SSU Transport (Deprecated)

Original UDP transport used prior to SSU2

Updated: 2024-01 Accurate for: 0.9.61

Deprecated: SSU (Secure Semi-Reliable UDP) has been replaced by SSU2. Java I2P removed SSU in release 2.4.0 (API 0.9.61) and i2pd removed it in 2.44.0 (API 0.9.56). This document is retained for historical reference only.

Highlights

  • UDP transport providing encrypted, authenticated point-to-point delivery of I2NP messages.
  • Relied on a 2048-bit Diffie–Hellman handshake (same prime as ElGamal).
  • Each datagram carried a 16-byte HMAC-MD5 (non-standard truncated variant) + 16-byte IV followed by AES-256-CBC encrypted payload.
  • Replay prevention and session state tracked within the encrypted payload.

Message Header

[16-byte MAC][16-byte IV][encrypted payload]

MAC calculation used: HMAC-MD5(ciphertext || IV || (len ^ version ^ ((netid-2)<<8))) with a 32-byte MAC key. Payload length was big-endian 16-bit appended inside the MAC calculation. Protocol version defaulted to 0; netId defaulted to 2 (main network).

Session & MAC Keys

Derived from the DH shared secret:

  1. Convert the shared value to a big-endian byte array (prepend 0x00 if high bit set).
  2. Session key: first 32 bytes (pad with zeros if shorter).
  3. MAC key: bytes 33–64; if insufficient, fall back to SHA-256 hash of the shared value.

Status

Routers no longer advertise SSU addresses. Clients should migrate to SSU2 or NTCP2 transports. Historical implementations can be found in older releases:

  • Java sources prior to 2.4.0 under router/transport/udp
  • i2pd sources prior to 2.44.0

For current UDP transport behaviour, refer to SSU2 specification.

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