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0.14 RC2 included full forms in SAS calc, clarified rep of big ints

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git-svn-id: file:///home/ksmith/gitmigration/svn/xmpp/trunk@659 4b5297f7-1745-476d-ba37-a9c6900126ab
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Ian Paterson 2007-03-15 17:14:41 +00:00
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xep-0116.xml
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@ -58,7 +58,7 @@
<spec>RFC 2104</spec>
<spec>RFC 2409</spec>
<spec>RFC 3526</spec>
<spec>RFC 3548</spec>
<spec>RFC 4648</spec>
<spec>SHA256</spec>
<spec>xml-c14n</spec>
<spec>XEP-0004</spec>
@ -74,6 +74,12 @@
&ianpaterson;
&stpeter;
&dizzyd;
<revision>
<version>0.14</version>
<date>2007-03-15</date>
<initials>ip</initials>
<remark><p>Clarified representation of Big Ints, incorporated whole forms into SAS calculation</p></remark>
</revision>
<revision>
<version>0.13</version>
<date>2006-11-27</date>
@ -216,8 +222,8 @@
</ul>
</section2>
<section2 topic="ESession Request" anchor='init-online-request'>
<p>In addition to the "accept", "security", "otr" and "disclosure" fields (see <link url='#sec-backdoor'>Back Doors</link>) specified in <cite>Chat Session Negotiation</cite>, Alice MUST send to Bob each of the ESession options (see list below) that she is willing to use, in her order of preference (see <link url='#sec-mandatory'>Mandatory to Implement Technologies</link>).</p>
<section2 topic="ESession Request (Alice)" anchor='init-online-request'>
<p>In addition to the "accept", "security", "otr" and "disclosure" fields (see <link url='#sec-backdoor'>Back Doors</link>) specified in <cite>Stanza Session Negotiation</cite>, Alice MUST send to Bob each of the ESession options (see list below) that she is willing to use, in her order of preference (see <link url='#sec-mandatory'>Mandatory to Implement Technologies</link>).</p>
<ol>
<li><p>The list of Modular Exponential (MODP) group numbers (as specified in &rfc2409; or &rfc3526;) that MAY be used for Diffie-Hellman key exchange (valid group numbers include 1,2,3,4,5,14,15,16,17 and 18)</p></li>
<li><p>Symmetric block cipher algorithm names</p></li>
@ -350,7 +356,7 @@
]]></example>
</section2>
<section2 topic="ESession Rejection" anchor='init-online-reject'>
<section2 topic="ESession Rejection (Bob)" anchor='init-online-reject'>
<p>If Bob does not want to reveal presence to Alice for whatever reason then Bob SHOULD return no response or error.</p>
<p>If Alice initiated a 3-message negotiation but Bob only supports 4-message negotiations (with Alice) then he SHOULD return a &feature; error specifying the 'dhkeys' field:</p>
<example caption='Bob Informs Alice that 3-message Negotiation is Not Supported'><![CDATA[
@ -388,8 +394,8 @@
</message>
]]></example>
<p>Either Bob or Alice MAY attempt to initiate a new ESession after any error during the negotiation process. However, both MUST consider the previous negotiation to have failed and MUST discard any information learned through the previous negotiation.</p>
<p>If Bob is unwilling to start an ESession, but he <em>is</em> ready to initiate a one-to-one chat session with Alice (see <cite>Chat Session Negotiation</cite>), and if Alice included an option for the "security" field with the value "none" or "c2s", then Bob SHOULD accept the Chat Session and terminate the ESession negotiation by specifying "none" or "c2s" for the value of the "security" field in his response.</p>
<example caption='Bob Accepts Chat Session'><![CDATA[
<p>If Bob is unwilling to start an ESession, but he <em>is</em> ready to initiate a one-to-one stanza session with Alice (see <cite>Stanza Session Negotiation</cite>), and if Alice included an option for the "security" field with the value "none" or "c2s", then Bob SHOULD accept the stanza session and terminate the ESession negotiation by specifying "none" or "c2s" for the value of the "security" field in his response.</p>
<example caption='Bob Accepts Stanza Session'><![CDATA[
<message from='bob@example.com/laptop' to='alice@example.org/pda'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<feature xmlns='http://jabber.org/protocol/feature-neg'>
@ -407,7 +413,7 @@
]]></example>
</section2>
<section2 topic="ESession Response" anchor='init-response'>
<section2 topic="ESession Response (Bob)" anchor='init-response'>
