On Saturday, March 30, 2019 4:09 PM, Jon Callas
<joncallas(_at_)icloud(_dot_)com> wrote:
On Mar 29, 2019, at 7:17 PM, Bart Butler
bartbutler=40protonmail(_dot_)com(_at_)dmarc(_dot_)ietf(_dot_)org wrote:
Hi Jon,
As others have noted, there is a lot of confusion on this thread, some of
which you touched in your AEAD Conundrum message, like when we say AEAD
should not release unauthenticated plaintext, do we mean the entire
message or the chunk?
That is precisely the question. But the bigger question is whether you care
about that. Sometimes it matters, sometimes it does not.
For example, let’s suppose that I have a very large blob on mass storage.
Media failures happen. If there’s a bad block on a disk, do you want to
lose the whole file because of it? Sometimes you want to throw your hands
up in the air. This is most common in an interactive protocol, and in
general the answer is yes. For example, an SSL connection to a server, if
there’s funny business going on, you want to blow up the connection and try
over again. On the other hand, if you had an archival thing (e.g. tar file
with some historic documents), you want to recover as much as you can.
OpenPGP is in general the latter case rather than the former. I believe
it’s less important to have strict semantics on failures because it’s
usually storage.
I agree. I would say my point is that with sufficiently small chunks, the
user/decrypter can choose what kind of failure behavior is appropriate. Large
chunks robs the decrypter of that.
Another piece of confusion is that Efail isn't a single vulnerability, it
was several vulnerabilities related (at best) thematically.
I understand Efail. Trust me.
I do. I was bit excessively pedantic defining everything because I felt that
this thread has suffered from ambiguity in several terms.
So to be very specific, for the purpose of the following discussion, the
advantage of smaller AEAD chunks is specifically to prevent Efail-style
ciphertext malleability/gadget attacks, and the prohibition on releasing
unauthenticated plaintext is applied to individual chunks, which is
sufficient to foil this kind of attack in email.
I disagree. If you want to prevent something like Efail, you want larger
chunks. Assuming that you believe that early release matters.
Let’s rewind here, and not talk about Efail, let’s talk about the real
issue. If you want the entire blob to have all-or-nothing semantics, then
you want the fewest number of chunks as is reasonable. If you have
attacker-controlled inputs, then every joint between the chunks is a
vulnerability.
OK, I think this is the part that I don't understand. Why does it matter what
chunking scheme is used here? If my app requires all-or-nothing semantics, I
would program my app to enforce that all chunks must pass and not release
plaintext unless that happened, with no truncation, etc. So why would every
joint be a vulnerability?
What value does large-chunk AEAD actually provide? What I'm getting from
the AEAD Conundrum message is that it's a way for the message encrypter
to leverage the "don't release unauthenticated chunks" prohibition to
force the decrypter to decrypt the whole message before releasing
anything. Why do we want to give the message creator this kind of power?
Why should the message creator be given the choice to force her recipient
to either decrypt the entire message before release or be less safe than
she would have been with smaller chunks?
Let me summarize the conundrum: If you want strict AEAD no-release
semantics, you want a fewer number of chunks.
I guess this is my fundamental question. You can force no-release semantics
at the application level for any chunk size scheme, right?
Coming back to Neal's point, it's really hard to see any sort of value in
really large AEAD chunks, because the performance overhead is negligible
at that point and the only security 'benefit' that I can see is the
encrypter trying to use the spec to force the decrypter to not stream,
which does not seem like something at all desirable.
Okay, here’s another thing that’s a pet peeve of mine. We’re arguing
security and you brought up performance. I never mentioned performance, the
people who want large chunks haven’t brought it up. They want large chunks
because they perceive it to be more secure..
If you respond to a security request with a performance answer, you
literally don’t know what you’re talking about. So let’s toss that aside.
I apologize, I was not trying to create a strawman here, but I am completely
at a loss for what the benefit of large chunks is.
Elided out of this, and possibly important is that “support” includes
chunks smaller than that size. I should have said that, but I wanted it to
be as stark as possible. So let me repeat it and abstract it with some
variables:
(1) MUST support up to <small-chunk-size> chunks.
(2) SHOULD support up to <larger-chunk-size> chunks, as these are common..
(3) MAY support larger up to the present very large size.
(4) MAY reject or error out on chunks larger than <small-chunk-size>, but
repeating ourselves, SHOULD support <larger-chunk-size>.
Clauses (3) and (4) set up a sandbox for the people who want very large
chunks. They can do whatever they want, and the rest of us can ignore
them.. Why get rid of that? It doesn’t add any complexity to the code. It
lets the people who want the huge ones do them in their own environment and
not bother other people.
My concern is over (1) and (2) and specifically that there’s both <small>
and <large> sizes.
I think that’s an issue. If there are two numbers we are apt to end up with
skew before settling on one, so it’s better to agree on just one. That’s
the real wart in my proposal.
I'm OK with eliminating (2) and just using the MAY part to take care of any
legacy 256K messages OpenPGP.js users might have. As I said, we don't have
any of these messages in production yet and I'd err on the side of a cleaner
spec. I just really want to understand the benefit of large chunks for
security and right now I clearly do not.
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