From c07444d3e6826933800a621f81f8da15d855dcc7 Mon Sep 17 00:00:00 2001 From: Richard van der Hoff Date: Thu, 22 Sep 2016 11:04:19 +0100 Subject: First stab at a megolm spec --- .gitignore | 1 + Makefile | 1 + docs/megolm.rst | 218 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 3 files changed, 220 insertions(+) create mode 100644 docs/megolm.rst diff --git a/.gitignore b/.gitignore index aece1fb..067636a 100644 --- a/.gitignore +++ b/.gitignore @@ -1,5 +1,6 @@ /build /CHANGELOG.html +/docs/megolm.html /docs/olm.html /olm-*.tgz /README.html diff --git a/Makefile b/Makefile index 9d08c50..9a84cf4 100644 --- a/Makefile +++ b/Makefile @@ -44,6 +44,7 @@ JS_POST := javascript/olm_outbound_group_session.js \ javascript/olm_inbound_group_session.js \ javascript/olm_post.js DOCS := tracing/README.html \ + docs/megolm.html \ docs/olm.html \ README.html \ CHANGELOG.html diff --git a/docs/megolm.rst b/docs/megolm.rst new file mode 100644 index 0000000..24bceed --- /dev/null +++ b/docs/megolm.rst @@ -0,0 +1,218 @@ +Megolm group ratchet +==================== + +An AES-based cryptographic ratchet intended for group communications. + +The Megolm algorithm +-------------------- + +Initial setup +~~~~~~~~~~~~~ + +Each participant in a conversation generates their own Megolm session. A +session consists of three parts: a 32 bit counter, :math:`i`; an `Ed25519`_ +keypair, :math:`K`; and a ratchet, :math:`R_i`. The ratchet consists of four +256-bit values, :math:`R_{i,j}` for :math:`j \in {0,1,2,3}`. + +The counter :math:`i` is initialised to :math:`0`. A new Ed25519 keypair is +generated for :math:`K`. The ratchet is simply initialised with 1024 bits of +cryptographically-secure random data. + +A single participant may use multiple sessions over the lifetime of a +conversation. The public part of :math:`K` is used as an identifier to +discriminate between sessions. + +Sharing session data +~~~~~~~~~~~~~~~~~~~~ + +To allow other participants in the conversation to decrypt messages, the +session data is formatted as described in `Session-sharing format`_. It is then +shared with other participants in the conversation via a secure peer-to-peer +channel (such as that provided by `Olm`_). + +When the session data is received from other participants, the recipient first +checks that the signature matches the public key. They then store their own +copy of the counter, ratchet, and public key. + +Message encryption +~~~~~~~~~~~~~~~~~~ + +Megolm uses AES-256_ in CBC_ mode with `PCKS#7`_ padding for and HMAC-SHA-256_ +(truncated to 64 bits). The 256 bit AES key, 256 bit HMAC key, and 128 bit AES +IV are derived from the megolm ratchet :math:`R_i`: + +.. math:: + + \begin{align} + AES\_KEY_{i}\;\parallel\;HMAC\_KEY_{i}\;\parallel\;AES\_IV_{i} + &= HKDF\left(0,\,R_{i},\text{"MEGOLM\_KEYS"},\,80\right) \\ + \end{align} + +where :math:`\parallel` represents string splitting, and +:math:`HKDF\left(salt,\,IKM,\,info,\,L\right)` refers to the `HMAC-based key +derivation function`_ using using `SHA-256`_ as the hash function +(`HKDF-SHA-256`_) with a salt value of :math:`salt`, input key material of +:math:`IKM`, context string :math:`info`, and output keying material length of +:math:`L` bytes. + +The plain-text is encrypted with AES-256, using the key :math:`AES\_KEY_{i}` +and the IV :math:`AES\_IV_{i}` to give the cipher-text, :math:`X_{i}`. + +The ratchet index :math:`i`, and the cipher-text :math:`X_{i}`, are then packed +into a message as described in `Message format`_. Then the entire message +(including the version bytes and all payload bytes) are