End-to-end encryption is a desirable feature for any communication technology. Ideally, such a technology would design encryption in from the beginning and would forbid unencrypted communications. Realistically, most communication technologies have not been designed in that manner, and Jabber/XMPP technologies are no exception. In particular, the original Jabber technologies developed in 1999 did not include end-to-end encryption by default. PGP-based encryption of message bodies and signing of presence information was added as an extension to the core protocols in the year 2000; this extension is documented in Current Jabber OpenPGP Usage (XEP-0027) [1]. When the core protocols were formalized within the Internet Standards Process by the IETF's XMPP Working Group in 2003 (see RFC 3920 [2] and RFC 3921 [3]), a different extension was defined using S/MIME-based signing and encryption of CPIM-formatted messages (see RFC 3862 [4]) and PIDF-formatted presence information (see RFC 3863 [5]); this extension is specified in RFC 3923 [6].
For reasons described in Requirements for Encrypted Sessions (XEP-0210) [7], the foregoing proposals (and others not mentioned) have not been widely implemented and deployed. This is unfortunate, since an open communication protocol needs to enable end-to-end encryption in order to be seriously considered for deployment by a broad range of users.
This document describes a different session-based approach to the end-to-end encryption of the full content of XMPP stanzas sent between two entities. The protocol assumes that the encrypted session parameters (initial keys, counters and algorithms etc.) have already been agreed, typically through a negotiation protocol such as Encrypted Session Negotiation (XEP-0116) [8], Simplified Encrypted Session Negotiation (XEP-0217) [9] or Offline Encrypted Sessions (XEP-0187) [10]. The session approach when combined with short-lived keys offers many important advantages over the existing "Object Encryption" proposals, including Perfect Forward Secrecy and Identity Protection.
The requirements and the consequent cryptographic design that underpin this protocol and its associated protocols are described in Requirements for Encrypted Sessions (XEP-0210) [7] and Cryptographic Design of Encrypted Sessions. The specific objectives of this protocol include:
Encryption of the full stanza content (except that which is required for stanza routing)
Minimise the Perfect Forward Secrecy window by enabling light-weight renegotiation of the short-term keys without requiring a full session renegotiation.
Minimise bandwidth overhead
No dependence on XML canonicalization
This document introduces two characters to help the reader follow the necessary exchanges:
While Alice and Bob are introduced as "end users", they are simply meant to be examples of XMPP entities. Any directly addressable XMPP entity may send or receive encrypted stanzas within an encrypted session or initiate a re-key.
Here we assume that Alice and Bob have already established an encrypted session. Either Alice or Bob MAY send encrypted stanzas within the encrypted session or initiate a re-key. The following sections describe the process where Alice sends Bob an encrypted stanza and initiates a re-key.
The following sections assume that the parameters in the tables below have already been agreed. For more details refer to an encrypted session negotiation protocol such as Encrypted Session Negotiation.
| Parameter | Description |
|---|---|
| CA, CB | The initial block cipher counter value for blocks sent by Alice and Bob |
| KCA, KCB | The initial secret cipher keys that Alice and Bob use to encrypt |
| KMA, KMB | The initial secret MAC keys that Alice and Bob use to protect the integrity of encrypted data |
| HASH | Agreed hash algorithm |
| CIPHER, DECIPHER | Agreed CTR-mode block cipher algorithm |
| COMPRESS, DECOMPRESS | Agreed compression algorithm |
| Parameter | Description |
|---|---|
| g | Diffie-Hellman generator |
| p | Diffie-Hellman prime |
| e, d | Alice and Bob's initial public Diffie-Hellman keys |
| x, y | *Alice and Bob's initial private Diffie-Hellman keys |
* x and y MUST be known only to Alice and Bob respectively, all other parameters MUST be known by both parties
All parameters except the algorithms are multi-precision integers, so implementations will need a Big Integer Math library to perform the necessary modular arithmetic. Note: A simple HMAC function and a cryptographic-strength pseudo-random number generator are also required, but no other cryptographic code is necessary.
