Table of Contents

1. Introduction

Noise is a framework for crypto protocols based on Diffie-Hellman key agreement. Noise can describe protocols that consist of a single message as well as interactive protocols.

2. Overview

2.1. Terminology

A Noise protocol begins with two parties exchanging handshake messages. During this handshake phase the parties exchange DH public keys and perform a sequence of DH operations, hashing the DH results into a shared secret key. After the handshake phase each party can use this shared key to send encrypted transport messages.

The Noise framework supports handshakes where each party has a long-term static key pair and/or an ephemeral key pair. A Noise handshake is described by a simple language. This language consists of tokens which are arranged into message patterns. Message patterns are arranged into handshake patterns.

A message pattern is a sequence of tokens that specifies the DH public keys that comprise a handshake message, and the DH operations that are performed when sending or receiving that message. A handshake pattern specifies the sequential exchange of messages that comprise a handshake.

A handshake pattern can be instantiated by DH functions, cipher functions, and a hash function to give a concrete Noise protocol.

2.2. Overview of handshake state machine

The core of Noise is a set of variables maintained by each party during a handshake, and rules for sending and receiving handshake messages by sequentially processing the tokens from a message pattern.

Each party maintains the following variables:

A handshake message consists of some DH public keys followed by a payload. The payload may contain certificates or other data chosen by the application. To send a handshake message, the sender specifies the payload and sequentially processes each token from a message pattern. The possible tokens are:

After processing the final token in a handshake message, the sender then writes the payload into the message buffer, encrypting it if k is non-empty, and hashes the output along with the old h to derive a new h.

As a simple example, an unauthenticated DH handshake is described by the handshake pattern:

  -> e
  <- e, ee

The initiator sends the first message, which is simply an ephemeral public key. The responder sends back its own ephemeral public key. Then a DH is performed and the output is hashed into a shared secret key.

Note that a cleartext payload is sent in the first message, after the cleartext ephemeral public key, and an encrypted payload is sent in the response message, after the cleartext ephemeral public key. The application may send whatever payloads it wants.

The responder can send its static public key (under encryption) and authenticate itself via a slightly different pattern:

  -> e
  <- e, ee, s, es

In this case, the final ck and k values are a hash of both DH results. Since the es token indicates a DH between the initiator's ephemeral key and the responder's static key, successful decryption by the initiator of the second message's payload serves to authenticate the responder to the initiator.

Note that the second message's payload may contain a zero-length plaintext, but the payload ciphertext will still contain authentication data (such as an authentication tag or "synthetic IV"), since encryption is with an AEAD mode. The second message's payload can also be used to deliver certificates for the responder's static public key.

The initiator can send its static public key (under encryption), and authenticate itself, using a handshake pattern with one additional message:

  -> e
  <- e, ee, s, es
  -> s, se

The following sections flesh out the details, and add some complications. However, the core of Noise is this simple system of variables, tokens, and processing rules, which allow concise expression of a range of protocols.

3. Message format

All Noise messages are less than or equal to 65535 bytes in length. Restricting message size has several advantages:

All Noise messages can be processed without parsing, since there are no type or length fields. Of course, Noise messages might be encapsulated within a higher-level protocol that contains type and length information. Noise messages might encapsulate payloads that require parsing of some sort, but payloads are handled by the application, not by Noise.

A Noise transport message is simply an AEAD ciphertext that is less than or equal to 65535 bytes in length, and that consists of an encrypted payload plus 16 bytes of authentication data. The details depend on the AEAD cipher function, e.g. AES256-GCM, or ChaCha20-Poly1305, but typically the authentication data is either a 16-byte authentication tag appended to the ciphertext, or a 16-byte synthetic IV prepended to the ciphertext.

A Noise handshake message is also less than or equal to 65535 bytes. It begins with a sequence of one or more DH public keys, as determined by its message pattern. Following the public keys will be a single payload which can be used to convey certificates or other handshake data, but can also contain a zero-length plaintext.

