the 'MAY' clauses from RFC4648 regarding the pad character
to make mime_decode() and mime_decode_to_string() functions more
tolerant of badly padded base64. The RFC is quoted below for easy
reference.

RFC4648 Section 3.3 with reference to MIME decoding:
Furthermore, such specifications MAY ignore the pad character, "=",
treating it as non-alphabet data, if it is present before the end of
the encoded data.  If more than the allowed number of pad characters
is found at the end of the string (e.g., a base 64 string terminated
with "==="), the excess pad characters MAY also be ignored.

1 files changed, 76 insertions, 10 deletions

diff --git a/lib/stdlib/test/base64_SUITE.erl b/lib/stdlib/test/base64_SUITE.erl
index 44742063b3..3790e7d4c0 100644
--- a/lib/stdlib/test/base64_SUITE.erl
+++ b/lib/stdlib/test/base64_SUITE.erl
@@ -29,7 +29,7 @@
 %% Test cases must be exported.
 -export([base64_encode/1, base64_decode/1, base64_otp_5635/1,
 	 base64_otp_6279/1, big/1, illegal/1, mime_decode/1,
-	 roundtrip/1]).
+	 mime_decode_to_string/1, roundtrip/1]).
 
 init_per_testcase(_, Config) ->
     Dog = test_server:timetrap(?t:minutes(2)),
@@ -50,7 +50,7 @@ all(doc) ->
 all(suite) ->
     [base64_encode, base64_decode, base64_otp_5635,
      base64_otp_6279, big, illegal, mime_decode,
-     roundtrip].
+     mime_decode_to_string, roundtrip].
 
