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X3DH Reference

• KeyServer - a service providing public key storage capabilities.

• Alice - a person wants to send a message.
• Bob - a person receiving the message.

All the keys are Curve 25519 keys.

• Identity Key (Ik) - long term key. Key is generated once when a participant is created.
• Signed Pre Key (Spk) - middle term key with it's id and signature. EdDSA scheme is used for generating a pre key signatures.
• One Time Pre Key (Opk) - short term key with it's id. It's supposed to have bunch of keys saved on the server. Each key could be used once.
• Ephemeral Key (Ek) - short term key. The key is generated during each protocol run. It's so named "session" key.

• Curve() - a function generating Curve25519 key pair.
• DH(k1, k2) - X25519 Elliptic Curve Diffie-Hellman function. Calculates a shared secret output from the passed keys.
• KDF(km, salt) - a function representing HKDF algorithm. km is a key material used as a concatenation of calculated shared secrets using DH function. The function calculates two type of keys:
  • root key is used to derive a new chain key.
  • chain key is used to derive a new message key.
• KDF_CK(chain) - an HKDF function for deriving message keys and a new chain key. A message key consists of four parameters:
  • cipher key - a key used for a plaintext encryption and ciphertext decryption.
  • mac key - a key used for HMAC calculation.
  • initialization vector (IV) - a parameter used for a chosen encryption algorithm.
  • index - a counter for an appropriate chain key. The counter is increased when chain key is updated. The counter is used for out-of-order messages processing.
• Encrypt(messageKey, plaintext) - returns encryption of plaintext using the given parameters from a message key.
• Decrypt(messageKey, ciphertext) - returns decryption of ciphertext using the given parameters from a message key.
• Sign(ikB, spkB) - an EdDSA function returning signature of the specified signed pre key.
• Verify(ikB, spkB, sign) - an EdDSA function used to verify the given signature of a signed pre key by the specified identity key.

Double ratchet