Static public keys and payloads will be in cleartext if they are sent in a handshake prior to a DH operation, and will be AEAD ciphertexts if they occur after a DH operation. (If Noise is being used with pre-shared symmetric keys, this rule is different: all static public keys and payloads will be encrypted; see Section 7). Like transport messages, AEAD ciphertexts will expand each encrypted field (whether static public key or payload) by 16 bytes.

For an example, consider the handshake pattern:

  -> e
  <- e, ee, s, es
  -> s, se

The first message consists of a cleartext public key ("e") followed by a cleartext payload (remember that a payload is implicit at the end of each message pattern). The second message consists of a cleartext public key ("e") followed by an encrypted public key ("s") followed by an encrypted payload. The third message consists of an encrypted public key ("s") followed by an encrypted payload.

Assuming each payload contains a zero-length plaintext, and DH public keys are 56 bytes, the message sizes will be:

  1. 56 bytes (one cleartext public key and a cleartext payload)
  2. 144 bytes (two public keys, the second encrypted, and encrypted payload)
  3. 88 bytes (one encrypted public key and encrypted payload)

If pre-shared symmetric keys are used, the first message grows in size to 72 bytes, since the first payload becomes encrypted.

4. Crypto functions

A Noise protocol is instantiated with a concrete set of DH functions, cipher functions, and a hash function. The signature for these functions is defined below. Some concrete functions are defined in Section 10.

The following notation will be used in algorithm pseudocode:

4.1. DH functions

Noise depends on the following DH functions (and an associated constant):

4.2. Cipher functions

Noise depends on the following cipher functions:

4.3. Hash functions

Noise depends on the following hash function (and associated constants):

Noise defines additional functions based on the above HASH() function:

Note that temp_key, output1, and output2 are all HASHLEN bytes in length. Also note that the HKDF() function is simply HKDF from [3] with the chaining_key as HKDF salt, and zero-length HKDF info.

5. Processing rules

To precisely define the processing rules we adopt an object-oriented terminology, and present three "objects" which encapsulate state variables and provide "methods" which implement processing logic. These three objects are presented as a hierarchy: each higher-layer object includes one instance of the object beneath it. From lowest-layer to highest, the objects are:

To execute a Noise protocol you Initialize() a HandshakeState. During initialization you specify the handshake pattern, any local key pairs, and any public keys for the remote party you have knowledge of. After Initialize() you call WriteMessage() and ReadMessage() on the HandshakeState to process each handshake message. If a decryption error occurs the handshake has failed and the HandshakeState is deleted without sending further messages.

Processing the final handshake message returns two CipherState objects, the first for encrypting transport messages from initiator to responder, and the second for messages in the other direction. At that point the HandshakeState may be deleted. Transport messages are then encrypted and decrypted by calling EncryptWithAd() and DecryptWithAd() on the relevant CipherState with zero-length associated data.

The below sections describe these objects in detail.

5.1 The CipherState object

A CipherState can encrypt and decrypt data based on its k and n variables:

A CipherState responds to the following methods. The ++ post-increment operator applied to n means "use the current n value, then increment it". The maximum n value (264-1) is reserved for future use and must not be used. If incrementing n results in 264-1 (the maximum value), then any further EncryptWithAd() or DecryptWithAd() calls will signal an error to the caller.

5.2. The SymmetricState object

A SymmetricState object contains a CipherState plus the following variables:

A SymmetricState responds to the following methods:

5.3. The HandshakeState object

A HandshakeState object contains a SymmetricState plus the following variables, any of which may be empty. Empty is a special value which indicates the variable has not yet been initialized.

A HandshakeState also has variables to track its role, and the remaining portion of the handshake pattern:

A HandshakeState responds to the following methods:

6. Prologue

Noise protocols have a prologue input which allows arbitrary data to be hashed into the h variable. If both parties do not provide identical prologue data, the handshake will fail due to a decryption error. This is useful when the parties engaged in negotiation prior to the handshake and want to ensure they share identical views of that negotiation.

For example, suppose Bob communicates to Alice a list of Noise protocols that he is willing to support. Alice will then choose and execute a single protocol. To ensure that a "man-in-the-middle" did not edit Bob's list to remove options, Alice and Bob could include the list as prologue data.

