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# Megolm group ratchet
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An AES-based cryptographic ratchet intended for group communications.
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## Background
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The Megolm ratchet is intended for encrypted messaging applications where there
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may be a large number of recipients of each message, thus precluding the use of
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peer-to-peer encryption systems such as [Olm][].
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It also allows a recipient to decrypt received messages multiple times. For
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instance, in client/server applications, a copy of the ciphertext can be stored
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on the (untrusted) server, while the client need only store the session keys.
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## Overview
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Each participant in a conversation uses their own outbound session for
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encrypting messages. A session consists of a ratchet and an [Ed25519][] keypair.
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Secrecy is provided by the ratchet, which can be wound forwards but not
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backwards, and is used to derive a distinct message key for each message.
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Authenticity is provided via Ed25519 signatures.
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The value of the ratchet, and the public part of the Ed25519 key, are shared
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with other participants in the conversation via secure peer-to-peer
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channels. Provided that peer-to-peer channel provides authenticity of the
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messages to the participants and deniability of the messages to third parties,
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the Megolm session will inherit those properties.
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## The Megolm ratchet algorithm
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The Megolm ratchet $`R_i`$ consists of four parts, $`R_{i,j}`$ for
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$`j \in {0,1,2,3}`$. The length of each part depends on the hash function
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in use (256 bits for this version of Megolm).
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The ratchet is initialised with cryptographically-secure random data, and
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advanced as follows:
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```math
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\begin{aligned}
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R_{i,0} &=
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\begin{cases}
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H_0\left(R_{2^24(n-1),0}\right) &\text{if }\exists n | i = 2^24n\\
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R_{i-1,0} &\text{otherwise}
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\end{cases}\\
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R_{i,1} &=
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\begin{cases}
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H_1\left(R_{2^24(n-1),0}\right) &\text{if }\exists n | i = 2^24n\\
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H_1\left(R_{2^16(m-1),1}\right) &\text{if }\exists m | i = 2^16m\\
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R_{i-1,1} &\text{otherwise}
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\end{cases}\\
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R_{i,2} &=
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\begin{cases}
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H_2\left(R_{2^24(n-1),0}\right) &\text{if }\exists n | i = 2^24n\\
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H_2\left(R_{2^16(m-1),1}\right) &\text{if }\exists m | i = 2^16m\\
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H_2\left(R_{2^8(p-1),2}\right) &\text{if }\exists p | i = 2^8p\\
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R_{i-1,2} &\text{otherwise}
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\end{cases}\\
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R_{i,3} &=
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\begin{cases}
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H_3\left(R_{2^24(n-1),0}\right) &\text{if }\exists n | i = 2^24n\\
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H_3\left(R_{2^16(m-1),1}\right) &\text{if }\exists m | i = 2^16m\\
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H_3\left(R_{2^8(p-1),2}\right) &\text{if }\exists p | i = 2^8p\\
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H_3\left(R_{i-1,3}\right) &\text{otherwise}
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\end{cases}
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\end{aligned}
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```
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where $`H_0`$, $`H_1`$, $`H_2`$, and $`H_3`$ are different hash
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functions. In summary: every $`2^8`$ iterations, $`R_{i,3}`$ is
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reseeded from $`R_{i,2}`$. Every $`2^16`$ iterations, $`R_{i,2}`$
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and $`R_{i,3}`$ are reseeded from $`R_{i,1}`$. Every $`2^24`$
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iterations, $`R_{i,1}`$, $`R_{i,2}`$ and $`R_{i,3}`$ are reseeded
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from $`R_{i,0}`$.
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The complete ratchet value, $`R_{i}`$, is hashed to generate the keys used
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to encrypt each message. This scheme allows the ratchet to be advanced an
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arbitrary amount forwards while needing at most 1023 hash computations. A
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client can decrypt chat history onwards from the earliest value of the ratchet
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it is aware of, but cannot decrypt history from before that point without
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reversing the hash function.
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This allows a participant to share its ability to decrypt chat history with
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another from a point in the conversation onwards by giving a copy of the
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ratchet at that point in the conversation.
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## The Megolm protocol
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### Session setup
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Each participant in a conversation generates their own Megolm session. A
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session consists of three parts:
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* a 32 bit counter, $`i`$.
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* an [Ed25519][] keypair, $`K`$.
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* a ratchet, $`R_i`$, which consists of four 256-bit values,
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$`R_{i,j}`$ for $`j \in {0,1,2,3}`$.
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The counter $`i`$ is initialised to $`0`$. A new Ed25519 keypair is
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generated for $`K`$. The ratchet is simply initialised with 1024 bits of
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cryptographically-secure random data.
