Re: [openpgp] review of the SOP draft

Antoine Beaupré <anarcat@torproject.org> Tue, 12 November 2019 17:26 UTC

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To: Daniel Kahn Gillmor <dkg@fifthhorseman.net>, openpgp@ietf.org
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Subject: Re: [openpgp] review of the SOP draft
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On 2019-11-11 18:00:17, Daniel Kahn Gillmor wrote:
> Hi Antoine--
>
> Thanks for the thoughtful review.  It was super long though!  I've
> opened a bunch of issues from the stuff you raised here, but more
> dicsussion follows inline below.

No problem! It's a long draft in the first place (24 pages PDF), and
talks about something I actually have (had) lots of ideas about in the
past, so I guess that's normal...

> On Mon 2019-11-11 12:57:51 -0500, Antoine Beaupré wrote:
>> https://gitlab.com/dkg/openpgp-stateless-cli/merge_requests/9
>
> I've reviewed and merged the changes requested there, definitely useful
> to have this clarifying editorial work done as patches, thanks!

Great!

>>> This separation should make it easier to provide interoperability testing for the object security work, and to allow implementations to consume and produce new cryptographic primitives as needed.
>>  
>> I don't understand the part after ", and allow..." what does that
>> actually mean? What do we mean by "new cryptographic primitives" here
>> exactly?
>
> The intent is that if every OpenPGP implementation provides a `sop`
> interface (or potentially a superset of it), then we can write
> interoperabiity tests and the like that drive all of them in a reliable
> way.
>
> We can, for example, generate new OpenPGP objects that incorporate new
> primitives, and feed them to a stable of `sop` implementations, to
> determine whether those implementations can consume them.
>
> Or, we can drive them with simple inputs, and see which cryptographic
> primitives they choose to use as they produce output.
>
> If you think this text is clearer, i can incorporate it directly in the
> draft.

That would be great, thanks! Providing examples like this would go a
long way...

>> [...]
>>
>>> Obviously, the user will need to manage their secret keys (and their
>>> peers' certificates) somehow, but the goal of this interface is to
>>> separate out that task from the task of interacting with OpenPGP
>>> messages.
>>  
>> It's unclear to me how or if the SOP specification takes into account
>> the current design of GnuPG, specifically the part where secrets are
>> handled by a separate process, gpg-agent, which is designed to be
>> separate from the other parts of gnupg. From what I understand, the
>> "agents" keep the secrets and do the operations on behalf of other parts
>> of gnupg. But here sop would do so. Are we designing an agent here?
>>
>> What about OpenPGP cards like the Yubikey? How does sop interoperate
>> with those?
>
> sop is not GnuPG, and it doesn't claim or intend to be.

thank you for that too. ;)

> An implementation of sop *may* choose to support other forms of secret
> key access (the "@FOO:" namespace is carved out to allow for that), but
> the goal of sop is that it is simple enough that every implementation
> that can handle OpenPGP secret keys and certificates should be capable
> of implementing the interface.

I guess what I'm wondering is how I would make this work with my yubikey
at all. Or maybe I got this backwards and the yubikey interface is what
should implement sop directly?

>> I find those examples confusing. Multiple arguments, in particular,
>> seems ambiguous. Is it "CERT DATA"? or "CERT DATA"?
>
> ???  i think those are the same thing, but i'll just assume you meant
> "DATA CERTS" at the end.  The answer is that there must be exactly one
> SIGNATURE object to verify and there may be multiple certs, so the only
> possible way to do it is SIGNATURE first, then CERTS.

Ah, yes, sorry about this. I was specifically refering to:

    sop verify announcement.txt.asc alice.pgp < announcement.txt

And `"CERT DATA" or "DATA CERT"`?

And I guess where we differ is I am not sure it's that clear that the
first argument of a series can be different from the rest...