Note that while the parties confirm their prologues are identical, they don't mix prologue data into encryption keys. If an input contains secret data that's intended to strengthen the encryption, a "PSK" handshake should be used instead (see next section).

7. Pre-shared symmetric keys

Noise provides an optional pre-shared symmetric key or PSK mode to support protocols where both parties already have a shared secret key. When using pre-shared symmetric keys, the following changes are made:

8. Handshake patterns

A message pattern is some sequence of tokens from the set ("e", "s", "ee", "es", "se", "ss").

A handshake pattern consists of:

The pre-messages represent an exchange of public keys that was somehow performed prior to the handshake, so these public keys must be inputs to Initialize() for the "recipient" of the pre-message.

The first actual handshake message is sent from the initiator to the responder, the next is sent by the responder, the next from the initiator, and so on in alternating fashion.

The following handshake pattern describes an unauthenticated DH handshake:

Noise_NN():
  -> e
  <- e, ee

The handshake pattern name is Noise_NN. This naming convention will be explained in Section 8.3. The empty parentheses indicate that neither party is initialized with any key pairs. The tokens "s" and/or "e" inside the parentheses would indicate that the initiator is initialized with static and/or ephemeral key pairs. The tokens "rs" and/or "re" would indicate the same thing for the responder.

Pre-messages are shown as patterns prior to the delimiter "...", with a right-pointing arrow for the initiator's pre-message, and a left-pointing arrow for the responder's pre-message. If both parties have a pre-message, the initiator's is listed first (and hashed first). During Initialize(), MixHash() is called on any pre-message public keys, as described in Section 5.3.

The following pattern describes a handshake where the initiator has pre-knowledge of the responder's static public key, and performs a DH with the responder's static public key as well as the responder's ephemeral public key. This pre-knowledge allows an encrypted payload to be sent in the first message, although full forward secrecy and replay protection is only achieved with the second message.

Noise_NK(rs):
  <- s
  ...
  -> e, es 
  <- e, ee

8.1. Pattern validity

Handshake patterns must be valid in the following senses:

  1. Parties can only send a static public key if they were initialized with a static key pair, and can only perform DH between private keys and public keys they possess.

  2. Parties must not send their static public key, or an ephemeral public key, more than once per handshake (i.e. ignoring the pre-messages, there must be no more than one occurrence of "e", and one occurrence of "s", in the messages sent by any party).

  3. Parties must send an ephemeral public key at the start of the first message they send (i.e. the first token of the first message pattern in each direction must be "e"). To support "compound protocols" (see Section 9.2) an exception is allowed if the party's ephemeral public key was used as a "pre-message". A handshake pattern that relies on this exception is a dependent pattern, and can only be used according to the rules in Section 9.2.

  4. After performing a DH between a remote public key and any local private key that is not an ephemeral private key, the local party must not send any encrypted data unless they have also performed a DH between an ephemeral private key and the remote public key.

Patterns failing the first check are obviously nonsense.

The second check outlaws redundant transmission of values to simplify implementation and testing.

The third and fourth checks are necessary because Noise uses DH outputs involving ephemeral keys to randomize the shared secret keys. Noise also uses ephemeral public keys to randomize PSK-based encryption. Patterns failing these checks could result in subtle but catastrophic security flaws.

Users are recommended to only use the handshake patterns listed below, or other patterns that have been vetted by experts to satisfy the above checks.

8.2. One-way patterns

The following example handshake patterns represent "one-way" handshakes supporting a one-way stream of data from a sender to a recipient. These patterns could be used to encrypt files, database records, or other non-interactive data streams.

Following a one-way handshake the sender can send a stream of transport messages, encrypting them using the first CipherState returned by Split(). The second CipherState from Split() is discarded - the recipient must not send any messages using it (as this would violate the rules in Section 8.1).