passed through +HMAC-SHA-256. The first 8 bytes of the MAC are appended to the message. + +Finally, the authenticated message is signed using the Ed25519 keypair; the 64 +byte signature is appended to the message. + +The complete signed message, together with the public part of :math:`K` (acting +as a session identifier), can then be sent over an insecure channel. The +message can then be authenticated and decrypted only by recipients who have +received the session data. + +Advancing the ratchet +~~~~~~~~~~~~~~~~~~~~~ + +After each message is encrypted, the ratchet is advanced. This is done as +follows: + +.. math:: + \begin{align} + R_{i,0} &= + \begin{cases} + HMAC\left(R_{2^24(n-1),0}, \text{"\textbackslash x00"}\right) + &\text{if }\exists n | i = 2^24n\\ + R_{i-1,0} &\text{otherwise} + \end{cases}\\ + R_{i,1} &= + \begin{cases} + HMAC\left(R_{2^24(n-1),0}, \text{"\textbackslash x01"}\right) + &\text{if }\exists n | i = 2^24n\\ + HMAC\left(R_{2^16(m-1),1}, \text{"\textbackslash x01"}\right) + &\text{if }\exists m | i = 2^16m\\ + R_{i-1,1} &\text{otherwise} + \end{cases}\\ + R_{i,2} &= + \begin{cases} + HMAC\left(R_{2^24(n-1),0}, \text{"\textbackslash x02"}\right) + &\text{if }\exists n | i = 2^24n\\ + HMAC\left(R_{2^16(m-1),1}, \text{"\textbackslash x02"}\right) + &\text{if }\exists m | i = 2^16m\\ + HMAC\left(R_{2^8(p-1),2}, \text{"\textbackslash x02"}\right) + &\text{if }\exists p | i = 2^8p\\ + R_{i-1,2} &\text{otherwise} + \end{cases}\\ + R_{i,3} &= + \begin{cases} + HMAC\left(R_{2^24(n-1),0}, \text{"\textbackslash x03"}\right) + &\text{if }\exists n | i = 2^24n\\ + HMAC\left(R_{2^16(m-1),1}, \text{"\textbackslash x03"}\right) + &\text{if }\exists m | i = 2^16m\\ + HMAC\left(R_{2^8(p-1),2}, \text{"\textbackslash x03"}\right) + &\text{if }\exists p | i = 2^8p\\ + HMAC\left(R_{i-1,3}, \text{"\textbackslash x03"}\right) + &\text{otherwise} + \end{cases} + \end{align} + +where :math:`HMAC(K, T)` is the HMAC-SHA-256_ of ``T``, using ``K`` as the +key. In summary: every :math:`2^8` iterations, :math:`R_{i,3}` is reseeded from +:math:`R_{i,2}`. Every :math:`2^16` iterations, :math:`R_{i,2}` and +:math:`R_{i,3}` are reseeded from :math:`R_{i,1}`. Every :math:`2^24` +iterations, :math:`R_{i,1}`, :math:`R_{i,2}` and :math:`R_{i,3}` are reseeded +from :math:`R_{i,0}`. + +This scheme allows the ratchet to be advanced an arbitrary amount forwards +while needing at most 1023 hash computations. A recipient can decrypt +conversation history onwards from the earliest value of the ratchet it is aware +of, but cannot decrypt history from before that point without reversing the +hash function. + +For outbound sessions, the updated ratchet and counter are stored in the +session. + +In order to maintain the ability to decrypt conversation history, inbound +sessions should store a copy of their earliet known ratchet value (unless they +explicitly want to drop the ability to decrypt that history). They may also +choose to cache calculated ratchet values, but the decision of which ratchet +states to cache is left to the application. + +Data exchange formats +--------------------- + +Session-sharing format +~~~~~~~~~~~~~~~~~~~~~~ + +The Megolm key-sharing format is as follows: + +.. code:: + + +---+----+--------+--------+--------+--------+------+-----------+ + | V | i | R(i,0) | R(i,1) | R(i,2) | R(i,3) | Kpub | Signature | + +---+----+--------+--------+--------+--------+------+-----------+ + 0 1 5 37 69 101 133 165 229 + +The version byte, ``V``, is ``"\x02"``. + +This is followed by the ratchet index, :math:`i`, which is encoded as a +big-endian 