mpOTR: Multi-party Off-the-Record Messaging
The mpOTR paper by Ian Goldberg et al. [1] is the basis for the cryptography in mpOTR. It is lacking in several areas and we felt it important to choose specifics for the handwaving parts of the paper carefully. Our implementation is mostly a toy but we believe it is a reasonable first stab at creating a simple mpOTR demonstration. With native implementations, we believe that mpOTR is a very reasonable encrypted chat protocol but our implementation is by no means peer-reviewed and should be considered an experiment.
We have chosen to use elliptic curve cryptography and specifically, we've chosen to use (all hail the NSA?) P256, for use with Elliptic curve Diffie–Hellman (ECDH) as our ephemeral key agreement protocol, Elliptic Curve Digital Signature Algorithm (ECDSA) over P256 as our digital signature algorithm (512 bits in length giving 128 bits of security), SHA-512 as our hash function (512 bits in length), HMAC as our MAC over our hash function (128 bits in length), AES-128 in CTR with a random IV for encryption (128 bits of key material, obviously), BASE64 for encoding of data, keys and HTTP for the transport - we assume HTTPS or Tor Hidden Service and consider those details out of scope. These choices should work with other protocols such as XMPP as the server is essentially just relaying messages - encoding will need to be considered for protocols other than HTTP.
The original mpOTR paper leaves the group key agreement protocol open. We have chosen to define the group session key as the hash of secrets from all the participants. The secrets are exchanged between each pair of users by using Elliptic Curve El Gamal, authenticated by the pair's ephemeral signing keys.
Each chat message is encrypted with AES in CTR mode with the group session key and signed with the sender's ephemeral ECDSA key.
Alice, Bob, Mallory, and Eve wish to have a chat using the XMPP chat server crypto.cat in the channel '#protocol-spec'.
Alice generates a long term private/public key pair for verification of her identity. She does this once and MAY store it for all time.
Currently, any time the list of participants changes, it is necessary to re-start the key-agreement protocol. In the future, it may be beneficial to periodically re-start the protocol to limit the window in which compromise of any principal can undermine deniability of messages sent after that point.
At the beginning of the key agreement protocol, each party generates an ephemeral ECDSA private/public key pair for the chat session. Each party then sends its nickname and a random number to the server, which are shared by the server and used to generate a unique, non-secret session ID which is shared between all parties.
XXX: Do we really want to reveal the nickname to the server?
XXX: TODO: Enforce binding that each client's value for the channel name is
consistent.Each principal performs a pair-wise deniable authenticated key-exchange, multiplexed by the Cryptocat server. Currently, this is an Elliptic Curve Diffie-Hellman key exchange authenticated with each principal's long-term ECDSA key.
After completing pair-wise key exchange, each party sends messages to confirm correct receipt of ephemeral keys, as described by the mpOTR protocol.
Finally, once all users have exchanged ephemeral keys, a group key exchange protocol is conducted to derive a group session key. Currently, each party generates a 128-bit random value, and exchanges it pairwise with all other participants using Elliptic Curve Diffie-Hellman authenticated by each principal's ephemeral private key. The group session key is then the hash of all principals' random values, arranged lexicographically.
XXX: TODO: Prior to chatting, each user must send an Attest() message committing to the session ID and any other parameters which should be globally agreed upon. This should include in our case the version of the protocol, the timeout (if any), and the name of the chat channel.
When Alice wishes to send a message to the channel - she encrypts it with AES in CTR mode using the group session key, signs it with her ephemeral ECDSA key, and sends it to the server.
Alice wants to share a message with Bob and so she does...
XXX: File sharing should be documented here
Alice leaves the chat and sends her private ephemeral signing key to the channel. Each party confirms receipt of the key and at that time, the entire chat channel re-keys.
XXX: We have not implemented the tear-down steps of the mpOTR paper.
Eve is not part of the chat but is able to intercept data sent to and from the server. Eve is able to learn the nickname of each chat member and to watch the frequency and size of the messages sent by each party. If she does not witness the setup phase, she will not learn the participants' nicknames.
Mallory can use Alice's ephemeral signing key to forge messages that appears to be from her. Alice can reasonably believe that her ephemeral signing key proves nothing about her statements after that point.
The server is able to learn the nickname, the number of parties, the chat channel name, the IP addresses of the client, the timestamps and the size of messages.
XXX: TODO - Document examples of each part of the protocol and process.
References
[1] "Multi-party Off-the-Record Messaging", Ian Goldberg, Berkant Ustaoglu, Matthew D. Van Gundy, Hao Chen. CCS '09, November 9-13, 2009, Chicago, Illinois. http://www.cypherpunks.ca/~iang/pubs/mpotr.pdf SHA1(mpotr.pdf) = f32d0371d82a295bd5437797f0494071f65342a1
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