<section3 topic="Diffie-Hellman Preparation (Bob)" anchor='init-online-bobprep'>
<p>If Bob supports one or more of each of Alice's ESession options and is willing to start an ESession with Alice, then he MUST select one of the options from each of the ESession fields he received from Alice including one hash algorithm ("HASH"), and one of the MODP groups (see &rfc3766; or <cite>RFC 3526</cite> for recommendations regarding balancing the sizes of symmetric cipher blocks and Diffie-Hellman moduli) and Alice's corresponding value of 'He' (for 4-message negotiations) or 'e' (for 3-message negotiations).</p>
@ -637,10 +643,6 @@
<field var="mac"><value> ** Integrity of identity ** </value></field>
</x>
</feature>
<c xmlns='urn:xmpp:crypt'>
<data> ** Base64 encoded m_final ** </data>
<mac> ** Base64 encoded a_mac ** </mac>
</c>
</message>
]]></example>
</section3>
@ -662,12 +664,12 @@
<code>&macA; = DECIPHER(&KCsubA;, &CsubA;, &IDA;) <em>OR</em> {&pubKeyA;, &signA;} = DECIPHER(&KCsubA;, &CsubA;, &IDA;)</code>
<code>&macA; = <em>HMAC</em>(HASH, &KSsubA;, {&NsubB;, &NsubA;, e, &pubKeyA;, &formA;, &formA2;})</code>
<code>VERIFY(&signA;, &pubKeyA;, &macA;)</code>
<p>In the case of a 3-message negotiation, the ESession negotiation is now complete.</p>
<p><em>In the case of a 3-message negotiation, the ESession negotiation is now complete.</em></p>
</section3>
<section3 topic="Short Authentication String" anchor='init-acceptbob-sas'>
<p>Note: The steps in this and all the following Online ESession Negotiation sections are only necessary for 4-message negotiations.</p>
<p>Bob and Alice MAY confirm out-of-band that the Short Authentication Strings (SAS) their clients generate for them (using the SAS generation algorithm that they agreed on) are the same. This out-of-band step MAY be performed at any time. However, if either Bob or Alice has not provided a public key, or if either of their public keys has never been authenticated by the other party, then they SHOULD confirm out-of-band that their SAS match as soon as they realise that the two clients have no retained secret in common (see <link url='#init-finalbob'>Generating Bob's Final Session Keys</link> below, or <link url='#init-finalalice'>Generating Alice's Final Session Keys</link>).</p>
<p><em>Note: The steps in this and all the following Online ESession Negotiation sections are only necessary for 4-message negotiations.</em></p>
<p>Bob and Alice MAY confirm out-of-band that the Short Authentication Strings (SAS) their clients generate for them (using the SAS generation algorithm that they agreed on) are the same. This out-of-band step MAY be performed at any time. However, if either Bob or Alice has not provided a public key, or if either of their public keys has never been authenticated by the other party, then they SHOULD confirm out-of-band that their SAS match as soon as they realise that the two clients have no retained secret in common (see <link url='#init-finalbob'>Generating Bob's Final Session Keys</link> below, or <link url='#init-finalalice'>Generating Alice's Final Session Keys</link>). However, if it is inconvenient for Bob and Alice to confirm the match immediately, both clients MAY remember (in a secure way) that a SAS match has not yet been confirmed and remind Bob and Alice at the start of each ESession that they should confirm the SAS match (even if they have a retained secret in common). Their clients should continue to remind them until they either confirm a SAS match, or indicate that security is not important enough for them to bother.</p>
</section3>
<section3 topic="Generating Bob's Final Session Keys" anchor='init-finalbob'>
@ -705,7 +707,7 @@
</section3>
</section2>
<section2 topic="ESession Accept (Alice)" anchor='init-complete'>
<section2 topic="Final Steps (Alice)" anchor='init-complete'>
<section3 topic="Generating Alice's Final Session Keys" anchor='init-finalalice'>
<p>Alice MUST identify the shared retained secret (SRS) by selecting from her client's list of the secrets it retained from sessions with Bob's clients (the most recent secret for each of the clients he has used to negotiate ESessions with Alice's client).</p>
