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This page should be used to discuses the XEP's and implementation of
See: https://wiki.xmpp.org/web/XEP-Remarks/XEP-0373:_OpenPGP_for_XMPP
[https://xmpp.org/extensions/xep-0373.html XEP-0373: OpenPGP for XMPP]. If you
think there are some things which are wrong, feel free to contact me.
 
= Fundamentals =
 
There are different people with different background knowledge about OpenPGP /
GnuPG. XMPP OX should be able to integrate OpenPGP in clients which hides the technical
details of OpenPGP to the user. But, XMPP OX should also useable for technical
people which may have a OpenPGP Smartcard or a Token.
 
At the end of the day, the different client applications should be able to share information and messages.
 
== OpenPGP via GnuPG ==
There are two use cases to create the key material.
 
Users with technical background may have his own key or prefer to generate his
own key via GnuP. This is '''not''' a question about trust the gnupg application or trust the xmpp
client application, this is more a habit of the user.
 
Non-technical users may prefer that the XMPP client acts on behalf of the user
and will generate the key material. Even, '''without''' asking the user a lot of questions.
 
=== Generating key ===
 
Basically both use cases are fine and the best way is, to let the user decides
which he would like to use. Generate a Key via gpg can be done by
<pre>
gpg --full-gen-key`
</pre>
 
Generating the key within the XMPP client is up to the developer.
It's recommend to have such a option, to make the on boarding for non technical
people easier.
 
=== UID ===
There may people which prefer one key pair for E-Mail and one dedicated key pair
for XMPP. There may also people ( independent of this knowledge ) which prefer
to have one key pair for all "services". In the most cases, at least I know, it
will be done via the OpenPGP UID.
 
The user should be able to manage his key pair. Also, the XMPP Client should be
able to assistant, by adding a UID for the XMPP Account.
 
A key can look like this:
 
<pre>
pub  rsa2048 2020-05-01 [SCEA]
      6D740FE2B1D55DCD74CAAD95AC1A1629095EEDAE
uid        [ unbekannt ] xmpp:alice@domain.tld
</pre>
 
or like this:
 
<pre>
pub  rsa4096 2019-05-14 [SC] [verfällt: 2021-05-13]
      A602F76893F138B4A8EFDDD5C2DC916F35751C24
uid        [ ultimativ ] Name <mailbox@domain.tld>
uid        [ ultimativ ] Name (FSFE) <mailbox@domain1.tld>
uid        [ ultimativ ] Name (devLUG) <mailbox@domain2.tld>
uid        [ ultimativ ] xmpp:user@domain.tld
uid        [ ultimativ ] xmpp:xmpp:alice@domain.tld
sub  rsa4096 2019-05-14 [E] [verfällt: 2021-05-13]
sub  rsa4096 2019-05-14 [A] [verfällt: 2021-05-13]
</pre>
 
* If there is a private key with an existing UID for the account, just use it.
* If there is a private key, but there is no private key with a UID of the account, ask the user to add a UID
* If there is no private key, create one for the XMPP account.
 
=== Export a public key ===
 
Sharing the public key is also a question about privacy. If the user has only
one key pair which is used by XMPP, only. There may not many problems to share
those key via XMPP PEP. If a user have a key with more UIDs and is using the
WoT, the user may prefer which information should / shouldn't included within
the public key.
 
By default, the public will include Name, E-Mail-Addresses, XMPP Addresses of
all OpenPGP UIDs in the public key. The public key also includes the signatures.
If the user prefer to publish his public key with minimal information, he can do
so by
 
<pre>
gpg --export --export-options export-minimal  --export-filter 'keep-uid=uid =~ xmpp:local@domain.tld' MY_FINGERPRINT > /tmp/test.gpg
</pre>
 
In this cases, just the UID with the xmpp-Address will be extracted and no
signatures.
I think it is important, that the user can decides if he creates the public key
export and push int on PEP or if the XMPP Client ( in think in most cases the
full export) should do it.
 
=== Trust-model ===
The user should decides which trust model the user prefers.
 
Users which just would like to use it and do not crate much of trust
fingerprints,  may should use trust-model TOFO (Trust On First Use).
 
Option trust-model and tofu-default-policy in .gnupg/gpg.conf.
Users which prefer trust-model pgp should be able to use the WoT (default in gnupg).
 
I think the WoT is '''not''' nonsense. There is maybe an issues, that not all clients supporting a friendly way to sign key and publish it.
 
One important part of the asymmetric cryptography is the exchange of the public
key and verify those key (certificate - singing of a key). With the singing of
the key and building the gnupg's trust db, I think the WoT is a powerful and
helpful concept to verify the keys of other persons.
 