One-way patterns are named with a single character, which indicates the status of the sender's static key:

Noise_N(rs):
  <- s
  ...
  -> e, es
Noise_K(s, rs):
  -> s
  <- s
  ...
  -> e, es, ss
Noise_X(s, rs):
  <- s
  ...
  -> e, es, s, ss

Noise_N is a conventional DH-based public-key encryption. The other patterns add sender authentication, where the sender's public key is either known to the recipient beforehand (Noise_K) or transmitted under encryption (Noise_X).

8.3. Interactive patterns

The following example handshake patterns represent interactive protocols.

Interactive patterns are named with two characters, which indicate the status of the initator and responder's static keys:

The first character refers to the initiator's static key:

The second character refers to the responder's static key:

Noise_NN():
  -> e
  <- e, ee
   Noise_KN(s):
     -> s
     ...
     -> e
     <- e, ee, se
Noise_NK(rs):
  <- s
  ...
   -> e, es
   <- e, ee
   Noise_KK(s, rs):
     -> s
     <- s
     ...
     -> e, es, ss
     <- e, ee, se
 Noise_NX(rs):
   -> e
   <- e, ee, s, es
    Noise_KX(s, rs):
      -> s
      ...
      -> e
      <- e, ee, se, s, es
 Noise_XN(s):
   -> e
   <- e, ee
   -> s, se
    Noise_IN(s):
      -> e, s
      <- e, ee, se
 Noise_XK(s, rs):
   <- s
   ...
   -> e, es
   <- e, ee
   -> s, se
    Noise_IK(s, rs):
      <- s
      ...
      -> e, es, s, ss
      <- e, ee, se
 Noise_XX(s, rs):
   -> e
   <- e, ee, s, es
   -> s, se
    Noise_IX(s, rs):
      -> e, s
      <- e, ee, se, s, es

The Noise_XX pattern is the most generically useful, since it is efficient and supports mutual authentication and transmission of static public keys.

All interactive patterns allow some encryption of handshake payloads:

The security properties for handshake payloads are usually weaker than the final security properties achieved by transport payloads, so these early encryptions must be used with caution.

In some patterns the security properties of transport payloads can also vary. In particular: patterns starting with "K" or "I" have the caveat that the responder is only guaranteed "weak" forward secrecy for the transport messages it sends until it receives a transport message from the initiator. After receiving a transport message from the initiator, the responder becomes assured of "strong" forward secrecy.

The next section provides more analysis of these payload security properties.

8.4. Payload security properties

The following table lists the security properties for Noise handshake and transport payloads for all the named patterns in Section 8.2 and Section 8.3. Each payload is assigned an "authentication" property regarding the degree of authentication of the sender provided to the recipient, and a "confidentiality" property regarding the degree of confidentiality provided to the sender.

The authentication properties are:

  1. No authentication. This payload may have been sent by any party, including an active attacker.

  2. Sender authentication vulnerable to key-compromise impersonation (KCI). The sender authentication is based on a static-static DH ("ss") involving both parties' static key pairs. If the recipient's long-term private key has been compromised, this authentication can be forged. Note that a future version of Noise might include signatures, which could improve this security property, but brings other trade-offs.

  3. 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.

The confidentiality properties are:

  1. No confidentiality. This payload is sent in cleartext.

  2. 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.

  3. 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.

  4. Encryption to a known recipient, weak forward secrecy. This payload is encrypted based on an ephemeral-ephemeral DH and also an ephemeral-static DH involving the recipient's static key pair. However, the binding between the recipient's alleged ephemeral public key and the recipient's static public key hasn't been verified by the sender, so the recipient's alleged ephemeral public key may have been forged by an active attacker. In this case, the attacker could later compromise the recipient's static private key to decrypt the payload. Note that a future version of Noise might include signatures, which could improve this security property, but brings other trade-offs.