<p>Alice does this by using each secret in the list in turn as the key to calculate the HMAC (with HASH) of the string "Shared Retained Secret", and comparing the calculated value with the value in the 'srshash' field she received from Bob (see <link url='#init-acceptbob-send'>Sending Bob's Identity</link>). Once she finds a match, and has confirmed that the secret has not expired (because it is older than an implementation-defined period of time), then she has found the SRS.</p>
@ -725,7 +727,7 @@
<section1 topic='ESession Termination' anchor='terminate'>
<p>Either entity MAY terminate an ESession at any time. Entities MUST terminate all open ESessions before they go offline. To terminate an ESession Alice MUST send an encrypted stanza to Bob including within the encrypted XML of the &lt;data/&gt; element a chat negotiation form with a "terminate" field (as specified in the Termination section of <cite>Chat Session Negotiation</cite>). Note: She MAY publish old values of &KMsubA; and/or &KMsubB; within her termination stanza as long as she is sure all the stanzas that MAY use the old values have been received and validated (see <cite>Stanza Encryption</cite>). She MUST then securely destroy all keys associated with the ESession.</p>
<p>Either entity MAY terminate an ESession at any time. Entities MUST terminate all open ESessions before they go offline. To terminate an ESession Alice MUST send an encrypted stanza to Bob including within the encrypted XML of the &lt;data/&gt; element a stanza session negotiation form with a "terminate" field (as specified in the Termination section of <cite>Stanza Session Negotiation</cite>). Note: She MAY publish old values of &KMsubA; and/or &KMsubB; within her termination stanza as long as she is sure all the stanzas that MAY use the old values have been received and validated (see <cite>Stanza Encryption</cite>). She MUST then securely destroy all keys associated with the ESession.</p>
<example caption='Alice Terminates an ESession'><![CDATA[
<message from='alice@example.org/pda' to='bob@example.com/laptop'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
@ -736,7 +738,7 @@
</c>
</message>
]]></example>
<p>When Bob receives a termination stanza he MUST verify the MAC (to be sure he received all the stanzas Alice sent him during the ESession) and immediately send an encrypted termination acknowledgement form (as specified in the Termination section of <cite>Chat Session Negotiation</cite>) back to Alice. Note: He MAY publish <em>any</em> old values of &KMsubA; or &KMsubB; within the acknowledgement stanza. He MUST then securely destroy all keys associated with the ESession.</p>
<p>When Bob receives a termination stanza he MUST verify the MAC (to be sure he received all the stanzas Alice sent him during the ESession) and immediately send an encrypted termination acknowledgement form (as specified in the Termination section of <cite>Stanza Session Negotiation</cite>) back to Alice. Note: He MAY publish <em>any</em> old values of &KMsubA; or &KMsubB; within the acknowledgement stanza. He MUST then securely destroy all keys associated with the ESession.</p>
<example caption='Bob Acknowledges ESession Termination'><![CDATA[
<message from='bob@example.com/laptop' to='alice@example.org/pda'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
@ -750,19 +752,25 @@
<p>When Alice receives the stanza she MUST verify the MAC to be sure she received all the stanzas Bob sent her during the ESession. Once an entity has sent a termination or termination acknowledgement stanza it MUST NOT send another stanza within the ESession.</p>
</section1>
<section1 topic='XML Normalization' anchor='sign-normal'>
<p>Before the signature or MAC of a block of XML is generated or verified, all character data <em>between</em> all elements MUST be removed and the XML MUST be converted to canonical form (see &w3canon;).</p>
<p>All the XML this protocol requires to be signed or MACed is very simple, so in this case, canonicalization SHOULD only require the following changes:</p>
<ul>
<li>Set attribute value delimiters to single quotation marks (i.e. simply replace all single quotes in the serialized XML with double quotes)</li>
<li>Impose lexicographic order on the attributes of "field" elements (i.e. ensure "type" is before "var")</li>
</ul>
<p>Implementations MAY conceivably also need to make the following changes. Note: Empty elements and special characters SHOULD NOT appear in the signed or MACed XML specified in this protocol.</p>
<ul>
<li>Ensure there are no character references</li>
<li>Convert empty elements to start-end tag pairs</li>