The main problem I see, there is less interest and the second problem, lot of
clients (not only XMPP client) don't integrate a nice UI for the users. Using
the WoT requires 4 steps:
 
* get the public key of the contact
* verify the fingerprint of the public key
* signing the public key
* sent the signed public key encrypted back to the contact
 
The 2nd step is the step which has to be managed by the human. I think all
other steps can be done by XMPP Clients.
 
=== Sharing the private key ===
This topic is not easy. I think, it should also be optional at all. In some
cases it may helpful and a nice feature, and in other cases it won't work at
all.
 
Sharing the private key is required to support multi devices.
 
Lot of non technical person moved to an "smartphone only mode". There are a lot
of people which are using IM / Mail only on the smartphone. For those people,
there is no need to share the private key. It would be included in the backup.
 
There are also people which are using a smartphone, Desktop and Laptop and don't
won't to care about key management. For those people the sharing of private key
on PEP is a very nice feature.
 
For people using Smartcard / Token, it don't make sense to share the key. The
key is on a hardware device and they just need the public key and the token.
 
= Discussions =
 
== Fundamentals ==
I tried to write the Fundamentals just as possible use cases. This don't mean that the XMPP Clients should implement all.
But, we should make sure, that we are able to support the use cases as much as possible, but it shouldn't be necessary to implement all.
(For example, it would be difficult to implement thinks like "--export-options export-minimal  --export-filter", I won't implement it - but we should be able to work with it, when the user would like to gpg to export it)
 
* Examples
If a XMPP Client has been implemented that it will check the PEP of the contact, first. The Clients OX will not work, if there is no public key.
This could be a problem, if others client doesn't require the publishing of public key on PEP.
 
If the XMPP Client doesn't check the UIDs / Sub-Key of the Mainkey, it could be an issue if there key looking different.
 
== History of version of public key ==
[https://xmpp.org/extensions/xep-0373.html#discover-pubkey Requesting Public Keys]
 
<blockquote>
Note that the result may contain multiple pubkey elements. Only the public keys found in the most recent item MUST be used. Requesters may want to limit the results to the most recent item using the 'max_items' attribute set to '1'.
</blockquote>
 
Is it required to have a versions for the public key?
 
A public key can be changed for
 
* Adding or removing UIDs - No need to have a version, just need the current public key
* Adding or removing (revoke) Subkeys - No need to have a version, just need the current public key
* Change the expiration date - No need to have a version, just need the current public key
* Adding Key signatures - No need to have a version, just need the current public key
 
I think there is no need to provide a history.
 
Other use cases which we should check?
 
== Key-lookup / GnuPG's Keyring / Homedir ==
How should the Sender fetch the public key and where should it be stored?
 
* The key-lookup can be done via a lookup of all known keys with the XMPP-URI as UID.
 
  sec  rsa3072 2020-05-01 [SC] [verfällt: 2022-05-01]
        7FA1EB8644BAC07E7F18E7C9F121E6A6F3A0C7A5
  uid        [ ultimativ ] Doctor Snuggles <doctor.snuggles@domain.tld>
  uid        [ ultimativ ] xmpp:doctor.snuggles@domain.tld
  ssb  rsa3072 2020-05-01 [E] [verfällt: 2022-05-01]
 
In this example there is a UID for the E-Mail and an additional UID with the XMPP-URI as Name (`gpg --quick-add-uid`).
I guess, most of the time the user has just one key per account. It could be, that a user has one key for his Desktop and one key for his laptop.
There is also the possibility - I didn't try yet - to create two subkeys [E] and have one key stored on the Desktop and one on the laptop.
 
  gpg --quick-add-key 7FA1EB8644BAC07E7F18E7C9F121E6A6F3A0C7A5 rsa3072 encr 2y
 
  sec  rsa3072 2020-05-01 [SC] [verfällt: 2022-05-01]
        7FA1 EB86 44BA C07E 7F18  E7C9 F121 E6A6 F3A0 C7A5
  uid        [ ultimativ ] Doctor Snuggles <doctor.snuggles@domain.tld>
  uid        [ ultimativ ] xmpp:doctor.snuggles@domain.tld
  ssb  rsa3072 2020-05-01 [E] [verfällt: 2022-05-01]
        AFBB 126C DC71 912E F5C0  F47D FC18 A7DA 7495 5B8A
  ssb  rsa3072 2020-06-02 [E] [verfällt: 2022-06-02]
        6C88 E0E5 3F69 A137 2F0F  A36A 3AED DD36 93E7 8BCE
 
There are two [E] subkeys.
 
* We shouldn't care how the user receives the public key. This should be done via Keyserver, WKD, E-Mail or XMPP PEP.
 
If I read a Mail via Mailinglist and get the public key via WKD, it would also be possible to use those key for XMPP. Also, if I'm going to update the public keys.
This is important for the WoT and to make sure that a key is valid, because of new signatures of the public key.
 