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A single participant may use multiple sessions over the lifetime of a
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conversation. The public part of $`K`$ is used as an identifier to
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discriminate between sessions.
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### Sharing session data
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To allow other participants in the conversation to decrypt messages, the
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session data is formatted as described in [Session-sharing format](#Session-sharing-format). It is then
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shared with other participants in the conversation via a secure peer-to-peer
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channel (such as that provided by [Olm][]).
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When the session data is received from other participants, the recipient first
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checks that the signature matches the public key. They then store their own
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copy of the counter, ratchet, and public key.
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### Message encryption
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This version of Megolm uses AES-256_ in CBC_ mode with [PKCS#7][] padding and
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HMAC-SHA-256_ (truncated to 64 bits). The 256 bit AES key, 256 bit HMAC key,
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and 128 bit AES IV are derived from the megolm ratchet $`R_i`$:
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```math
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\begin{aligned}
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AES\_KEY_{i}\;\parallel\;HMAC\_KEY_{i}\;\parallel\;AES\_IV_{i}
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&= HKDF\left(0,\,R_{i},\text{"MEGOLM\_KEYS"},\,80\right) \\
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\end{aligned}
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```
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where $`\parallel`$ represents string splitting, and
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$`HKDF\left(salt,\,IKM,\,info,\,L\right)`$ refers to the [HMAC-based key
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derivation function][] using using [SHA-256][] as the hash function
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([HKDF-SHA-256][]) with a salt value of $`salt`$, input key material of
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$`IKM`$, context string $`info`$, and output keying material length of
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$`L`$ bytes.
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The plain-text is encrypted with AES-256, using the key $`AES\_KEY_{i}`$
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and the IV $`AES\_IV_{i}`$ to give the cipher-text, $`X_{i}`$.
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The ratchet index $`i`$, and the cipher-text $`X_{i}`$, are then packed
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into a message as described in [Message format](#message-format). Then the entire message
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(including the version bytes and all payload bytes) are passed through
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HMAC-SHA-256. The first 8 bytes of the MAC are appended to the message.
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Finally, the authenticated message is signed using the Ed25519 keypair; the 64
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byte signature is appended to the message.
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The complete signed message, together with the public part of $`K`$ (acting
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as a session identifier), can then be sent over an insecure channel. The
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message can then be authenticated and decrypted only by recipients who have
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received the session data.
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### Advancing the ratchet
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After each message is encrypted, the ratchet is advanced. This is done as
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described in [The Megolm ratchet algorithm](#the-megolm-ratchet-algorithm), using the following definitions:
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```math
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\begin{aligned}
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H_0(A) &\equiv HMAC(A,\text{"\x00"}) \\
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H_1(A) &\equiv HMAC(A,\text{"\x01"}) \\
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H_2(A) &\equiv HMAC(A,\text{"\x02"}) \\
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H_3(A) &\equiv HMAC(A,\text{"\x03"}) \\
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\end{aligned}
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```
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where $`HMAC(A, T)`$ is the HMAC-SHA-256 of ``T``, using ``A`` as the
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key.
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For outbound sessions, the updated ratchet and counter are stored in the
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session.
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In order to maintain the ability to decrypt conversation history, inbound
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sessions should store a copy of their earliest known ratchet value (unless they
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explicitly want to drop the ability to decrypt that history - see [Partial
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Forward Secrecy](#partial-forward-secrecy)). They may also choose to cache calculated ratchet values,
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but the decision of which ratchet states to cache is left to the application.
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## Data exchange formats
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### Session-sharing format
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The Megolm key-sharing format is as follows:
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```
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+---+----+--------+--------+--------+--------+------+-----------+
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| V | i | R(i,0) | R(i,1) | R(i,2) | R(i,3) | Kpub | Signature |
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+---+----+--------+--------+--------+--------+------+-----------+
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0 1 5 37 69 101 133 165 229 bytes
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```
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The version byte, ``V``, is ``"\x02"``.
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This is followed by the ratchet index, $`i`$, which is encoded as a
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big-endian 32-bit integer; the ratchet values $`R_{i,j}`$; and the public
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part of the Ed25519 keypair $`K`$.
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The data is then signed using the Ed25519 keypair, and the 64-byte signature is
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appended.
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### Message format
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Megolm messages consist of a one byte version, followed by a variable length
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payload, a fixed length message authentication code, and a fixed length
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signature.
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```
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+---+------------------------------------+-----------+------------------+
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| V | Payload Bytes | MAC Bytes | Signature Bytes |
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+---+------------------------------------+-----------+------------------+
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0 1 N N+8 N+72 bytes
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```
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The version byte, ``V``, is ``"\x03"``.
