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I recently saw the BIP 324 proposal. It shows us there can be some privacy enhancement by using ECDH secrets and encryption algorithms such as ChaCha20 and AEAD-ChaCha20-Poly1305. The proposal includes:

  • The length decriptor size is reduced to 3 Bytes.
  • The command could be expressed as an ID (1 Byte) OR as characters (13 Bytes, by including the 0x00 byte selector).
  1. However, I don't fully understand the order in data will be structured. Can it be explained ?
  2. What algorithm will be applied to which data structure ? Because I can suppose the ChaCha20 is used in order to encrypt the payload size and AEAD-ChaCha20-Poly1305 to encrypt payload.
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  • What do you mean with "the order"? Aug 20, 2023 at 13:17
  • I mean the structure of sent data. For example: CommandId (1 Byte) + Size (4 Bytes)...
    – Loopite
    Aug 20, 2023 at 13:23

1 Answer 1

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In the new P2P transport protocol proposed in BIP324, after the handshake (which among other things establishes encryption keys), packets have the following structure:

  • A 3-byte length descriptor (encrypted with FSChaCha20)
  • An arbitrary-length ciphertext which is the encryption of a plaintext (encrypted with FSChaCha20Poly1305, which expands the data by 16 bytes in the process by adding a Poly1305 authentication tag); the plaintext consists of:
    • A 1-byte header (which is used for signalling decoy data)
    • A variable-length contents (whose length is described by the length descriptor). The contents consists of:
      • A 1-byte or 13-byte message type (e.g. version, verack, tx, inv, ...).
      • A variable-length payload (the actual data sent in the message).

So if you want everything flattened out, you get:

  • 3-byte encrypted length descriptor
  • 1-byte encrypted header
  • 1-byte or 13-byte encrypted message type
  • variable length encrypted payload
  • 16-byte Poly1305 authentication tag

However, only 2 separate encryption algorithm invocations are used per packet:

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  • So, I have a new question: Why the command field isn't directly encrypted with the length through ChaCha20 ?
    – Loopite
    Aug 20, 2023 at 15:05
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    @Loopite Because we want as much as possible to be covered by the Poly1305 authentication tag (as that allows modifications in flight to be detected). Only the length isn't, because we need to know the length of a packet before we can decrypt/authenticate the packet. It would have been possible in theory to have a separate authentication tag for just the length, but that'd come at fairly high cost (an additional 16 bytes per packet), and not actually achieve anything (see footnote 23 in the BIP). Aug 20, 2023 at 15:28

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