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-rw-r--r--lib/ssl/src/ssl_cipher.erl80
1 files changed, 55 insertions, 25 deletions
diff --git a/lib/ssl/src/ssl_cipher.erl b/lib/ssl/src/ssl_cipher.erl
index 72f02a4362..95a5efd6d0 100644
--- a/lib/ssl/src/ssl_cipher.erl
+++ b/lib/ssl/src/ssl_cipher.erl
@@ -154,18 +154,23 @@ decipher(?AES, HashSz, CipherState, Fragment, Version) ->
block_decipher(Fun, #cipher_state{key=Key, iv=IV} = CipherState0,
HashSz, Fragment, Version) ->
- try Fun(Key, IV, Fragment) of
- Text ->
- GBC = generic_block_cipher_from_bin(Text, HashSz),
- case is_correct_padding(GBC, Version) of
- true ->
- Content = GBC#generic_block_cipher.content,
- Mac = GBC#generic_block_cipher.mac,
- CipherState1 = CipherState0#cipher_state{iv=next_iv(Fragment, IV)},
- {Content, Mac, CipherState1};
- false ->
- ?ALERT_REC(?FATAL, ?BAD_RECORD_MAC)
- end
+ try
+ Text = Fun(Key, IV, Fragment),
+ GBC = generic_block_cipher_from_bin(Text, HashSz),
+ Content = GBC#generic_block_cipher.content,
+ Mac = GBC#generic_block_cipher.mac,
+ CipherState1 = CipherState0#cipher_state{iv=next_iv(Fragment, IV)},
+ case is_correct_padding(GBC, Version) of
+ true ->
+ {Content, Mac, CipherState1};
+ false ->
+ %% decryption failed or invalid padding,
+ %% intentionally break Content to make
+ %% sure a packet with a an invalid padding
+ %% but otherwise correct data will fail
+ %% the MAC test later
+ {<<16#F0, Content/binary>>, Mac, CipherState1}
+ end
catch
_:_ ->
%% This is a DECRYPTION_FAILED but
@@ -500,14 +505,38 @@ hash_size(md5) ->
hash_size(sha) ->
20.
+%% RFC 5246: 6.2.3.2. CBC Block Cipher
+%%
+%% Implementation note: Canvel et al. [CBCTIME] have demonstrated a
+%% timing attack on CBC padding based on the time required to compute
+%% the MAC. In order to defend against this attack, implementations
+%% MUST ensure that record processing time is essentially the same
+%% whether or not the padding is correct. In general, the best way to
+%% do this is to compute the MAC even if the padding is incorrect, and
+%% only then reject the packet. For instance, if the pad appears to be
+%% incorrect, the implementation might assume a zero-length pad and then
+%% compute the MAC. This leaves a small timing channel, since MAC
+%% performance depends to some extent on the size of the data fragment,
+%% but it is not believed to be large enough to be exploitable, due to
+%% the large block size of existing MACs and the small size of the
+%% timing signal.
+%%
+%% implementation note:
+%% We return the original (possibly invalid) PadLength in any case.
+%% A invalid PadLength will be cought by is_correct_padding/2
+%%
generic_block_cipher_from_bin(T, HashSize) ->
Sz1 = byte_size(T) - 1,
- <<_:Sz1/binary, ?BYTE(PadLength)>> = T,
+ <<_:Sz1/binary, ?BYTE(PadLength0)>> = T,
+ PadLength = if
+ PadLength0 >= Sz1 -> 0;
+ true -> PadLength0
+ end,
CompressedLength = byte_size(T) - PadLength - 1 - HashSize,
<<Content:CompressedLength/binary, Mac:HashSize/binary,
- Padding:PadLength/binary, ?BYTE(PadLength)>> = T,
+ Padding:PadLength/binary, ?BYTE(PadLength0)>> = T,
#generic_block_cipher{content=Content, mac=Mac,
- padding=Padding, padding_length=PadLength}.
+ padding=Padding, padding_length=PadLength0}.
generic_stream_cipher_from_bin(T, HashSz) ->
Sz = byte_size(T),
@@ -516,17 +545,18 @@ generic_stream_cipher_from_bin(T, HashSz) ->
#generic_stream_cipher{content=Content,
mac=Mac}.
-is_correct_padding(_, {3, 0}) ->
- true;
-%% For interoperability reasons we do not check the padding in TLS 1.0 as it
-%% is not strictly required and breaks interopability with for instance
-%% Google.
-is_correct_padding(_, {3, 1}) ->
- true;
+%% For interoperability reasons we do not check the padding content in
+%% SSL 3.0 and TLS 1.0 as it is not strictly required and breaks
+%% interopability with for instance Google.
+is_correct_padding(#generic_block_cipher{padding_length = Len,
+ padding = Padding}, {3, N})
+ when N == 0; N == 1 ->
+ Len == byte_size(Padding);
%% Padding must be check in TLS 1.1 and after
-is_correct_padding(#generic_block_cipher{padding_length = Len, padding = Padding}, _) ->
- list_to_binary(lists:duplicate(Len, Len)) == Padding.
-
+is_correct_padding(#generic_block_cipher{padding_length = Len,
+ padding = Padding}, _) ->
+ Len == byte_size(Padding) andalso
+ list_to_binary(lists:duplicate(Len, Len)) == Padding.
get_padding(Length, BlockSize) ->
get_padding_aux(BlockSize, Length rem BlockSize).