Loading docs/megolm.rst→docs/megolm.md +325 −0 Original line number Diff line number Diff line .. 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 ==================== # Megolm group ratchet An AES-based cryptographic ratchet intended for group communications. .. contents:: Background ---------- ## 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`_. peer-to-peer encryption systems such as [Olm][]. It also allows a recipient 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 -------- ## Overview Each participant in a conversation uses their own outbound session for encrypting messages. A session consists of a ratchet and an `Ed25519`_ keypair. 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. Loading @@ -48,18 +28,17 @@ 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 ratchet 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 The Megolm ratchet $`R_i`$ consists of four parts, $`R_{i,j}`$ for $`j \in {0,1,2,3}`$. 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} ```math \begin{aligned} R_{i,0} &= \begin{cases} H_0\left(R_{2^24(n-1),0}\right) &\text{if }\exists n | i = 2^24n\\ Loading @@ -85,16 +64,17 @@ advanced as follows: 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} \end{aligned} ``` 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}`. where $`H_0`$, $`H_1`$, $`H_2`$, and $`H_3`$ are different hash functions. In summary: every $`2^8`$ iterations, $`R_{i,3}`$ is reseeded from $`R_{i,2}`$. Every $`2^16`$ iterations, $`R_{i,2}`$ and $`R_{i,3}`$ are reseeded from $`R_{i,1}`$. Every $`2^24`$ iterations, $`R_{i,1}`$, $`R_{i,2}`$ and $`R_{i,3}`$ are reseeded from $`R_{i,0}`$. The complete ratchet value, :math:`R_{i}`, is hashed to generate the keys used The complete ratchet value, $`R_{i}`$, is hashed to generate the keys used 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 Loading @@ -106,92 +86,88 @@ another from a point in the conversation onwards by giving a copy of the ratchet at that point in the conversation. The Megolm protocol ------------------- ## The Megolm protocol Session setup ~~~~~~~~~~~~~ ### 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`. * a ratchet, :math:`R_i`, which consists of four 256-bit values, :math:`R_{i,j}` for :math:`j \in {0,1,2,3}`. * a 32 bit counter, $`i`$. * an [Ed25519][] keypair, $`K`$. * a ratchet, $`R_i`$, which consists of four 256-bit values, $`R_{i,j}`$ for $`j \in {0,1,2,3}`$. 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 The counter $`i`$ is initialised to $`0`$. A new Ed25519 keypair is generated for $`K`$. 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 conversation. The public part of $`K`$ is used as an identifier to discriminate between sessions. Sharing session data ~~~~~~~~~~~~~~~~~~~~ ### 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 session data is formatted as described in [Session-sharing format](#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`_). 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 ~~~~~~~~~~~~~~~~~~ ### Message encryption This version of Megolm uses AES-256_ in CBC_ mode with `PKCS#7`_ padding and This version of Megolm uses AES-256_ in CBC_ mode with [PKCS#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:: and 128 bit AES IV are derived from the megolm ratchet $`R_i`$: \begin{align} ```math \begin{aligned} AES\_KEY_{i}\;\parallel\;HMAC\_KEY_{i}\;\parallel\;AES\_IV_{i} &= HKDF\left(0,\,R_{i},\text{"MEGOLM\_KEYS"},\,80\right) \\ \end{align} \end{aligned} ``` 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. where $`\parallel`$ represents string splitting, and $`HKDF\left(salt,\,IKM,\,info,\,L\right)`$ refers to the [HMAC-based key derivation function][] using using [SHA-256][] as the hash function ([HKDF-SHA-256][]) with a salt value of $`salt`$, input key material of $`IKM`$, context string $`info`$, and output keying material length of $`L`$ 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 plain-text is encrypted with AES-256, using the key $`AES\_KEY_{i}`$ and the IV $`AES\_IV_{i}`$ to give the cipher-text, $`X_{i}`$. 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 The ratchet index $`i`$, and the cipher-text $`X_{i}`$, are then packed into a message as described in [Message format](#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 The complete signed message, together with the public part of $`K`$ (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 ~~~~~~~~~~~~~~~~~~~~~ ### Advancing the ratchet After each message is encrypted, the ratchet is advanced. This is done as described in `The Megolm ratchet algorithm`_, using the following definitions: .. math:: \begin{align} 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(A, T)` is the HMAC-SHA-256_ of ``T``, using ``A`` as the described in [The Megolm ratchet algorithm](#the-megolm-ratchet-algorithm), using the following definitions: ```math \begin{aligned} H_0(A) &\equiv HMAC(A,\text{"\x00"}) \\ H_1(A) &\equiv HMAC(A,\text{"\x01"}) \\ H_2(A) &\equiv HMAC(A,\text{"\x02"}) \\ H_3(A) &\equiv HMAC(A,\text{"\x03"}) \\ \end{aligned} ``` where $`HMAC(A, T)`$ is the HMAC-SHA-256 of ``T``, using ``A`` as the key. For outbound sessions, the updated ratchet and counter are stored in the Loading @@ -199,59 +175,54 @@ session. In order to maintain the ability to decrypt conversation history, inbound sessions should store a copy of their earliest known ratchet value (unless they explicitly want to drop the ability to decrypt that history - see `Partial Forward Secrecy`_\ ). They may also choose to cache calculated ratchet values, explicitly want to drop the ability to decrypt that history - see [Partial Forward Secrecy](#partial-forward-secrecy)). 