Number
111
Author
EinMByte, orignal, psi, str4d, zzz
Editor
manas, str4d, zzz
Created
Thread
http://zzz.i2p/topics/1577
Last updated
Status
Closed
Supercedes
106

Note

Proposal phase is closed. See [SPEC] for the official specification. This proposal may still be referenced for background information.

Overview

This proposal describes an authenticated key agreement protocol to improve the resistance of [NTCP] to various forms of automated identification and attacks.

The proposal is organized as follows: the security goals are presented, followed by a discussion of the basic protocol. Next, a complete specification of all protocol messages is given. Finally, router addresses and version identification are discussed. An appendix discussing a generic attack on common padding schemes is also included, as well as an appendix containing a number of candidates for the authenticated cipher.

As with other I2P transports, NTCP2 is defined solely for point-to-point (router-to-router) transport of I2NP messages. It is not a general-purpose data pipe.

Motivation

[NTCP] data is encrypted after the first message (and the first message appears to be random data), thus preventing protocol identification through "payload analysis". It is still vulnerable to protocol identification through "flow analysis". That's because the first 4 messages (i.e. the handshake) are fixed length (288, 304, 448, and 48 bytes).

By adding random amounts of random data to each of the messages, we can make it a lot harder.

The authors acknowledge that standard security practices would suggest to use an existing protocol such as TLS, but this is [Prop104] and it has problems of its own. Wherever appropriate, "future work" paragraphs have been added to indicate missing features or subjects of discussion.

Design Goals

  • Support NTCP 1 and 2 on a single port, auto-detect, and published as a single "transport" (i.e. [RouterAddress]) in the [NetDB].
  • Publish support for version 1 only, 2 only, or 1+2 in the NetDB in a separate field, and default to version 1 only (don't bind version support to a particular router version)
  • Ensure that all implementations (Java/i2pd/Kovri/go) can add version 2 support (or not) on their own schedules
  • Add random padding to all NTCP messages including handshake and data messages (i.e. length obfuscation so all messages aren't a multiple of 16 bytes) Provide options mechanism for both sides to request min and max padding and/or padding distribution. Specifics of the padding distribution are implementation-dependent and may or may not be specified in the protocol itself.
  • Obfuscate the contents of messages that aren't encrypted (1 and 2), sufficiently so that DPI boxes and AV signatures can't easily classify them. Also ensure that the messages going to a single peer or set of peers do not have a similar pattern of bits.
  • Fix loss of bits in DH due to Java format [Ticket1112], possibly (probably?) by switching to X25519.
  • Switch to a real key derivation function (KDF) rather than using the DH result as-is?
  • Add "probing resistance" (as Tor calls it); this includes replay resistance.
  • Maintain 2-way authenticated key exchange (2W-AKE). 1W-AKE is not sufficient for our application.
  • Continue to use the variable-type, variable-length signatures (from the published [RouterIdentity] signing key) as a part of authentication. Rely on a static public key published in the RouterInfo as another part of authentication.
  • Add options/version in handshake for future extensibility.
  • Add resistance to malicious MitM TCP segmentation if possible.
  • Don't add significantly to CPU required for connection setup; if possible, reduce it significantly.
  • Add message authentication (MAC), possibly HMAC-SHA256 and Poly1305, and remove Adler checksum.
  • Shorten and simplify I2NP header: Shorten expiration to 4 bytes, as in SSU. Remove one-byte truncated SHA256 checksum.
  • If possible, reduce the 4-message, two-round-trip handshake to a 3-message, one-round-trip handshake, as in [SSU]. This would require moving Bob's signature in message 4 to message 2. Research the reason for 4 messages in the ten-year-old email/status/meeting archives.
  • Minimize protocol overhead before padding. While padding will be added, and possibly lots of it, overhead before padding is still overhead. Low-bandwidth nodes must be able to use NTCP2.
  • Maintain timestamps for replay and skew detection.
  • Avoid any year 2038 issues in timestamps, must work until at least 2106.
  • Increase max message size from 16K to 32K or 64K.
  • Any new cryptographic primitives should be readily available in libraries for use in Java (1.7), C++, and Go router implementations.
  • Include representatives of Java, C++, and Go router developers in the design.
  • Minimize changes (but there will still be a lot).
  • Support both versions in a common set of code (this may not be possible and is implementation-dependent in any case).

Non-Goals

  • Bullet-proof DPI resistance... that would be pluggable transports, [Prop109].
  • A TLS-based (or HTTPS-lookalike) transport... that would be [Prop104].
  • It's OK to change the symmetric stream cryptography.
  • Timing-based DPI resistance (inter-message timing/delays can be implementation-dependent; intra-message delays can be introduced at any point, including before sending the random padding, for example). Artificial delays (what obfs4 calls IAT or inter-arrival time) are independent of the protocol itself.
  • Deniability of participating in a session (there's signatures in there).

Non-goals that may be partially reconsidered or discussed:

  • The degree of protection against Deep Packet Inspection (DPI)
  • Post-Quantum (PQ) security
  • Deniability

Security Goals

We consider three parties:

  • Alice, who wishes to establish a new session.
  • Bob, with whom Alice wishes to establish a session.
  • Mallory, the "man in the middle" between Alice and Bob.

At most two participants can engage in active attacks.

Alice and Bob are both in possession of a static key pair, which is contained in their [RouterIdentity].

The proposed protocol attempts to allow Alice and Bob to agree on a shared secret key (K) under the following requirements:

  1. Private key security: neither Bob nor Mallory learns anything about Alice's static private key. Symmetrically, Alice does not learn anything about Bob's static private key.
  2. The session key K is only known by Alice and Bob.
  3. Perfect forward secrecy: the agreed upon session key remains secret in the future, even when the static private keys of Alice and/or Bob are revealed after the key has been agreed upon.
  4. Two-way authentication: Alice is certain that she has established a session with Bob, and vice versa.
  5. Protection against online DPI: Ensure that it is not trivial to detect that Alice and Bob are engaged in the protocol using only straightforward deep packet inspection (DPI) techniques. See below.
  6. Limited deniability: neither Alice nor Bob can deny participation in the protocol, but if either leaks the shared key the other party can deny the authenticity of the contents of the transmitted data.

The present proposal attempts to provide all five requirements based on the Station-To-Station (STS) protocol [STS]. Note that this protocol is also the basis for the [SSU] protocol.

Additional DPI Discussion

We assume two DPI components:

1) Online DPI

Online DPI inspecting all flows in real-time. Connections may be blocked or otherwise tampered with. Connection data or metadata may be identified and stored for offline analysis. The online DPI does not have access to the I2P network database. The online DPI has only limited real-time computational capability, including length calculation, field inspection, and simple calculations such as XOR. The online DPI does have the capability of fast real-time cryptographic functions such as AES, AEAD, and hashing, but these would be too expensive to apply to most or all flows. Any application of these cryptographic operations would apply only to flows on IP/Port combinations previously identified by offline analysis. The online DPI does not have the capability of high-overhead cryptographic functions such as DH or elligator2. The online DPI is not designed specifically to detect I2P, although it may have limited classification rules for that purpose.

It is a goal to prevent protocol identification by an online DPI.

The notion of online or "straightforward" DPI is here taken to include the following adversary capabilities:

  1. The ability to inspect all data sent or received by the target.
  2. The ability to perform operations on the observed data, such as applying block ciphers or hash functions.
  3. The ability to store and compare with previously sent messages.
  4. The ability to modify, delay or fragment packets.

However, the online DPI is assumed to have the following restrictions:

  1. The inability to map IP addresses to router hashes. While this is trivial with real-time access to the network database, it would require a DPI system specifically designed to target I2P.
  2. The inability to use timing information to detect the protocol.
  3. Generally speaking, the online DPI toolbox does not contain any built-in tools that are specifically designed for I2P detection. This includes creating "honeypots", which would for example include nonrandom padding in their messages. Note that this does not exclude machine learning systems or highly configurable DPI tools as long as they meet the other requirements.

To counter payload analysis, it is ensured that all messages are indistinguishable from random. This also requires their length to be random, which is more complicated than just adding random padding. In fact, in Appendix A, the authors argue that a naive (i.e. uniform) padding scheme does not resolve the problem. Appendix A therefore proposes to include either random delays or to develop an alternate padding scheme that can provide reasonable protection for the proposed attack.

To protect against the sixth entry above, implementations should include random delays in the protocol. Such techniques are not covered by this proposal, but they could also resolve the padding length issues. In summary, the proposal provides good protection against payload analysis (when the considerations in Appendix A are taken into account), but only limited protection against flow analysis.

2) Offline DPI

Offline DPI inspecting data stored by the online DPI for later analysis. The offline DPI may be designed specifically to detect I2P. The offline DPI does have real-time access to the I2P network database. The offline DPI does have access to this and other I2P specifications. The offline DPI has unlimited computational capability, including all cryptographic functions defined in this specification.

The offline DPI does not have the ability to block existing connections. The offline DPI does have the capability to do near-realtime (within minutes of setup) sending to host/port of parties, for example TCP RST. The offline DPI does have the capability to do near-realtime (within minutes of setup) replay of previous messages (modified or not) for "probing" or other reasons.