  5. Encryption to a known recipient, weak forward secrecy if the sender's private key has been compromised. This payload is encrypted based on an ephemeral-ephemeral DH, and also based on an ephemeral-static DH involving the recipient's static key pair. However, the binding between the recipient's alleged ephemeral public and the recipient's static public key has only been verified based on DHs involving both those public keys and the sender's static private key. Thus, if the sender's static private key was previously compromised, the recipient's alleged ephemeral public key may have been forged by an active attacker. In this case, the attacker could later compromise the intended recipient's static private key to decrypt the payload (this is a variant of a "KCI" attack enabling a "weak forward secrecy" attack). Note that a future version of Noise might include signatures, which could improve this security property, but brings other trade-offs.

  6. 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.

For one-way handshakes, the below-listed security properties apply to the handshake payload as well as transport payloads.

For interactive handshakes, security properties are listed for each handshake payload. Transport payloads are listed as arrows without a pattern. Transport payloads are only listed if they have different security properties than the previous handshake payload sent from the same party. If two transport payloads are listed, the security properties for the second only apply if the first was received.

                     Authentication   Confidentiality
Noise_N                     0                2
Noise_K                     1                2
Noise_X                     1                2
Noise_NN
  -> e                      0                0
  <- e, ee                  0                1
  ->                        0                1
Noise_NK
  <- s
  ...
  -> e, es                  0                2
  <- e, ee                  2                1
  ->                        0                5
Noise_NX
  -> e                      0                0
  <- e, ee, s, es           2                1
  ->                        0                5
Noise_XN
  -> e                      0                0
  <- e, ee                  0                1
  -> s, se                  2                1
  <-                        0                5
Noise_XK
  <- s
  ...
  -> e, es                  0                2
  <- e, ee                  2                1
  -> s, se                  2                5
  <-                        2                5
Noise_XX
 -> e                       0                0
 <- e, ee, s, es            2                1
 -> s, se                   2                5
 <-                         2                5
Noise_KN
  -> s
  ...
  -> e                      0                0
  <- e, ee, se              0                3
  ->                        2                1
  <-                        0                5
Noise_KK
  -> s
  <- s
  ...
  -> e, es, ss              1                2
  <- e, ee, se              2                4
  ->                        2                5
  <-                        2                5
Noise_KX
  -> s
  ...
  -> e                      0                0
  <- e, ee, se, s, es       2                3
  ->                        2                5
  <-                        2                5
Noise_IN
  -> e, s                   0                0
  <- e, ee, se              0                3
  ->                        2                1
  <-                        0                5
Noise_IK
  <- s
  ...
  -> e, es, s, ss           1                2
  <- e, ee, se              2                4
  ->                        2                5
  <-                        2                5
Noise_IX
  -> e, s                   0                0
  <- e, ee, se, s, es       2                3
  ->                        2                5
  <-                        2                5

8.5. Identity hiding

The following table lists the identity hiding properties for all the named patterns in Section 8.2 and Section 8.3. 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.

The properties for the relevant public key are:

  1. Transmitted in clear.

  2. Encrypted with forward secrecy, but can be probed by an anonymous initiator.

  3. Encrypted with forward secrecy, but sent to an anonymous responder.

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

  5. Encrypted to responder's static public key, without forward secrecy. If an attacker learns the responder's private key they can decrypt the initiator's public key.

  6. Not transmitted, but a passive attacker can check candidates for the responder's private key and initiator's public key and determine if both candidates are correct.

  7. Encrypted but with weak forward secrecy. An active attacker who pretends to be the initiator without the initiator's static private key, then later learns the initiator private key, can then decrypt the responder's public key.

  8. Not transmitted, but an active attacker who pretends to be the initator without the initiator's static private key, then later learns a candidate for the initiator private key, can then check whether the candidate is correct.

  9. Encrypted with forward secrecy to an authenticated party.

           Initiator      Responder
Noise_N        -              3
Noise_K        5              5
Noise_X        4              3
Noise_NN       -              -
Noise_NK       -              3
Noise_NX       -              1
Noise_XN       2              -
Noise_XK       8              3
Noise_XX       8              1
Noise_KN       7              -
Noise_KK       5              5
Noise_KX       7              6
Noise_IN       0              -
Noise_IK       4              3
Noise_IX       0              6

8.6. More patterns

The patterns in the previous sections are the best option for most scenarios.

