noise.md

title	author	revision	date	bibliography	link-citations	csl
The Noise Protocol Framework	Trevor Perrin ([email protected])	32draft	2017-02-25	my.bib	true	ieee-with-url.csl

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:

• s, e: The local party's static and ephemeral key pairs (which may be empty).

• rs, re: The remote party's static and ephemeral public keys (which may be empty).

• h: A handshake hash value that hashes all the handshake data that's been sent and received.

• ck: A chaining key that hashes all previous DH outputs. Once the handshake completes, the chaining key will be used to derive the encryption keys for transport messages.

• k, n: An encryption key k (which may be empty) and a counter-based nonce n. Whenever a new DH output causes a new ck to be calculated, a new k is also calculated. The key k and nonce n are used to encrypt static public keys and handshake payloads. Encryption with k uses some AEAD cipher mode (in the sense of Rogaway [@Rogaway:2002]) and includes the current h value as "associated data" which is covered by the AEAD authentication. Encryption of static public keys and payloads provides some confidentiality and key confirmation during the handshake phase.

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:

• "e": The sender 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.

• "s": The sender writes its static public key from the s variable 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.

• "ee", "se", "es", "ss": A DH is performed between the initiator's key pair (whether static or ephemeral is determined by the first letter) and the responder's key pair (whether static or ephemeral is determined by the second letter). The result is hashed along with the old ck to derive a new ck and k, and n is set to zero.

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:

• Simpler testing, since it's easy to test the maximum sizes.

• Reduces the likelihood of errors in memory handling, or integer overflow.

• Enables support for streaming decryption and random-access decryption of large data streams.

• Enables higher-level protocols that encapsulate Noise messages to use an efficient standard length field of 16 bits.

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.

\pagebreak

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:

• The || operator concatenates byte sequences.
• The byte() function constructs a single byte.

4.1. DH functions

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

• GENERATE_KEYPAIR(): Generates a new DH key pair. A DH key pair consists of public_key and private_key elements. A public_key represents an encoding of a DH public key into a byte sequence of length DHLEN. The public_key encoding details are specific to each set of DH functions.

• DH(key_pair, public_key): Performs a DH calculation between the private key in key_pair and public_key and returns an output sequence of bytes of length DHLEN. If the function detects an invalid public_key, the output may be all zeros or any other value that doesn't leak information about the private key. For reasons discussed in Section 9.1 it is recommended for the function to have a null public key value that always yields the same output, regardless of private key. For example, the DH functions in Section 10 always map a DH public key of all zeros to an output of all zeros.

• DHLEN = A constant specifying the size in bytes of public keys and DH outputs. For security reasons, DHLEN must be 32 or greater.

4.2. Cipher functions

Noise depends on the following cipher functions:

• ENCRYPT(k, n, ad, plaintext): Encrypts plaintext using the cipher key k of 32 bytes and an 8-byte unsigned integer nonce n which must be unique for the key k. Returns the ciphertext. Encryption must be done with an "AEAD" encryption mode with the associated data ad (using the terminology from [@Rogaway:2002]) and returns a ciphertext that is the same size as the plaintext plus 16 bytes for authentication data. The entire ciphertext must be indistinguishable from random if the key is secret.

• DECRYPT(k, n, ad, ciphertext): Decrypts ciphertext using a cipher key k of 32 bytes, an 8-byte unsigned integer nonce n, and associated data ad. Returns the plaintext, unless authentication fails, in which case an error is signaled to the caller.

4.3. Hash functions

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

• HASH(data): Hashes some arbitrary-length data with a collision-resistant cryptographic hash function and returns an output of HASHLEN bytes.

• HASHLEN = A constant specifying the size in bytes of the hash output. Must be 32 or 64.

• BLOCKLEN = A constant specifying the size in bytes that the hash function uses internally to divide its input for iterative processing. This is needed to use the hash function with HMAC (BLOCKLEN is B in [@rfc2104]).

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

• HMAC-HASH(key, data): Applies HMAC from [@rfc2104] using the HASH() function. This function is only called as part of HKDF(), below.

• HKDF(chaining_key, input_key_material): Takes a chaining_key byte sequence of length HASHLEN, and an input_key_material byte sequence with length either zero bytes, 32 bytes, or DHLEN bytes. Returns two byte sequences of length HASHLEN, as follows:

  • Sets temp_key = HMAC-HASH(chaining_key, input_key_material).
  • Sets output1 = HMAC-HASH(temp_key, byte(0x01)).
  • Sets output2 = HMAC-HASH(temp_key, output1 || byte(0x02)).
  • Returns the pair (output1, output2).