It is not a goal to prevent protocol identification by an offline DPI. All decoding of obfuscated data in the first two messages, which is implemented by I2P routers, may also be implemented by the offline DPI.

It is a goal to reject attempted connections using replay of previous messages.

Future work

  • Consider the behavior of the protocol when packets are dropped or reordered by an attacker. Recent interesting work in this area can be found in [IACR-1150].
  • Provide a more accurate classification of DPI systems, taking into account the existing literature related to the subject.
  • Discuss the formal security of the proposed protocol, ideally taking into account the DPI attacker model.

Noise Protocol Framework

This proposal provides the requirements based on the Noise Protocol Framework [NOISE] (Revision 33, 2017-10-04). Noise has similar properties to the Station-To-Station protocol [STS], which is the basis for the [SSU] protocol. In Noise parlance, Alice is the initiator, and Bob is the responder.

NTCP2 is based on the Noise protocol Noise_XK_25519_ChaChaPoly_SHA256. (The actual identifier for the initial key derivation function is "Noise_XKaesobfse+hs2+hs3_25519_ChaChaPoly_SHA256" to indicate I2P extensions - see KDF 1 section below) This Noise protocol uses the following primitives:

  • Handshake Pattern: XK Alice transmits her key to Bob (X) Alice knows Bob's static key already (K)
  • DH Function: X25519 X25519 DH with a key length of 32 bytes as specified in [RFC-7748].
  • Cipher Function: ChaChaPoly AEAD_CHACHA20_POLY1305 as specified in [RFC-7539] section 2.8. 12 byte nonce, with the first 4 bytes set to zero.
  • Hash Function: SHA256 Standard 32-byte hash, already used extensively in I2P.

Additions to the Framework

This proposal defines the following enhancements to Noise_XK_25519_ChaChaPoly_SHA256. These generally follow the guidelines in [NOISE] section 13.

  1. Cleartext ephemeral keys are obfuscated with AES encryption using a known key and IV. This is quicker than elligator2.
  2. Random cleartext padding is added to messages 1 and 2. The cleartext padding is included in the handshake hash (MixHash) calculation. See the KDF sections below for message 2 and message 3 part 1. Random AEAD padding is added to message 3 and data phase messages.
  3. A two-byte frame length field is added, as is required for Noise over TCP, and as in obfs4. This is used in the data phase messages only. Message 1 and 2 AEAD frames are fixed length. Message 3 part 1 AEAD frame is fixed length. Message 3 part 2 AEAD frame length is specified in message 1.
  4. The two-byte frame length field is obfuscated with SipHash-2-4, as in obfs4.
  5. The payload format is defined for messages 1,2,3, and the data phase. Of course, this is not defined in Noise.

New Cryptographic Primitives for I2P

Existing I2P router implementations will require implementations for the following standard cryptographic primitives, which are not required for current I2P protocols:

  1. X25519 key generation and DH
  2. AEAD_ChaCha20_Poly1305 (abbreviated as ChaChaPoly below)
  3. SipHash-2-4

Processing overhead estimate

Message sizes for the 3 messages:

  1. 64 bytes + padding (NTCP was 288 bytes)
  2. 64 bytes + padding (NTCP was 304 bytes)
  3. approx. 64 bytes + Alice router info + padding Average router info is about 750 bytes Total average 814 bytes before padding (NTCP was 448 bytes)
  4. not required in NTCP2 (NTCP was 48 bytes)

Total before padding: NTCP2: 942 bytes NTCP: 1088 bytes Note that if Alice connected to Bob for the purpose of sending a DatabaseStore Message of her RouterInfo, that message is not required, saving approximately 800 bytes.

The following cryptographic operations are required by each party to complete the handshake and start the data phase:

  • AES: 2
  • SHA256: 7 (Alice), 6 (Bob) (not including 1 Alice, 2 Bob precalculated for all connections) (not including HMAC-SHA256)
  • HMAC-SHA256: 19
  • ChaChaPoly: 4
  • X25519 key generation: 1
  • X25519 DH: 3
  • Signature verification: 1 (Bob) (Alice previously signed when generating her RI) Presumably Ed25519 (dependent on RI signature type)

The following cryptographic operations are required by each party for each data phase message:

  • SipHash: 1
  • ChaChaPoly: 1

Messages

All NTCP2 messages are less than or equal to 65537 bytes in length. The message format is based on Noise messages, with modifications for framing and indistinguishability. Implementations using standard Noise libraries may need to pre-process received messages to/from the Noise message format. All encrypted fields are AEAD ciphertexts.

The establishment sequence is as follows:

Alice                           Bob

SessionRequest ------------------->
<------------------- SessionCreated
SessionConfirmed ----------------->

Using Noise terminology, the establishment and data sequence is as follows: (Payload Security Properties)

XK(s, rs):           Authentication   Confidentiality
  <- s
  ...
  -> e, es                  0                2
  <- e, ee                  2                1
  -> s, se                  2                5
  <-                        2                5

Once a session has been established, Alice and Bob can exchange Data messages.

All message types (SessionRequest, SessionCreated, SessionConfirmed, Data and TimeSync) are specified in this section.

Some notations:

- RH_A = Router Hash for Alice (32 bytes)
- RH_B = Router Hash for Bob (32 bytes)

Authenticated Encryption

There are three separate authenticated encryption instances (CipherStates). One during the handshake phase, and two (transmit and receive) for the data phase. Each has its own key from a KDF.

Encrypted/authenticated data will be represented as

+----+----+----+----+----+----+----+----+
|                                       |
+                                       +
|   Encrypted and authenticated data    |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

ChaCha20/Poly1305

Encrypted and authenticated data format.

Inputs to the encryption/decryption functions:

k :: 32 byte cipher key, as generated from KDF

nonce :: Counter-based nonce, 12 bytes.
         Starts at 0 and incremented for each message.
         First four bytes are always zero.
         Last eight bytes are the counter, little-endian encoded.
         Maximum value is 2**64 - 2.
         Connection must be dropped and restarted after
         it reaches that value.
         The value 2**64 - 1 must never be sent.

ad :: In handshake phase:
      Associated data, 32 bytes.
      The SHA256 hash of all preceding data.
      In data phase:
      Zero bytes

data :: Plaintext data, 0 or more bytes

Output of the encryption function, input to the decryption function:

+----+----+----+----+----+----+----+----+
|Obfs Len |                             |
+----+----+                             +
|       ChaCha20 encrypted data         |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+
|  Poly1305 Message Authentication Code |
+              (MAC)                    +
|             16 bytes                  |
+----+----+----+----+----+----+----+----+

Obfs Len :: Length of (encrypted data + MAC) to follow, 16 - 65535
            Obfuscation using SipHash (see below)
            Not used in message 1 or 2, or message 3 part 1, where the length is fixed
            Not used in message 3 part 1, as the length is specified in message 1

encrypted data :: Same size as plaintext data, 0 - 65519 bytes

MAC :: Poly1305 message authentication code, 16 bytes

For ChaCha20, what is described here corresponds to [RFC-7539], which is also used similarly in TLS [RFC-7905].

Notes

  • Since ChaCha20 is a stream cipher, plaintexts need not be padded. Additional keystream bytes are discarded.
  • The key for the cipher (256 bits) is agreed upon by means of the SHA256 KDF. The details of the KDF for each message are in separate sections below.
  • ChaChaPoly frames for messages 1, 2, and the first part of message 3, are of known size. Starting with the second part of message 3, frames are of variable size. The message 3 part 1 size is specified in message 1. Starting with the data phase, frames are prepended with a two-byte length obfuscated with SipHash as in obfs4.
  • Padding is outside the authenticated data frame for messages 1 and 2. The padding is used in the KDF for the next message so tampering will be detected. Starting in message 3, padding is inside the authenticated data frame.

AEAD Error Handling

  • In messages 1, 2, and message 3 parts 1 and 2, the AEAD message size is known in advance. On an AEAD authentication failure, recipient must halt further message processing and close the connection without responding. This should be an abnormal close (TCP RST).
  • For probing resistance, in message 1, after an AEAD failure, Bob should set a random timeout (range TBD) and then read a random number of bytes (range TBD) before closing the socket. Bob should maintain a blacklist of IPs with repeated failures.
  • In the data phase, the AEAD message size is "encrypted" (obfuscated) with SipHash. Care must be taken to avoid creating a decryption oracle. On a data phase AEAD authentication failure, the recipient should set a random timeout (range TBD) and then read a random number of bytes (range TBD). After the read, or on read timeout, the recipient should send a payload with a termination block containing an "AEAD failure" reason code, and close the connection.
  • Take the same error action for an invalid length field value in the data phase.

Key Derivation Function (KDF) (for handshake message 1)

The KDF generates a handshake phase cipher key k from the DH result, using HMAC-SHA256(key, data) as defined in [RFC-2104]. These are the InitializeSymmetric(), MixHash(), and MixKey() functions, exactly as defined in the Noise spec.