> But that doesn't address your larger point:
>
>> There should be *mandatory* commandline *options* instead, that clearly
>> state the purpose of (say) the "CERT" argument. It's a common in APIs,
>> to rely on the order of arguments for meaning, and I think it's often a
>> mistake. We should explicitely use *options* instead of *arguments* (as
>> in `--foo=bar` instead of just `bar`) for critical parameters like
>> secret or verification keys.
>
> I would welcome a MR that makes this change across the entire spec, to
> see whether it looks reasonable.  If people on this list prefer that
> structure, i would be fine with adopting it, though i think it makes
> the CLI more verbose than i would like.
>
> I've trimmed out a lot of your comments below that were to do with
> whether positional arguments are sensible or not.  that's not because i
> don't care, but i don't think arguing about it in this one message is
> helpful.
>
> i've opened https://gitlab.com/dkg/openpgp-stateless-cli/issues/7 to
> record the overall concern.  if you decide to make an MR for it, please
> reference that issue in the MR!

I have a patch for this and will submit it. It was more invasive than
the others so I figured I would bring it up as a discussion first.

>> In general, I feel using the numeric error codes in the document make it
>> (needlessly?) harder to read. When i got to this section, my first
>> reaction was: "69?? why 69? and why 37? where the heck do those come
>> from and why do they matter?" We should at least include a reference to
>> the "Failure modes section" in the Introduction section. In Terminology
>> maybe? And maybe refer to it here.
>>
>> In general, I'm worried there might be inconsistencies between the table
>> in the "Failure modes" section and the various hardcoded integers
>> peppered through the document. This practice also makes the document
>> more difficult to review and maintain in the future. We might instead
>> use constant names like `SUCCESS`, `NO_GOOD_SIG` that *then* have
>> integer values in the later section. This could also provide for good
>> constants to use in a library implementation.
>
> I really like this idea, and i encourage someone™ who also likes it to
> make a MR that implements it.
>
> i've opened https://gitlab.com/dkg/openpgp-stateless-cli/issues/1 to
> keep track of it.

I can try to tackle this as well.

>>> For all commands that have an `--armor|--no-armor` option, it defaults
>>> to `--armor`, meaning that any output OpenPGP material should be
>>> ASCII-armored (section 6 of {{I-D.ietf-openpgp-rfc4880bis}})
>>> by default.
>>  
>> Is this on input or output? or both? It's clarified later, I
>> think, but it should be made explicit here as well.
>
> the text you've quoted says "any output OpenPGP material".  I'm not sure
> how to make that more visible, but i welcome proposed edits.

I made this MR for this:

https://gitlab.com/dkg/openpgp-stateless-cli/merge_requests/10

>> How do we generate purpose-specific subkeys?
>
> With `sop`, you do not ;)

Sad.

> If you want to do fancy OpenPGP certificate generation, you do that with
> your toolkit's own fancy features.
>
> I've opened https://gitlab.com/dkg/openpgp-stateless-cli/issues/2 to
> track that maybe we do want some rough guidance about what kinds of
> secret key capabilities we want any `sop` to be able to generate here
> though.

Commented on that. Would still love to see a more decent way to handle
subkeys because that's a really hard thing to do in existing
implementations.

At least creating split subkeys by default would be a great start, IMHO.

>>> sign: Create a Detached Signature {#sign}
>> […]
>>> If `--as=text` and the input `DATA` is
>>> not valid `UTF-8`, `sop sign` fails with a return code of 53.
>>
>> Why do we mandate UTF-8 here? Explain.
>
> We don't mandate UTF-8 unless the signer claims that the thing being
> signed is text.  If so, it really does need to be UTF-8.  I have no
> patience for non-UTF-8-encoded text in 2019.
>
> OpenPGP embeds UTF-8 explicitly in its User ID formatting.  Any OpenPGP
> implementation must already handle UTF-8.
>
> if anyone thinks that dealing with different character encodings is a
> good idea, please consider that the character encoding is not recorded
> in the signature itself, leading charset-switching attacks like those in
> https://dkg.fifthhorseman.net/notes/inline-pgp-harmful/
>
> Do you think this information belongs in this document?

Absolutely, otherwise it looks like an arbitrary decision.

I am not, for the record, arguing for other encodings. UTF-8 is fine and
diverging is a recipe for disaster, indeed.