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The payload uses a format based on the [Protocol Buffers encoding][]. It
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consists of the following key-value pairs:
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**Name**|**Tag**|**Type**|**Meaning**
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:-----:|:-----:|:-----:|:-----:
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Message-Index|0x08|Integer|The index of the ratchet, i
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Cipher-Text|0x12|String|The cipher-text, Xi, of the message
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Within the payload, integers are encoded using a variable length encoding. Each
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integer is encoded as a sequence of bytes with the high bit set followed by a
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byte with the high bit clear. The seven low bits of each byte store the bits of
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the integer. The least significant bits are stored in the first byte.
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Strings are encoded as a variable-length integer followed by the string itself.
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Each key-value pair is encoded as a variable-length integer giving the tag,
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followed by a string or variable-length integer giving the value.
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The payload is followed by the MAC. The length of the MAC is determined by the
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authenticated encryption algorithm being used (8 bytes in this version of the
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protocol). The MAC protects all of the bytes preceding the MAC.
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The length of the signature is determined by the signing algorithm being used
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(64 bytes in this version of the protocol). The signature covers all of the
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bytes preceding the signature.
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## Limitations
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### Message Replays
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A message can be decrypted successfully multiple times. This means that an
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attacker can re-send a copy of an old message, and the recipient will treat it
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as a new message.
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To mitigate this it is recommended that applications track the ratchet indices
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they have received and that they reject messages with a ratchet index that
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they have already decrypted.
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### Lack of Transcript Consistency
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In a group conversation, there is no guarantee that all recipients have
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received the same messages. For example, if Alice is in a conversation with Bob
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and Charlie, she could send different messages to Bob and Charlie, or could
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send some messages to Bob but not Charlie, or vice versa.
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Solving this is, in general, a hard problem, particularly in a protocol which
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does not guarantee in-order message delivery. For now it remains the subject of
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future research.
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### Lack of Backward Secrecy
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Once the key to a Megolm session is compromised, the attacker can decrypt any
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future messages sent via that session.
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In order to mitigate this, the application should ensure that Megolm sessions
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are not used indefinitely. Instead it should periodically start a new session,
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with new keys shared over a secure channel.
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<!-- TODO: Can we recommend sensible lifetimes for Megolm sessions? Probably
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depends how paranoid we're feeling, but some guidelines might be useful. -->
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### Partial Forward Secrecy
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Each recipient maintains a record of the ratchet value which allows them to
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decrypt any messages sent in the session after the corresponding point in the
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conversation. If this value is compromised, an attacker can similarly decrypt
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those past messages.
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To mitigate this issue, the application should offer the user the option to
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discard historical conversations, by winding forward any stored ratchet values,
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or discarding sessions altogether.
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### Dependency on secure channel for key exchange
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The design of the Megolm ratchet relies on the availability of a secure
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peer-to-peer channel for the exchange of session keys. Any vulnerabilities in
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the underlying channel are likely to be amplified when applied to Megolm
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session setup.
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For example, if the peer-to-peer channel is vulnerable to an unknown key-share
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attack, the entire Megolm session become similarly vulnerable. For example:
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Alice starts a group chat with Eve, and shares the session keys with Eve. Eve
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uses the unknown key-share attack to forward the session keys to Bob, who
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believes Alice is starting the session with him. Eve then forwards messages
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from the Megolm session to Bob, who again believes they are coming from
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Alice. Provided the peer-to-peer channel is not vulnerable to this attack, Bob
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will realise that the key-sharing message was forwarded by Eve, and can treat
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the Megolm session as a forgery.
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A second example: if the peer-to-peer channel is vulnerable to a replay
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attack, this can be extended to entire Megolm sessions.
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## License
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The Megolm specification (this document) is licensed under the Apache License,
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Version 2.0 http://www.apache.org/licenses/LICENSE-2.0.
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[Ed25519]: http://ed25519.cr.yp.to/
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[HMAC-based key derivation function]: https://tools.ietf.org/html/rfc5869
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[HKDF-SHA-256]: https://tools.ietf.org/html/rfc5869
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[HMAC-SHA-256]: https://tools.ietf.org/html/rfc2104
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[SHA-256]: https://tools.ietf.org/html/rfc6234
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[AES-256]: http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf
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[CBC]: http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf
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[PKCS#7]: https://tools.ietf.org/html/rfc2315
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[Olm]: https://gitlab.matrix.org/matrix-org/olm/blob/master/docs/olm.md
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[Protocol Buffers encoding]: https://developers.google.com/protocol-buffers/docs/encoding
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.. Copyright 2016 OpenMarket Ltd
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..
