%ents; ]>
OpenPGP for XMPP Specifies end-to-end encryption and authentication of data with the help of OpenPGP, announcement, discovery and retrieval of public keys and a mechanism to synchronize secret keys over multiple devices. &LEGALNOTICE; 0373 Experimental Standards Track Standards Council XMPP Core XEP-0030 XEP-0082 XEP-0163 XEP-0223 XEP-0334 ox &flow; Dominik Schürmann dominik@dominikschuermann.de dominik@dominikschuermann.de Vincent Breitmoser look@my.amazin.horse valodim@stratum0.org 0.2.1 2017-11-13 fs
  • Recommend setting the PubSub configuration field 'send_last_published_item' to 'on_sub'.
  • Only recommend persistent PubSub nodes.
0.2 2017-09-11 XEP Editor (jwi) Defer due to lack of activity. 0.1.3 2016-07-15 fs (XEP Editor: ssw)

Update acknowledgements.

0.1.2 2016-07-11 bjc (XEP Editor: ssw)

Minior editorial fixes.

0.1.1 2016-06-04 fs

Minior editorial fixes.

0.1 2016-05-10 XEP Editor (ssw)

Initial published version approved by the XMPP Council.

0.0.1 2016-03-25 fs

First draft.

This XMPP extension protocol specifies the foundations of end-to-end encryption and authentication, based on digital signatures, of data with the help of OpenPGP. Additional XEPs will use this extension protocol as building block when specifying their own OpenPGP profile suiting their use case. One such profile is the Instant Messaging Profile specified in &xep0374;.

XMPP provides the mechanisms to solve a lot of issues that come with modern day OpenPGP usage. For example, based on &xep0163; this specification describes a standardized way to discover OpenPGP public keys of other entities. But unlike the OpenPGP keyservers, this process establishes a strong relation between the key and the key's owning entity (usually a human user). A similar mechanism described herein allows to synchronize the secret key(s) across multiple devices.

OpenPGP in return allows for end-to-end encrypted data to be exchanged between one, two or even multiple entities (multi-end-to-multi-end encryption). Therefore this XEP can be used for example to implement end-to-end encrypted &xep0045;.

OpenPGP element
An XMPP extension element: &openpgp; qualified by the 'urn:xmpp:openpgp:0' namespace
OpenPGP content element
An element embedded via OpenPGP in a &openpgp; element. Either one of &signcrypt;, &sign; or &crypt;, qualified by the 'urn:xmpp:openpgp:0' namespace.
PEP
Personal Eventing Protocol (XEP-0163)
Public key PEP node
A PEP node containing an entity's public OpenPGP key.
Secret key PEP node
A PEP node containing an entity's encrypted secret OpenPGP key.

The &openpgp; extension element qualified by the 'urn:xmpp:openpgp:0' namespace is used in order to exchange encrypted and signed data.

BASE64_OPENPGP_MESSAGE ]]>

The text content of &openpgp; ("BASE64_OPENPGP_MESSAGE") is a Base64 encoded (&rfc4648; § 4) OpenPGP message as specified in &rfc4880; which contains an encrypted and/or signed UTF-8 (&rfc3629;) encoded string. This string MUST correspond to exactly one OpenPGP content element, that is, it represents either a &signcrypt;, a &sign; or a &crypt; extension element qualified by the 'urn:xmpp:openpgp:0' namespace. Note that OpenPGP's ASCII Armor is not used, instead the XMPP client MUST encode the raw bytes of OpenPGP message using Base64.

In case of a &signcrypt; element, the OpenPGP message embedded in the &openpgp; element MUST be encrypted and signed, and SHOULD also be encrypted to self. In case of a &sign; element, the OpenPGP message MUST be signed and MUST NOT be encrypted. In case of &crypt; the OpenPGP message MUST NOT be signed, but MUST be encrypted.