This is the "e" message pattern:

// Define protocol_name.
Set protocol_name = "Noise_XKaesobfse+hs2+hs3_25519_ChaChaPoly_SHA256"
 (48 bytes, US-ASCII encoded, no NULL termination).

// Define Hash h = 32 bytes
h = SHA256(protocol_name);

Define ck = 32 byte chaining key. Copy the h data to ck.
Set ck = h

Define rs = Bob's 32-byte static key as published in the RouterInfo

// MixHash(null prologue)
h = SHA256(h);

// up until here, can all be precalculated by Alice for all outgoing connections

// Alice must validate that Bob's static key is a valid point on the curve here.

// Bob static key
// MixHash(rs)
// || below means append
h = SHA256(h || rs);

// up until here, can all be precalculated by Bob for all incoming connections

This is the "e" message pattern:

Alice generates her ephemeral DH key pair e.

// Alice ephemeral key X
// MixHash(e.pubkey)
// || below means append
h = SHA256(h || e.pubkey);

// h is used as the associated data for the AEAD in message 1
// Retain the Hash h for the message 2 KDF


End of "e" message pattern.

This is the "es" message pattern:

// DH(e, rs) == DH(s, re)
Define input_key_material = 32 byte DH result of Alice's ephemeral key and Bob's static key
Set input_key_material = X25519 DH result

// MixKey(DH())

Define temp_key = 32 bytes
Define HMAC-SHA256(key, data) as in [RFC-2104]_
// Generate a temp key from the chaining key and DH result
// ck is the chaining key, defined above
temp_key = HMAC-SHA256(ck, input_key_material)
// overwrite the DH result in memory, no longer needed
input_key_material = (all zeros)

// Output 1
// Set a new chaining key from the temp key
// byte() below means a single byte
ck =       HMAC-SHA256(temp_key, byte(0x01)).

// Output 2
// Generate the cipher key k
Define k = 32 bytes
// || below means append
// byte() below means a single byte
k =        HMAC-SHA256(temp_key, ck || byte(0x02)).
// overwrite the temp_key in memory, no longer needed
temp_key = (all zeros)

// retain the chaining key ck for message 2 KDF


End of "es" message pattern.

1) SessionRequest

Alice sends to Bob.

Noise content: Alice's ephemeral key X Noise payload: 16 byte option block Non-noise payload: Random padding

(Payload Security Properties)

XK(s, rs):           Authentication   Confidentiality
  -> e, es                  0                2

  Authentication: None (0).
  This payload may have been sent by any party, including an active attacker.

  Confidentiality: 2.
  Encryption to a known recipient, forward secrecy for sender compromise
  only, vulnerable to replay.  This payload is encrypted based only on DHs
  involving the recipient's static key pair.  If the recipient's static
  private key is compromised, even at a later date, this payload can be
  decrypted.  This message can also be replayed, since there's no ephemeral
  contribution from the recipient.

  "e": Alice generates a new ephemeral key pair and stores it in the e
       variable, writes the ephemeral public key as cleartext into the
       message buffer, and hashes the public key along with the old h to
       derive a new h.

  "es": A DH is performed between the Alice's ephemeral key pair and the
        Bob's static key pair.  The result is hashed along with the old ck to
        derive a new ck and k, and n is set to zero.

The X value is encrypted to ensure payload indistinguishably and uniqueness, which are necessary DPI countermeasures. We use AES encryption to achieve this, rather than more complex and slower alternatives such as elligator2. Asymmetric encryption to Bob's router public key would be far too slow. AES encryption uses Bob's router hash as the key and Bob's IV as published in the network database.

AES encryption is for DPI resistance only. Any party knowing Bob's router hash, and IV, which are published in the network database, may decrypt the X value in this message.

The padding is not encrypted by Alice. It may be necessary for Bob to decrypt the padding, to inhibit timing attacks.

Raw contents:

+----+----+----+----+----+----+----+----+
|                                       |
+        obfuscated with RH_B           +
|       AES-CBC-256 encrypted X         |
+             (32 bytes)                +
|                                       |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
|                                       |
+                                       +
|   ChaChaPoly frame                    |
+             (32 bytes)                +
|   k defined in KDF for message 1      |
+   n = 0                               +
|   see KDF for associated data         |
+----+----+----+----+----+----+----+----+
|     unencrypted authenticated         |
~         padding (optional)            ~
|     length defined in options block   |
+----+----+----+----+----+----+----+----+

X :: 32 bytes, AES-256-CBC encrypted X25519 ephemeral key, little endian
        key: RH_B
        iv: As published in Bobs network database entry

padding :: Random data, 0 or more bytes.
           Total message length must be 65535 bytes or less.
           Total message length must be 287 bytes or less if
           Bob is publishing his address as NTCP
           (see Version Detection section below).
           Alice and Bob will use the padding data in the KDF for message 2.
           It is authenticated so that any tampering will cause the
           next message to fail.

Unencrypted data (Poly1305 authentication tag not shown):

+----+----+----+----+----+----+----+----+
|                                       |
+                                       +
|                   X                   |
+              (32 bytes)               +
|                                       |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
|               options                 |
+              (16 bytes)               +
|                                       |
+----+----+----+----+----+----+----+----+
|     unencrypted authenticated         |
+         padding (optional)            +
|     length defined in options block   |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

X :: 32 bytes, X25519 ephemeral key, little endian

options :: options block, 16 bytes, see below

padding :: Random data, 0 or more bytes.
           Total message length must be 65535 bytes or less.
           Total message length must be 287 bytes or less if
           Bob is publishing his address as "NTCP"
           (see Version Detection section below)
           Alice and Bob will use the padding data in the KDF for message 2.
           It is authenticated so that any tampering will cause the
           next message to fail.

Options block: Note: All fields are big-endian.

+----+----+----+----+----+----+----+----+
|Rsvd| ver|  padLen | m3p2len | Rsvd(0) |
+----+----+----+----+----+----+----+----+
|        tsA        |   Reserved (0)    |
+----+----+----+----+----+----+----+----+

Reserved :: 7 bytes total, set to 0 for compatibility with future options

ver :: 1 byte, protocol version (currently 2)

padLen :: 2 bytes, length of the padding, 0 or more
          Min/max guidelines TBD. Random size from 0 to 31 bytes minimum?
          (Distribution to be determined, see Appendix A.)

m3p2Len :: 2 bytes, length of the the second AEAD frame in SessionConfirmed
           (message 3 part 2) See notes below

tsA :: 4 bytes, Unix timestamp, unsigned seconds.
       Wraps around in 2106

Reserved :: 4 bytes, set to 0 for compatibility with future options

Notes

  • When the published address is "NTCP", Bob supports both NTCP and NTCP2 on the same port. For compatibility, when initiating a connection to an address published as "NTCP", Alice must limit the maximum size of this message, including padding, to 287 bytes or less. This facilitates automatic protocol identification by Bob. When published as "NTCP2", there is no size restriction. See the Published Addresses and Version Detection sections below.
  • The unique X value in the initial AES block ensure that the ciphertext is different for every session.
  • Bob must reject connections where the timestamp value is too far off from the current time. Call the maximum delta time "D". Bob must maintain a local cache of previously-used handshake values and reject duplicates, to prevent replay attacks. Values in the cache must have a lifetime of at least 2*D. The cache values are implementation-dependent, however the 32-byte X value (or its encrypted equivalent) may be used.
  • Diffie-Hellman ephemeral keys may never be reused, to prevent cryptographic attacks, and reuse will be rejected as a replay attack.
  • The "KE" and "auth" options must be compatible, i.e. the shared secret K must be of the appropriate size. If more "auth" options are added, this could implicitly change the meaning of the "KE" flag to use a different KDF or a different truncation size.
  • Bob must validate that Alice's ephemeral key is a valid point on the curve here.
  • Padding should be limited to a reasonable amount. Bob may reject connections with excessive padding. Bob will specify his padding options in message 2. Min/max guidelines TBD. Random size from 0 to 31 bytes minimum? (Distribution to be determined, see Appendix A.)
  • On any error, including AEAD, DH, timestamp, apparent replay, or key validation failure, Bob must halt further message processing and close the connection without responding. This should be an abnormal close (TCP RST). For probing resistance, after an AEAD failure, Bob should set a random timeout (range TBD) and then read a random number of bytes (range TBD), before closing the socket.
  • DoS Mitigation: DH is a relatively expensive operation. As with the previous NTCP protocol, routers should take all necessary measures to prevent CPU or connection exhaustion. Place limits on maximum active connections and maximum connection setups in progress. Enforce read timeouts (both per-read and total for "slowloris"). Limit repeated or simultaneous connections from the same source. Maintain blacklists for sources that repeatedly fail. Do not respond to AEAD failure.
  • To facilitate rapid version detection and handshaking, implementations must ensure that Alice buffers and then flushes the entire contents of the first message at once, including the padding. This increases the likelihood that the data will be contained in a single TCP packet (unless segmented by the OS or middleboxes), and received all at once by Bob. Additionally, implementations must ensure that Bob buffers and then flushes the entire contents of the second message at once, including the padding. and that Bob buffers and then flushes the entire contents of the third message at once. This is also for efficiency and to ensure the effectiveness of the random padding.
  • "ver" field: The overall Noise protocol, extensions, and NTCP protocol including payload specifications, indicating NTCP2. This field may be used to indicate support for future changes.
  • Message 3 part 2 length: This is the size of the second AEAD frame (including 16-byte MAC) containing Alice's Router Info and optional padding that will be sent in the SessionConfirmed message. As routers periodically regenerate and republish their Router Info, the size of the current Router Info may change before message 3 is sent. Implementations must choose one of two strategies: a) save the current Router Info to be sent in message 3, so the size is known, and optionally add room for padding; b) increase the specified size enough to allow for possible increase in the Router Info size, and always add padding when message 3 is actually sent. In either case, the "m3p2len" length included in message 1 must be exactly the size of that frame when sent in message 3.
  • Bob must fail the connection if any incoming data remains after validating message 1 and reading in the padding. There should be no extra data from Alice, as Bob has not responded with message 2 yet.

