EFail mitigations for S/MIME
martijn.list at gmail.com
Wed May 16 16:15:45 CEST 2018
On 15-05-18 14:31, Andre Heinecke wrote:
> It think Bernhards mail can be summed up further. To check that the encrypted
> data was not manipulated we only need:
> - Any hash over the plaintext.
> To get such a hash we can most easily use a signature, regardless of any trust
> in the signature. The hash does not need to be encrypted.
> If we have no hash we won't offer to save a decrypted file from a GUI or show
> it in an HTML enabled mail client. This would disallow encrypt, then sign
> schemes but in practice everyone uses sign then encrypt anyway.
Adding a hash will not help in the general case because other S/MIME
clients will not support it.
I have done some experiments with the "Generic exfiltration" attack and
have been able to replicate the attack. Injecting new blocks is easy.
However after every injected block, there is a block of random data.
This block of random data can be used to detect whether the message was
attacked with EFAIL in most cases. The S/MIME RFCs strongly suggest that
every MIME part should be 7-bit. If a decrypted message therefore
contains data > 127 or < 32 excluding CR/LF/TAB, the message might have
been injected with additional blocks. For the "Generic exfiltration"
EFAIL attack, you need at least 2 blocks of data so there will be at
least 32 bytes of random data. The changes that all those bytes fall
within the 7-bit range is slim so I think this check would work to
detect most (if not all) EFAIL attacks. The only problem you might run
into is if a sender encrypted a message containing 8-bit characters
(i.e., the message was not canonicalized to 7-bit).
I have written a short blog item about this here
Any comments on whether this will work?
As we speak I'm working on such a check.
More information about the Gnupg-devel