32-bit integer; the ratchet values :math:`R_{i,j}`; and the public +part of the Ed25519 keypair :math:`K`. + +The data is then signed using the Ed25519 keypair, and the 64-byte signature is +appended. + +Message format +~~~~~~~~~~~~~~ + +Megolm messages consist of a one byte version, followed by a variable length +payload, a fixed length message authentication code, and a fixed length +signature. + +.. code:: + + +---+------------------------------------+-----------+------------------+ + | V | Payload Bytes | MAC Bytes | Signature Bytes | + +---+------------------------------------+-----------+------------------+ + 0 1 N N+8 N+72 + +The version byte, ``V``, is ``"\x03"``. + +The payload consists of key-value pairs where the keys are integers and the +values are integers and strings. The keys are encoded as a variable length +integer tag where the 3 lowest bits indicates the type of the value: +0 for integers, 2 for strings. If the value is an integer then the tag is +followed by the value encoded as a variable length integer. If the value is +a string then the tag is followed by the length of the string encoded as +a variable length integer followed by the string itself. + +Olm uses a variable length encoding for integers. Each integer is encoded as a +sequence of bytes with the high bit set followed by a byte with the high bit +clear. The seven low bits of each byte store the bits of the integer. The least +significant bits are stored in the first byte. + +============= ===== ======== ================================================ + Name Tag Type Meaning +============= ===== ======== ================================================ +Message-Index 0x08 Integer The index of the ratchet, :math:`i` +Cipher-Text 0x12 String The cipher-text, :math:`X_{i}`, of the message +============= ===== ======== ================================================ + +The length of the MAC is determined by the authenticated encryption algorithm +being used (8 bytes in this version of the protocol). The MAC protects all of +the bytes preceding the MAC. + +The length of the signature is determined by the signing algorithm being used +(64 bytes in this version of the protocol). The signature covers all of the +bytes preceding the signaure. + + +.. _`Ed25519`: http://ed25519.cr.yp.to/ +.. _`HMAC-based key derivation function`: https://tools.ietf.org/html/rfc5869 +.. _`HKDF-SHA-256`: https://tools.ietf.org/html/rfc5869 +.. _`HMAC-SHA-256`: https://tools.ietf.org/html/rfc2104 +.. _`SHA-256`: https://tools.ietf.org/html/rfc6234 +.. _`AES-256`: http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf +.. _`CBC`: http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf +.. _`PCKS#7`: https://tools.ietf.org/html/rfc2315 +.. _`Olm`: ./olm.html -- cgit v1.2.3 From ec91dd45705020c02edfe0d2593f8fb39085cb6a Mon Sep 17 00:00:00 2001 From: Richard van der Hoff Date: Thu, 22 Sep 2016 11:35:23 +0100 Subject: Megolm.rst: add Background and footer --- docs/megolm.rst | 39 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 39 insertions(+) diff --git a/docs/megolm.rst b/docs/megolm.rst index 24bceed..9d49aef 100644 --- a/docs/megolm.rst +++ b/docs/megolm.rst @@ -3,6 +3,35 @@ Megolm group ratchet An AES-based cryptographic ratchet intended for group communications. +Background +---------- + +The Megolm ratchet is intended for encrypted messaging applications where there +may be a large number of recipients of each message, thus precluding the use of +peer-to-peer encryption systems such as `Olm`_. + +It also allows a receipient to decrypt received messages