Issues

  • Is the fixed-size option block big enough?

Key Derivation Function (KDF) (for handshake message 2 and message 3 part 1)

  // take h saved from message 1 KDF
// MixHash(ciphertext)
h = SHA256(h || 32 byte encrypted payload from message 1)

// MixHash(padding)
// Only if padding length is nonzero
h = SHA256(h || random padding from message 1)

This is the "e" message pattern:

Bob generates his ephemeral DH key pair e.

// h is from KDF for handshake message 1
// Bob ephemeral key Y
// MixHash(e.pubkey)
// || below means append
h = SHA256(h || e.pubkey);

// h is used as the associated data for the AEAD in message 2
// Retain the Hash h for the message 3 KDF

End of "e" message pattern.

This is the "ee" message pattern:

// DH(e, re)
Define input_key_material = 32 byte DH result of Alice's ephemeral key and Bob's ephemeral key
Set input_key_material = X25519 DH result
// overwrite Alice's ephemeral key in memory, no longer needed
// Alice:
e(public and private) = (all zeros)
// Bob:
re = (all zeros)

// MixKey(DH())

Define temp_key = 32 bytes
Define HMAC-SHA256(key, data) as in [RFC-2104]_
// Generate a temp key from the chaining key and DH result
// ck is the chaining key, from the KDF for handshake message 1
temp_key = HMAC-SHA256(ck, input_key_material)
// overwrite the DH result in memory, no longer needed
input_key_material = (all zeros)

// Output 1
// Set a new chaining key from the temp key
// byte() below means a single byte
ck =       HMAC-SHA256(temp_key, byte(0x01)).

// Output 2
// Generate the cipher key k
Define k = 32 bytes
// || below means append
// byte() below means a single byte
k =        HMAC-SHA256(temp_key, ck || byte(0x02)).
// overwrite the temp_key in memory, no longer needed
temp_key = (all zeros)

// retain the chaining key ck for message 3 KDF

End of "es" message pattern.

2) SessionCreated

Bob sends to Alice.

Noise content: Bob's ephemeral key Y Noise payload: 16 byte option block Non-noise payload: Random padding

(Payload Security Properties)

XK(s, rs):           Authentication   Confidentiality
  <- e, ee                  2                1

  Authentication: 2.
  Sender authentication resistant to key-compromise impersonation (KCI).
  The sender authentication is based on an ephemeral-static DH ("es" or "se")
  between the sender's static key pair and the recipient's ephemeral key pair.
  Assuming the corresponding private keys are secure, this authentication cannot be forged.

  Confidentiality: 1.
  Encryption to an ephemeral recipient.
  This payload has forward secrecy, since encryption involves an ephemeral-ephemeral DH ("ee").
  However, the sender has not authenticated the recipient,
  so this payload might be sent to any party, including an active attacker.


  "e": Bob generates a new ephemeral key pair and stores it in the e variable,
  writes the ephemeral public key as cleartext into the message buffer,
  and hashes the public key along with the old h to derive a new h.

  "ee": A DH is performed between the Bob's ephemeral key pair and the Alice's ephemeral key pair.
  The result is hashed along with the old ck to derive a new ck and k, and n is set to zero.

The Y value is encrypted to ensure payload indistinguishably and uniqueness, which are necessary DPI countermeasures. We use AES encryption to achieve this, rather than more complex and slower alternatives such as elligator2. Asymmetric encryption to Alice's router public key would be far too slow. AES encryption uses Bob's router hash as the key and the AES state from message 1 (which was initialized with Bob's IV as published in the network database).

AES encryption is for DPI resistance only. Any party knowing Bob's router hash and IV, which are published in the network database, and captured the first 32 bytes of message 1, may decrypt the Y value in this message.

Raw contents:

+----+----+----+----+----+----+----+----+
|                                       |
+        obfuscated with RH_B           +
|       AES-CBC-256 encrypted Y         |
+              (32 bytes)               +
|                                       |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
|   ChaChaPoly frame                    |
+   Encrypted and authenticated data    +
|   32 bytes                            |
+   k defined in KDF for message 2      +
|   n = 0; see KDF for associated data  |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
|     unencrypted authenticated         |
+         padding (optional)            +
|     length defined in options block   |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

Y :: 32 bytes, AES-256-CBC encrypted X25519 ephemeral key, little endian
        key: RH_B
        iv: Using AES state from message 1

Unencrypted data (Poly1305 auth tag not shown):

+----+----+----+----+----+----+----+----+
|                                       |
+                                       +
|                  Y                    |
+              (32 bytes)               +
|                                       |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
|               options                 |
+              (16 bytes)               +
|                                       |
+----+----+----+----+----+----+----+----+
|     unencrypted authenticated         |
+         padding (optional)            +
|     length defined in options block   |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

Y :: 32 bytes, X25519 ephemeral key, little endian

options :: options block, 16 bytes, see below

padding :: Random data, 0 or more bytes.
           Total message length must be 65535 bytes or less.
           Alice and Bob will use the padding data in the KDF for message 3 part 1.
           It is authenticated so that any tampering will cause the
           next message to fail.

Notes

  • Alice must validate that Bob's ephemeral key is a valid point on the curve here.
  • Padding should be limited to a reasonable amount. Alice may reject connections with excessive padding. Alice will specify her padding options in message 3. Min/max guidelines TBD. Random size from 0 to 31 bytes minimum? (Distribution to be determined, see Appendix A.)
  • On any error, including AEAD, DH, timestamp, apparent replay, or key validation failure, Alice must halt further message processing and close the connection without responding. This should be an abnormal close (TCP RST).
  • To facilitate rapid handshaking, implementations must ensure that Bob buffers and then flushes the entire contents of the first message at once, including the padding. This increases the likelihood that the data will be contained in a single TCP packet (unless segmented by the OS or middleboxes), and received all at once by Alice. This is also for efficiency and to ensure the effectiveness of the random padding.
  • Alice must fail the connection if any incoming data remains after validating message 2 and reading in the padding. There should be no extra data from Bob, as Alice has not responded with message 3 yet.

Options block: Note: All fields are big-endian.

+----+----+----+----+----+----+----+----+
| Rsvd(0) | padLen  |   Reserved (0)    |
+----+----+----+----+----+----+----+----+
|        tsB        |   Reserved (0)    |
+----+----+----+----+----+----+----+----+

Reserved :: 10 bytes total, set to 0 for compatibility with future options

padLen :: 2 bytes, big endian, length of the padding, 0 or more
          Min/max guidelines TBD. Random size from 0 to 31 bytes minimum?
          (Distribution to be determined, see Appendix A.)

tsB :: 4 bytes, big endian, Unix timestamp, unsigned seconds.
       Wraps around in 2106

Notes

  • Alice must reject connections where the timestamp value is too far off from the current time. Call the maximum delta time "D". Alice must maintain a local cache of previously-used handshake values and reject duplicates, to prevent replay attacks. Values in the cache must have a lifetime of at least 2*D. The cache values are implementation-dependent, however the 32-byte Y value (or its encrypted equivalent) may be used.

Issues

  • Include min/max padding options here?

Encryption for for handshake message 3 part 1, using message 2 KDF)

  // take h saved from message 2 KDF
// MixHash(ciphertext)
h = SHA256(h || 24 byte encrypted payload from message 2)

// MixHash(padding)
// Only if padding length is nonzero
h = SHA256(h || random padding from message 2)
// h is used as the associated data for the AEAD in message 3 part 1, below

This is the "s" message pattern:

Define s = Alice's static public key, 32 bytes

// EncryptAndHash(s.publickey)
// EncryptWithAd(h, s.publickey)
// AEAD_ChaCha20_Poly1305(key, nonce, associatedData, data)
// k is from handshake message 1
// n is 1
ciphertext = AEAD_ChaCha20_Poly1305(k, n++, h, s.publickey)
// MixHash(ciphertext)
// || below means append
h = SHA256(h || ciphertext);

// h is used as the associated data for the AEAD in message 3 part 2

End of "s" message pattern.

