Otrv2

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This document describes version 2 of the Off-the-Record Messaging protocol. The main changes over version 1 include:. OTR assumes a network model which provides in-order delivery of messages, but that some messages may not get delivered at all for example, if the user disconnects. There may be an active attacker, who is allowed to perform a Denial of Service attack, but not to learn the contents of messages. There are two ways Alice can inform Bob that she is willing to use the OTR protocol to speak with him: Each method also includes a way for Alice to communicate to Bob which versions of the OTR protocol she is willing to speak with him. On the other hand, the semantics of the whitespace tag are that Alice is merely indicating to Bob that she is willing and able to have an OTR conversation with him. All exponentiations are done modulo a particular bit prime, and g is a generator of that group, as indicated in the detailed description below. The general idea is that Alice and Bob do an unauthenticated Diffie-Hellman D-H key exchange to set up an encrypted channel, and then do mutual authentication inside that channel. This section outlines the method used to protect data being exchanged between Alice and Bob. As above, all exponentiations are done modulo a particular bit prime, and g is a generator of that group, as indicated in the detailed description below. While data messages are being exchanged, either Alice or Bob may run SMP to detect impersonation or man-in-the-middle attacks. As above, all exponentiations are done modulo a particular bit prime, and g 1 is a generator of that group. All sent values include zero-knowledge proofs that they were generated according to this protocol, as indicated in the detailed description below. The random exponents are bit numbers. If Alice wishes to communicate to Bob that she would like to use OTR, she sends a message containing the string '? The version string is constructed as follows:. If Alice wishes to communicate to Bob that she is willing to use OTR, she can attach a special whitespace tag to any plaintext message she sends him. This tag may occur anywhere in the message, and may be hidden from the user as in the Query Messages, above. The tag consists of the following 16 bytes, followed by one or more sets of 8 bytes indicating the version of OTR Alice is willing to use:. On the other hand, if Alice receives a plaintext message from Bob rather than an initiation of the AKE , she should stop sending him the whitespace tag. Any message containing the string '? The following part of the message should contain human-readable details of the error. This section describes the byte-level format of the base encoded binary OTR messages. The binary form of each of the messages is described below. For the Diffie-Hellman group computations, the group is the one defined in RFC with bit modulus hex, big-endian:. Note that this means that whenever you see a Diffie-Hellman exponentiation in this document, it always means that the exponentiation is done modulo the above bit number. OTR users have long-lived public keys that they use for authentication but not encryption. The current version of the OTR protocol only supports DSA public keys, but there is a key type marker for future extensibility. OTR public keys are used to generate signatures ; different types of keys produce signatures in different formats. The format for a signature made by a DSA public key is as follows:. OTR public keys have fingerprints , which are hex strings that serve as identifiers for the public key. The fingerprint is calculated by taking the SHA-1 hash of the byte-level representation of the public key. However, there is an exception for backwards compatibility: The encoding assures that, assuming the hash function itself has no useful collisions, and DSA keys have length less than bits times larger than most DSA keys , no two public keys will have the same fingerprint. This is the first message of the AKE. Bob sends it to Alice to commit to a choice of D-H encryption key but the key itself is not yet revealed. This allows the secure session id to be much shorter than in OTR version 1, while still preventing a man-in-the-middle attack on it. This is the second message of the AKE. This is the third message of the AKE. Bob sends it to Alice, revealing his D-H encryption key and thus opening an encrypted channel , and also authenticating himself and the parameters of the channel, preventing a man-in-the-middle attack on the channel itself to Alice. This is the final message of the AKE. Alice sends it to Bob, authenticating herself and the channel parameters to him. This message is used to transmit a private message to the correspondent. It is also used to reveal old MAC keys. The plaintext message either before encryption, or after decryption consists of a human-readable message encoded in UTF-8, optionally with HTML markup , optionally followed by:. A message with an empty human-readable part the plaintext is of zero length, or starts with a NUL is a 'heartbeat' packet, and should not be displayed to the user. A full justification for its use in OTR is made by Alexander and Goldberg, in a paper published in The following is a technical account of what is transmitted during the course of the protocol. The secret information x and y compared during this protocol contains not only information entered by the users, but also information unique to the conversation in which SMP takes place. Specifically, the format is:. The additional fields insure that not only do both parties know the same secret input string, but no man-in-the-middle is capable of reading their communication either. The SMP state consists of one main variable, as well as information from the partial computations at each protocol step. This variable has no effect on type 0 or type 1 TLVs, which are always allowed. The following sections outline what is to be done in each case. In the following actions, there are many places where a SHA hash of an integer followed by one or two MPIs is taken. The input to this hash function is:. It contains the following mpi values:. SMP message 2 is sent by Bob to complete the DH exchange to determine the new generators, g 2 and g 3. It also begins the construction of the values used in the final comparison of the protocol. Note that all DH key pairs should have a private part that is at least bits long. Otherwise, calculate the session keys as outlined below. If it does not verify, reject the message. Check that the counter in the Data message is strictly larger than the last counter you saw using this pair of keys. If not, reject the message. OTR uses Diffie-Hellman to calculate shared secrets in the usual way: Alice calculates g x y , and Bob calculates g y x. During the AKE, Alice and Bob each calculate s in this way, and then they each compute seven values based on s:. If the user transmitted the Reveal Signature message during the AKE that produced this ssid, then display the first 32 bits in bold, and the second 32 bits in non-bold. If the user transmitted the Signature message instead, display the first 32 bits in non-bold, and the second 32 bits in bold. This in done to allow the forgeability of OTR transcripts: Some networks may have a maximum message size that is too small to contain an encoded OTR message. In that event, the sender may choose to split the message into a number of fragments. This section describes the format of the fragments. All OTR version 2 clients must be able to assemble received fragments, but performing fragmentation on outgoing messages is optional. If you receive a non-OTR message, or an unfragmented message, forget any stored fragment you may have, store '' as F and store 0,0 as K,N. For example, here is a Data Message we would like to transmit over a network with an unreasonably small maximum message size:. An OTR client maintains separate state for every correspondent. For example, Alice may have an active OTR conversation with Bob, while having an unprotected conversation with Charlie. This state consists of two main state variables, as well as some other information such as encryption keys. The two main state variables are:. The message state variable, msgstate, controls what happens to outgoing messages typed by the user. It can take one of three values:. The authentication state variable, authstate, can take one of four values plus one extra for OTR version 1 compatibility:. OTR clients can set different policies for different correspondents. For example, Alice could set up her client so that it speaks only OTR version 2, except with Charlie, who she knows has only an old client; so that it will opportunistically start an OTR conversation whenever it detects the correspondent supports it; or so that it refuses to send non-encrypted messages to Bob, ever. The policies that can be set on a global or per-correspondent basis are any combination of the following boolean flags:. The four old version 1 policies correspond to the following combinations of flags adding an allowance for version 2 of the protocol:. Note that it is possible for UIs simply to offer the old 'combinations' of options, and not ask about each one separately. The following sections will outline what actions to take in each case.

Методон в Костереве

Otrv2

Безопасное общение при помощи Jabber + OTR.

Как шифровать переписку в Jabber: пошаговая инструкция

Руководство по OTR для Windows

Off-the-Record Messaging

ОТР онлайн в хорошем качестве.

Jabber + шифрование переписки OTR + шифрование трафика Tor на примере Pidgin

Настройка OTR шифрование Psi

Off-the-Record Messaging Protocol version 2

Настраиваем Pidgin + OTR + TOR (На примере Windows)

Безопасное общение при помощи Jabber + OTR.

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