OpenPGP content elements MUST possess exactly one 'time' element as direct child elements. The &signcrypt; and &crypt; content elements MUST contain at least one 'to' element(s), which MUST have a 'jid' attribute containing the intended recipient's XMPP address of the signed and/or encrypted data to prevent Surreptitious Forward AttacksJee Hea An, Yevgeniy Dodis, and Tal Rabin. 2002. On the Security of Joint Signature and Encryption. In Proceedings of the International Conference on the Theory and Applications of Cryptographic Techniques: Advances in Cryptology (EUROCRYPT '02), Lars R. Knudsen (Ed.). Springer-Verlag, London, UK, UK, 83-107. <https://www.iacr.org/archive/eurocrypt2002/23320080/adr.pdf>. The XMPP address found in the 'to' element's 'jid' attribute SHOULD be without Resourcepart (i.e., a bare JID). A &sign; content element may not carry a 'to' attribute. The 'time' element MUST have a 'stamp' attribute which contains the timestamp when the OpenPGP content element was signed and/or encrypted in the DateTime format as specified in &xep0082; § 3.2. The &signcrypt; and &crypt; elements SHOULD furthermore contain a 'rpad' element which text content is a random-length random-content padding.

Content Element 'to' Element 'time' Element <rpad/> Element <payload/> Element
&signcrypt; MUST have at least one MUST have exactly one SHOULD have exaclty one MUST have exactly one
&sign; MAY NOT contain one MUST have exaclty one NOT REQUIRED MUST have exactly one
&crypt; MUST have at least one MUST have exaclty one SHOULD have exaclty one MUST have exactly one

OpenPGP content elements MUST possess exactly one &payload; element. The child elements of &payload; can be seen as OpenPGP secured Stanza extension elements which are encrypted and/or signed. After the &openpgp; element and the including &signcrypt;, &sign; or &crypt; element was verified, they are processed according to the specification of the relevant OpenPGP for XMPP profile (see for example &xep0374;).

Recipients MUST verify that the signature is valid, that the signature's key corresponds to the sender's key, and that the sender's key has a User ID containing the sender's XMPP address in the form "xmpp:juliet@example.org" (for details see "OpenPGP User IDs"). Thus, the recipient may need to retrieve the key from the Personal Eventing Protocol node as described above. At least one of the XMPP addresses found in the 'to' elements contained in OpenPGP content element MUST correspond to the outer 'to' of the XMPP &MESSAGE;. Furthermore, recipients are RECOMMENDED to verify the 'time' element for plausibility or to display it to a user for verification.

Parties interested in exchanging encrypted data between each other via OpenPGP need to know the public key(s) of the recipients. The following section specifies a mechanism to announce and discover public keys.

In order to announce the public key, the client needs to store it in a PEP node. The public key data, as specified in RFC 4880, is stored within a <pubkey/> element which is a child element of the <pubkeys/> element qualified by the 'urn:xmpp:openpgp:0' namespace. Note that OpenPGP's ASCII Armor is not used, instead the XMPP client MUST encode the public key using Base64. Clients MAY only try to store the public key if the Personal Eventing Protocol service supports persistent-items, thus they possibly check if the service reports the 'http://jabber.org/protocol/pubsub#persistent-items' feature.

BASE64_OPENPGP_PUBLIC_KEY ]]>

In order to discover the public key of an XMPP entity, clients send a PubSub &IQ; request to the entity's bare JID of which it wants to know the public key.

]]> BASE64_OPENPGP_PUBLIC_KEY ]]>

Note that the result may contain multiple pubkey elements. Only the public keys found in the most recent MUST be used. Requesters may want to limit the results to the most recent item using the 'max_items' attribute set to '1'. Clients could alternatively use &xep0059; as an alternative to 'max_items' but accoding to XEP-0060 RSM is not (yet) mandatory for PubSub services.

Some XMPP services may not provide the Personal Eventing Protocol feature required to provide the mechanism described here. If so, they will return an &IQ; error of type service-unavailable.

A private PEP node is used to allow XMPP clients to synchronize the user's secret OpenPGP key. Where private PEP node is defined: A PEP node in whitelist mode where only the bare JID of the key owner is whitelisted as described in &xep0223;. The secret key is additionally encrypted.

The used PEP server MUST support PEP and the whitelist access model. It SHOULD also support persistent items.

]]>

The service discovery result must contain a PEP identity '<identity category='pubsub' type='pep'/>, and the 'http://jabber.org/protocol/pubsub#access-whitelist' feature. Ideally it also contains the 'http://jabber.org/protocol/pubsub#persistent-items' feature

... ]]>

In order to synchronize the secret key over a private PEP node, clients first need to discover and verify the node for the correct settings.