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 --------------------- ## Data exchange formats Session-sharing format ~~~~~~~~~~~~~~~~~~~~~~ ### 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 bytes ``` 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`. This is followed by the ratchet index, $`i`$, which is encoded as a big-endian 32-bit integer; the ratchet values $`R_{i,j}`$; and the public part of the Ed25519 keypair $`K`$. The data is then signed using the Ed25519 keypair, and the 64-byte signature is appended. Message format ~~~~~~~~~~~~~~ ### 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 bytes ``` The version byte, ``V``, is ``"\x03"``. The payload uses a format based on the `Protocol Buffers encoding`_. It The payload uses a format based on the [Protocol Buffers encoding][]. It consists of the following key-value pairs: ============= ===== ======== ================================================ 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 ============= ===== ======== ================================================ **Name**|**Tag**|**Type**|**Meaning** :-----:|:-----:|:-----:|:-----: Message-Index|0x08|Integer|The index of the ratchet, i Cipher-Text|0x12|String|The cipher-text, Xi, of the message 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 Loading @@ -271,11 +242,9 @@ 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 signature. Limitations ----------- ## Limitations Message Replays --------------- ### Message Replays A message can be decrypted successfully multiple times. This means that an attacker can re-send a copy of an old message, and the recipient will treat it Loading @@ -285,8 +254,7 @@ To mitigate this it is recommended that applications track the ratchet indices they have received and that they reject messages with a ratchet index that they have already decrypted. Lack of Transcript Consistency ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ### Lack of Transcript Consistency In a group conversation, there is no guarantee that all recipients have received the same messages. For example, if Alice is in a conversation with Bob Loading @@ -297,8 +265,7 @@ Solving this is, in general, a hard problem, particularly in a protocol which does not guarantee in-order message delivery. For now it remains the subject of future research. Lack of Backward Secrecy ~~~~~~~~~~~~~~~~~~~~~~~~ ### Lack of Backward Secrecy Once the key to a Megolm session is compromised, the attacker can decrypt any future messages sent via that session. Loading @@ -307,11 +274,10 @@ In order to mitigate this, the application should ensure that Megolm sessions are not used indefinitely. Instead it should periodically start a new session, with new keys shared over a secure channel. .. TODO: Can we recommend sensible lifetimes for Megolm sessions? Probably depends how paranoid we're feeling, but some guidelines might be useful. <!-- TODO: Can we recommend sensible lifetimes for Megolm sessions? Probably depends how paranoid we're feeling, but some guidelines might be useful. --> Partial Forward Secrecy ~~~~~~~~~~~~~~~~~~~~~~~ ### Partial Forward Secrecy Each recipient maintains a record of the ratchet value which allows them to decrypt any messages sent in the session after the corresponding point in the Loading @@ -322,8 +288,7 @@ To mitigate this issue, the application should offer the user the option to discard historical conversations, by winding forward any stored ratchet values, or discarding sessions altogether. Dependency on secure channel for key exchange ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ### Dependency on secure channel for key exchange The design of the Megolm ratchet relies on the availability of a secure peer-to-peer channel for the exchange of session keys. Any vulnerabilities in Loading @@ -343,20 +308,18 @@ the Megolm session as a forgery. A second example: if the peer-to-peer channel is vulnerable to a replay attack, this can be extended to entire Megolm sessions. License ------- The Megolm specification (this document) is licensed under the `Apache License, Version 2.0 <http://www.apache.org/licenses/LICENSE-2.0>`_. .. _`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 .. _`PKCS#7`: https://tools.ietf.org/html/rfc2315 .. _`Olm`: ./olm.html .. _`Protocol Buffers encoding`: https://developers.google.com/protocol-buffers/docs/encoding ## License The Megolm specification (this document) is licensed under the Apache License, Version 2.0 http://www.apache.org/licenses/LICENSE-2.0. [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 [PKCS#7]: https://tools.ietf.org/html/rfc2315 [Olm]: https://gitlab.matrix.org/matrix-org/olm/blob/master/docs/olm.md [Protocol Buffers encoding]: https://developers.google.com/protocol-buffers/docs/encoding Loading