Key Derivation Function (KDF) (for handshake message 3 part 2)

This is the "se" message pattern:

// DH(s, re) == DH(e, rs)
Define input_key_material = 32 byte DH result of Alice's static key and Bob's ephemeral key
Set input_key_material = X25519 DH result
// overwrite Bob's ephemeral key in memory, no longer needed
// Alice:
re = (all zeros)
// Bob:
e(public and private) = (all zeros)

// MixKey(DH())

Define temp_key = 32 bytes
Define HMAC-SHA256(key, data) as in [RFC-2104]_
// Generate a temp key from the chaining key and DH result
// ck is the chaining key, from the KDF for handshake message 1
temp_key = HMAC-SHA256(ck, input_key_material)
// overwrite the DH result in memory, no longer needed
input_key_material = (all zeros)

// Output 1
// Set a new chaining key from the temp key
// byte() below means a single byte
ck =       HMAC-SHA256(temp_key, byte(0x01)).

// Output 2
// Generate the cipher key k
Define k = 32 bytes
// || below means append
// byte() below means a single byte
k =        HMAC-SHA256(temp_key, ck || byte(0x02)).

// h from message 3 part 1 is used as the associated data for the AEAD in message 3 part 2

// EncryptAndHash(payload)
// EncryptWithAd(h, payload)
// AEAD_ChaCha20_Poly1305(key, nonce, associatedData, data)
// n is 0
ciphertext = AEAD_ChaCha20_Poly1305(k, n++, h, payload)
// MixHash(ciphertext)
// || below means append
h = SHA256(h || ciphertext);

// retain the chaining key ck for the data phase KDF
// retain the hash h for the data phase Additional Symmetric Key (SipHash) KDF

End of "se" message pattern.

// overwrite the temp_key in memory, no longer needed
temp_key = (all zeros)

3) SessionConfirmed

Alice sends to Bob.

Noise content: Alice's static key Noise payload: Alice's RouterInfo and random padding Non-noise payload: none

(Payload Security Properties)

XK(s, rs):           Authentication   Confidentiality
  -> s, se                  2                5

  Authentication: 2.
  Sender authentication resistant to key-compromise impersonation (KCI).  The
  sender authentication is based on an ephemeral-static DH ("es" or "se")
  between the sender's static key pair and the recipient's ephemeral key
  pair.  Assuming the corresponding private keys are secure, this
  authentication cannot be forged.

  Confidentiality: 5.
  Encryption to a known recipient, strong forward secrecy.  This payload is
  encrypted based on an ephemeral-ephemeral DH as well as an ephemeral-static
  DH with the recipient's static key pair.  Assuming the ephemeral private
  keys are secure, and the recipient is not being actively impersonated by an
  attacker that has stolen its static private key, this payload cannot be
  decrypted.

  "s": Alice writes her static public key from the s variable into the
  message buffer, encrypting it, and hashes the output along with the old h
  to derive a new h.

  "se": A DH is performed between the Alice's static key pair and the Bob's
  ephemeral key pair.  The result is hashed along with the old ck to derive a
  new ck and k, and n is set to zero.

This contains two ChaChaPoly frames. The first is Alice's encrypted static public key. The second is the Noise payload: Alice's encrypted RouterInfo, optional options, and optional padding. They use different keys, because the MixKey() function is called in between.

Raw contents:

+----+----+----+----+----+----+----+----+
|                                       |
+   ChaChaPoly frame (48 bytes)         +
|   Encrypted and authenticated         |
+   Alice static key S                  +
|      (32 bytes)                       |
+                                       +
|     k defined in KDF for message 2    |
+     n = 1                             +
|     see KDF for associated data       |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
|                                       |
+     Length specified in message 1     +
|                                       |
+   ChaChaPoly frame                    +
|   Encrypted and authenticated         |
+                                       +
|       Alice RouterInfo                |
+       using block format 2            +
|       Alice Options (optional)        |
+       using block format 1            +
|       Arbitrary padding               |
+       using block format 254          +
|                                       |
+                                       +
| k defined in KDF for message 3 part 2 |
+     n = 0                             +
|     see KDF for associated data       |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

S :: 32 bytes, ChaChaPoly encrypted Alice's X25519 static key, little endian
     inside 48 byte ChaChaPoly frame

Unencrypted data (Poly1305 auth tags not shown):

+----+----+----+----+----+----+----+----+
|                                       |
+                                       +
|              S                        |
+       Alice static key                +
|          (32 bytes)                   |
+                                       +
|                                       |
+                                       +
+----+----+----+----+----+----+----+----+
|                                       |
+                                       +
|                                       |
+                                       +
|       Alice RouterInfo block          |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+
|                                       |
+       Optional Options block          +
|                                       |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+
|                                       |
+       Optional Padding block          +
|                                       |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

S :: 32 bytes, Alice's X25519 static key, little endian

Notes

  • Bob must perform the usual Router Info validation. Ensure the signature type is supported, verify the signature, verify the timestamp is within bounds, and any other checks necessary.
  • Bob must verify that Alice's static key received in the first frame matches the static key in the Router Info. Bob must first search the Router Info for a NTCP or NTCP2 Router Address with a matching version (v) option. See Published Router Info and Unpublished Router Info sections below.
  • If Bob has an older version of Alice's RouterInfo in his netdb, verify that the static key in the router info is the same in both, if present, and if the older version is less than XXX old (see key rotate time below)
  • Bob must validate that Alice's static key is a valid point on the curve here.
  • Options should be included, to specify padding parameters.
  • On any error, including AEAD, RI, DH, timestamp, or key validation failure, Bob must halt further message processing and close the connection without responding. This should be an abnormal close (TCP RST).
  • To facilitate rapid handshaking, implementations must ensure that Alice buffers and then flushes the entire contents of the third message at once, including both AEAD frames. This increases the likelihood that the data will be contained in a single TCP packet (unless segmented by the OS or middleboxes), and received all at once by Bob. This is also for efficiency and to ensure the effectiveness of the random padding.
  • Message 3 part 2 frame length: The length of this frame (including MAC) is sent by Alice in message 1. See that message for important notes on allowing enough room for padding.
  • Message 3 part 2 frame content: This format of this frame is the same as the format of data phase frames, except that the length of the frame is sent by Alice in message 1. See below for the data phase frame format. The frame must contain 1 to 3 blocks in the following order: 1) Alice's Router Info block (required) 2) Options block (optional) 3) Padding block (optional) This frame must never contain any other block type.
  • Message 3 part 2 padding is not required if Alice appends a data phase frame (optionally containing padding) to the end of message 3 and sends both at once, as it will appear as one big stream of bytes to an observer. As Alice will generally, but not always, have an I2NP message to send to Bob (that's why she connected to him), this is the recommended implementation, for efficiency and to ensure the effectiveness of the random padding.
  • Total length of both Message 3 AEAD frames (parts 1 and 2) is 65535 bytes; part 1 is 48 bytes so part 2 max frame length is 65487; part 2 max plaintext length excluding MAC is 65471.

Key Derivation Function (KDF) (for data phase)

The data phase uses a zero-length associated data input.

The KDF generates two cipher keys k_ab and k_ba from the chaining key ck, using HMAC-SHA256(key, data) as defined in [RFC-2104]. This is the Split() function, exactly as defined in the Noise spec.

ck = from handshake phase

// k_ab, k_ba = HKDF(ck, zerolen)
// ask_master = HKDF(ck, zerolen, info="ask")

// zerolen is a zero-length byte array
temp_key = HMAC-SHA256(ck, zerolen)
// overwrite the chaining key in memory, no longer needed
ck = (all zeros)

// Output 1
// cipher key, for Alice transmits to Bob (Noise doesn't make clear which is which, but Java code does)
k_ab =   HMAC-SHA256(temp_key, byte(0x01)).

// Output 2
// cipher key, for Bob transmits to Alice (Noise doesn't make clear which is which, but Java code does)
k_ba =   HMAC-SHA256(temp_key, k_ab || byte(0x02)).


KDF for SipHash for length field:
Generate an Additional Symmetric Key (ask) for SipHash
SipHash uses two 8-byte keys (big endian) and 8 byte IV for first data.

// "ask" is 3 bytes, US-ASCII, no null termination
ask_master = HMAC-SHA256(temp_key, "ask" || byte(0x01))
// sip_master = HKDF(ask_master, h || "siphash")
// "siphash" is 7 bytes, US-ASCII, no null termination
// overwrite previous temp_key in memory
// h is from KDF for message 3 part 2
temp_key = HMAC-SHA256(ask_master, h || "siphash")
// overwrite ask_master in memory, no longer needed
ask_master = (all zeros)
sip_master = HMAC-SHA256(temp_key, byte(0x01))

Alice to Bob SipHash k1, k2, IV:
// sipkeys_ab, sipkeys_ba = HKDF(sip_master, zerolen)
// overwrite previous temp_key in memory
temp_key = HMAC-SHA256(sip_master, zerolen)
// overwrite sip_master in memory, no longer needed
sip_master = (all zeros)

sipkeys_ab = HMAC-SHA256(temp_key, byte(0x01)).
sipk1_ab = sipkeys_ab[0:7], little endian
sipk2_ab = sipkeys_ab[8:15], little endian
sipiv_ab = sipkeys_ab[16:23]

Bob to Alice SipHash k1, k2, IV:

sipkeys_ba = HMAC-SHA256(temp_key, sipkeys_ab || byte(0x02)).
sipk1_ba = sipkeys_ba[0:7], little endian
sipk2_ba = sipkeys_ba[8:15], little endian
sipiv_ba = sipkeys_ba[16:23]

// overwrite the temp_key in memory, no longer needed
temp_key = (all zeros)

4) Data Phase

Noise payload: As defined below, including random padding Non-noise payload: none

Starting with the 2nd part of message 3, all messages are inside an authenticated and encrypted ChaChaPoly "frame" with a prepended two-byte obfuscated length. All padding is inside the frame. Inside the frame is a standard format with zero or more "blocks". Each block has a one-byte type and a two-byte length. Types include date/time, I2NP message, options, termination, and padding.

