The bundle SHOULD always contain around 100 pre-keys.
minimum number of pre-keys to provide in the bundle
The bundle MUST always contain at least 25 pre-keys.
associated data
The associated data is created by concatenating the identity keys of Alice and Bob: AD = Encode(IK_A) || Encode(IK_B)
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XEdDSA
To reduce the amount of bytes that have to be transferred, the key exchange uses XEdDSA on curves X25519/Ed25519 (aka XEd25519) to derive signing keys from encryption keys.
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XEdDSA
To reduce the amount of bytes that have to be transferred, the key exchange uses XEdDSA on curves X25519/Ed25519 (aka XEd25519) to build and verify signatures using encryption key pairs.
The key exchange is done just-in-time when sending the first message to a device. Thus, each key exchange message always also contains encrypted content as produced by the Double Ratchet encryption scheme below.