<li>Ensure there is no whitespace except for single spaces before attributes</li>
</ul>
<section1 topic='Implementation Notes' anchor='implement'>
<section2 topic='Multiple-Precision Integers' anchor='sign-normal'>
<p>Before Base-64 encoding, hashing or HMACing an arbitrary-length integer, the integer MUST first be converted to a "big endian" bitstring. The bitstring MUST then be padded with leading zero bits so that there are an integral number of octets. Finally, if the integer is not of fixed bit-length (i.e. not a hash or HMAC result) and the bitstring contains leading octets that are zero, these MUST be removed (so the high-order octet is non-zero).</p>
</section2>
<section2 topic='XML Normalization' anchor='sign-normal'>
<p>Before the signature or MAC of a block of XML is generated or verified, all character data <em>between</em> all elements MUST be removed and the XML MUST be converted to canonical form (see &w3canon;).</p>
<p>All the XML this protocol requires to be signed or MACed is very simple, so in this case, canonicalization SHOULD only require the following changes:</p>
<ul>
<li>Set attribute value delimiters to single quotation marks (i.e. simply replace all single quotes in the serialized XML with double quotes)</li>
<li>Impose lexicographic order on the attributes of "field" elements (i.e. ensure "type" is before "var")</li>
</ul>
<p>Implementations MAY conceivably also need to make the following changes. Note: Empty elements and special characters SHOULD NOT appear in the signed or MACed XML specified in this protocol.</p>
<ul>
<li>Ensure there are no character references</li>
<li>Convert empty elements to start-end tag pairs</li>
<li>Ensure there is no whitespace except for single spaces before attributes</li>
</ul>
</section2>
</section1>
<section1 topic='Security Considerations' anchor='sec'>
@ -784,12 +792,12 @@
<p>Since Alice MAY use many different JIDs to talk to Bob, but always identify herself to him with the same public key, Entities SHOULD associate a "petname" with each public key fingerprint they store. Entities MUST present any public key petnames clearly to their users, and more prominently than any petname or nickname associated with the JID or the JID itself.</p>
</section2>
<section2 topic='Unencrypted ESessions' anchor='sec-unencrypted'>
<p>Organisations with full disclosure policies may require entities to disable encryption (see <link url='#sec-backdoor'>Back Doors</link>) to enable the logging of all messages on their server. Unencrypted ESessions meet all the Security Requirements (see <cite>Cryptographic Design of Encrypted Sessions</cite>) except for Confidentiality. Unencrypted ESessions enable Alice to to confirm <em>securely</em> with Bob that both client-server connections are secure. i.e. that the value of the 'security' option (as specified in <cite>Chat Session Negotiation</cite>) has not been tampered with.</p>
<p>Organisations with full disclosure policies may require entities to disable encryption (see <link url='#sec-backdoor'>Back Doors</link>) to enable the logging of all messages on their server. Unencrypted ESessions meet all the Security Requirements (see <cite>Cryptographic Design of Encrypted Sessions</cite>) except for Confidentiality. Unencrypted ESessions enable Alice to to confirm <em>securely</em> with Bob that both client-server connections are secure. i.e. that the value of the 'security' option (as specified in <cite>Stanza Session Negotiation</cite>) has not been tampered with.</p>
</section2>
<section2 topic="Back Doors" anchor="sec-backdoor">
<p>The authors and the XSF would like to discourage the deliberate inclusion of "back doors" in implementations of this protocol. However, we recognize that some organizations must monitor chats or record chats in decryptable form for legal compliance reasons, or may choose to monitor chats for quality assurance purposes. In these cases it is important to inform the other entity of the (potential for) disclosure before starting the ESession (if only to maintain public confidence in this protocol).</p>
<p>The authors and the XSF would like to discourage the deliberate inclusion of "back doors" in implementations of this protocol. However, we recognize that some organizations must monitor stanza sessions or record stanza sessions in decryptable form for legal compliance reasons, or may choose to monitor stanza sessions for quality assurance purposes. In these cases it is important to inform the other entity of the (potential for) disclosure before starting the ESession (if only to maintain public confidence in this protocol).</p>