>> In general, I find the `--as` arguments to be a little confusing and I
>> don't undrestand what they bring to the table.
>
> OpenPGP has two different forms of cryptographic signature, which are
> dealt with by recipients differently.  The textual form canonicalizes
> line-endings and the binary does not.  When signing a document, you need
> to indicate to the thing doing the signing which form you are expecting
> to create.

The intricacies of OpenPGP never cease to amaze me.

> If you have a different suggestion, i'm happy to hear it.

I wish we didn't have to deal with this distinction, but if so, maybe we
should clarify the source of it here. Otherwise it comes as a surprise
to me, an experience OpenPGP user.

>>> Example:
>>> 
>>>     $ sop sign --as=text alice.sec < message.txt >message.txt.asc
>>>     $ head -n1 < message.txt.asc
>>>     -----BEGIN PGP SIGNATURE-----
>>>     $
>>
>> Another good example of the "argument vs option" problem. If I would see
>> a `sop sign` command, the first thing I would try would be:
>>
>>     sop sign document
>>
>> and expect it to find the right private key to sign the document
>> with. Of course, we don't do this in sop, which is fine, but I'll note
>> that we allow implementations to do so.
>
> We deliberately *do not* allow implementations to do so.  sop requires
> that you indicate which secret keys you want to sign with.  it is one or
> more KEY objects, not zero or more.
>
> If you think that `sop` is supposed to sign with other objects, then
> i've done a bad job at drafting this proposal.  I've opened
> https://gitlab.com/dkg/openpgp-stateless-cli/issues/5 to try to clarify this.

I understand where you're coming from here, and it's good to make sure a
stateless implementation.

What I'm saying is the `sop sign` example is error prone. Forget the `<`
and the mandated order and you might reverse the signing key and the
message.

>> By forcing the arguments here to be the signing key, we make it
>> difficult to let the implementation pick the right key.
>
> The entire point of `sop` is that `sop` *does not know* anything about
> the keys.  it is stateless, and you have to tell it explicitly which key
> to use.

I guess I'm wondering if implementations would be allow to diverge on
that matter at all.

[...]

>>> `--session-key-out` can be used to learn the session key on
>>> successful decryption.
>>
>> "learn"? What does that mean? It seems it means "write to a file". 
>> If so that should be said explicitely here.
>
> *What* the user of sop does is that they learn of the session key that
> was used for the encrypted message.  *How* they learn it is by receiving
> it from the program, either via the filesystem, or via @FD:NNN.
>
> If you think that's unclear, i'd welcome a clarifying patch.

https://gitlab.com/dkg/openpgp-stateless-cli/merge_requests/11

>>> If `sop decrypt` fails for any reason and the identified `--session-key-out`
>>> file already exists in the filesystem, the file will be unlinked.
>>  
>> This seems dangerous! Why do we delete a file we haven't created?
>> Explain.
>
> We don't want the user to run `sop`, and then inspect a file that was
> already in the filesystem thinking that it is `sop`s output.  If you
> think that's a bad decision, please suggest what we should do
> differently.

Maybe we should not overwrite existing files at all and fail earlier?

>>> [`--with-session-key`] enables decryption of the `CIPHERTEXT` using the session key directly against the `SEIPD` packet.
>>> This option can be used multiple times if several possible session keys should be tried.
>>
>> What happens if both "in" and "out" are provided? I can venture a guess,
>> but it would be important to make that explicit as there can be horrible
>> bugs there.
>
> Please do venture a guess, in the form of proposed text! I'd also love
> to hear what the horrible bugs are.  I don't see them.

I would argue that both options should not be provided at once. One
implementation that could come up would be that the program attempts to
read the file as it's writing it, truncating the precious key before it
has time to read it.

[...]

