Protocol

Ring account creation

A Ring account is defined by an RSA key pair with a key length of at least 4096 bits.

The standard x509 160-bits fingerprint of the account public key is called the RingID.

The account public key is used as the subject of an x509 certificate that must be valid, have the Certificate Authority flag set, and can be self-signed. This certificate is called the Ring account certificate.

The subject UID field of the account certificate must be the hexadecimal form of the public key fingerprint. The issuer UID field must be the hexadecimal form of the issuer public key fingerprint.

Persisting the account

Persisting a Ring account private key and certificate is implementation defined.

Access to a saved Ring account private key must be authenticated and authorized. Authentication and authorization method to access the account private key is implementation defined.

Adding a device to a Ring account

See RFC 5280

A device is defined by an RSA key pair with a key length of at least 4096 bits.

A device certificate is defined as an x509 certificate whose subject is a device public key, signed with an account private key. The certificate MUST be valid. The issuer UID field MUST be the hexadecimal form of the account public key fingerprint.

Persisting a device private key and certificate is implementation defined. Access to a saved device private key should be authenticated. Authentication method to access the device private key is implementation defined.

Removing a device from a Ring account

A device can be “removed” from a Ring account through revocation of the device certificate. Revoked device certificates are added to one or more standard x509 Certificate Revocation List (CRL). CRLs for revoked device must be valid and signed with the corresponding CA key, which is the Ring account private key.

Account transmission format

The account archive format defines how to serialize an account private key for transmission, for instance to sign a new device certificate.

The account archive is an encrypted JSON object with the following structure:

{
    "ringAccountKey": (PEM-encoded account private key string),
    "ringAccountCert": (PEM-encoded account certificate string),
    "ringAccountCRL": (PEM-encoded account CRL string)
}

The JSON object can contain additional implementation-defined key-value pairs. Implementation-defined key names shouldn’t start with “ring”.

The string JSON object is encrypted using a key defined as :

salt = PIN + timestamp
key = argon2(password, salt)

Where PIN is a random 32bits number in hexadecimal form, “+” is string concatenation, timestamp is the current UNIX timestamp divided by 1200 (20 minutes) and password is a user-chosen password.

The PIN should be shown to the user to be copied manually on the new physical device along with the password.

Contacting another account

ICE descriptor exchange over OpenDHT

  • Listening for incoming calls

A device listens for incoming call by performing a listen OpenDHT operation on

h("callto"+deviceID)

where h is SHA1, “+” is the string concatenation and deviceID is the hexadecimal form of the deviceID.

Received OpenDHT values that are not encrypted or not properly signed must be dropped. The value must be encrypted with the called device public key and signed with the calling device private key according to OpenDHT specifications.

  • Sending the Initial Offer

See RFC 5245

RFC 5245 defines ICE (Interactive Connectivity Establishment), a protocol for NAT traversal.

ICE is used in Ring to establish a peer-to-peer communication between two devices.

The calling device gathers candidates and build an Initial Offer according to the ICE specifications and starts the ICE negotiation process.

The calling device puts the encrypted ICE offer (the Initial Offer) on the DHT at h(“callto”+deviceID) where deviceID is the hexadecimal form of the called deviceID.

  • ICE serialization format

ICE messages exchanged between peers during a call setup use following format. An ICE message is a chunk of binary data, following msgpack data format.

This protocol is a compound of msgpack values, successively packed in this order:

  • an integer giving the version of ICE message format protocol used for the rest of the data. Current defined protocol version is 1.

  • a 2-elements array of strings of the ICE local session ufrag and the ICE local session password

  • an integer giving the number of components in the ICE session

  • an array of string, of the previous number entries, where each string describe the ICE candidate, formated as an “a=” line (without the “a=” header) described in rfc5245, section 4.3

  • Sending the Answer

Upon reception of the encrypted and signed Initial ICE Offer (through the listen operation), a called device should perform authorization checks of the calling device, identified as the Initial Offer signer. Authorization rules are implementation defined, but a typical implementation would authorize known or trusted contacts.

If the calling device is not authorized or if for any implementation defined reason the called device refuses the incoming connection request, the called device must ignore the Initial Offer and may log the event.

If the called device authorizes the caller and wish to accept the connection it must build an ICE answer, start the ICE negotiation process and send the encrypted and signed ICE answer at the same DHT key.

DTLS negotiation

Once a peer-to-peer communication channel has been established, the called device listens on it for incoming DTLS connections (acting as a DTLS server) while the caller initiates an outgoing DTLS connection (acting as a DTLS client).

The DTLS communication must be RFC6347 compliant (1).

Peers must only support PFS cypher suites. The set of supported cypher suites is implementation defined but should include at least ECDHE-AES-GCM (TODO: specify the exact suites recommended to support).

During the DTLS handshake, both peers must provide their respective device certificate chain and must authenticate the other peer, checking that its public key is the same used during the DHT ICE exchange.

SIP call

See Important_RFC

Once an encrypted and authenticated peer-to-peer communication channel is available, the SIP protocol 2 must be used to place a call and send messages. The caller might send a SIP INVITE as soon as the DTLS channel is established.

The SIP implementation must support ICE and SRTP.

Supported codecs are implementation defined, but Ring clients should support the Opus audio coded and the H264 video codec.

SRTP must be used when negotiating media with SIP, using a new random key for each media and each negotiation. ICE should be used when negotiating media with SIP.

Cryptographic primitives

Password stretching

See Argon2 specifications

Passwords are stretched using argon2i using t_cost = 16, m_cost = 2^16 (64 MiB), mono-threaded, to generate a 512 bits hash.

The result is then hashed again using SHA{1, 256, 512} depending on the requested key size.

Encryption

Using a provided key (128, 192 or 256 bits)

Encryption uses standard AES-GCM as implemented by Nettle using a random IV for each encryption.

Using a text password

The password is stretched to generate a 256 bits key and a random salt of 128 bits.

The input data is encrypted using AES-GCM (see above) and the salt is appended at the beginning of the resulting cypher-text.

During a call

Audio/video data are exchanged using encrypted RTP channels between peers.

The protocol is a classic SRTP, with following supported crypto suites:

  • Ring account force AES_CM_128_HMAC_SHA1_80

  • SIP can use AES_CM_128_HMAC_SHA1_80 or AES_CM_128_HMAC_SHA1_32

The master key and salt is a random number, different for each call. On call’s master key is constant during the full live of a call.

The keys are exchanged using SDES method: keys are written into the SIP SDP messages during the SIP INVITE negotiation. When SDES is used, Ring forces the underlaying transport to be secure (encrypted) to not disclose these keys. Ring supports DTLS natively for SIP and Ring accounts for such. The call cannot be done if this condition is not fulfilled.