multiple times. For +instance, in client/server applications, a copy of the ciphertext can be stored +on the (untrusted) server, while the client need only store the session keys. + +Overview +-------- + +Each participant in a conversation uses their own session, which consists of a +ratchet, and an Ed25519 keypair. + +Secrecy is provided by the ratchet, which can be wound forwards, via hash +functions, but not backwards, and is used to derive a distinct message key +for each message. + +Authenticity is provided via the Ed25519 key. + +The value of the ratchet, and the public part of the Ed25519 key, are shared +with other participants in the conversation via secure peer-to-peer +channels. Provided that peer-to-peer channel provides authenticity of the +messages to the participants and deniability of the messages to third parties, +the Megolm session will inherit those properties. + The Megolm algorithm -------------------- @@ -206,6 +235,16 @@ The length of the signature is determined by the signing algorithm being used (64 bytes in this version of the protocol). The signature covers all of the bytes preceding the signaure. +IPR +--- + +The Megolm specification (this document) is hereby placed in the public domain. + +Feedback +-------- + +Can be sent to richard at matrix.org. + .. _`Ed25519`: http://ed25519.cr.yp.to/ .. _`HMAC-based key derivation function`: https://tools.ietf.org/html/rfc5869 -- cgit v1.2.3 From 182eccc624b8f520c9815f0441a64313c218b777 Mon Sep 17 00:00:00 2001 From: Richard van der Hoff Date: Thu, 22 Sep 2016 11:38:33 +0100 Subject: megolm.rst: linkify Ed25519 --- docs/megolm.rst | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/docs/megolm.rst b/docs/megolm.rst index 9d49aef..4043357 100644 --- a/docs/megolm.rst +++ b/docs/megolm.rst @@ -18,7 +18,7 @@ Overview -------- Each participant in a conversation uses their own session, which consists of a -ratchet, and an Ed25519 keypair. +ratchet, and an `Ed25519`_ keypair. Secrecy is provided by the ratchet, which can be wound forwards, via hash functions, but not backwards, and is used to derive a distinct message key -- cgit v1.2.3 From fc6688c4c841c45dd1057e50d91b4e858def9540 Mon Sep 17 00:00:00 2001 From: Richard van der Hoff Date: Thu, 22 Sep 2016 13:32:03 +0100 Subject: megolm.rst: review feedback Split ratchet algorithm out to a separate section. Also clean up some phrasing and correct a typo or two. --- docs/megolm.rst | 140 ++++++++++++++++++++++++++++++++------------------------ 1 file changed, 79 insertions(+), 61 deletions(-) diff --git a/docs/megolm.rst b/docs/megolm.rst index 4043357..a103e97 100644 --- a/docs/megolm.rst +++ b/docs/megolm.rst @@ -3,6 +3,8 @@ Megolm group ratchet An AES-based cryptographic ratchet intended for group communications. +.. contents:: + Background ---------- @@ -18,13 +20,12 @@ Overview -------- Each participant in a conversation uses their own session, which consists of a -ratchet, and an `Ed25519`_ keypair. +ratchet and an `Ed25519`_ keypair. -Secrecy is provided by the ratchet, which can be wound forwards, via hash -functions, but not backwards, and is used to derive a distinct message key -for each message. +Secrecy is provided by the ratchet, which can be wound forwards but not +backwards, and is used to derive a distinct message key for each message. -Authenticity is provided via the Ed25519 key. +Authenticity is provided via Ed25519 signatures. The value of the ratchet, and the public part of the Ed25519 key, are shared with other participants in the conversation via secure peer-to-peer @@ -32,10 +33,68 @@ channels. Provided that peer-to-peer channel provides authenticity of the messages to the participants and deniability of the messages to third parties, the Megolm session will inherit those properties. -The Megolm algorithm --------------------- +The Megolm ratchet algorithm +---------------------------- + +The Megolm ratchet :math:`R_i` consists of four parts, :math:`R_{i,j}` for +:math:`j \in {0,1,2,3}`. The length of each part depends on the hash function +in use (256 bits for this version of Megolm). + +The ratchet is initialised with cryptographically-secure random data, and +advanced as follows: + +.. math:: + \begin{align} + R_{i,0} &= + \begin{cases} + H_0\left(R_{2^24(n-1),0}\right) &\text{if }\exists n | i = 2^24n\\ + R_{i-1,0} &\text{otherwise} + \end{cases}\\ + R_{i,1} &= + \begin{cases} + H_1\left(R_{2^24(n-1),0}\right) &\text{if }\exists n | i = 2^24n\\ + H_1\left(R_{2^16(m-1),1}\right) &\text{if }\exists m | i = 2^16m\\ + R_{i-1,1} &\text{otherwise} + \end{cases}\\ + R_{i,2} &= + \begin{cases} + H_2\left(R_{2^24(n-1),0}\right) &\text{if }\exists n | i = 2^24n\\ + H_2\left(R_{2^16(m-1),1}\right) &\text{if }\exists m | i = 2^16m\\ + H_2\left(R_{2^8(p-1),2}\right) &\text{if }\exists p | i = 2^8p\\ + R_{i-1,2} &\text{otherwise} + \end{cases}\\ + R_{i,3} &= + \begin{cases} + H_3\left(R_{2^24(n-1),0}\right) &\text{if }\exists n | i = 2^24n\\ + H_3\left(R_{2^16(m-1),1}\right) &\text{if }\exists m | i = 2^16m\\ + H_3\left(R_{2^8(p-1),2}\right) &\text{if }\exists p | i = 2^8p\\ + H_3\left(R_{i-1,3}\right) &\text{otherwise} + \end{cases} + \end{align} -Initial setup +where :math:`H_0`, :math:`H_1`, :math:`H_2`, and :math:`H_3` are different hash +functions. In summary: every :math:`2^8` iterations, :math:`R_{i,3}` is +reseeded from :math:`R_{i,2}`. Every :math:`2^16` iterations, :math:`R_{i,2}` +and :math:`R_{i,3}` are reseeded from :math:`R_{i,1}`. Every :math:`2^24` +iterations, :math:`R_{i,1}`, :math:`R_{i,2}` and :math:`R_{i,3}` are reseeded +from :math:`R_{i,0}`. + +The complete ratchet value, :math:`R_{i}`, is hashed to generate the keys used +to encrypt each mesage. This scheme allows the ratchet to be advanced an +arbitrary amount forwards while needing at most 1023 hash computations. A +client can decrypt chat history onwards from the earliest value of the ratchet +it is aware of, but cannot decrypt history from before that point without +reversing the hash function. + +This allows a participant to share its ability to decrypt chat history with +another from a point in the conversation onwards by giving a copy of the +ratchet at that point in the conversation. + + +The Megolm protocol +------------------- + +Session setup ~~~~~~~~~~~~~ Each participant in a conversation generates their own Megolm session. A @@ -66,9 +125,9 @@ copy of the counter, ratchet, and public key. Message encryption ~~~~~~~~~~~~~~~~~~ -Megolm uses AES-256_ in CBC_ mode with `PCKS#7`_ padding for and HMAC-SHA-256_ -(truncated to 64 bits). The 256 bit AES key, 256 bit HMAC key, and 128 bit AES -IV are derived from the megolm ratchet :math:`R_i`: +This version of Megolm uses AES-256_ in CBC_ mode with `PCKS#7`_ padding and +HMAC-SHA-256_ (truncated to 64 bits). The 256 bit AES key, 256 bit HMAC key, +and 128 bit AES IV are derived from the megolm ratchet :math:`R_i`: .. math:: @@ -104,59 +163,18 @@ Advancing the ratchet ~~~~~~~~~~~~~~~~~~~~~ After each message is encrypted, the ratchet is advanced. This is done as -follows: +described in `The Megolm ratchet algorithm`_, using the following definitions: .. math:: \begin{align} - R_{i,0} &= - \begin{cases} - HMAC\left(R_{2^24(n-1),0}, \text{"\textbackslash x00"}\right) - &\text{if }\exists n | i = 2^24n\\ - R_{i-1,0} &\text{otherwise} - \end{cases}\\ - R_{i,1} &= - \begin{cases} - HMAC\left(R_{2^24(n-1),0}, \text{"\textbackslash x01"}\right) - &\text{if }\exists n | i = 2^24n\\ - HMAC\left(R_{2^16(m-1),1}, \text{"\textbackslash x01"}\right) - &\text{if }\exists m | i = 2^16m\\ - R_{i-1,1} &\text{otherwise} - \end{cases}\\ - R_{i,2} &= - \begin{cases} - HMAC\left(R_{2^24(n-1),0}, \text{"\textbackslash x02"}\right) - &\text{if }\exists n | i = 2^24n\\ - HMAC\left(R_{2^16(m-1),1}, \text{"\textbackslash x02"}\right) - &\text{if }\exists m | i = 2^16m\\ - HMAC\left(R_{2^8(p-1),2}, \text{"\textbackslash x02"}\right) - &\text{if }\exists p | i = 2^8p\\ - R_{i-1,2} &\text{otherwise} - \end{cases}\\ - R_{i,3} &= - \begin{cases} - HMAC\left(R_{2^24(n-1),0}, \text{"\textbackslash x03"}\right) - &\text{if }\exists n | i = 2^24n\\ - HMAC\left(R_{2^16(m-1),1}, \text{"\textbackslash x03"}\right) - &\text{if }\exists m | i = 2^16m\\ - HMAC\left(R_{2^8(p-1),2}, \text{"\textbackslash x03"}\right) - &\text{if }\exists p | i = 2^8p\\ - HMAC\left(R_{i-1,3}, \text{"\textbackslash x03"}\right) - &\text{otherwise} - \end{cases} + H_0(A) &\equiv HMAC(A,\text{"\textbackslash x00"}) \\ + H_1(A) &\equiv HMAC(A,\text{"\textbackslash x01"}) \\ + H_2(A) &\equiv HMAC(A,\text{"\textbackslash x02"}) \\ + H_3(A) &\equiv HMAC(A,\text{"\textbackslash x03"}) \\ \end{align} -where :math:`HMAC(K, T)` is the HMAC-SHA-256_ of ``T``, using ``K`` as the -key. In summary: every :math:`2^8` iterations, :math:`R_{i,3}` is reseeded from -:math:`R_{i,2}`. Every :math:`2^16` iterations, :math:`R_{i,2}` and -:math:`R_{i,3}` are reseeded from :math:`R_{i,1}`. Every :math:`2^24` -iterations, :math:`R_{i,1}`, :math:`R_{i,2}` and :math:`R_{i,3}` are reseeded -from :math:`R_{i,0}`. - -This scheme allows the ratchet to be advanced an arbitrary amount forwards -while needing at most 1023 hash computations. A recipient can decrypt -conversation history onwards from the earliest value of the ratchet it is aware -of, but cannot decrypt history from before that point without reversing the -hash function. +where :math:`HMAC(A, T)` is the HMAC-SHA-256_ of ``T``, using ``A`` as the +key. For outbound sessions, the updated ratchet and counter are stored in the session. @@ -215,8 +233,8 @@ followed by the value encoded as a variable length integer. If the value is a string then the tag is followed by the length of the string encoded as a variable length integer followed by the string itself. -Olm uses a variable length encoding for integers. Each integer is encoded as a -sequence of bytes with the high bit set followed by a byte with the high bit +Megolm uses a variable length encoding for integers. Each integer is encoded as +a sequence of bytes with the high bit set followed by a byte with the high bit clear. The seven low bits of each byte store the bits of the integer. The least significant bits are stored in the first byte. -- cgit v1.2.3 From 6449c90d8170577d3c488cd3722f3f4048632c24 Mon Sep 17 00:00:00 2001 From: Richard van der Hoff Date: Thu, 22 Sep 2016 18:23:43 +0100 Subject: More review feedback --- docs/megolm.rst | 77 +++++++++++++++++++++++++++++++++------------------------ 1 file changed, 45 insertions(+), 32 deletions(-) diff --git a/docs/megolm.rst b/docs/megolm.rst index a103e97..6d20cff 100644 --- a/docs/megolm.rst +++ b/docs/megolm.rst @@ -1,3 +1,18 @@ +.. Copyright 2016 OpenMarket Ltd +.. +.. Licensed under the Apache License, Version 2.0 (the "License"); +.. you