>>> If `sop decrypt` tries and fails to use a supplied `PASSWORD`, and it
>>> observes that there is trailing `UTF-8` whitespace at the end of the
>>> `PASSWORD`, it will retry with the trailing whitespace stripped.
>>  
>> Explain why we do magic things with whitespace. Consider not doing magic
>> at all as magic can be evil.
>
> I expect the following use case to be common:
>
>     echo correct horse battery staple > password.txt
>     sop decrypt --with-password=password.txt < ciphertext > cleartext
>
> If we don't strip whitespace, it will fail on one side or the other.
>
> sop tries to define what sensible magic should be here, and i think i've
> gotten it right.

We can continue the discussion in issue #13, but the TL;DR: is that I
agree that stripping trailing control characters is a good idea, but
disagree about whitespace in general.

>>> `--verify-out` produces signature verification status to the
>>> designated file.
>>> 
>>> `sop decrypt` does not fail (that is, the return code is not modified)
>>> based on the results of signature verification.  The caller MUST check
>>> the returned `VERIFICATIONS` to confirm signature status.  An empty
>>> `VERIFICATIONS` output indicates that no valid signatures were found.
>>> If `sop decrypt` itself fails for any reason, and the identified
>>> `VERIFICATIONS` file already exists in the filesystem, the file will
>>> be unlinked.
>>> 
>>> `--verify-with` identifies a set of certificates whose signatures would be
>>> acceptable for signatures over this message.
>>
>> Not failing explicitely on verification seems very dangerous. It relies
>> on callers properly reading the spec and realizing this is the only
>> exception where exit codes don't suffice in providing a general state of
>> the program. I would strongly recommend failing here, just like regular
>> verify.
>
> if the person invokes `sop decrypt` with no arguments, should it fail?

no.

> if the person invokes `sop decrypt` with `--verify-out` and
> `--verify-with`, should `sop decrypt` produce any output?

yes, except if verification fails.

> I'm trying to imagine using this in a MUA.  In my MUA i have what i
> think are the keys for my peer, and i see a message from them.
>
> I don't yet know whether it's signed.
>
> I want to decrypt the message to read it, and in the process, i want to
> find out whether it has been signed.  I'd prefer to avoid two passes in
> this common use case.
>
> If i supply the --verify-* arguments, and sop fails, then i don't get
> the cleartext of the message (not in any reliable way, anyhow).  If i
> don't supply the --verify-* arguments, and sop succeeds, i've lost any
> signature data.

I don't know how OpenPGP packets are built. Can't we show the signature
on the output of decrypt?

> The subcommand is "decrypt", so `sop` treats successful decryption as a
> success.  If you want to understand some additional state, you have to
> inspect that state somewhere else, and that's in the contents of
> `--verify-out`.
>
> does that make sense?  do you think any of that explanation belongs in
> the document itself?

Maybe there are some obvious assumptions about the OpenPGP message
structure that make it clear this option is necessary. It's not to me.

>> As an aside, why can't we compose verify and decrypt here and just keep
>> "verify" out of "decrypt" altogether? I would guess that's (a
>> limitation?) part of the OpenPGP standard, but maybe it would be nice to
>> explicitely expand on this here as well.
>
> Do you want to propose a way to compose them?  I suppose we could have
> `sop decrypt` offer a `--signatures-to` argument, which the sender could
> then use for `sop verify`, if anything comes out.  That's kind of an
> interesting suggestion, and it might reduce the overall complexity of
> `sop`.
>
> However:
>
>  * It means that a caller would need to handle the data twice (a minor
>    inefficiency)
>
>  * the signatures might not be verifiable if the thing that is signed is
>    not a simple literal data packet, but is, say, a compressed data
>    packet wrapped around a literal data packet.
>
> So i don't see how to do this safely (or efficiently, but the
> verifiability of the signature is more important than the efficiency
> argument)

You very likely know better. ;) It was just an idea, and probably
impractical in reality.