docs/megolm.rst→docs/megolm.md +325 −0 Original line number Diff line number Diff line .. 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 ==================== # Megolm group ratchet An AES-based cryptographic ratchet intended for group communications. .. contents:: Background ---------- ## 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`_. peer-to-peer encryption systems such as [Olm][]. It also allows a recipient 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 -------- ## Overview Each participant in a conversation uses their own outbound session for encrypting messages. A session consists of a ratchet and an `Ed25519`_ keypair. 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. Loading @@ -48,18 +28,17 @@ 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 ratchet 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 The Megolm ratchet $`R_i`$ consists of four parts, $`R_{i,j}`$ for $`j \in {0,1,2,3}`$. 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} ```math \begin{aligned} R_{i,0} &= \begin{cases} H_0\left(R_{2^24(n-1),0}\right) &\text{if }\exists n | i = 2^24n\\ Loading @@ -85,16 +64,17 @@ advanced as follows: 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} \end{aligned} ``` 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}`. where $`H_0`$, $`H_1`$, $`H_2`$, and $`H_3`$ are different hash functions. In summary: every $`2^8`$ iterations, $`R_{i,3}`$ is reseeded from $`R_{i,2}`$. Every $`2^16`$ iterations, $`R_{i,2}`$ and $`R_{i,3}`$ are reseeded from $`R_{i,1}`$. Every $`2^24`$ iterations, $`R_{i,1}`$, $`R_{i,2}`$ and $`R_{i,3}`$ are reseeded from $`R_{i,0}`$. The complete ratchet value, :math:`R_{i}`, is hashed to generate the keys used The complete ratchet value, $`R_{i}`$, is hashed to generate the keys used 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 Loading @@ -106,92 +86,88 @@ another from a point in the conversation onwards by giving a copy of the ratchet at that point in the conversation. The Megolm protocol ------------------- ## The Megolm protocol Session setup ~~~~~~~~~~~~~ ### 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`. * a ratchet, :math:`R_i`, which consists of four 256-bit values, :math:`R_{i,j}` for :math:`j \in {0,1,2,3}`. * a 32 bit counter, $`i`$. * an [Ed25519][] keypair, $`K`$. * a ratchet, $`R_i`$, which consists of four 256-bit values, $`R_{i,j}`$ for $`j \in {0,1,2,3}`$. 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 The counter $`i`$ is initialised to $`0`$. A new Ed25519 keypair is generated for $`K`$. 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 conversation. The public part of $`K`$ is used as an identifier to discriminate between sessions. Sharing session data ~~~~~~~~~~~~~~~~~~~~ ### 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 session data is formatted as described in [Session-sharing format](#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`_). 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 ~~~~~~~~~~~~~~~~~~ ### Message encryption This version of Megolm uses AES-256_ in CBC_ mode with `PKCS#7`_ padding and This version of Megolm uses AES-256_ in CBC_ mode with [PKCS#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:: and 128 bit AES IV are derived from the megolm ratchet $`R_i`$: \begin{align} ```math \begin{aligned} AES\_KEY_{i}\;\parallel\;HMAC\_KEY_{i}\;\parallel\;AES\_IV_{i} &= HKDF\left(0,\,R_{i},\text{"MEGOLM\_KEYS"},\,80\right) \\ \end{align} \end{aligned} ``` 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. where $`\parallel`$ represents string splitting, and $`HKDF\left(salt,\,IKM,\,info,\,L\right)`$ refers to the [HMAC-based key derivation function][] using using [SHA-256][] as the hash function ([HKDF-SHA-256][]) with a salt value of $`salt`$, input key material of $`IKM`$, context string $`info`$, and output keying material length of $`L`$ 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 plain-text is encrypted with AES-256, using the key $`AES\_KEY_{i}`$ and the IV $`AES\_IV_{i}`$ to give the cipher-text, $`X_{i}`$. 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 The ratchet index $`i`$, and the cipher-text $`X_{i}`$, are then packed into a message as described in [Message format](#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 The complete signed message, together with the public part of $`K`$ (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 ~~~~~~~~~~~~~~~~~~~~~ ### Advancing the ratchet After each message is encrypted, the ratchet is advanced. This is done as described in `The Megolm ratchet algorithm`_, using the following definitions: .. math:: \begin{align} 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(A, T)` is the HMAC-SHA-256_ of ``T``, using ``A`` as the described in [The Megolm ratchet algorithm](#the-megolm-ratchet-algorithm), using the following definitions: ```math \begin{aligned} H_0(A) &\equiv HMAC(A,\text{"\x00"}) \\ H_1(A) &\equiv HMAC(A,\text{"\x01"}) \\ H_2(A) &\equiv HMAC(A,\text{"\x02"}) \\ H_3(A) &\equiv HMAC(A,\text{"\x03"}) \\ \end{aligned} ``` where $`HMAC(A, T)`$ is the HMAC-SHA-256 of ``T``, using ``A`` as the key. For outbound sessions, the updated ratchet and counter are stored in the Loading @@ -199,59 +175,54 @@ session. In order to maintain the ability to decrypt conversation history, inbound sessions should store a copy of their earliest known ratchet value (unless they explicitly want to drop the ability to decrypt that history - see `Partial Forward Secrecy`_\ ). They may also choose to cache calculated ratchet values, explicitly want to drop the ability to decrypt that history - see [Partial Forward Secrecy](#partial-forward-secrecy)). 