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A Sender is the Person sending a message. In case of M2M it is the Operating Entity of the Device sending.
A Device is an entitiy, which is sending representative of a Sender.
A Device is in state Sole when it is not member of a Device group and when it is not part of a Negotiation.
A Device is in state Grouped when it is member of a Device group and when it is not part of a Negotiation.
A TID (transaction ID) is a UUID version 4 variant 1.
The Challenge is identified by a TID. The Challenge is being set by each Beacon and must be repeated in a corresponding Negotiation Request. The Challenge has two functions:
A Negotiation is a Transaction identified by a TID. The Negotiation's TID is the XOR of the two Challenge TIDs of the two devices, respectively.
The key with which the Sender of the message is signing. In case of trusted messages this is signalled within the encrypted message. This is signalled by by opt_field pEp-sender-sign, which is not reflected to the outer message.
Transports can opt to use HMAC or OMAC instead of digital signatures.
Keys with which others and not the Sender are signing a message.
A Sender Group Key is a Sender's signing key, which is used to update the Device Group information. If it is reset the Device Groups breaks.