However, to construct new patterns we can apply some transformation to an existing pattern, and name the resulting pattern by appending the transformation name to the existing pattern's name.

For example, if you don't care about identity hiding, you could apply a "noidh" transformation which moves static public keys earlier in messages, so they are sent in cleartext where possible. This transforms the patterns from the left column to the right column:

Noise_X(s, rs):
  <- s
  ...
  -> e, es, s, ss
 Noise_Xnoidh(s, rs):
   <- s
   ...
   -> e, s, es, ss
Noise_NX(rs):
  -> e
  <- e, ee, s, es
 Noise_NXnoidh(rs):
   -> e
   <- e, s, ee, es
Noise_XX(s, rs):
  -> e
  <- e, ee, s, es
  -> s, se
 Noise_XXnoidh(s, rs):
   -> e
   <- e, s, ee, es
   -> s, se
Noise_KX(s, rs):
  -> s
  ...
  -> e
  <- e, ee, se, s, es
 Noise_KXnoidh(s, rs):
   -> s
   ...
   -> e
   <- e, s, ee, se, es
Noise_IK(s, rs):
  <- s
  ...
  -> e, es, s, ss
  <- e, ee, se
 Noise_IKnoidh(s, rs):
   <- s
   ...
   -> e, s, es, ss
   <- e, ee, se
Noise_IX(s, rs):
  -> e, s
  <- e, ee, se, s, es
 Noise_IXnoidh(s, rs):
   -> e, s
   <- e, s, ee, se, es

Other tranformations might add or remove "ss" operations, or defer DH operations until later.

9. Advanced uses

9.1. Dummy static public keys

Consider a protocol where an initiator will authenticate herself if the responder requests it. This could be viewed as the initiator choosing between patterns like Noise_NX and Noise_XX based on some value inside the responder's first handshake payload.

Noise doesn't directly support this. Instead, this could be simulated by always executing Noise_XX. The initiator can simulate the Noise_NX case by sending a dummy static public key if authentication is not requested. The value of the dummy public key doesn't matter. For efficiency, the initiator can send a null public key value per Section 4 (e.g. an all-zeros 25519 value that is guaranteed to produce an all-zeros output).

This technique is simple, since it allows use of a single handshake pattern. It also doesn't reveal which option was chosen from message sizes. It could be extended to allow a Noise_XX pattern to support any permutation of authentications (initiator only, responder only, both, or none).

9.2. Compound protocols and "Noise Pipes"

Consider a protocol where the initiator can attempt zero-RTT encryption based on the responder's static public key. If the responder has changed his static public key, the parties will need to switch to a "fallback" handshake where the responder transmits the new static public key and the initiator resends the zero-RTT data.

This can be handled by both parties re-initializing their HandshakeState and executing a different handshake. Using handshake re-initialization to switch from one "simple" Noise protocol to another results in a compound protocol.

Public keys that were exchanged in the first handshake can be represented as pre-messages in the second handshake. If an ephemeral public key was sent in the first handshake and used as a pre-message in the second handshake, that party can avoid sending a new ephemeral by using a "dependent" pattern (see Section 8.1).

Re-initializing with a dependent pattern is only allowed if the new handshake and old handshake have different protocol names (see Section 11) and the rules for handling PSKs are followed (see Section 7).

If any negotiation occurred in the first handshake, the first handshake's h variable should be provided as prologue to the second handshake.

By way of example, this section defines the Noise Pipe compound protocol. This protocol uses three handshake patterns - two defined in the previous section, and a new one:

Below are the three patterns used for Noise Pipes:

Noise_XX(s, rs):  
  -> e
  <- e, ee, s, es
  -> s, se

Noise_IK(s, rs):                   
  <- s                         
  ...
  -> e, es, s, ss          
  <- e, ee, se
                                    
Noise_XXfallback(s, rs, re):                   
  <- e
  ...
  -> e, ee, s, se
  <- s, es

Note that in the fallback case, the initiator and responder roles are switched: If Alice initiates a Noise_IK handshake with Bob, Bob might initiate a Noise_XXfallback handshake.

