Tech pages/OX
This page should be used to discuses the XEP's and implementation of 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
gpg --full-gen-key`
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:
pub rsa2048 2020-05-01 [SCEA]
6D740FE2B1D55DCD74CAAD95AC1A1629095EEDAE
uid [ unbekannt ] xmpp:alice@domain.tld
or like this:
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]
- 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
gpg --export --export-options export-minimal --export-filter 'keep-uid=uid =~ xmpp:local@domain.tld' MY_FINGERPRINT > /tmp/test.gpg
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
History of version of public key
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'.
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 The OpenPGP Public-Key Data Node Example 3 is a date attribute in the pubkey tag. In Section 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.
"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?
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 Affix Elements rpad is defined like
"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?"
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 (!%^).
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 = '=';
Example:
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
Verification
Table 1: Overview about different crypto property elements
Receiving clients MUST check, if the JID matches the to attribute of the enclosing stanza and otherwise alert the user/reject the message
This could be full JID or bare JID, depends on the Envelope Element? Should be a plain string compare?
Receiving clients MUST check whether the difference between the timestamp and the sending time derived from the stanza itself lays within a reasonable margin.
???