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.. Licensed under the Apache License, Version 2.0 (the "License");
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.. you may not use this file except in compliance with the License.
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.. You may obtain a copy of the License at
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..
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.. http://www.apache.org/licenses/LICENSE-2.0
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..
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.. Unless required by applicable law or agreed to in writing, software
|
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.. distributed under the License is distributed on an "AS IS" BASIS,
|
||||
.. WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
|
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.. See the License for the specific language governing permissions and
|
||||
.. limitations under the License.
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Megolm group ratchet
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====================
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An AES-based cryptographic ratchet intended for group communications.
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.. contents::
|
||||
|
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Background
|
||||
----------
|
||||
|
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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 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
|
||||
--------
|
||||
|
||||
Each participant in a conversation uses their own outbound session for
|
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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.
|
||||
|
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Authenticity is provided via Ed25519 signatures.
|
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|
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The value of the ratchet, and the public part of the Ed25519 key, are shared
|
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with other participants in the conversation via secure peer-to-peer
|
||||
channels. Provided that peer-to-peer channel provides authenticity of the
|
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messages to the participants and deniability of the messages to third parties,
|
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the Megolm session will inherit those properties.
|
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|
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The Megolm ratchet algorithm
|
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----------------------------
|
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The Megolm ratchet :math:`R_i` consists of four parts, :math:`R_{i,j}` for
|
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:math:`j \in {0,1,2,3}`. The length of each part depends on the hash function
|
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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}
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R_{i,0} &=
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||||
\begin{cases}
|
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H_0\left(R_{2^24(n-1),0}\right) &\text{if }\exists n | i = 2^24n\\
|
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R_{i-1,0} &\text{otherwise}
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||||
\end{cases}\\
|
||||
R_{i,1} &=
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\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\\
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||||
R_{i-1,1} &\text{otherwise}
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||||
\end{cases}\\
|
||||
R_{i,2} &=
|
||||
\begin{cases}
|
||||
H_2\left(R_{2^24(n-1),0}\right) &\text{if }\exists n | i = 2^24n\\
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||||
H_2\left(R_{2^16(m-1),1}\right) &\text{if }\exists m | i = 2^16m\\
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||||
H_2\left(R_{2^8(p-1),2}\right) &\text{if }\exists p | i = 2^8p\\
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R_{i-1,2} &\text{otherwise}
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||||
\end{cases}\\
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R_{i,3} &=
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\begin{cases}
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||||
H_3\left(R_{2^24(n-1),0}\right) &\text{if }\exists n | i = 2^24n\\
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||||
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}
|
||||
|
||||
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 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
|
||||
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
|
||||
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
|
||||
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
|
||||
~~~~~~~~~~~~~~~~~~
|
||||
|
||||
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::
|
||||
|
||||
\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
|
||||
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
|
||||
key.
|
||||
|
||||
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 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,
|
||||
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 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`.
|
||||
|
||||
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 bytes
|
||||
|
||||
The version byte, ``V``, is ``"\x03"``.
|
||||
|
||||
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
|
||||
============= ===== ======== ================================================
|
||||
|
||||
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 signature.
|
||||
|
||||
Limitations
|
||||
-----------
|
||||
|
||||
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
|
||||
as a new message.
|
||||
|
||||
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
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
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
|
||||
and Charlie, she could send different messages to Bob and Charlie, or could
|
||||
send some messages to Bob but not Charlie, or vice versa.
|
||||
|
||||
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
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
Once the key to a Megolm session is compromised, the attacker can decrypt any
|
||||
future messages sent via that session.
|
||||
|
||||
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.
|
||||
|
||||
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
|
||||
conversation. If this value is compromised, an attacker can similarly decrypt
|
||||
those past messages.
|
||||
|
||||
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
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
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
|
||||
the underlying channel are likely to be amplified when applied to Megolm
|
||||
session setup.
|
||||
|
||||
For example, if the peer-to-peer channel is vulnerable to an unknown key-share
|
||||
attack, the entire Megolm session become similarly vulnerable. For example:
|
||||
Alice starts a group chat with Eve, and shares the session keys with Eve. Eve
|
||||
uses the unknown key-share attack to forward the session keys to Bob, who
|
||||
believes Alice is starting the session with him. Eve then forwards messages
|
||||
from the Megolm session to Bob, who again believes they are coming from
|
||||
Alice. Provided the peer-to-peer channel is not vulnerable to this attack, Bob
|
||||
will realise that the key-sharing message was forwarded by Eve, and can treat
|
||||
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
|
Loading…
Reference in a new issue