Note: Bob may, but is not required, to send his RouterInfo to Alice as his first message to Alice in the data phase.

(Payload Security Properties)

XK(s, rs):           Authentication   Confidentiality
  <-                        2                5
  ->                        2                5

  Authentication: 2.
  Sender authentication resistant to key-compromise impersonation (KCI).
  The sender authentication is based on an ephemeral-static DH ("es" or "se")
  between the sender's static key pair and the recipient's ephemeral key pair.
  Assuming the corresponding private keys are secure, this authentication cannot be forged.

  Confidentiality: 5.
  Encryption to a known recipient, strong forward secrecy.
  This payload is encrypted based on an ephemeral-ephemeral DH as well as
  an ephemeral-static DH with the recipient's static key pair.
  Assuming the ephemeral private keys are secure, and the recipient is not being actively impersonated
  by an attacker that has stolen its static private key, this payload cannot be decrypted.

Notes

  • For efficiency and to minimize identification of the length field, implementations must ensure that the sender buffers and then flushes the entire contents of data messages at once, including the length field and the AEAD frame. This increases the likelihood that the data will be contained in a single TCP packet (unless segmented by the OS or middleboxes), and received all at once the other party. This is also for efficiency and to ensure the effectiveness of the random padding.
  • The router may choose to terminate the session on AEAD error, or may continue to attempt communications. If continuing, the router should terminate after repeated errors.

SipHash obfuscated length

Reference: [SipHash]

Once both sides have completed the handshake, they transfer payloads that are then encrypted and authenticated in ChaChaPoly "frames".

Each frame is preceded by a two-byte length, big endian. This length specifies the number of encrypted frame bytes to follow, including the MAC. To avoid transmitting identifiable length fields in stream, the frame length is obfuscated by XORing a mask derived from SipHash, as initialized from the data phase KDF. Note that the two directions have unique SipHash keys and IVs from the KDF.

      sipk1, sipk2 = The SipHash keys from the KDF.  (two 8-byte long integers)
    IV[0] = sipiv = The SipHash IV from the KDF. (8 bytes)
    length is big endian.
    For each frame:
      IV[n] = SipHash-2-4(sipk1, sipk2, IV[n-1])
      Mask[n] = First 2 bytes of IV[n]
      obfuscatedLength = length ^ Mask[n]

    The first length output will be XORed with with IV[1].

The receiver has the identical SipHash keys and IV. Decoding the length is done by deriving the mask used to obfsucate the length and XORing the truncated digest to obtain the length of the frame. The frame length is the total length of the encrypted frame including the MAC.

Notes

  • If you use a SipHash library function that returns an unsigned long integer, use the least significant two bytes as the Mask. Convert the long integer to the next IV as little endian.

Raw contents

+----+----+----+----+----+----+----+----+
|obf size |                             |
+----+----+                             +
|                                       |
+   ChaChaPoly frame                    +
|   Encrypted and authenticated         |
+   key is k_ab for Alice to Bob        +
|   key is k_ba for Bob to Alice        |
+   as defined in KDF for data phase    +
|   n starts at 0 and increments        |
+   for each frame in that direction    +
|   no associated data                  |
+   16 bytes minimum                    +
|                                       |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

obf size :: 2 bytes length obfuscated with SipHash
            when de-obfuscated: 16 - 65535

Minimum size including length field is 18 bytes.
Maximum size including length field is 65537 bytes.
Obfuscated length is 2 bytes.
Maximum ChaChaPoly frame is 65535 bytes.

Unencrypted data

There are zero or more blocks in the encrypted frame. Each block contains a one-byte identifier, a two-byte length, and zero or more bytes of data.

For extensibility, receivers must ignore blocks with unknown identifiers, and treat them as padding.

Encrypted data is 65535 bytes max, including a 16-byte authentication header, so the max unencrypted data is 65519 bytes.

(Poly1305 auth tag not shown):

+----+----+----+----+----+----+----+----+
|blk |  size   |       data             |
+----+----+----+                        +
|                                       |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+
|blk |  size   |       data             |
+----+----+----+                        +
|                                       |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+
~               .   .   .               ~

blk :: 1 byte
       0 for datetime
       1 for options
       2 for RouterInfo
       3 for I2NP message
       4 for termination
       224-253 reserved for experimental features
       254 for padding
       255 reserved for future extension
size :: 2 bytes, big endian, size of data to follow, 0 - 65516
data :: the data

Maximum ChaChaPoly frame is 65535 bytes.
Poly1305 tag is 16 bytes
Maximum total block size is 65519 bytes
Maximum single block size is 65519 bytes
Block type is 1 byte
Block length is 2 bytes
Maximum single block data size is 65516 bytes.

Block Ordering Rules

In the handshake message 3 part 2, order must be: RouterInfo, followed by Options if present, followed by Padding if present. No other blocks are allowed.

In the data phase, order is unspecified, except for the following requirements: Padding, if present, must be the last block. Termination, if present, must be the last block except for Padding.

There may be multiple I2NP blocks in a single frame. Multiple Padding blocks are not allowed in a single frame. Other block types probably won't have multiple blocks in a single frame, but it is not prohibited.

DateTime

Special case for time synchronization:

+----+----+----+----+----+----+----+
| 0  |    4    |     timestamp     |
+----+----+----+----+----+----+----+

blk :: 0
size :: 2 bytes, big endian, value = 4
timestamp :: Unix timestamp, unsigned seconds.
             Wraps around in 2106

Options

Pass updated options. Options include: Min and max padding.

Options block will be variable length.

+----+----+----+----+----+----+----+----+
| 1  |  size   |tmin|tmax|rmin|rmax|tdmy|
+----+----+----+----+----+----+----+----+
|tdmy|  rdmy   |  tdelay |  rdelay |    |
~----+----+----+----+----+----+----+    ~
|              more_options             |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

blk :: 1
size :: 2 bytes, big endian, size of options to follow, 12 bytes minimum

tmin, tmax, rmin, rmax :: requested padding limits
    tmin and rmin are for desired resistance to traffic analysis.
    tmax and rmax are for bandwidth limits.
    tmin and tmax are the transmit limits for the router sending this options block.
    rmin and rmax are the receive limits for the router sending this options block.
    Each is a 4.4 fixed-point float representing 0 to 15.9375
    (or think of it as an unsigned 8-bit integer divided by 16.0).
    This is the ratio of padding to data. Examples:
    Value of 0x00 means no padding
    Value of 0x01 means add 6 percent padding
    Value of 0x10 means add 100 percent padding
    Value of 0x80 means add 800 percent (8x) padding
    Alice and Bob will negotiate the minimum and maximum in each direction.
    These are guidelines, there is no enforcement.
    Sender should honor receiver's maximum.
    Sender may or may not honor receiver's minimum, within bandwidth constraints.

tdmy: Max dummy traffic willing to send, 2 bytes big endian, bytes/sec average
rdmy: Requested dummy traffic, 2 bytes big endian, bytes/sec average
tdelay: Max intra-message delay willing to insert, 2 bytes big endian, msec average
rdelay: Requested intra-message delay, 2 bytes big endian, msec average

Padding distribution specified as additional parameters?
Random delay specified as additional parameters?

more_options :: Format TBD

Options Issues

  • Options format is TBD.
  • Options negotiation is TBD.

RouterInfo

Pass Alice's RouterInfo to Bob. Used in handshake message 3 part 2. Pass Alice's RouterInfo to Bob, or Bob's to Alice. Used optionally in the data phase.

+----+----+----+----+----+----+----+----+
| 2  |  size   |flg |    RouterInfo     |
+----+----+----+----+                   +
| (Alice RI in handshake msg 3 part 2)  |
~ (Alice, Bob, or third-party           ~
|  RI in data phase)                    |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

blk :: 2
size :: 2 bytes, big endian, size of flag + router info to follow
flg :: 1 byte flags
       bit order: 76543210
       bit 0: 0 for local store, 1 for flood request
       bits 7-1: Unused, set to 0 for future compatibility
routerinfo :: Alice's or Bob's RouterInfo

Notes

  • When used in the data phase, receiver (Alice or Bob) shall validate that it's the same Router Hash as originally sent (for Alice) or sent to (for Bob). Then, treat it as a local I2NP DatabaseStore Message. Validate signature, validate more recent timestamp, and store in the local netdb. If the flag bit 0 is 1, and the receiving party is floodfill, treat it as a DatabaseStore Message with a nonzero reply token, and flood it to the nearest floodfills.
  • The Router Info is NOT compressed with gzip (unlike in a DatabaseStore Message, where it is)
  • Flooding must not be requested unless there are published RouterAddresses in the RouterInfo. The receiving router must not flood the RouterInfo unless there are published RouterAddresses in it.
  • Implementers must ensure that when reading a block, malformed or malicious data will not cause reads to overrun into the next block.
  • This protocol does not provide an acknowledgement that the RouterInfo was received, stored, or flooded (either in the handshake or data phase). If acknowledgement is desired, and the receiver is floodfill, the sender should instead send a standard I2NP DatabaseStoreMessage with a reply token.