There needs to be some way for the recipient of a message to distinguish whether it's the next message in the current handshake pattern, or requires re-initialization for a new pattern. For example, each handshake message could be preceded by a type byte (see Section 12). This byte is not part of the Noise message proper, but simply signals when re-initialization is needed. It could have the following meanings:

Note that the type byte doesn't need to be explicitly authenticated (as prologue, or additional AEAD data), since it's implicitly authenticated if the message is processed succesfully.

9.3. Protocol indistinguishability

Parties may wish to hide what protocol they are executing from an eavesdropper. For example, suppose parties are using Noise Pipes, and want to hide whether they are performing a full handshake, abbreviated handshake, or fallback handshake.

This is fairly easy:

This leaves the Noise ephemerals in the clear, so an eavesdropper might suspect the parties are using Noise, even if it can't distinguish the handshakes. To make the ephemerals indistinguishable from random, techniques like Elligator [4] could be used.

9.4. Channel binding

Parties may wish to execute a Noise protocol, then perform authentication at the application layer using signatures, passwords, or something else.

To support this, Noise libraries should expose the final value of h to the application as a handshake hash which uniquely identifies the Noise session.

Parties can then sign the handshake hash, or hash it along with their password, to get an authentication token which has a "channel binding" property: the token can't be used by the receiving party with a different sesssion.

10. DH functions, cipher functions, and hash functions

10.1. The 25519 DH functions

10.2. The 448 DH functions

10.3. The ChaChaPoly cipher functions

10.4. The AESGCM cipher functions

10.5. The SHA256 hash function

10.6. The SHA512 hash function

10.7. The BLAKE2s hash function

10.8. The BLAKE2b hash function

11. Protocol names

To produce a Noise protocol name for Initialize() you concatenate the ASCII names for the handshake pattern, the DH functions, the cipher functions, and the hash function, with underscore separators. For example:

If a pre-shared symmetric key is in use, then the prefix "NoisePSK_" is used instead of "Noise_":

12. Application responsibilities

An application built on Noise must consider several issues:

13. Security considerations

This section collects various security considerations:

14. Rationale

This section collects various design rationale:

Nonces are 64 bits in length because:

The recommended hash function families are SHA2 and BLAKE2 because:

Hash output lengths of both 256 bits and 512 bits are supported because:

Cipher keys and pre-shared symmetric keys are 256 bits because:

The authentication data in a ciphertext is 128 bits because:

The GCM security limit is 256 bytes because:

Big-endian length fields are recommended because:

Cipher nonces are big-endian for AES-GCM, and little-endian for ChaCha20, because:

The MixKey() design uses HKDF because:

MixHash() is used instead of sending all inputs through MixKey() because:

The h value hashes handshake ciphertext instead of plaintext because:

Session termination is left to the application because:

Explicit random nonces (like TLS "Random" fields) are not used because:

15. IPR

The Noise specification (this document) is hereby placed in the public domain.

16. Acknowledgements

Noise is inspired by:

General feedback on the spec and design came from: Moxie Marlinspike, Jason Donenfeld, Rhys Weatherley, Tiffany Bennett, Jonathan Rudenberg, Stephen Touset, Tony Arcieri, and Alex Wied.

Thanks to Tom Ritter, Karthikeyan Bhargavan, David Wong, and Klaus Hartke for editorial feedback.

Moxie Marlinspike, Hugo Krawczyk, Samuel Neves, Christian Winnerlein, J.P. Aumasson, and Jason Donenfeld provided helpful input and feedback on the key derivation design.

The BLAKE2 team (in particular J.P. Aumasson, Samuel Neves, and Zooko) provided helpful discussion on using BLAKE2 with Noise.

Jeremy Clark, Thomas Ristenpart, and Joe Bonneau gave feedback on much earlier versions.