may not use this file except in compliance with the License. +.. You may obtain a copy of the License at +.. +.. http://www.apache.org/licenses/LICENSE-2.0 +.. +.. Unless required by applicable law or agreed to in writing, software +.. distributed under the License is distributed on an "AS IS" BASIS, +.. WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +.. See the License for the specific language governing permissions and +.. limitations under the License. + + Megolm group ratchet ==================== @@ -19,8 +34,8 @@ on the (untrusted) server, while the client need only store the session keys. Overview -------- -Each participant in a conversation uses their own session, which consists of a -ratchet and an `Ed25519`_ keypair. +Each participant in a conversation uses their own outbound session for +encrypting messages. A session consists of a ratchet and an `Ed25519`_ keypair. Secrecy is provided by the ratchet, which can be wound forwards but not backwards, and is used to derive a distinct message key for each message. @@ -80,7 +95,7 @@ iterations, :math:`R_{i,1}`, :math:`R_{i,2}` and :math:`R_{i,3}` are reseeded from :math:`R_{i,0}`. The complete ratchet value, :math:`R_{i}`, is hashed to generate the keys used -to encrypt each mesage. This scheme allows the ratchet to be advanced an +to encrypt each message. This scheme allows the ratchet to be advanced an arbitrary amount forwards while needing at most 1023 hash computations. A client can decrypt chat history onwards from the earliest value of the ratchet it is aware of, but cannot decrypt history from before that point without @@ -98,9 +113,12 @@ Session setup ~~~~~~~~~~~~~ Each participant in a conversation generates their own Megolm session. A -session consists of three parts: a 32 bit counter, :math:`i`; an `Ed25519`_ -keypair, :math:`K`; and a ratchet, :math:`R_i`. The ratchet consists of four -256-bit values, :math:`R_{i,j}` for :math:`j \in {0,1,2,3}`. +session consists of three parts: + +* a 32 bit counter, :math:`i`. +* an `Ed25519`_ keypair, :math:`K`. +* a ratchet, :math:`R_i`, which consists of four 256-bit values, + :math:`R_{i,j}` for :math:`j \in {0,1,2,3}`. The counter :math:`i` is initialised to :math:`0`. A new Ed25519 keypair is generated for :math:`K`. The ratchet is simply initialised with 1024 bits of @@ -180,7 +198,7 @@ For outbound sessions, the updated ratchet and counter are stored in the session. In order to maintain the ability to decrypt conversation history, inbound -sessions should store a copy of their earliet known ratchet value (unless they +sessions should store a copy of their earliest known ratchet value (unless they explicitly want to drop the ability to decrypt that history). They may also choose to cache calculated ratchet values, but the decision of which ratchet states to cache is left to the application. @@ -198,7 +216,7 @@ The Megolm key-sharing format is as follows: +---+----+--------+--------+--------+--------+------+-----------+ | V | i | R(i,0) | R(i,1) | R(i,2) | R(i,3) | Kpub | Signature | +---+----+--------+--------+--------+--------+------+-----------+ - 0 1 5 37 69 101 133 165 229 + 0 1 5 37 69 101 133 165 229 bytes The version byte, ``V``, is ``"\x02"``. @@ -221,22 +239,11 @@ signature. +---+------------------------------------+-----------+------------------+ | V | Payload Bytes | MAC Bytes | Signature Bytes | +---+------------------------------------+-----------+------------------+ - 0 1 N N+8 N+72 + 0 1 N N+8 N+72 bytes The version byte, ``V``, is ``"\x03"``. -The payload consists of key-value pairs where the keys