Issues

  • Could also be used in data phase, instead of a I2NP DatabaseStoreMessage. For example, Bob could use it to start off the data phase.
  • Is it allowed for this to contain the RI for routers other than the originator, as a general replacement for DatabaseStoreMessages, e.g. for flooding by floodfills?

I2NP Message

An single I2NP message with a modified header. I2NP messages may not be fragmented across blocks or across ChaChaPoly frames.

This uses the first 9 bytes from the standard NTCP I2NP header, and removes the last 7 bytes of the header, as follows: truncate the expiration from 8 to 4 bytes, remove the 2 byte length (use the block size - 9), and remove the one-byte SHA256 checksum.

+----+----+----+----+----+----+----+----+
| 3  |  size   |type|    msg id         |
+----+----+----+----+----+----+----+----+
|   short exp       |     message       |
+----+----+----+----+                   +
|                                       |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

blk :: 3
size :: 2 bytes, big endian, size of type + msg id + exp + message to follow
        I2NP message body size is (size - 9).
type :: 1 byte, I2NP msg type, see I2NP spec
msg id :: 4 bytes, big endian, I2NP message ID
short exp :: 4 bytes, big endian, I2NP message expiration, Unix timestamp, unsigned seconds.
             Wraps around in 2106
message :: I2NP message body

Notes

  • Implementers must ensure that when reading a block, malformed or malicious data will not cause reads to overrun into the next block.

Termination

Noise recommends an explicit termination message. Original NTCP doesn't have one. Drop the connection. This must be the last non-padding block in the frame.

+----+----+----+----+----+----+----+----+
| 4  |  size   |    valid data frames   |
+----+----+----+----+----+----+----+----+
    received   | rsn|     addl data     |
+----+----+----+----+                   +
~               .   .   .               ~
+----+----+----+----+----+----+----+----+

blk :: 4
size :: 2 bytes, big endian, value = 9 or more
valid received :: The number of valid AEAD data phase frames received
                              (current receive nonce value)
                              0 if error occurs in handshake phase
                              8 bytes, big endian
rsn :: reason, 1 byte:
       0: normal close or unspecified
       1: termination received
       2: idle timeout
       3: router shutdown
       4: data phase AEAD failure
       5: incompatible options
       6: incompatible signature type
       7: clock skew
       8: padding violation
       9: AEAD framing error
       10: payload format error
       11: message 1 error
       12: message 2 error
       13: message 3 error
       14: intra-frame read timeout
       15: RI signature verification fail
       16: s parameter missing, invalid, or mismatched in RouterInfo
       17: banned
addl data :: optional, 0 or more bytes, for future expansion, debugging,
             or reason text.
             Format unspecified and may vary based on reason code.

Notes

Not all reasons may actually be used, implementation dependent. Handshake failures will generally result in a close with TCP RST instead. See notes in handshake message sections above. Additional reasons listed are for consistency, logging, debugging, or if policy changes.

Padding

This is for padding inside AEAD frames. Padding for messages 1 and 2 are outside AEAD frames. All padding for message 3 and the data phase are inside AEAD frames.

Padding inside AEAD should roughly adhere to the negotiated parameters. Bob sent his requested tx/rx min/max parameters in message 2. Alice sent her requested tx/rx min/max parameters in message 3. Updated options may be sent during the data phase. See options block information above.

If present, this must be the last block in the frame.

+----+----+----+----+----+----+----+----+
|254 |  size   |      padding           |
+----+----+----+                        +
|                                       |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

blk :: 254
size :: 2 bytes, big endian, size of padding to follow
padding :: random data

Notes

  • Padding strategies TBD.
  • Minimum padding TBD.
  • Padding-only frames are allowed.
  • Padding defaults TBD.
  • See options block for padding parameter negotiation
  • See options block for min/max padding parameters
  • Noise limits messages to 64KB. If more padding is necessary, send multiple frames.
  • Router response on violation of negotiated padding is implementation-dependent.

Other block types

Implementations should ignore unknown block types for forward compatibility, except in message 3 part 2, where unknown blocks are not allowed.

Future work

  • The padding length is either to be decided on a per-message basis and estimates of the length distribution, or random delays should be added. These countermeasures are to be included to resist DPI, as message sizes would otherwise reveal that I2P traffic is being carried by the transport protocol. The exact padding scheme is an area of future work, Appendix A provides more information on the topic.

5) Termination

Connections may be terminated via normal or abnormal TCP socket close, or, as Noise recommends, an explicit termination message. The explicit termination message is defined in the data phase above.

Upon any normal or abnormal termination, routers should zero-out any in-memory ephemeral data, including handshake ephemeral keys, symmetric crypto keys, and related information.

Published Router Info

Published Addresses

The published RouterAddress (part of the RouterInfo) will have a protocol identifier of either "NTCP" or "NTCP2".

The RouterAddress must contain "host" and "port" options, as in the current NTCP protocol.

The RouterAddress must contain three options to indicate NTCP2 support:

  • s=(Base64 key) The current Noise static public key (s) for this RouterAddress. Base 64 encoded using the standard I2P Base 64 alphabet. 32 bytes in binary, 44 bytes as Base 64 encoded, little-endian X25519 public key.
  • i=(Base64 IV) The current IV for encrypting the X value in message 1 for this RouterAddress. Base 64 encoded using the standard I2P Base 64 alphabet. 16 bytes in binary, 24 bytes as Base 64 encoded, big-endian.
  • v=2 The current version (2). When published as "NTCP", additional support for version 1 is implied. Support for future versions will be with comma-separated values, e.g. v=2,3 Implementation should verify compatibility, including multiple versions if a comma is present. Comma-separated versions must be in numerical order.

Alice must verify that all three options are present and valid before connecting using the NTCP2 protocol.

When published as "NTCP" with "s", "i", and "v" options, the router must accept incoming connections on that host and port for both NTCP and NTCP2 protocols, and automatically detect the protocol version.

When published as "NTCP2" with "s", "i", and "v" options, the router accepts incoming connections on that host and port for the NTCP2 protocol only.

If a router supports both NTCP1 and NTCP2 connections but does not implement automatic version detection for incoming connections, it must advertise both "NTCP" and "NTCP2" addresses, and include the NTCP2 options in the "NTCP2" address only. The router should set a lower cost value (higher priority) in the "NTCP2" address than the "NTCP" address, so NTCP2 is preferred.

If multiple NTCP2 RouterAddresses (either as "NTCP" or "NTCP2") are published in the same RouterInfo (for additional IP addresses or ports), all addresses specifying the same port must contain the identical NTCP2 options and values. In particular, all must contain the same static key and iv.

Unpublished NTCP2 Address

If Alice does not publish her NTCP2 address (as "NTCP" or "NTCP2") for incoming connections, she must publish a "NTCP2" router address containing only her static key and NTCP2 version, so that Bob may validate the key after receiving Alice's RouterInfo in message 3 part 2.

  • s=(Base64 key) As defined above for published addresses.
  • v=2 As defined above for published addresses.

This router address will not contain "i", "host" or "port" options, as these are not required for outbound NTCP2 connections. The published cost for this address does not strictly matter, as it is inbound only; however, it may be helpful to other routers if the cost is set higher (lower priority) than other addresses. The suggested value is 14.

Alice may also simply add the "s" and "v" options to an existing published "NTCP" address.

Public Key and IV Rotation

Due to caching of RouterInfos, routers must not rotate the static public key or IV while the router is up, whether in a published address or not. Routers must persistently store this key and IV for reuse after an immediate restart, so incoming connections will continue to work, and restart times are not exposed. Routers must persistently store, or otherwise determine, last-shutdown time, so that the previous downtime may be calculated at startup.

Subject to concerns about exposing restart times, routers may rotate this key or IV at startup if the router was previously down for some time (a couple hours at least).

If the router has any published NTCP2 RouterAddresses (as NTCP or NTCP2), the minimum downtime before rotation should be much longer, for example one month, unless the local IP address has changed or the router "rekeys".

If the router has any published SSU RouterAddresses, but not NTCP2 (as NTCP or NTCP2) the minimum downtime before rotation should be longer, for example one day, unless the local IP address has changed or the router "rekeys". This applies even if the published SSU address has introducers.

If the router does not have any published RouterAddresses (NTCP, NTCP2, or SSU), the minimum downtime before rotation may be as short as two hours, even if the IP address changes, unless the router "rekeys".

If the router "rekeys" to a different Router Hash, it should generate a new noise key and IV as well.