Alice MAY use this protocol to encrypt only that part of the content of one-to-one <message/>, <presence/> and <iq/> stanzas that would normally be ignored by the intermediate servers. She MUST NOT encrypt:
Stanza wrapper element tags (only stanza content)
<error/> elements [11]
<defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> child elements of <error/> elements. [12]
<thread/> elements [13]
<amp/> elements (see Advanced Message Processing (XEP-0079) [14])
A stanza MUST NOT contain more than one <c xmlns='http://www.xmpp.org/extensions/xep-0200.html#ns'/> element, and it MUST be an immediate child of the stanza wrapper element. There is only one exception to those two rules, if the stanza is type 'error' then its <error/> child element MAY also contain a <c xmlns='http://www.xmpp.org/extensions/xep-0200.html#ns'/> element.
<message from='alice@example.org/pda'
to='bob@example.com/laptop'
type='chat'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<body>Hello, Bob!</body>
<amp xmlns='http://jabber.org/protocol/amp'>
<rule action='error' condition='match-resource' value='exact'/>
</amp>
<active xmlns='http://jabber.org/protocol/chatstates'/>
</message>
<body>Hello, Bob!</body> <active xmlns='http://jabber.org/protocol/chatstates'/>
<presence from='alice@example.org/pda'
to='bob@example.com/laptop'>
<show>dnd</show>
<status>Working</status>
<c xmlns='http://jabber.org/protocol/caps'
node='http://exodus.jabberstudio.org/caps'
ver='0.9'
ext='jingle ftrans xhtml'/>
<geoloc xmlns='http://jabber.org/protocol/geoloc'>
<alt>1609</alt>
<description>Jabber, Inc.</description>
<error>10</error>
<lat>39.75477</lat>
<lon>-104.99768</lon>
<timestamp>2004-02-19T21:12Z</timestamp>
</geoloc>
</presence>
<presence from='alice@example.org/pda'
to='bob@example.com/laptop'>
<c xmlns='http://www.xmpp.org/extensions/xep-0200.html#ns'>
<data> ** Base64 encoded m_final ** </data>
<mac> ** Base64 encoded a_mac ** </mac>
</c>
</presence>
<iq from='alice@example.org/pda'
to='bob@example.com/laptop'
id='publish1'
type='error'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<publish node='princely_musings'>
<item id='ae890ac52d0df67ed7cfdf51b644e901'>
...
</item>
</publish>
</pubsub>
<error type='modify'>
<not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<payload-too-big xmlns='http://jabber.org/protocol/pubsub#errors'/>
</error>
</iq>
<iq from='alice@example.org/pda'
to='bob@example.com/laptop'
id='publish1'
type='error'>
<c xmlns='http://www.xmpp.org/extensions/xep-0200.html#ns'>
<data> ** Base64 encoded m_final ** </data>
<mac> ** Base64 encoded a_mac ** </mac>
</c>
<error type='modify'>
<not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<c xmlns='http://www.xmpp.org/extensions/xep-0200.html#ns'>
<data> ** Base64 encoded m_final ** </data>
<mac> ** Base64 encoded a_mac ** </mac>
</c>
</error>
</iq>
Alice MUST perform the following steps to encrypt the XML content. Note: if there is no XML content to be encrypted (e.g. if this is an empty Re-Key Exchange stanza), then CA MUST be incremented by 1 (see below), and only the last two steps (normalization and MAC calculation) should be performed.
