[Mailman-Developers] OpenPGP Mailman integration discussion [was: Re: GSoc - Requirement from Mentor to complete the project]

[Abhilash Raj]

On Thu, May 9, 2013 at 10:47 PM, Daniel Kahn Gillmor
<dkg at fifthhorseman.net> wrote:
> As i about this, i wonder if the OpenPGP integration project shouldn't
> be broken into two phases, so that the tricky nuances could be handled
> more simply.
>
> I'm imagining a first phase ("message authentication") would not expect
> or handle encrypted messages, but would just use cryptographic
> signatures to verify the authenticity of origin of the incoming message,
> enhancing (and/or replacing) some of the current checks mailman uses to
> decide if a message should be passed on to the list.
>
> Then the second phase ("encrypted messages") would address the
> possibility of having encrypted message content.
>
> If you can get the "message authentication" phase done properly, you'll
> have a good argument that your keyring management is effective, without
> getting sidetracked into issues of exactly where the content of the
> message might leak (since the messages aren't encrypted at all).
>
> When the first phase is done, you could focus on the "encrypted
> messages" phase, based on the assumption that you have decent keyring
> management, and handle the peculiarities and concerns there separately.
>
> I'm not a mailman developer; but I'd be curious to hear what mailman
> devs think of this breakdown.
>
> More comments and questions below, based on the workflow you've outlined
> that mixes "message authentication" with "encrypted messages" and tries
> to solve them all at once:
>

After the Barry's comment on my proposal I decided to cut down the
proposal to implement use of OpenPGP  signatures for posting
privileges instead of both signed and encrypted list.
Most of the infrastructure for encrypted list will be created along
with this project so that later on it can also be implemented easily.

Though I would like to continue this discussion on encrypted list for
future reference. Some of these questions are already answered in my
proposal here[1].

> On 05/09/2013 01:31 AM, Abhilash Raj wrote:
>> Consider this flow of message from one user to all list-subscribers:
>>
>>  * A user sends a message( signed as well as encrypted )which as soon as
>> received by mailman is first queued in "IN" queue.
>
> who does the sender encrypt the message to?  are we talking about a
> public key associated with the list?  how does the sender learn of that
> public key?

Sender encrypts the message to list's pub key. List's pub-private
keypair is uploaded by the list's owner when he creates the list.
Subscribers gets to download the list's pub-key when he subscribes to
it.

>
>>  * When the incoming runner wakes up the message is checked if it is for
>> owners, posting or encrypted.An encrypted chain is created  for
>> encrypted messages.
>
> what if it is an encrypted message sent to "owners"?  how does that get
> handled?

So for both regular posting and owners we can add separate chains say
encypted_posting, encrypted_owners.

>>  * Now according to the use case there can be two encrypted chain:
>>
>>       1) Where the posters are fully trusted and thus this rules only checks
>> the signature of a message. If a message signed by a member is received
>> it is sent to pipeline queue in a encrypted form for further processing.
>
> when you say "fully trusted" it might be worth saying what they are
> trusted for.  I think here you mean "fully trusted to post messages
> without moderation".
>
Yes.

> What happens if the message cannot be decrypted by the mailing list and
> the signature is inside the encryption?  does the message bounce?
>
Yes, it would bounce.

> what if the decryption succeeds but there is no message signature inside it?
>

In that case a notification is sent to the sender with only the
headers of the decrypted message that the signature of the message
does not match. If it is not sent by him he is asked to ignore the
notification.

> what if the signature appears to be valid, but was made 3 years ago (or
> in the future)?

Is that a problem if he has not revoked his signature yet?

> what if the signature appears to be valid, and recent, but is the same
> signature that was used on a previous e-mail to the list?

Can there be same signatures for different messages? I think the
digital signature is created by encrypting the message-hash, so if the
contents of the message are changed then signature cannot be valid.

> what if the signature is cryptographically valid, but the list member's
> key has expired?