]]> BASE64_OPENPGP_ENCRYPTED_SECRET_KEY ]]>

If the node does not exist the service will return an &IQ; error indicating the item-not-found error condition. The client MUST then create it with an whitelist access model.

]]>

The service will return a service-unavailable error &IQ; if it does not support PEP.

]]>
http://jabber.org/protocol/pubsub#node_config whitelist on_sub ]]> ]]>

The node is now created and the only affiliated entity is the bare JID of the user, who created the node, with an affiliation as 'owner'.

In order to set a new secret key, clients store the encrypted secret key as Base64 encoded raw OpenPGP message within an <secretkey/> element qualified by the 'urn:xmpp:openpgp:0' namespace. These secret key backups are created as follows:

  1. All secret keys that should be included in the backup MUST be concatenated in their transferable key format (RFC 4880 § 11.1).
  2. A backup code is generated from secure random: The backup code consists of 24 upper case characters from the Latin alphabet and numbers without 'O' ("LATIN CAPITAL LETTER O") and '0' ("DIGIT ZERO") (alphabet: 123456789ABCDEFGHIJKLMNPQRSTUVWXYZ) grouped into 4-character chunks, e.g., TWNK-KD5Y-MT3T-E1GS-DRDB-KVTW. The characters MUST be generated from cryptographically secure random. For example getrandom(2), SecureRandom or /dev/urandom. More information about the randomness requirements for security can be found in &rfc4086;
  3. The whole backup code including the dashes is directly used as a string to encrypt the concatenated transferable keys as an OpenPGP message. More precisely: It is used as the symmetric-key for a Symmetric-Key Encrypted Session Key Packet according to RFC 4880 § 5.3; the symmetric-key is thus 29 characters long including the dashes. The encryption algorithm MUST be one of the standardized OpenPGP symmetric algorithms, e.g, AES-128.

Implementations of this XEP MUST generate and accept only version 4 (or higher) OpenPGP packets. Lower version OpenPGP packets are insecure in many aspects (see for example RFC 4880 § 5.5.2.).

The PubSub nodes specified by herein SHOULD be configured to either never send the latest item, or to send the latest item only when a new entity subscribed. Thus the nodes 'send_last_published_item' configuration option SHOULD be set to either 'never' or 'on_sub' (see XEP-0060 § 16.4.4).

OpenPGP implementations have a sad history of being not very user-friendly which results in users either not using OpenPGP or in users wrongly using OpenPGP. Implementors of this XEP, and additional future XEPs based on this XEP, therefore should read STEEDKoch, Werner, and Marcus Brinkman "STEED — Usable End-to-End Encryption", White Paper, g10 GmbH, 2011-10-17. <http://g10code.com/steed.html> and "Why Johnny can't encrypt"Whitten, Alma, and J. Doug Tygar. "Why Johnny Can't Encrypt: A Usability Evaluation of PGP 5.0." Usenix Security. Vol. 1999. 1999. <https://www.cs.berkeley.edu/~tygar/papers/Why_Johnny_Cant_Encrypt/OReilly.pdf>. Implementors of this XEP are encouraged to provide the concepts described in STEED:

  • Automatic key generation
  • Automatic key distribution
  • Opportunistic encryption
  • Trust upon first contact

Furthermore implementors should design the user interface for effective security by following the design principles and techniques for security mentioned in "Why Johnny Can't Encrypt".

Implementors should be aware that the size OpenPGP public and secret keys is somewhere in the range of tens of kilobytes. Applying Base64 encoding on keys, as it is described herein, further increases the size. The formula to determine the Base64 encoded size is: ceil(bytes / 3) * 4. Thus the lower bound for the maximum stanza size of 10000 bytes, as specified in RFC 6120 § 13.12. 4., is usually exceeded. However all XMPP server implementations, the authors are aware of, follow the recommendation of the RFC and do not blindly set the maximum stanza size to such a low value, but use a much higher threshold. Therefore, this should hardly be an issue for implementations. Nevertheless, it is advised to keep the size of OpenPGP keys small by removing all signatures except the most recent self-signature on each User ID before exporting the key (cf. GnuPG's --export-options export-minimal). In addition, implementors are advised to handle <policy-violation/> error responses when trying to transmit Base64 encoded keys.