Serialize the XML content she wishes to send into an array of UTF-8 bytes, m. [15]
Compress m using the negotiated algorithm. If a compression algorithm other than 'none' was agreed, the compression context is typically initialized after key exchange and passed from one stanza to the next, with only a partial flush at the end of each stanza. [16]
m_compressed = COMPRESS(m)
Encrypt the data with the agreed algorithm in counter mode, using the encryption key KCA. Note: CA MUST be incremented by 1 for each encrypted block or partial block (i.e. CA = (CA + 1) mod 2n, where n is the number of bits per cipher block for the agreed block cipher algorithm). Note: if the block cipher algorithm 'none' was agreed then encryption MUST NOT be performed and CA MUST be incremented by 1 (for replay protection).
m_final = CIPHER(KCA, CA, m_compressed)
Generate the whole serialized content of the <c/> element:
If there is encrypted XML content, the XML MUST include the Base64 encoded (even if the block cipher algorithm 'none' was agreed [17], in accordance with Section 4 of RFC 4648 [18]) value of m_final wrapped in a <data/> element.
Only if Alice has received stanzas containing a <key/> element (see Re-Key Exchange) from Bob since she sent her last stanza then the XML MUST include the (positive, non-zero) number of such stanzas she has received (since she sent her last stanza) wrapped in a <new/> element.
The content may also contain one <key/> element (see Re-Key Exchange) and one or more <old/> elements (see Publishing Old MAC Values).
Alice MUST normalize the content by removing any whitespace from the serialized content (i.e. remove all character data from between all elements). Note: <c/> elements are so simple that there should never be a need to convert the XML to canonical form.
For example:
m_content = '<data> ** Base64 encoded m_final ** </data><new>1</new>'
Process the XML content, concatenated with the value of Alice's block cipher counter CA before the data was encrypted, through the HMAC algorithm (as defined in Section 2 of RFC 2104 [19]), along with the agreed hash algorithm ("HASH") and the integrity key KMA.
a_mac = HMAC(HASH, KMA, m_content | CA)
Before sending the stanza to Bob, Alice MUST wrap the (unnormalized) content and the Base64 encoded value of a_mac (wrapped in a <mac/> element) inside an <c/> element and insert it into the stanza in place of the original content.
<message from='alice@example.org/pda'
to='bob@example.com/laptop'
type='chat'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<c xmlns='http://www.xmpp.org/extensions/xep-0200.html#ns'>
<data> ** Base64 encoded m_final ** </data>
<new>1</new>
<mac> ** Base64 encoded a_mac ** </mac>
</c>
<amp xmlns='http://jabber.org/protocol/amp' per-hop='true'>
<rule action='error' condition='match-resource' value='exact'/>
</amp>
</message>
When Bob receives the stanza from Alice, he MUST extract and Base64 decode the values of m_final and a_mac from the content and perform the following steps.
Use any <new/> element the stanza may (or may not) contain to determine which of his <key/> elements (see Re-Key Initiation) Alice had received before she sent him the stanza. He MUST use this information to determine which of his stored sets of values {KCA KMA y} he should use to decrypt and verify the stanza. Bob MUST then securely destroy any of his sets of values that are older than the selected set.
Remove the <mac/> element from the serialized content of the <c/> element, and normalize the remaining content by removing all whitespace. Calculate the Message Authentication Code (MAC) for the content concatenated with Alice's block cipher counter using the agreed hash algorithm ("HASH") and the integrity key KMA.
b_mac = HMAC(HASH, KMA, m_content | CA)
Verify that b_mac and a_mac match. If they are not identical, the content has been tampered with and Bob MUST terminate the encrypted session, he MAY send a <not-acceptable/> error to Alice. [20]
Decrypt m_final using the agreed algorithm, KCA and CA. Note: CA MUST be incremented by 1 for each decrypted block (see Encrypting a Stanza). Note: if the block cipher algorithm 'none' was agreed decryption MUST NOT be performed and CA MUST be incremented by 1.
m_compressed = DECIPHER(KCA, CA, m_final)
Decompress m_compressed using the negotiated algorithm (usually 'none').
m = DECOMPRESS(m_compressed)
Replace the <c/> element in the serialized XML stanza with m and feed the stanza into an XML parser. If the parser returns an XML format error then Bob MUST terminate the encrypted session, he MAY send a <not-acceptable/> error to Alice. [21]
Once an attacker has discovered an encryption key it could be used to decrypt all stanzas within a session, including stanzas that were intercepted before the key was discovered. To reduce the window of vulnerability, both Alice and Bob SHOULD use the Diffie-Hellman key exchange described below to agree a new encryption key as regularly as possible. They MUST also destroy all copies of keys as soon as they are no longer needed.