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 --------------------- ## Data exchange formats Session-sharing format ~~~~~~~~~~~~~~~~~~~~~~ ### 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 bytes ``` 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`. This is followed by the ratchet index, $`i`$, which is encoded as a big-endian 32-bit integer; the ratchet values $`R_{i,j}`$; and the public part of the Ed25519 keypair $`K`$. The data is then signed using the Ed25519 keypair, and the 64-byte signature is appended. Message format ~~~~~~~~~~~~~~ ### 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 bytes ``` The version byte, ``V``, is ``"\x03"``. The payload uses a format based on the `Protocol Buffers encoding`_. It The payload uses a format based on the [Protocol Buffers encoding][]. It consists of the following key-value pairs: ============= ===== ======== ================================================ 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 ============= ===== ======== ================================================ **Name**|**Tag**|**Type**|**Meaning** :-----:|:-----:|:-----:|:-----: Message-Index|0x08|Integer|The index of the ratchet, i Cipher-Text|0x12|String|The cipher-text, Xi, of the message 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 Loading @@ -271,11 +242,9 @@ 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 signature. Limitations ----------- ## Limitations Message Replays --------------- ### Message Replays A message can be decrypted successfully multiple times. This means that an attacker can re-send a copy of an old message, and the recipient will treat it Loading @@ -285,8 +254,7 @@ To mitigate this it is recommended that applications track the ratchet indices they have received and that they reject messages with a ratchet index that they have already decrypted. Lack of Transcript Consistency ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ### Lack of Transcript Consistency In a group conversation, there is no guarantee that all recipients have received the same messages. For example, if Alice is in a conversation with Bob Loading @@ -297,8 +265,7 @@ Solving this is, in general, a hard problem, particularly in a protocol which does not guarantee in-order message delivery. For now it remains the subject of future research. Lack of Backward Secrecy ~~~~~~~~~~~~~~~~~~~~~~~~ ### Lack of Backward Secrecy Once the key to a Megolm session is compromised, the attacker can decrypt any future messages sent via that session. Loading @@ -307,11 +274,10 @@ In order to mitigate this, the application should ensure that Megolm sessions are not used indefinitely. Instead it should periodically start a new session, with new keys shared over a secure channel. .. TODO: Can we recommend sensible lifetimes for Megolm sessions? Probably depends how paranoid we're feeling, but some guidelines might be useful. <!-- TODO: Can we recommend sensible lifetimes for Megolm sessions? Probably depends how paranoid we're feeling, but some guidelines might be useful. --> Partial Forward Secrecy ~~~~~~~~~~~~~~~~~~~~~~~ ### Partial Forward Secrecy Each recipient maintains a record of the ratchet value which allows them to decrypt any messages sent in the session after the corresponding point in the Loading @@ -322,8 +288,7 @@ To mitigate this issue, the application should offer the user the option to discard historical conversations, by winding forward any stored ratchet values, or discarding sessions altogether. Dependency on secure channel for key exchange ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ### Dependency on secure channel for key exchange The design of the Megolm ratchet relies on the availability of a secure peer-to-peer channel for the exchange of session keys. Any vulnerabilities in Loading @@ -343,20 +308,18 @@ the Megolm session as a forgery. A second example: if the peer-to-peer channel is vulnerable to a replay attack, this can be extended to entire Megolm sessions. License ------- The Megolm specification (this document) is licensed under the `Apache License, Version 2.0 <http://www.apache.org/licenses/LICENSE-2.0>`_. .. _`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 .. _`PKCS#7`: https://tools.ietf.org/html/rfc2315 .. _`Olm`: ./olm.html .. _`Protocol Buffers encoding`: https://developers.google.com/protocol-buffers/docs/encoding ## License The Megolm specification (this document) is licensed under the Apache License, Version 2.0 http://www.apache.org/licenses/LICENSE-2.0. [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 [PKCS#7]: https://tools.ietf.org/html/rfc2315 [Olm]: https://gitlab.matrix.org/matrix-org/olm/blob/master/docs/olm.md [Protocol Buffers encoding]: https://developers.google.com/protocol-buffers/docs/encoding
Aaron Raimist <aaron@raim.ist>Aaron Raimist <aaron@raim.ist>