>>> If the caller is interested in signature verification, both
>>> `--verify-out` and at least one `--verify-with` must be supplied.  If
>>> only one of these arguments is supplied, `sop decrypt` fails with a
>>> return code of 23.
>>  
>> Another argument for failing on bad signatures: if we fail on bad
>> arguments of --verify, why don't we fail on bad signatures?
>
> failing on bad arguments is "you've asked an ill-formed question".
> That's legitimate and useful to let the operator know that things are
> not what they seem.
>
> Failing on a bad signature would be "the answer is no".
>
> If the primary operation you're asking for is verification, then failing
> on bad signatures is a reasonable outcome -- only succeed if you succeed
> in verifying.
>
> But if the primary operation is decryption, i don't think we should fail
> on signature validity for reasons outlined above.

But that assumes decryption is the primary operation. In the context
where all my email traffic is encrypted with OpenPGP, for example,
decryption is not the primary operation anymore. I *do* want to fail
properly on signature validity, it becomes a primary operation when
encryption is "default"...

>>> armor: armor: Add ASCII Armor
>>> -----------------------------
>>
>> [...]
>>
>>> If the incoming data is already armored, and the `--allow-nested` flag
>>> is not specified, the data MUST be output with no modifications.
>>> Data is considered ASCII armored iff the first 14 bytes are exactly
>>> `-----BEGIN PGP`. This operation is thus idempotent by default.
>>  
>> Explain why we want idempotent and why we want to do this guessing game.
>
> I'm at a bit of a loss here, because these seem obvious to me.
>
> We want a guessing game because users who don't know what they want are
> going to guess anyway, and they're likely to guess wrong.  We might as
> well guess on their behalf if they don't know.
>
> We want idempotent because "ensure this thing is armored" is a
> reasonable semantic request -- indeed, it's probably the *only*
> reasonable semantic request.  Perhaps we could drop --allow-nested?

I am now also confused here and ready to drop this argument. :)

>> This @ENV: and @FD: stuff really makes me uncomfortable. It's a neat
>> hack for commandline applications, but it would break down when
>> designing a library API, as the "type" of data passed around the API
>> would be ambiguous, or at least with possible side effects. That feels
>> like "design smell" here and I would like this to be changed.
>
> FWIW, the @ENV: and @FD: conventions are specifically CLI conventions
> and i don't expect them to be translated to any programmatic API.

Of course.

> for example, in the pythonic variant of this framework that i'm working
> on, i've treated these objects as labeled bytestreams. ("labeled"
> meaning -- if you get a set of these objects, each one has a bytes-like
> thing, and a textual name that you can use in error reporting)
>
> I admit that i'm struggling a bit with whether i want to pass around
> bytes objects directly (simple, what i've got now) or some sort of
> asyncio handle that the bytes are readable from (fancier, probably nicer
> to use in the kinds of programs that i will eventually want to write
> with this). https://gitlab.com/dkg/python-sop/issues/1 But that has
> nothing to do with @ENV and @FD, as those aren't exposed to the python
> functions at all.

I'm not sure asyncio is useful here. Callers can wrap around the byte
stream as they see fit, no?

(I really dislike this "parallel universe" thing they built in Python 3
for asyncio, it makes every API more complicated than it needs to
be. But that's another story...)

>> I would recommend using equivalent environment variables to the
>> parameters instead, for example SIGN_WITH for --sign-with and so
>> on. This would, of course, require switching positional arguments to
>> options but I already explain why that would be a good idea anyways
>> earlier.
>
> environment variables won't work for arguments that can be supplied
> multiple times, because then we have to invent a new delimiting scheme.
> I definitely don't want to do that.

True. That is a significant limitation. I am not sure how to work around
that problem.

>> File descriptors could be passable as distinct options, like
>> --sign-with-fd for --sign-with.
>
> This is an interesting proposal, though i don't see how --sign-with=@FD:3
> is much different from --sign-with-fd=3  -- i guess it lets you use
> files that are literally named @FD:3 ?  Is that important?

It's less magic, more explicit, and correlates better with other
commandline APIs I have encountered.

> If you could open a merge request that proposes this change i'd be happy
> to consider it.
>
> I've opened https://gitlab.com/dkg/openpgp-stateless-cli/issues/14 to
> keep track of it.

I'll see what I can do.

