diff options
Diffstat (limited to 'lib/ssl/src/ssl_cipher.erl')
| -rw-r--r-- | lib/ssl/src/ssl_cipher.erl | 80 |
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). |