  Note that temp_key, output1, and output2 are all HASHLEN bytes in length. Also note that the HKDF() function is simply HKDF from [@rfc5869] 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:

• A CipherState object contains k and n variables, which it uses to encrypt and decrypt ciphertexts. During the handshake phase each party has a single CipherState, but during the transport phase each party has two CipherState objects: one for sending, and one for receiving.

• A SymmetricState object contains a CipherState plus ck and h variables. It is so-named because it encapsulates all the "symmetric crypto" used by Noise. During the handshake phase each party has a single SymmetricState, which can be deleted once the handshake is finished.

• A HandshakeState object contains a SymmetricState plus DH variables (s, e, rs, re) and a variable representing the handshake pattern. During the handshake phase each party has a single HandshakeState, which can be deleted once the handshake is finished.

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:

• k: A cipher key of 32 bytes (which may be empty). Empty is a special value which indicates k has not yet been initialized.

• n: An 8-byte (64-bit) unsigned integer nonce.

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 (2^64^-1) is reserved for future use and must not be used. If incrementing n results in 2^64^-1 (the maximum value), then any further EncryptWithAd() or DecryptWithAd() calls will signal an error to the caller.

• InitializeKey(key): Sets k = key. Sets n = 0.

• HasKey(): Returns true if k is non-empty, false otherwise.

• EncryptWithAd(ad, plaintext): If k is non-empty returns ENCRYPT(k, n++, ad, plaintext). Otherwise returns plaintext.

• DecryptWithAd(ad, ciphertext): If k is non-empty returns DECRYPT(k, n++, ad, ciphertext). Otherwise returns ciphertext. If an authentication failure occurs in DECRYPT() the error is signaled to the caller.

5.2. The SymmetricState object

A SymmetricState object contains a CipherState plus the following variables:

• ck: A chaining key of HASHLEN bytes.
• h: A hash output of HASHLEN bytes.

A SymmetricState responds to the following methods:

• InitializeSymmetric(protocol_name): Takes an arbitrary-length protocol_name byte sequence (see Section 11). Executes the following steps:

  • If protocol_name is less than or equal to HASHLEN bytes in length, sets h equal to protocol_name with zero bytes appended to make HASHLEN bytes. Otherwise sets h = HASH(protocol_name).

  • Sets ck = h.

  • Calls InitializeKey(empty).

• MixKey(input_key_material): Sets ck, temp_k = HKDF(ck, input_key_material). If HASHLEN is 64, then truncates temp_k to 32 bytes. Calls InitializeKey(temp_k).

• MixHash(data): Sets h = HASH(h || data).

• EncryptAndHash(plaintext): Sets ciphertext = EncryptWithAd(h, plaintext), calls MixHash(ciphertext), and returns ciphertext. Note that if k is empty, the EncryptWithAd() call will set ciphertext equal to plaintext.

• DecryptAndHash(ciphertext): Sets plaintext = DecryptWithAd(h, ciphertext), calls MixHash(ciphertext), and returns plaintext. Note that if k is empty, the DecryptWithAd() call will set plaintext equal to ciphertext.

• Split(): Returns a pair of CipherState objects for encrypting transport messages. Executes the following steps, where zerolen is a zero-length byte sequence:

  • Sets temp_k1, temp_k2 = HKDF(ck, zerolen).
  • If HASHLEN is 64, then truncates temp_k1 and temp_k2 to 32 bytes.
  • Creates two new CipherState objects c1 and c2.
  • Calls c1.InitializeKey(temp_k1) and c2.InitializeKey(temp_k2).
  • Returns the pair (c1, c2).

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.

• s: The local static key pair
• e: The local ephemeral key pair
• rs: The remote party's static public key
• re: The remote party's ephemeral public key

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

• initiator: A boolean indicating the initiator or responder role.

• message_patterns: A sequence of message patterns. Each message pattern is a sequence of tokens from the set ("s", "e", "ee", "es", "se", "ss").

A HandshakeState responds to the following methods:

• Initialize(handshake_pattern, initiator, prologue, s, e, rs, re): Takes a valid handshake_pattern (see Section 8) and an initiator boolean specifying this party's role as either initiator or responder.

  Takes a prologue byte sequence which may be zero-length, or which may contain context information that both parties want to confirm is identical (see Section 6).