Note: Although most entities are capable of exchanging new keys after every stanza, clients running in constrained runtime environments may require a few seconds of full-load CPU time in order to re-key. During encrypted session negotiation (when using Encrypted Session Negotiation for example) these clients MAY negotiate the minimum number of stanzas to be exchanged between re-keys at the cost of a larger window of vulnerability. Entities MUST NOT initiate key re-exchanges more frequently than the agreed limit.
Either Alice or Bob MAY initiate a key re-exchange. Here we describe the process initiated by Alice.
First Alice MUST calculate new values for the encryption parameters:
Generate: a secret random number x (where 22n-1 < x < p - 1, where n is the number of bits per cipher block for CIPHER)
Calculate: e = gx mod p
Calculate: K = dx mod p (the new shared secret)
Alice's Encryption key: KCA = HMAC(HASH, K, "Rekey Initiator Crypt")
Bob's Encryption key: KCB = HMAC(HASH, K, "Rekey Acceptor Crypt")
Alice's Integrity key: KMA = HMAC(HASH, K, "Rekey Initiator MAC")
Bob's Integrity key: KMB = HMAC(HASH, K, "Rekey Acceptor MAC")
Note: Once KCA, KMA, KCB and KMB have been calculated the value of K MUST be securely destroyed. When calculating those keys, as many bits of key data as are needed for each key MUST be taken from the least significant bits of the output of HMAC. For algorithms with variable-length keys the maximum length (up to the output length of HMAC) SHOULD be used.
The new value of e SHOULD be wrapped in a <key/> element and sent to Bob. To avoid unnecessary network traffic, it SHOULD be sent together with encrypted content (see Encrypting a Stanza). Alice MUST use her old KCA and KMA to encrypt and calculate the MAC of this stanza, after which she MUST securely destroy all copies of the old value of KCA. She MUST use her new KCA and KMA when sending subsequent stanzas.
Note: There is no need for Alice to provide a signature because the calculation of the MAC includes the new value of e (see Encrypting a Stanza).
<message from='alice@example.org/pda' to='bob@example.com/laptop'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<c xmlns='http://www.xmpp.org/extensions/xep-0200.html#ns'>
<data> ** Base64 encoded m_final ** </data>
<key> ** Base64 encoded value of new e ** </key>
<mac> ** Base64 encoded a_mac ** </mac>
</c>
</message>
Note: Bob may send one of more stanzas before he receives Alice's <key/> element (i.e. the stanzas may be in transit at the same time). So, before destroying her old values of KCB and KMB, Alice MUST wait until either she receives a stanza encrypted with her new key, or a reasonable time has passed (60 seconds should cover a network round-trip and calculations by a constrained client). Similarly she MUST wait before destroying her old value of x, in case Bob sends one or more stanzas including a <key/> element before he receives Alice's new key. Consequently, if Alice sends several <key/> elements to Bob within a reasonable time without receiving a stanza from him, then she MUST remember several "sets" of the three values: {KCB KMB x}.
Note: Alice never remembers more than one copy of KCA and KMA.
After receiving and decrypting a stanza that contains a <key/> element Bob MUST extract the new value of e and confirm that it is greater than one. Then he MUST calculate K using the new value of e and the value of y from his oldest stored set of values {KCA KMA y} (i.e. the set that also contains the value of KCA used to decrypt the stanza):
K = ey mod pBob MUST replace the values of KCA and KMA in all his stored sets of values {KCA KMA y} with values that are derived from K in exactly the same way as Alice did (see Re-Key Initiation).
