AirPlay.V2.SecureChannel (AirPlay v0.2.0)

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The encrypted RTSP control channel used after AirPlay 2 pairing — a port of EncryptionContext. Once paired, every RTSP message is wrapped: split into ≤1024-byte blocks, each emitted as [len:u16-le][ciphertext][tag:16], ChaCha20-Poly1305 with a 12-byte nonce <<0::32, counter::little-64>> (separate write/read counters) and AAD = the 2-byte length prefix.

Write/read keys are HKDF-SHA512(session_key) with the HomeKit control salts: write = info "Control-Write-Encryption-Key", read = "Control-Read-Encryption-Key", both salt "Control-Salt". (From the controller side: we write with the Write key and read the device's replies with the Read key.)

Summary

Types

t()

Functions

decrypt(ch, data)

Decrypt as many complete blocks as are buffered (incl. carried inbuf). Returns {plaintext, channel} — plaintext may be empty if no full block yet; leftover partial bytes stay in channel.inbuf.

encrypt(ch, plaintext)

Encrypt plaintext into the block-framed wire form; returns {bytes, channel}.

new(session_key)

Derive a control channel from the pairing session_key.

Types

t()

@type t() :: %AirPlay.V2.SecureChannel{
  inbuf: term(),
  read_ctr: term(),
  read_key: term(),
  write_ctr: term(),
  write_key: term()
}

Functions

decrypt(ch, data)

@spec decrypt(t(), binary()) :: {binary(), t()}

Decrypt as many complete blocks as are buffered (incl. carried inbuf). Returns {plaintext, channel} — plaintext may be empty if no full block yet; leftover partial bytes stay in channel.inbuf.

encrypt(ch, plaintext)

@spec encrypt(t(), binary()) :: {binary(), t()}

Encrypt plaintext into the block-framed wire form; returns {bytes, channel}.

new(session_key)

@spec new(binary()) :: t()

Derive a control channel from the pairing session_key.