<p>Both implementations MUST immediately and clearly inform their users if the negotiated value of the 'disclose' field is not 'never'.</p>
<p>Before disclosing any chat, an entity SHOULD either negotiate the value of the 'disclose' field to be 'enabled' or terminate the negotiation unsuccessfully. It MUST NOT negotiate the value of the 'disclose' field to be 'disabled' unless it would be illegal for it to divulge the disclosure to the other entity.</p>
<p>Before disclosing any stanza session, an entity SHOULD either negotiate the value of the 'disclose' field to be 'enabled' or terminate the negotiation unsuccessfully. It MUST NOT negotiate the value of the 'disclose' field to be 'disabled' unless it would be illegal for it to divulge the disclosure to the other entity.</p>
<p>In any case an implementation MUST NOT negotiate the value of the 'disclose' field to be 'never' unless it implements no feature or mechanism (not even a disabled feature or mechanism) that could be used directly or indirectly to divulge to <em>any</em> third-party either the identites of the participants, or the keys, or the content of <em>any</em> ESession (or information that could be used to recover any of those items). If an implementation deliberately fails to observe this last point (or fails to correct an accidental back door) then it is not compliant with this protocol and MUST NOT either claim or imply any compliance with this protocol or any of the other protocols developed by the authors or the XSF. In this case the authors and the XSF reserve all rights regarding the names of the protocols.</p>
<p>The expectation is that this legal requirement will persuade many implementors either to tell the users of their products that a back door exists, or not to implement a back door at all (if, once informed, the market demands that).</p>
</section2>
@ -868,14 +876,14 @@
</section2>
</section1>
<section3 topic="The sas28x5 SAS Algorithm" anchor='sas'>
<p>Given the multi-precision integers e and d (each a big-endian byte array) and the hash function "HASH", the following steps can be used to calculate a 5-character SAS with over 16 million possible values that is easy to read and communicate verbally:</p>
<section1 topic="The sas28x5 SAS Algorithm" anchor='sas'>
<p>Given the multi-precision integer &MsubA; (a big-endian byte array), the UTF-8 byte string &formB; (see <link url='#init-hide'>Hiding Bob's Identity</link>) and the hash function "HASH", the following steps can be used to calculate a 5-character SAS with over 16 million possible values that is easy to read and communicate verbally:</p>
<ol>
<li><p>Concatenate e, d and the string "Short Authentication String" into a string of bytes</p></li>
<li><p>Concatenate &MsubA;, &formB; and the UTF-8 byte string "Short Authentication String" into a string of bytes</p></li>
<li><p>Calculate the least significant 24-bits of the HASH of the string</p></li>
<li><p>Convert the 24-bit integer into a base-28 <note>Base-28 was used instead of Base-36 because some characters are often confused when communicated verbally (n, s, b, t, z, j), and because zero is often read as the letter 'o', and the letter 'l' is often read as the number '1'.</note> 5-character string using the following digits (values 0-27): acdefghikmopqruvwxy123456789</p></li>
</ol>
</section3>
</section1>
<section1 topic='IANA Considerations' anchor='iana'>
<p>This document requires no interaction with &IANA;. </p>
@ -890,7 +898,7 @@
</ul>
</section2>
<section2 topic='Field Standardization' anchor='registrar-formtype'>
<p>&xep0068; defines a process for standardizing the fields used within Data Forms qualified by a particular namespace. The following fields shall be registered for use in <em>both</em> Encrypted Session Negotiation and Chat Session Negotiation:</p>
<p>&xep0068; defines a process for standardizing the fields used within Data Forms qualified by a particular namespace. The following fields shall be registered for use in <em>both</em> Encrypted Session Negotiation and Stanza Session Negotiation:</p>
<code caption='Registry Submission'><![CDATA[
<form_type>
<name>urn:xmpp:esession</name>