>> I've dealt with commandline applications that have special meanings with
>> @, and in retrospect, it was a bad idea. In particular, Python's
>> argparse module supports using a prefix argument to mean "read options
>> from this file" and I've used it to implement crude configuration file
>> support for monkeysign and other programs. It's confusing for users and
>> does not work very well.
>
> afaict, this is not mandatory in argparse, and i wouldn't recommend it
> for any sop implementation:
> https://docs.python.org/3/library/argparse.html#fromfile-prefix-chars
>
> I think the way it's specified in `sop` right now is pretty pincipled
> and hard to screw up, but i could be wrong.
>
>> Specifically in this case, I would also worry about security
>> vulnerabilities with untrusted filenames being passed to the program.
>
> can you explain this more?

Say you think you are in a trusted directory with "CERTS" that you want
to encrypt to. You call:

  sop encrypt * < /tmp/file > /tmp/file.pgp

Except you made a mistake and the attacker has control of the current
directory, and injects a file named (say) @ENV:SOMETHING. Assuming they
have control over the SOMETHING environment, they can now add an
encryption key to the message.

Control of the environment is kind of a stretch, I must admit, but in
certain environments (most notably web servers), a *lot* of stuff can
end up there and it shouldn't be completely trusted this way.

>>> CERTS {#certs}
>>> -----
>>> 
>>> One or more OpenPGP certificates (section 11.1 of {{I-D.ietf-openpgp-rfc4880bis}}), aka "Transferable Public Key".
>>> May be armored.
>>> 
>>> Although some existing workflows may prefer to use one `CERTS` object with multiple certificates in it (a "keyring"), supplying exactly one certificate per `CERTS` input will make error reporting clearer and easier.
>>
>> This last bit is in contradiction with `extract-cert` command
>> documentation which says it will "only contain one cert". Maybe we
>> should just pick one and stick with it here?
>
> I have carefully considered this, and i do not think these are in
> contradiction.
>
> extract-cert explicitly says it will contain only one cert *because* the
> other places where `CERTS` might be supplied could contain more than
> one.
>
> The fact is that people use "keyrings" today, including some that
> contain hundreds or thousands of keys.  If a distro, for example, wants
> to use `sop` to verify that a package is signed by one of their
> developers, i don't want the distro to need to put each developer's key
> in a separate file.

Understood.

>> That last part doesn't *look* like "arbitrary text" to me. It looks like
>> some explanatory message of the operation. If that's the case, we should
>> make that explicit and say why the text is present at all. Calling it a
>> "note" or "message" would already be an improvement.
>
> patches welcome, particularly for this kind of editorial cleanup :)

https://gitlab.com/dkg/openpgp-stateless-cli/merge_requests/12

[...]

>> It would also be great if we could explain where those magic numbers
>> come from in the first place. I suspect they were chosen to not overlap
>> with existing error codes, but that's just a guess.
>
> Justus picked 69 in his OpenPGP Interoperability Test Suite.  I chose
> the others as "reasonable-sized primes" just for fun.  I don't think
> this information belongs in this document, as it doesn't matter.

I love this kind of information in text, it makes it less dull. :p

[...]

>>> FIXME: if an encrypted OpenPGP message arrives without metadata, it is difficult to know which signers to consider when decrypting.
>>> How do we do this efficiently without invoking `sop decrypt` twice, once without `--verify-*` and again with the expected identity material?
>>  
>> Maybe we could use a "sop probe" command for this and other things?
>
> I don't understand what you think a "sop probe" command would do.  if
> you'd like to propose it as an MR, i'd consider it, though.

`sop probe` would do the minimal amount of work required to determine
which keys ("signers") to consider  when decrypting, then call `decrypt`
properly.

`sop probe` could also do the general task of parsing OpenPGP messages
into packets and stuff like that.

>>> Compression {#compression}
> […]
>> How about decryption? Do we attempt decompression during decrypt?
>
> It will be interesting to see what implementers do!  I've left `sop`
> deliberately agnostic there, and i would like to learn from test suites
> what the answer is.

Should we make that decision clearer in the document?

Thanks again!

-- 
Antoine Beaupré
torproject.org system administration