 
 %%-------------------------------------------------------------------------
@@ -59,7 +59,7 @@ base64_encode(doc) ->
 base64_encode(suite) ->
     [];
 base64_encode(Config) when is_list(Config) ->
-    %% Two pads 
+    %% Two pads
     <<"QWxhZGRpbjpvcGVuIHNlc2FtZQ==">> =
 	base64:encode("Aladdin:open sesame"),
     %% One pad
@@ -77,8 +77,8 @@ base64_decode(doc) ->
 base64_decode(suite) ->
     [];
 base64_decode(Config) when is_list(Config) ->
-    %% Two pads 
-    <<"Aladdin:open sesame">> = 
+    %% Two pads
+    <<"Aladdin:open sesame">> =
 	base64:decode("QWxhZGRpbjpvcGVuIHNlc2FtZQ=="),
     %% One pad
     <<"Hello World">> = base64:decode(<<"SGVsbG8gV29ybGQ=">>),
@@ -138,20 +138,85 @@ illegal(Config) when is_list(Config) ->
     {'EXIT',{function_clause, _}} = (catch base64:decode("()")),
     ok.
 %%-------------------------------------------------------------------------
+%% mime_decode and mime_decode_to_string have different implementations
+%% so test both with the same input separately. Both functions have
+%% the same implementation for binary/string arguments.
 mime_decode(doc) ->
     ["Test base64:mime_decode/1."];
 mime_decode(suite) ->
     [];
 mime_decode(Config) when is_list(Config) ->
-    %% Two pads 
-    <<"Aladdin:open sesame">> = 
+    %% Test correct padding
+    <<"one">> = base64:mime_decode(<<"b25l">>),
+    <<"on">>  = base64:mime_decode(<<"b24=">>),
+    <<"o">>   = base64:mime_decode(<<"bw==">>),
+    %% Test 1 extra padding
+    <<"one">> = base64:mime_decode(<<"b25l= =">>),
+    <<"on">>  = base64:mime_decode(<<"b24== =">>),
+    <<"o">>   = base64:mime_decode(<<"bw=== =">>),
+    %% Test 2 extra padding
+    <<"one">> = base64:mime_decode(<<"b25l===">>),
+    <<"on">>  = base64:mime_decode(<<"b24====">>),
+    <<"o">>   = base64:mime_decode(<<"bw=====">>),
+    %% Test misc embedded padding
+    <<"one">> = base64:mime_decode(<<"b2=5l===">>),
+    <<"on">>  = base64:mime_decode(<<"b=24====">>),
+    <<"o">>   = base64:mime_decode(<<"b=w=====">>),
+    %% Test misc white space and illegals with embedded padding
+    <<"one">> = base64:mime_decode(<<" b~2=\r\n5()l===">>),
+    <<"on">>  = base64:mime_decode(<<"\tb =2\"�4=�=   ==">>),
+    <<"o">>   = base64:mime_decode(<<"\nb=w=====">>),
+    %% Two pads
+    <<"Aladdin:open sesame">> =
 	base64:mime_decode("QWxhZGRpbjpvc()GVuIHNlc2FtZQ=="),
-    %% One pad, followed by ignored text
-    <<"Hello World">> = base64:mime_decode(<<"SGVsb)(G8gV29ybGQ=apa">>),
+    %% One pad to ignore, followed by more text
+    <<"Hello World!!">> = base64:mime_decode(<<"SGVsb)(G8gV29ybGQ=h IQ= =">>),
+    %% No pad
+    <<"Aladdin:open sesam">> =
+	base64:mime_decode("QWxhZGRpbjpvcG�\")(VuIHNlc2Ft"),
+    %% Encoded base 64 strings may be divided by non base 64 chars.
+    %% In this cases whitespaces.
+    <<"0123456789!@#0^&*();:<>,. []{}">> =
+	base64:mime_decode(
+	  <<"MDEy MzQ1Njc4 \tOSFAIzBeJ \nio)(oKTs6 PD4sLi \r\nBbXXt9">>),
+    ok.
+
+%%-------------------------------------------------------------------------
+
+%% Repeat of mime_decode() tests
+mime_decode_to_string(doc) ->
+    ["Test base64:mime_decode_to_string/1."];
+mime_decode_to_string(suite) ->
+    [];
+mime_decode_to_string(Config) when is_list(Config) ->
+    %% Test correct padding
+    "one" = base64:mime_decode_to_string(<<"b25l">>),
+    "on"  = base64:mime_decode_to_string(<<"b24=">>),
+    "o"   = base64:mime_decode_to_string(<<"bw==">>),
+    %% Test 1 extra padding
+    "one" = base64:mime_decode_to_string(<<"b25l= =">>),
+    "on"  = base64:mime_decode_to_string(<<"b24== =">>),
+    "o"   = base64:mime_decode_to_string(<<"bw=== =">>),
+    %% Test 2 extra padding
+    "one" = base64:mime_decode_to_string(<<"b25l===">>),
+    "on"  = base64:mime_decode_to_string(<<"b24====">>),
+    "o"   = base64:mime_decode_to_string(<<"bw=====">>),
+    %% Test misc embedded padding
+    "one" = base64:mime_decode_to_string(<<"b2=5l===">>),
+    "on"  = base64:mime_decode_to_string(<<"b=24====">>),
+    "o"   = base64:mime_decode_to_string(<<"b=w=====">>),
+    %% Test misc white space and illegals with embedded padding
+    "one" = base64:mime_decode_to_string(<<" b~2=\r\n5()l===">>),
+    "on"  = base64:mime_decode_to_string(<<"\tb =2\"�4=�=   ==">>),
+    "o"   = base64:mime_decode_to_string(<<"\nb=w=====">>),
+    %% Two pads
+    "Aladdin:open sesame" =
+	base64:mime_decode_to_string("QWxhZGRpbjpvc()GVuIHNlc2FtZQ=="),
+    %% One pad to ignore, followed by more text
+    "Hello World!!" = base64:mime_decode_to_string(<<"SGVsb)(G8gV29ybGQ=h IQ= =">>),
     %% No pad
     "Aladdin:open sesam" = 
 	base64:mime_decode_to_string("QWxhZGRpbjpvcG�\")(VuIHNlc2Ft"),
-
     %% Encoded base 64 strings may be divided by non base 64 chars.
     %% In this cases whitespaces.
     "0123456789!@#0^&*();:<>,. []{}" =
@@ -159,6 +224,7 @@ mime_decode(Config) when is_list(Config) ->
 	  <<"MDEy MzQ1Njc4 \tOSFAIzBeJ \nio)(oKTs6 PD4sLi \r\nBbXXt9">>),
     ok.
 
+%%-------------------------------------------------------------------------
 
 roundtrip(Config) when is_list(Config) ->
     Sizes = lists:seq(1, 255) ++ lists:seq(2400-5, 2440),