The format of XMPP addresses, sometimes called JIDs, is well defined. Thus they need to be normalized, as defined in &rfc7622;. When implementations are required to compare XMPP addresses for equality, as it is the case in "Verification of &openpgp; Content", then they also have to compare the normalized versions of the addresses.

This specification intentionally does not specify if the used OpenPGP key should be a primary key or a subkey. It is even possible to announce multiple public keys in the Personal Eventing Protocol node. Implementations MUST be prepared to find multiple public keys. The authors however believe that for ease of use only one OpenPGP key specially crafted for the XMPP use case should be created, announced and used.

The &openpgp; and OpenPGP content elements are container elements for arbitrary signed and encrypted data and can thus act as building blocks for encrypted data included in Message, IQ and Presence stanzas. For example, future specifications may use them to implement encrypted versions of &xep0047; or &xep0261;.

Note that signed OpenPGP messages already contain a timestamp as per the OpenPGP specification. OpenPGP content elements nevertheless require the 'time' element because not every OpenPGP API may provide access to the embedded OpenPGP timestamp.

The 'rpad' element of the OpenPGP content elements exists to prevent length-based side channel attacks.

This specification addresses all relevant issues of &xep0027; (§ 4, § 5). It mitigates replay attacks by including the recipient's address and a timestamp in the OpenPGP content elementFull Replay attack prevention would require a counter based approach.. It allows for both, signing and encrypting of the element. The scope of the specification was deliberately limited to OpenPGP.

Features like signed presences, which is provided by XEP-0027, may be added later on as add-on XEP to this.

We decided against OpenPGP ASCII Armor (which contains an additional checksum) and in favor for Base64, because encoding should be part of the network application rather than the crypto layer. Also XMPP, needs no additional error correction of payload. In "MIME Security with OpenPGP" (&rfc3156;), ASCII Armor has only been chosen to be backwards compatible with legacy applications supporting non-MIME OpenPGP emails only.

OpenPGP User IDs normally consist of a name - email address pair, e.g., "Juliet <juliet@example.org>" (RFC 4880 § 5.11). For this XEP, we require User IDs of the format "xmpp:juliet@example.org". First, it is required to have at least one User ID indicating the use of this OpenPGP key. When doing certification of keys (key signing), the partner must know what User ID she actually certifies. Second, this format uses the standardized URI from XEP-0147 to indicate that this User ID corresponds to a key that is used for XMPP. Third, having the Real Name inside provides no additional security or guideline if this key should be certified. The XMPP address is the only trust anchor here.

The scope of this XEP is intentionally limited, so that the specification just defines way for XMPP entities to discover, announce and synchronize OpenPGP keys, and how to exchange signed and encrypted data between two or more parties. Everything else is outside its scope. For example, how 'secure' the key material is protected on the endpoints is up to the implementation.

And while this XEP specifies a mechanism how to discover and retrieve a public key, it does not define how the trust relation to this key should be established. Even if key discovery and retrieval over XMPP provides a stronger coupling between the possessing entity (the XMPP address) and the key, as compared to the OpenPGP keyservers, how a XMPP server authenticates a remote server is a server policy, which does vary from server to server. Implementation MUST provide a way for the user to establish and assign trust to a public key. For example by using a QR code shown on the recipient's device screen.

Besides the protocol defined herein, OpenPGP implementations are another big attack surface. Needless to say that the security of encrypted data exchanged using this protocol depends on the security of the used OpenPGP implementation. It is strongly RECOMMENED to use existing implementations instead of writing your own. OpenPGP implementations have suffered from various vulnerabilities in the past which opened up DoS attack vectors. For example CVE-2013-4402 and CVE-2014-4717.

This document requires no interaction with &IANA;.

The ®ISTRAR; includes 'urn:xmpp:openpgp:0' in its registry of protocol namespaces (see &NAMESPACES;).

TODO: Add after the XEP leaves the 'experimental' state.

Thanks to Emmanuel Gil Peyrot, Sergei Golovan, Marc Laporte, Georg Lukas, Adithya Abraham Philip and Brian Cully for their feedback.

The first draft of this specification was worked out and written on the wall of the 'Kymera' room in one of Google's buildings by the authors, consisting of members of the XMPP Standards Foundation and the OpenKeychain project, at the GSOC Mentors Summit 2015. The authors would like to thank Google for making it possible by bringing the right people together.