17. References

[1] P. Rogaway, “Authenticated-encryption with Associated-data,” in Proceedings of the 9th ACM Conference on Computer and Communications Security, 2002. http://web.cs.ucdavis.edu/~rogaway/papers/ad.pdf

[2] H. Krawczyk, M. Bellare, and R. Canetti, “HMAC: Keyed-Hashing for Message Authentication.” Internet Engineering Task Force; RFC 2104 (Informational); IETF, Feb-1997. http://www.ietf.org/rfc/rfc2104.txt

[3] H. Krawczyk and P. Eronen, “HMAC-based Extract-and-Expand Key Derivation Function (HKDF).” Internet Engineering Task Force; RFC 5869 (Informational); IETF, May-2010. http://www.ietf.org/rfc/rfc5869.txt

[4] D. J. Bernstein, M. Hamburg, A. Krasnova, and T. Lange, “Elligator: Elliptic-curve points indistinguishable from uniform random strings.” Cryptology ePrint Archive, Report 2013/325, 2013. http://eprint.iacr.org/2013/325

[5] A. Langley, M. Hamburg, and S. Turner, “Elliptic Curves for Security.” Internet Engineering Task Force; RFC 7748 (Informational); IETF, Jan-2016. http://www.ietf.org/rfc/rfc7748.txt

[6] Y. Nir and A. Langley, “ChaCha20 and Poly1305 for IETF Protocols.” Internet Engineering Task Force; RFC 7539 (Informational); IETF, May-2015. http://www.ietf.org/rfc/rfc7539.txt

[7] M. J. Dworkin, “SP 800-38D. Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC,” National Institute of Standards & Technology, Gaithersburg, MD, United States, 2007. http://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38d.pdf

[8] NIST, “FIPS 180-4. Secure Hash Standard (SHS),” National Institute of Standards & Technology, Gaithersburg, MD, United States, 2012. http://csrc.nist.gov/publications/fips/fips180-4/fips-180-4.pdf

[9] M.-J. Saarinen and J.-P. Aumasson, “The BLAKE2 Cryptographic Hash and Message Authentication Code (MAC).” Internet Engineering Task Force; RFC 7693 (Informational); IETF, Nov-2015. http://www.ietf.org/rfc/rfc7693.txt

[10] D. J. Bernstein, T. Lange, and P. Schwabe, “NaCl: Networking and Cryptography Library.”. https://nacl.cr.yp.to/

[11] D. J. Bernstein, “CurveCP: Usable security for the Internet.”. https://curvecp.org

[12] H. Krawczyk, “SIGMA: The ‘SIGn-and-MAc’ Approach to Authenticated Diffie-Hellman and Its Use in the IKE Protocols,” in Advances in Cryptology - CRYPTO 2003, 2003. http://webee.technion.ac.il/~hugo/sigma.html

[13] S. Halevi and H. Krawczyk, “One-Pass HMQV and Asymmetric Key-Wrapping.” Cryptology ePrint Archive, Report 2010/638, 2010. http://eprint.iacr.org/2010/638

[14] I. Goldberg, D. Stebila, and B. Ustaoglu, “Anonymity and One-way Authentication in Key Exchange Protocols,” Design, Codes, and Cryptography, vol. 67, no. 2, May 2013. http://cacr.uwaterloo.ca/techreports/2011/cacr2011-11.pdf

[15] M. Di Raimondo, R. Gennaro, and H. Krawczyk, “Secure Off-the-record Messaging,” in Proceedings of the 2005 ACM Workshop on Privacy in the Electronic Society, 2005. http://www.dmi.unict.it/diraimondo/web/wp-content/uploads/papers/otr.pdf

[16] C. Kudla and K. G. Paterson, “Modular Security Proofs for Key Agreement Protocols,” in Advances in Cryptology - ASIACRYPT 2005: 11th International Conference on the Theory and Application of Cryptology and Information Security, 2005. http://www.isg.rhul.ac.uk/~kp/ModularProofs.pdf

[17] S. Blake-Wilson, D. Johnson, and A. Menezes, “Key agreement protocols and their security analysis,” in Crytography and Coding: 6th IMA International Conference Cirencester, UK, December 17–19, 1997 Proceedings, 1997. http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.25.387