[149216] in cryptography@c2.net mail archive
Re: [Cryptography] Does PGP use sign-then-encrypt or
daemon@ATHENA.MIT.EDU (Jerry Leichter)
Thu Jan 23 12:56:08 2014
X-Original-To: cryptography@metzdowd.com
From: Jerry Leichter <leichter@lrw.com>
In-Reply-To: <lbo5hv$2u5$1@ger.gmane.org>
Date: Thu, 23 Jan 2014 09:31:27 -0500
To: Yuriy Kaminskiy <yumkam@gmail.com>
Cc: cryptography@metzdowd.com
Errors-To: cryptography-bounces+crypto.discuss=bloom-picayune.mit.edu@metzdowd.com
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On Jan 22, 2014, at 5:12 AM, Yuriy Kaminskiy <yumkam@gmail.com> wrote:
>>> I am distinguishing MACs from "signatures", as at least in my =
nomenclature
>>> digital signature systems are an inherently pubkey system.
>> MAC's and digital signature systems are different in a more =
fundamental way:
>> With a signature system, Bob can prove to anyone that a message was =
signed by
>> Alice without himself being able to produce messages with Alice's =
signature
>> on them. With a MAC, Bob has everything needed to produce messages =
"MAC'ed"
>> by Alice. But that's fine, because the entire purpose of a MAC is =
for Bob to
>> be able to prove *to himself* that Alice produced a message. There's =
not
>> much point in him forging a message and then proving to himself that =
he
>> forged it!
>=20
> Yet *there are*. If Bob private key leaked and he is not aware about =
that,
> hijacker can decrypt message from Alice, change contents, and =
re-encrypt to Bob it with corrected MAC, and pass it to Bob. Not =
possible with signed message.
Let's see if I understand the attack you're describing: No one uses =
asymmetric crypto for bulk encryption for many very good reasons, so =
Alice and Bob share a symmetric key K that Alice uses to encrypt =
messages to Bob. They also share a MAC key K' that Alice uses to =
compute MAC's on messages sent to Bob. If Charles gets is hands on both =
K and K', yes, he can synthesize messages to Bob that Bob will accept as =
genuine. (He can also intercept messages from Alice to Bob and read =
them, but he doesn't need to do that to send his own messages.)
If Alice instead signs here messages using an asymmetric system with =
private key P_A, Charles will need to get hold of K and P_A. Granted, =
Charles can get both K and K' from Bob, but he can only get P_A from =
Alice. But is this really enough of a difference to matter? At least =
K' is per-session; if the per-session keys are chosen correctly, it will =
become worthless when the next session starts. P_A, on the other hand, =
is a long-term secret. If Alice loses it, she'd better hope that she =
(a) finds out quickly; (b) has access to an actual working key =
revocation system.
BTW, one might argue that using a combined encryption/authentication =
mode is less secure because then effectively K =3D=3D K'. But again, is =
it really all that different? If you can't keep your session keys =
secure, you're dead.
-- Jerry
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