Only if Bob is storing exactly one set of values {KCA KMA y} then he MUST also replace his values of KCB and KMB with values that are derived from K in exactly the same way as Alice did.
Once the expired MAC keys have been published, anyone could create valid arbitrary stanzas with them. This prevents anyone being able to prove the authenticity of a transcript of the encrypted session in the future.
Either entity MAY publish old values of KMA and/or KMB within any encrypted stanza as long as it knows that all the stanzas that MAY use the old values have been received and validated. Note: A 'man-in-the-middle' could delay the delivery of stanzas indefinitely. So, before Alice publishes KMA (and KMB), she MUST wait until she has both sent a re-key to Bob and received a stanza from Bob encrypted with her new key. (She MAY also publish KMB after she has received a re-key from Bob.)
<message from='alice@example.org/pda' to='bob@example.com/laptop'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<c xmlns='http://www.xmpp.org/extensions/xep-0200.html#ns'>
<data> ** Base64 encoded m_final ** </data>
<old> ** Base64 encoded old MAC key ** </old>
<old> ** Base64 encoded old MAC key ** </old>
<mac> ** Base64 encoded a_mac ** </mac>
</c>
</message>
Entities SHOULD ignore any <old/> elements they receive.
Weak pseudo-random number generators (PRNG) enable successful attacks. Implementors MUST use a cryptographically strong PRNG to generate all random numbers (see RFC 1750 [22]).
If either entity stores a (re-encrypted) transcript of an encrypted session for future consultation then the Perfect Forward Secrecy offered by this protocol is lost. If the negotiated value of the 'otr' Stanza Session Negotiation field is 'true' the entities MUST NOT store any part of the encrypted session content (not even in encrypted form).
The block cipher counters maintained implicitly by Alice and Bob (CA and CB) prevent stanzas being replayed within any encrypted session. They ensure that the MAC will be different for all stanzas, even if the HMAC key and the content of the stanza are identical.
Alice and Bob MUST ensure that every value of x and y (and therefore e and d) they generate is unique. This prevents complete online encrypted sessions being replayed.
After each key exchange an entity MUST NOT exchange a total of 232 encrypted blocks (not stanzas) before it initiates a key re-exchange (see RFC 4344 [23]). Note: This limitation also ensures the same key and counter values are never used to encrypt two different blocks using counter mode (thus preventing simple attacks).
In order to reduce the Perfect Forward Secrecy window of vulnerability, after an extended period of inactivity entities SHOULD re-key (or terminate the encrypted session).
Cryptography plays only a small part in an entity's security. Even if it implements this protocol perfectly it may still be vulnerable to other attacks. For examples, an implementation might store encrypted session keys on swap space or save private keys to a file in cleartext! Implementors MUST take very great care when developing applications with secure technologies.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [24].
Until this specification advances to a status of Draft, its associated namespace shall be "http://www.xmpp.org/extensions/xep-0200.html#ns"; upon advancement of this specification, the XMPP Registrar [25] shall issue a permanent namespace in accordance with the process defined in Section 4 of XMPP Registrar Function (XEP-0053) [26].
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='http://www.xmpp.org/extensions/xep-0200.html#ns'
xmlns='http://www.xmpp.org/extensions/xep-0200.html#ns'
elementFormDefault='qualified'>
<xs:element name='data' type='xs:string'/>
<xs:element name='c'>
<xs:complexType>
<xs:sequence>
<xs:element ref='data' minOccurs='0' maxOccurs='1'/>
<xs:element ref='key' minOccurs='0' maxOccurs='1'/>
<xs:element ref='mac' minOccurs='0' maxOccurs='1'/>
<xs:element ref='new' minOccurs='0' maxOccurs='1'/>
<xs:element ref='old' minOccurs='0' maxOccurs='unbounded'/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name='key' type='xs:string'/>
<xs:element name='mac' type='xs:string'/>
<xs:element name='new' type='xs:positiveInteger'/>
<xs:element name='old' type='xs:string'/>
</xs:schema>
This document in other formats: XML PDF
This XMPP Extension Protocol is copyright © 1999 – 2020 by the XMPP Standards Foundation (XSF).
Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.
## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##
In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.
This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).
The HTML representation (you are looking at) is maintained by the XSF. It is based on the YAML CSS Framework, which is licensed under the terms of the CC-BY-SA 2.0 license.
The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.
Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.
Given that this XMPP Extension Protocol normatively references IETF technologies, discussion on the <xsf-ietf@xmpp.org> list might also be appropriate.
Errata can be sent to <editor@xmpp.org>.
The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
1. XEP-0027: Current Jabber OpenPGP Usage <https://xmpp.org/extensions/xep-0027.html>.
2. RFC 3920: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc3920>.
3. RFC 3921: Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence <http://tools.ietf.org/html/rfc3921>.
4. RFC 3862: Common Presence and Instant Messaging (CPIM): Message Format <http://tools.ietf.org/html/rfc3862>.
5. RFC 3863: Presence Information Data Format (PIDF) <http://tools.ietf.org/html/rfc3863>.
6. RFC 3923: End-to-End Signing and Object Encryption for the Extensible Messaging and Presence Protocol (XMPP) <http://tools.ietf.org/html/rfc3923>.
7. XEP-0210: Requirements for Encrypted Sessions <https://xmpp.org/extensions/xep-0210.html>.
8. XEP-0116: Encrypted Session Negotiation <https://xmpp.org/extensions/xep-0116.html>.
9. XEP-0217: Simplified Encrypted Session Negotiation <https://xmpp.org/extensions/xep-0217.html>.
10. XEP-0187: Offline Encrypted Sessions <https://xmpp.org/extensions/xep-0187.html>.
11. RFC 6120 requires that stanzas of type 'error' contain an <error/> child element.
12. RFC 6120 requires that <error/> elements contain a <defined-condition/> child element.
13. Applications typically use <thread/> elements internally to route stanzas to the process handling a session. The content of thread elements MUST be opaque with no semantic meaning and only exact comparisons MAY be made against it.
14. XEP-0079: Advanced Message Processing <https://xmpp.org/extensions/xep-0079.html>.
15. Although counter mode encryption requires no padding, implementations MAY still disguise the length of m by appending a random number of white-space characters.
16. If Bob were to receive a stanza out-of-order, then he would fail to decrypt the stanza and be forced to terminate the encrypted session.
17. The content is encoded even if no encryption is used to avoid triggering namespace errors when entities parse the XML.
18. RFC 4648: The Base16, Base32, and Base64 Data Encodings <http://tools.ietf.org/html/rfc4648>.
19. RFC 2104: HMAC: Keyed-Hashing for Message Authentication <http://tools.ietf.org/html/rfc2104>.
20. If Bob were to receive a stanza out-of-order, then the MACs would not match because the values of CA would not be synchronized.
21. Bob MUST NOT send a stream error to his server since intermediate entities are not responsible for encoded content.
22. RFC 1750: Randomness Recommendations for Security <http://tools.ietf.org/html/rfc1750>.
23. RFC 4344: SSH Transport Layer Encryption Modes <http://tools.ietf.org/html/rfc4344>.
24. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.
25. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.
26. XEP-0053: XMPP Registrar Function <https://xmpp.org/extensions/xep-0053.html>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Added reference to Simplified Encrypted Session Negotiation; modified namespaces to reflect XMPP Registrar procedures regarding URN issuance.
Initial version (extracted from XEP-0116 version 0.12); clarified Re-Key Exchange section; removed canonicalization requirement; employed HMAC instead of hash; added new sections; changed element name and namespace
END