Implementations must be aware that changing the static public key or IV will prohibit incoming NTCP2 connections from routers that have cached an older RouterInfo. RouterInfo publishing, tunnel peer selection (including both OBGW and IB closest hop), zero-hop tunnel selection, transport selection, and other implementation strategies must take this into account.

IV rotation is subject to identical rules as key rotation, except that IVs are not present except in published RouterAddresses, so there is no IV for hidden or firewalled routers. If anything changes (version, key, options?) it is recommended that the IV change as well.

Note: The minimum downtime before rekeying may be modified to ensure network health, and to prevent reseeding by a router down for a moderate amount of time.

Identity Hiding

Deniability is not a goal. See overview above.

Each pattern is assigned properties describing the confidentiality supplied to the initiator's static public key, and to the responder's static public key. The underlying assumptions are that ephemeral private keys are secure, and that parties abort the handshake if they receive a static public key from the other party which they don't trust.

This section only considers identity leakage through static public key fields in handshakes. Of course, the identities of Noise participants might be exposed through other means, including payload fields, traffic analysis, or metadata such as IP addresses.

Alice: (8) Encrypted with forward secrecy to an authenticated party.

Bob: (3) Not transmitted, but a passive attacker can check candidates for the responder's private key and determine whether the candidate is correct.

Bob publishes his static public key in the netdb. Alice may or may not?

Issues

  • If Bob changes his static key, could fallback to a "XX" pattern?

Version Detection

When published as "NTCP", the router must automatically detect the protocol version for incoming connections.

This detection is implementation-dependent, but here is some general guidance.

To detect the version of an incoming NTCP connection, Bob proceeds as follows:

  • Wait for at least 64 bytes (minimum NTCP2 message 1 size)

  • If the initial received data is 288 or more bytes, the incoming connection is version 1.

  • If less than 288 bytes, either

    • Wait for a short time for more data (good strategy before widespread NTCP2 adoption) if at least 288 total received, it's NTCP 1.

    • Try the first stages of decoding as version 2, if it fails, wait a short time for more data (good strategy after widespread NTCP2 adoption)

      • Decrypt the first 32 bytes (the X key) of the SessionRequest packet using AES-256 with key RH_B.
      • Verify a valid point on the curve. If it fails, wait a short time for more data for NTCP 1
      • Verify the AEAD frame. If it fails, wait a short time for more data for NTCP 1

Note that changes or additional strategies may be recommended if we detect active TCP segmentation attacks on NTCP 1.

To facilitate rapid version detection and handshaking, implementations must ensure that Alice buffers and then flushes the entire contents of the first message at once, including the padding. This increases the likelihood that the data will be contained in a single TCP packet (unless segmented by the OS or middleboxes), and received all at once by Bob. This is also for efficiency and to ensure the effectiveness of the random padding. This applies to both NTCP and NTCP2 handshakes.

Variants, Fallbacks, and General Issues

  • If Alice and Bob both support NTCP2, Alice should connect with NTCP2.
  • If Alice fails to connect to Bob using NTCP2 for any reason, the connection fails. Alice may not retry using NTCP 1.
  • Fallback to XX pattern if Bob changes his keys? This would require a type byte prepended?
  • "Fall forward" to KK pattern if Alice reconnects, assuming Bob still has her static key? This doesn't save any round trips and uses 4 DH rounds compared to 3 for XK. Probably not.
    KK(s, rs):
    -> s
    <- s
    ...
    -> e, es, ss
    <- e, ee, se

Appendix A: Padding Scheme

This section discusses an attack on typical padding schemes that allows attackers to discover the probability distribution of the length of the unpadded messages, by only observing the length of the padded messages. Let N be a random variable describing the number of unpadded bytes, and P likewise for the number of padding bytes. The total message size is then N + P.

Assume that for an unpadded size of n, at least P_min(n) >= 0 and at most P_max(n) >= P_min(n) bytes of padding are added in a padding scheme. The obvious scheme uses padding of length P uniformly chosen at random:

Pr[P = p | N = n] = 1 / (P_max(n) - P_min(n)) if P_min(n) <= p <= P_max(n),
                    0                         otherwise.

A naive padding scheme would simply ensure that the size of the padded message does not exceed N_max:

P_max(n) = N_max - n, n <= N_max
P_min(n) = 0.

However, this leaks information about the unpadded length.

An attacker can easily estimate Pr[x <= N + P <= y], for example by means of a histogram.

  • From this, he can also try to estimate Pr[n_1 <= N <= n_2], indeed:
Pr[N + P = m] = Σ_n Pr[N = n] Pr[P = m - n | N = n].

In the naive scheme,

Pr[N + P = m] = Σ_{n <= m} Pr[N = n] / (N_max - n).

It's pretty obvious, as it was before doing the above calculation, that this leaks information about Pr[N = n]: if the length of packets is almost always more than m, then N + P <= m will almost never be observed. This is not the largest issue though, although being able to observe the minimum message length can be considered to be a problem by itself.

A bigger issue is that it is possible to determine Pr[N = n] exactly:

Pr[N + P = m] - Pr[N + P = m-1] = Pr[N = m] / (N_max - m),

that is

Pr[N = n] = (N_max - n)(Pr[N + P = n] - Pr[N + P = n - 1])

To distinguish NTCP2, then, the attacker can use any of the following:

  • Estimate Pr[kB <= N <= (k + 1)B - 1] for positive integers k. It will always be zero for NTCP2.
  • Estimate Pr[N = kB] and compare with a standard I2P profile.

This simple attack hence partially destroys the purpose of padding, which attempts to obfuscate the size distribution of the unpadded messages. The amount of messages that the attacker has to observe to distinguish the protocol depends on the desired accuracy and on the minimum and maximum unpadded message sizes that occur in practice. Note that it is easy to gather many messages for the attacker, since he can use all traffic sent from and to the particular port that the target is using.

In some forms (e.g. estimation of Pr[kB <= N <= (k + 1)B - 1]) the attack requires only a few bytes of memory (one integer is enough) and it could be argued that such an attack might be included in many slightly more advanced but nevertheless standard DPI frameworks.

This proposal suggests using one of the following countermeasures:

  • Develop an alternate padding scheme that takes into account the (estimated) distribution of N by using a non-uniform padding length distribution. A good padding scheme would probably require maintaining a histogram of the number of blocks per message.
  • Add random delays between (randomly sized) fragments of messages.

The second option is more generally preferred, because it can be simultaneously used as a countermeasure against flow analysis. However, such delays may be out of scope for the NTCP2 protocol, such that the first option, which is also easier to implement, may be preferred instead.

Appendix B: Random Delays

Timing-based DPI resistance (inter-message timing/delays can be implementation-dependent; intra-message delays can be introduced at any point, including before sending the random padding, for example). Artificial delays (what obfs4 calls IAT or inter-arrival time) are independent of the protocol itself.

References

[IACR-1150]https://eprint.iacr.org/2015/1150
[NetDB]https://geti2p.net/en/docs/how/network-database
[NOISE](1, 2) http://noiseprotocol.org/noise.html
[NTCP](1, 2) https://geti2p.net/en/docs/transport/ntcp
[Prop104](1, 2) https://geti2p.net/spec/proposals/104-tls-transport
[Prop109]https://geti2p.net/spec/proposals/109-pt-transport
[RFC-2104](1, 2) https://tools.ietf.org/html/rfc2104
[RFC-3526]https://tools.ietf.org/html/rfc3526
[RFC-6151]https://tools.ietf.org/html/rfc6151
[RFC-7539](1, 2) https://tools.ietf.org/html/rfc7539
[RFC-7748]https://tools.ietf.org/html/rfc7748
[RFC-7905]https://tools.ietf.org/html/rfc7905
[RouterAddress]https://geti2p.net/spec/common-structures#struct-routeraddress
[RouterIdentity](1, 2) https://geti2p.net/spec/common-structures#struct-routeridentity
[SIDH]De Feo, Luca; Jao, Plut., Towards quantum-resistant cryptosystems from supersingular elliptic curve isogenies
[SigningPublicKey]https://geti2p.net/spec/common-structures#type-signingpublickey
[SipHash]https://www.131002.net/siphash/
[SPEC]https://geti2p.net/en/docs/spec/ntcp2
[SSU](1, 2, 3) https://geti2p.net/en/docs/transport/ssu
[STS](1, 2) Diffie, W.; van Oorschot P. C.; Wiener M. J., Authentication and Authenticated Key Exchanges
[Ticket1112]https://trac.i2p2.de/ticket/1112
[Ticket1849]https://trac.i2p2.de/ticket/1849
[1]http://www.chesworkshop.org/ches2009/presentations/01_Session_1/CHES2009_ekasper.pdf
[2]https://www.blackhat.com/docs/us-16/materials/us-16-Devlin-Nonce-Disrespecting-Adversaries-Practical-Forgery-Attacks-On-GCM-In-TLS.pdf
[3]https://eprint.iacr.org/2014/613.pdf
[4]https://www.imperialviolet.org/2013/10/07/chacha20.html
[5]https://tools.ietf.org/html/rfc7539