I2P Tunnel Participant Protocol upgrade

THIS IS A DRAFT

Proposal for 4 new i2np message types and 2 new data structures.

Only applies to tunnel participants, tunnel gateway is unchanged.

Data Structures

BiDiTunnelRequestRecord

Function: replace tunnel request records, add certificates for future protocol expansion

Encrypted Format:

224 bytes encrypted data (encrypted to tunnel participants ed25519 key in router identity)

Cleartext Format:

16 bytes truncated sha256 of the router identity of this hop
8 bytes big endian integer expiration timestamp as seconds since unix epoch
4 bytes big endian integer send tunnel id 
4 bytes big endian integer recv tunnel id
32 bytes sha256 of router identity for send tunnel
32 bytes sha256 of router identity for recv tunnel
32 bytes chacha20 symmetric key for send tunnel
32 bytes chacha20 symmetric key for recv tunnel
32 bytes ed25519 public encryption key to encrypt reply to, also used as signing key for revocation
i2p certificate (currently NULL, 3 bytes)
N bytes random padding such that this record is 224 bytes total

BiDiTunnelResponseRecord

Function: replace BuildResponseRecord

Encrypted Format:

224 bytes encrypted data

Decrypted Format:

32 bytes sha256 hash of the next 192 bytes
191 bytes random badding
1 byte reply, same as from BuildResponseRecord

I2NP Messages

BiDiTunnelBuild

I2NP Type: 29

Function: request bidirectional tunnel participation, replaces VariableTunnelBuild

Composed of N encrypted/fake BiDiTunnelRequestRecords

Format:

224 * N bytes encrypted data

Tunnel Participant attempts to decrypt each BiDiTunnelRequestRecord contained.

This message can contain multiple BiDiTunnelRequestRecords for this participant.

BiDiTunnelBuildResult

I2NP Type: 30

Function: Result of a BiDiTunnelBuild , replaces VariableTunnelBuildReply

Same format as BiDiTunnelBuild but instead is filled with BiDiTunnelResponseRecords.

TunnelCancel

I2NP Type: 31

Function: Cancel a BiDiTunnel tunnel previously requested.

Sent from the creator of the tunnel, signed with the revocation key provided in the corresponding BiDiTunnelBuild Message.

Used to Prematurely revoke tunnel commitment to a tunnel participant.

Size: >= 96 bytes (32 bytes aligned)

Format:

32 bytes sha265 of router identity for send tunnel
32 bytes sha256 of router identity for recv tunnel
4 bytes big endian integer send tunnel id
4 bytes big endian integer recv tunnel id
i2p certificate (currently NULL, 3 bytes)
N bytes random padding such that message is 32 bytes aligned
32 bytes ed25519 signature of bytes before start of signature

Note: no limits are imposed on this message's padding other than the resulting message must be 32 bytes aligned and is less than 65536 bytes, the limit imposed by I2NP.

BiDiTunnelData

I2NP Type: 32

Function: TunnelDataMessage for bidirectional tunnels

Size: 2048 bytes

Format:

16 bytes poly1305 mac
8 bytes nonce
4 bytes send tunnel id 
4 bytes recv tunnel id
2016 bytes payload

Justification

• i2p's tunnel crypto lacks mac, possible tagging attack

• elgamal is too slow, possible DoS attack

• chacha20poly1305 is fast

• as of 2016 most of the network supports ed25519

• 32 bytes aligned implies 16 bytes aligned, messages are aligned in memory such that performance boosts can be achieved.