are integers and the -values are integers and strings. The keys are encoded as a variable length -integer tag where the 3 lowest bits indicates the type of the value: -0 for integers, 2 for strings. If the value is an integer then the tag is -followed by the value encoded as a variable length integer. If the value is -a string then the tag is followed by the length of the string encoded as -a variable length integer followed by the string itself. - -Megolm uses a variable length encoding for integers. Each integer is encoded as -a sequence of bytes with the high bit set followed by a byte with the high bit -clear. The seven low bits of each byte store the bits of the integer. The least -significant bits are stored in the first byte. +The payload consists of the following key-value pairs: ============= ===== ======== ================================================ Name Tag Type Meaning @@ -245,23 +252,29 @@ Message-Index 0x08 Integer The index of the ratchet, :math:`i` Cipher-Text 0x12 String The cipher-text, :math:`X_{i}`, of the message ============= ===== ======== ================================================ -The length of the MAC is determined by the authenticated encryption algorithm -being used (8 bytes in this version of the protocol). The MAC protects all of -the bytes preceding the MAC. +Within the payload, integers are encoded using a variable length encoding. Each +integer is encoded as a sequence of bytes with the high bit set followed by a +byte with the high bit clear. The seven low bits of each byte store the bits of +the integer. The least significant bits are stored in the first byte. + +Strings are encoded as a variable-length integer followed by the string itself. + +Each key-value pair is encoded as a variable-length integer giving the tag, +followed by a string or variable-length integer giving the value. + +The payload is followed by the MAC. The length of the MAC is determined by the +authenticated encryption algorithm being used (8 bytes in this version of the +protocol). The MAC protects all of the bytes preceding the MAC. The length of the signature is determined by the signing algorithm being used (64 bytes in this version of the protocol). The signature covers all of the bytes preceding the signaure. -IPR ---- - -The Megolm specification (this document) is hereby placed in the public domain. - -Feedback --------- +License +------- -Can be sent to richard at matrix.org. +The Megolm specification (this document) is licensed under the `Apache License, +Version 2.0 `_. .. _`Ed25519`: http://ed25519.cr.yp.to/ -- cgit v1.2.3 From 4049261867fc1a4d9e7689635788d538a09aab31 Mon Sep 17 00:00:00 2001 From: Richard van der Hoff Date: Fri, 23 Sep 2016 11:54:35 +0100 Subject: megolm.rst: link to protobuf spec --- docs/megolm.rst | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/docs/megolm.rst b/docs/megolm.rst index 6d20cff..7853963 100644 --- a/docs/megolm.rst +++ b/docs/megolm.rst @@ -243,7 +243,8 @@ signature. The version byte, ``V``, is ``"\x03"``. -The payload consists of the following key-value pairs: +The payload uses a format based on the `Protocol Buffers encoding`_. It +consists of the following key-value pairs: ============= ===== ======== ================================================ Name Tag Type Meaning @@ -286,3 +287,4 @@ Version 2.0 `_. .. _`CBC`: http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf .. _`PCKS#7`: https://tools.ietf.org/html/rfc2315 .. _`Olm`: ./olm.html +.. _`Protocol Buffers encoding`: https://developers.google.com/protocol-buffers/docs/encoding -- cgit v1.2.3