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Subject: Re: [Cfrg] OpenPGP security analysis
Trevor Perrin <Tperrin(_at_)sigaba(_dot_)com> forwarded a note from...
From: David Wagner [mailto:daw(_at_)cs(_dot_)berkeley(_dot_)edu]
[quoted here for those new to the conversation:]
I'm not sure I understand what you mean, but there's an attack.
Suppose you want to make the receiver think message M was sent,
even thought the sender would never authorize this. Then you
should construct M' = M || H(M) || X, where X is arbitrary and
where || denotes concatenation (it better be at block boundaries).
Ask the sender to transmit M'; he will form M' || H(M'), encrypt
it with CBC mode, and transmit the result. Now you can truncate
the ciphertext, snipping it just before the "X" part, and the
receiver will think M was sent, which is an integrity failure.
First, I should point out that, M *was* "sent", just not by the
apparent sender. An MDC is not a signature. The receiver should
not treat it as such.
Trevor went on to write:
I don't see any complications that would trip this attack up in OpenPGP's
encryption/integrity packet type.
Assuming the usual PGP public key model, the attacker doesn't need the
sender to participate in this plan. In this scenario, the attacker
can alter message traffic from sender to receiver, so he could simply
replace a message with is own well-formed PGP message. Nothing new here.
This assumes that the receiver's public key is really public.
If it's not, but you can convince the sender to encrypt arbitrary
messages to it, then it's not really very private anyway.
But if this is the scenario, then two facts complicate the attack:
M and M' must be formatted as OpenPGP packets; and,
the OpenPGP OFB scheme includes two redundant check bytes.
(I'll explain in more detail in a separate note when I get time.)
The MDC in OpenPGP is really no more than a checksum, intended to
detect accidental damage. (In fact, during the discussion, I believe
that some folks suggested that it be just a checksum.) Before the
MDC, there was little ability to detect *any* sort of accident -- not
just truncation, but fairly arbitrary internal damage as well. (When
the payload is a compressed packet, the decompression might discover
damage, but then again, it might not.) There was discussion of a
stronger MAC, but I think this was dropped given that OpenPGP has a
strong signature mechanism.
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