Merge branch 'markdown' into 'master'

Contributing code to libolm

===========================

# Contributing code to libolm

To contribute code to this library, the preferred way is to clone the git

repository, create a git patch series (for example via ``git

Naturally, you must be willing to license your contributions under the same

license as the project itself - in this case, Apache Software License v2 (see

`<LICENSE>`_).

[LICENSE](LICENSE)).

Sign off

--------

## Sign off

In order to have a concrete record that your contribution is intentional and

you agree to license it under the same terms as the project's license, we've

adopted the same lightweight approach that the

`Linux Kernel <https://www.kernel.org/doc/Documentation/SubmittingPatches>`_,

`Docker <https://github.com/docker/docker/blob/master/CONTRIBUTING.md>`_,

and many other projects use: the DCO

(`Developer Certificate of Origin <http://developercertificate.org/>`_).

[Linux Kernel](https://www.kernel.org/doc/html/latest/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin),

[Docker](https://github.com/docker/docker/blob/master/CONTRIBUTING.md),

and many other projects use: the DCO ([Developer Certificate of Origin](http://developercertificate.org/)).

This is a simple declaration that you wrote the contribution or otherwise have

the right to contribute it to Matrix::

Olm

===

# Olm

An implementation of the Double Ratchet cryptographic ratchet described by

https://whispersystems.org/docs/specifications/doubleratchet/, written in C and

C++11 and exposed as a C API.

The specification of the Olm ratchet can be found in `<docs/olm.rst>`_.

The specification of the Olm ratchet can be found in [docs/olm.md](docs/olm.md).

This library also includes an implementation of the Megolm cryptographic

ratchet, as specified in `<docs/megolm.rst>`_.

ratchet, as specified in [docs/megolm.md](docs/megolm.md).

Building

--------

## Building

To build olm as a shared library run either:

.. code:: bash

```bash

cmake . -Bbuild

cmake --build build

```

or:

.. code:: bash

```bash

make

```

Using cmake is the preferred method for building the shared library; the

Makefile may be removed in the future.

To run the tests when using cmake, run:

.. code:: bash

```bash

cd build/tests

ctest .

```

To run the tests when using make, run:

.. code:: bash

```bash

make test

```

To build the JavaScript bindings, install emscripten from http://kripken.github.io/emscripten-site/ and then run:

.. code:: bash

```bash

make js

```

Note that if you run emscripten in a docker container, you need to pass through

the EMCC_CLOSURE_ARGS environment variable.

To build the android project for Android bindings, run:

.. code:: bash

```bash

cd android

./gradlew clean assembleRelease

```

To build the Xcode workspace for Objective-C bindings, run:

.. code:: bash

```bash

cd xcode

pod install

open OLMKit.xcworkspace

```

To build the Python bindings, first build olm as a shared library as above, and

then run:

.. code:: bash

```bash

cd python

make

```

to make both the Python 2 and Python 3 bindings.  To make only one version, use

``make olm-python2`` or ``make olm-python3`` instead of just ``make``.

To build olm as a static library (which still needs libstdc++ dynamically) run

either:

.. code:: bash

```bash

cmake . -Bbuild -DBUILD_SHARED_LIBS=NO

cmake --build build

```

or

.. code:: bash

```bash

make static

```

The library can also be used as a dependency with CMake using:

.. code:: cmake

```cmake

find_package(Olm::Olm REQUIRED)

target_link_libraries(my_exe Olm::Olm)

```

Release process

---------------

## Release process

First: bump version numbers in ``common.mk``, ``CMakeLists.txt``,

``javascript/package.json``, ``python/olm/__version__.py``, ``OLMKit.podspec``,

It's probably sensible to do the above on a release branch (``release-vx.y.z``

by convention), and merge back to master once the release is complete.

.. code:: bash

```bash

make clean

# build and test C library

pod trunk push OLMKit.podspec --use-libraries --allow-warnings

# Check the pod has been successully published with:

pod search OLMKit

```

Design

------

## Design

Olm is designed to be easy port to different platforms and to be easy

to write bindings for.

advantage, and new functionality is being added in C, with C++ parts being

rewritten as the need ariases.

Error Handling

~~~~~~~~~~~~~~

### Error Handling

All C functions in the API for olm return ``olm_error()`` on error.

This makes it easy to check for error conditions within the language bindings.

Random Numbers

~~~~~~~~~~~~~~

### Random Numbers

Olm doesn't generate random numbers itself. Instead the caller must

provide the random data. This makes it easier to port the library to different

platforms since the caller can use whatever cryptographic random number

generator their platform provides.

Memory

~~~~~~

### Memory

Olm avoids calling malloc or allocating memory on the heap itself.

Instead the library calculates how much memory will be needed to hold the

output and the caller supplies a buffer of the appropriate size.

Output Encoding

~~~~~~~~~~~~~~~

### Output Encoding

Binary output is encoded as base64 so that languages that prefer unicode

strings will find it easier to handle the output.

Dependencies

~~~~~~~~~~~~

### Dependencies

Olm uses pure C implementations of the cryptographic primitives used by

the ratchet. While this decreases the performance it makes it much easier

to compile the library for different architectures.

Contributing

------------

Please see `<CONTRIBUTING.rst>`_ when making contributions to the library.

## Contributing

Security assessment

-------------------

Please see [CONTRIBUTING.md](CONTRIBUTING.md) when making contributions to the library.

## Security assessment

Olm 1.3.0 was independently assessed by NCC Group's Cryptography Services

Practive in September 2016 to check for security issues: you can read all

https://www.nccgroup.trust/us/our-research/matrix-olm-cryptographic-review/

and https://matrix.org/blog/2016/11/21/matrixs-olm-end-to-end-encryption-security-assessment-released-and-implemented-cross-platform-on-riot-at-last/

Bug reports

-----------

## Bug reports

Please file bug reports at https://github.com/matrix-org/olm/issues

What's an olm?

--------------

## What's an olm?

It's a really cool species of European troglodytic salamander.

http://www.postojnska-jama.eu/en/come-and-visit-us/vivarium-proteus/

Legal Notice

------------

## Legal Notice

The software may be subject to the U.S. export control laws and regulations

and by downloading the software the user certifies that he/she/it is

# 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 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

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.

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

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 $`R_i`$ 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{aligned}

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{aligned}

```

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, $`R_{i}`$, 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 1020 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, $`i`$.

* 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 $`i`$ 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 $`K`$ 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](#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 $`R_i`$:

```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{aligned}

```

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 $`AES\_KEY_{i}`$

and the IV $`AES\_IV_{i}`$ to give the cipher-text, $`X_{i}`$.

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 $`K`$ (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](#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

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](#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:

```

+---+----+--------+--------+--------+--------+------+-----------+

| 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, $`i`$, 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

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.

```

+---+------------------------------------+-----------+------------------+

| 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, 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

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]: https://gitlab.matrix.org/matrix-org/olm/blob/master/docs/olm.md

[Protocol Buffers encoding]: https://developers.google.com/protocol-buffers/docs/encoding