Well a expired key cannot be used to sign messages. If this situation
happens this means it was signed in past some time ago. We can bounce
the message and notify the user about this.

> what if only part of the message is signed?
My guess is that you are referring to inline-pgp. I read this article
which talks about the inline-pgp as a bad messaging format. I think we
should not support this. So if the message is partially signed it
should probably bounce back.

>>       2) Where the posters are not fully trusted. In this case there needs to
>> be various checks on decrypted message like current default-posting-chain.
>
> One common thing that might happen here is that the message is being
> held for moderation.  if the message is encrypted, how is a moderator
> supposed to review it and make a decision?

Yes this might be an issue, will have to think for something for this.

>>  * The messaged is moved in between queues in the encrypted form
>> encrypted by a symmetric key algorithm which uses list's secret key to
>> encrypt the message.
>
> this doesn't make sense to me.  when i hear "list's secret key" i tend
> to think people are talking about asymmetric encryption, but it seems
> here that you're talking about symmetric encryption.  Maybe it would
> help to enumerate exactly what secrets each entity holds (and maybe what
> public data is associated with those secrets, in the case of the
> public/private keypairs used by asymmetric crypto).  If you give those
> secrets names, you can refer to them more concisely and precisely.

Sorry to sound so confusing. What I actually meant was that we use
public-private key encryption via OpenPGP so send and receive signed
and encrypted messages. Now each list has a public-private keypair
which the list owner uploads when he creates the list.

During processing of the message inside mailman it is decrypted and
sometimes stored on disk as python pickles in various queues which is
bad for security reasons. I propose that when it is being stored on
disk it can be encrypted using a symmetric key encryption algorithm so
that only one who has access to list's secret key can decrypt it. When
the message is being sent to the subscribers it is encrypted with the
user's pub-key so that only he can decrypt it.

>>  * When the pipeline handler wakes up it decides which pipeline the
>> message is to passed through. In this case an encrypted_posting_pipeline
>> is created to process an encrypted message.
>
> which case is "this case" are we talking here about every encrypted
> message or just a subset of them?

Currently pipeline runner( not handler, typo last time ) decides which
pipeline will process the message. For the encrypted message we add
another pipeline encrypted_posting_pipeline to process all encrypted
message that are to be posted. For message to the owners we can have
another different pipeline.

>
>>  * This encrypted_posting_pipeline is used to keep the message in
>> plain-text for a minimal time. The message is decrypted, headers and
>> footers are added and the message is signed by list's pub-key. It is
>> then again (symmetrically) encrypted and stored in the outgoing queue.
>
> public keys don't sign messages; secret keys can sign messages, and
> public keys are used to verify those signatures.

Yes, sorry again a typo!

> what happens to the original sender's signature?  is it stripped?  or
> does it remain, nested within the list's signature?  How are list
> members expected to verify these signatures?

 The original sender's signature is stripped. I guess the list
subscribers just have to trust that the list owner. We cannot expect
each member of the list to have every other member's pub-key to
separately verify himself that the message is indeed from the sender.

> i'm again confused by the symmetrical encryption proposal here.
>
>>  * The outgoing runner personalizes each message by encrypting it with
>> each user's pub key and forwards it to MTA using VERP(Variable Envelop
>> Return Path).
>
> What happens if a user's public key is found to be expired or revoked?
> How does mailman become aware of revocations, of extensions of the
> expiration date, or of new keys or certificates for any given user?

If the user has his key uploaded to a public key server and wants( he
will be given option of whether ot not to) mailman to check , mailman
can periodically check for revocation and expiration of the key.
Though i don't know how will that be done.

 New keys will be added by user only from postorius after a
confirmation link is sent to him on the address the key verifies.

> i hope these questions are useful,

Yes, I had not though about revocation and expiration issues. Thanks for it.
>
>         --dkg
>
>
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[1] : https://gist.github.com/maxking/5455462

--
Abhilash Raj