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[openpgp] OpenPGP SEIP downgrade attack

2015-10-05 09:08:02

I've recently been analysing the OpenPGP standard and have found that it
is vulnerable to a chosen-ciphertext attack to downgrade an SEIP packet
to a plain SE packet. Due to the properties of CFB mode and OpenPGP’s
predictable message structure, it is possible to switch the SEIP tag to
SE, strip the MDC (and signature), and align and manipulate the
encrypted packet. The implications are, among others, that an encrypted
and signed message can be stripped of its signature and modified
arbitrarily, with certain restrictions, by an attacker without knowing
the key. This attack also resurrects attacks from as early as 2000
[4,5]. Part of the reason SEIP and MDC was introduced ~15 years ago was
to deal with exactly this problem. Looking back at the OpenPGP
mailinglist arhive it seems that the correctness of the integrity
protection has been questioned now and then over the years [1,2], but
it's been maintained that it is secure against this kind of attack [3].
It also seems like thoughts along this line is not new [6], so question
remains why it has not been dealt with before.

Different implementations handle SE packets differently. GPG throws a
warning [3] that the message could have been modified, but other
implementations do not differentiate between SE and SEIP. A quick fix
for this issue would be to throw an error for all SE packets, though I
understand the legacy issues it would bring (as was also suggested [3]).
A large part of the problem here is due to CFB mode, but it seems we're
stuck with that. It would make sense to use a different mode, but again
I understand the legacy issues.

To try it out in practice and to see how hard it would be for someone
else to come to the same conclusion, I created this challenge:
I thought it would be tough to crack, but to my great surprise it was
solved in less than 12 hours. So far only one person has solved it
though. I was going to submit a paper about the attack, but considering
how quickly the challenge was cracked I realised the urgency to report this.

The attack in more detail:

To start out, some important properties of CFB mode. (Sorry for
restating the obvious, it's just for completeness.)

    1) A modification of the length of the ciphertext will cause the
    same effect on the plaintext. Addition of blocks to the ciphertext
    will extend the plaintext equally. Any number of bytes cut off the
    end of the ciphertext will cut the same number of bytes from the

    2) The decryption of a block depends only on the preceding block,
    regardless of where in the ciphertext it appears. Sequences of
    blocks can be cut, duplicated, or injected amidst two blocks. (When
    injecting the surrounding blocks will fail to decrypt correctly, but
    under the right circumstances that does not matter.)

    3) If part of the plaintext is known, the corresponding part of the
    ciphertext can be modified to produce arbitrary text.

It is important to note that none of these properties depend on the
encryption key, which implies that knowing the key is not required to
abuse them.

Now, turning a SEIP packet into a SE packet, can be done in three steps:

    1) Changing the tag.

    2) Aligning the encrypted blocks to match the resynchronized
    structure of the SE packet.

    3) Ensure correct parsing of the decrypted text into packets.

The first step is obvious. To deal with the second step we can abuse
property 1 and 2, and simply prepend two bytes to the encrypted string.
Because of the resynchronization step in SE packet encryption, these two
bytes will only affect decryption of the last two bytes of the IV. The
rest of the blocks will now be aligned with the SE structure and decrypt
correctly. Because the blocks have been shifted the decrypted text will
however not parse correctly due to the now missplaced checkbytes.

The last step is to make the decrypted text parse as a set of packets.
To parse correctly, the first two bytes of the plaintext must match the
header of a packet such that it allows the rest of the text to parse as
the original packets. This can be achieved by abusing property 2 and 3.
Either, if the first two bytes of plaintext of any block is known, then
that block and the preceeding one can be repurposed to create such a
packet, or if no plaintext is known it can be bruteforced (given a
decryption oracle). Unfortunately OpenPGP packets are very predictable
and there are a number of ways this can be done. In fact, if the message
is signed it is pretty straight forward because of the predictable one
pass signature packet.

This is supposed to convey the idea of the attack, but not be a complete
recipe on how to do it. If more info is needed on the specifics of the
attack in order to fix it, then this can be provided.

Best regards,

Jonas Magazinius
Assured AB, Sweden

[1] Is there any published analysis of OpenPGP's MDC?
[2] security fixes (KDF, MDC->MAC)?
[3] Re: security fixes (KDF, MDC->MAC)? -
[4] A Chosen Ciphertext Attack against Several E-Mail Encryption
Protocols - USENIX'00
[5] Implementation of Chosen-Ciphertext Attacks against PGP and GnuPG -
[6] OpenPGP security analysis -

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