* The user should be able to use his own key. For instance, if the user would like to use his OpenPGP Smartcard / Token for E-Mail and XMPP.
* The user should be able to manage his public keys like all other keys GnuPG's `--update-trustdb`and `refresh-keys`
* The user should be able to use WoT pgp or tofu or tofu+pgp mode
* The WoT is important for messages which has been signed or signcypted
 
I think it will be better to use the same keyring and homedir like GnuPG is using.
 
= XEP Review =
 
== XEP-0373: OpenPGP for XMPP ==
 
=== Date of a public key ===
In Section [https://xmpp.org/extensions/xep-0373.html#announcing-pubkey The OpenPGP Public-Key Data Node] Example 3 is a date attribute in the pubkey tag.
In Section [https://xmpp.org/extensions/xep-0373.html#discover-pubkey  Requesting Public Keys]  Example 8. is no date attribute in the response.
The Text "Note that the result may contain multiple pubkey elements. Only the public keys found in the most recent item MUST be used." may reference to the date attribute.
<blockquote>
"Requesters may want to limit the results to the most recent item using the 'max_items' attribute set to '1'." Does this constrain guarantee that the response is always the most recent item?
</blockquote>
I think there is no need to have more than one version of a public key on the PEP. Within OpenPGP (local Keyring, Keyserver, Web Key Directory, doesn't support a version of public key AFAIK).
The user should be able to update the item with his current version of the key.
 
== XEP-0420: Stanza Content Encryption ==
 
=== rpad ===
 
In Section [https://xmpp.org/extensions/xep-0420.html#affix_elements Affix Elements] rpad is defined like
 
<blockquote>
"Prevent known ciphertext and message length correlation attacks. The content of this element is a randomly generated sequence of base64 characters of random length between 0 and 200 characters. TODO: sane boundaries?"
</blockquote>
 
In Example 1 is rpad
 
  <rpad>C1DHN9HK-9A25tSmwK4hU!Jji9%GKYK^syIlHJT9TnI4</rpad>
 
This is not the The RFC-4648 - The Base 64 Alphabet defined in: https://tools.ietf.org/html/rfc4648#section-4 (!%^).
 
<pre>
static const char rfc_4648_base64_alphabet[] = {
'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P',
'Q','R','S','T','V','V','W','X','Y','Z','a','b','c','d','e','f',
'g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v',
'w','x','y','z','0','1','2','3','4','5','6','7','8','9','+','/',
};
 
static const char* rfc_4648_base64_pad = '=';
</pre>
 
Example:
 
<pre>
rpad: 6mWBrmp77XWx/srrjVNwS32eXf5
rpad: B1LXIxB9xPZwzD5mXr6jiIiPyr0FpworMt5GpniK8JnyRVVVnWJzYbyS6kVMEbTmxDsgmwCOV7dpczyl1SiBO1DhzKhdxHCW6gsMH1nyAOC9dHW/JJM2L/fklEge4d+ktrC0
rpad: ebeJpeWb4KLt
rpad: P7ZgMDX6b4gFN4TN0/VSl6315bgvXxnHz2+AVmSVZVzl57ztMVQ5NtRZb2yVEZA
rpad: BQCJ/1e8iY2nJTW0RdP8bhg9NOaNZr/O/xvgTYCY+t0zFpEJMBaCY9sKWKfFePtB3BLva2HQ17zw2kFVDn50Kp8n+VKhctSfwofda51NQq8Giug+A5057Ma9NSjMZt
rpad: PyOyTdHT2WiXeTuevJdS5yQLzLtxrf3g6GbN4qKrzPKLgQMGOo8cd3bayBO9o1yZHy341KoCtDMhyhwhDl/aMTXNh2bPrjIpeAJ6xDLjL6Ph/OB32GMTvMqtpy9vbygXo3
</pre>
 
 
=== Verification ===
Table 1: Overview about different crypto property elements
 
<blockquote>
Receiving clients MUST check, if the JID matches the to attribute of the enclosing stanza and otherwise alert the user/reject the message
</blockquote>
 
This could be full JID or bare JID, depends on the Envelope Element? Should be a plain string compare?
 
<blockquote>
Receiving clients MUST check whether the difference between the timestamp and the sending time derived from the stanza itself lays within a reasonable margin.
</blockquote>
 
???
 
= Links =
 
* https://xmpp.org/extensions/xep-0373.html - OX
* https://xmpp.org/extensions/xep-0374.html - OXIM
* https://xmpp.org/extensions/xep-0420.html - SCE

Latest revision as of 06:56, 28 March 2021

See: https://wiki.xmpp.org/web/XEP-Remarks/XEP-0373:_OpenPGP_for_XMPP

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