  Takes a set of DH key pairs (s, e) and public keys (rs, re) for initializing local variables, any of which may be empty. Public keys are only passed in if the handshake_pattern uses pre-messages (see Section 8). The ephemeral values (e, re) are typically left empty, since they are created and exchanged during the handshake; but there are exceptions (see Section 9.2).

  • Derives a protocol_name byte sequence by combining the names for the handshake pattern and crypto functions, as specified in Section 11. Calls InitializeSymmetric(protocol_name).

  • Calls MixHash(prologue).

  • Sets the initiator, s, e, rs, and re variables to the corresponding arguments.

  • Calls MixHash() once for each public key listed in the pre-messages from handshake_pattern, with the specified public key as input (see Section 8 for an explanation of pre-messages). If both initiator and responder have pre-messages, the initiator's public keys are hashed first.

  • Sets message_patterns to the message patterns from handshake_pattern.

• WriteMessage(payload, message_buffer): Takes a payload byte sequence which may be zero-length, and a message_buffer to write the output into.

  • Fetches and deletes the next message pattern from message_patterns, then sequentially processes each token from the message pattern:

    • For "e": Sets e = GENERATE_KEYPAIR(), overwriting any previous value for e. Appends e.public_key to the buffer. Calls MixHash(e.public_key).

    • For "s": Appends EncryptAndHash(s.public_key) to the buffer.

    • For "xy": Calls MixKey(DH(x, ry)) if initiator, otherwise MixKey(DH(y, rx)).

  • Appends EncryptAndHash(payload) to the buffer.

  • If there are no more message patterns returns two new CipherState objects by calling Split().

• ReadMessage(message, payload_buffer): Takes a byte sequence containing a Noise handshake message, and a payload_buffer to write the message's plaintext payload into.

  • Fetches and deletes the next message pattern from message_patterns, then sequentially processes each token from the message pattern:

    • For "e": Sets re to the next DHLEN bytes from the message, overwriting any previous value for re. Calls MixHash(re.public_key).

    • For "s": Sets temp to the next DHLEN + 16 bytes of the message if HasKey() == True, or to the next DHLEN bytes otherwise. Sets rs to DecryptAndHash(temp).

    • For "xy": Calls MixKey(DH(x, ry)) if initiator, otherwise MixKey(DH(y, rx)).

  • Calls DecryptAndHash() on the remaining bytes of the message and stores the output into payload_buffer.

  • If there are no more message patterns returns two new CipherState objects by calling Split().

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:

• Protocol names (Section 11) use the prefix "NoisePSK_" instead of "Noise_".

• Initialize() takes an additional psk argument, which is a sequence of 32 bytes. Immediately after MixHash(prologue) it sets ck, temp = HKDF(ck, psk), then calls MixHash(temp). This mixes the pre-shared key into the chaining key, and also mixes a one-way function of the pre-shared key into the h value to ensure that h is a function of all handshake inputs.

• WriteMessage() and ReadMessage() are modified when processing the "e" token to call MixKey(e.public_key) as the final step. Because the initial messages in a handshake pattern are required to start with "e" (Section 8.1), this ensures k is initialized from the pre-shared key. This also uses the ephemeral public key's value as a random nonce to prevent re-using the same k and n for different messages.

• Initialize() is modified when processing an "e" token in the initiator's pre-message. A handshake pattern with such a pre-message is a fallback pattern (see Section 8.1). In this case, MixKey() is called on the ephemeral public key immediately after calling MixHash() on it. This accomplishes the same randomization as the previous bullet.

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:

• A pattern for the initiator's pre-message that is either:

  • "e"
  • "s"
  • "e, s"
  • empty

• A pattern for the responder's pre-message that takes the same range of values as the initiator's pre-message.

• A sequence of message patterns for the actual handshake messages

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 (with one exception - see next paragraph). The next message is sent by the responder, the next from the initiator, and so on in alternating fashion.

As will be described later, Noise allows compound protocols where the responder switches to a different "fallback" pattern than the initiator started with. If the initiator's pre-message contains an "e" token, then this handshake pattern is a fallback pattern (see Section 9.2). In the case of a fallback pattern, the first actual handshake message is sent by the responder, the next from the initiator, and so on.

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", "e", or "e, s" inside the parentheses would indicate that the initiator is initialized with static and/or ephemeral key pairs. The tokens "rs", "re", or "re, rs" would indicate the same thing for the responder.

Right-pointing arrows show messages sent by the initiator. Left-pointing arrows show messages sent by 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"). An exception is allowed for the initiator if the initiator's ephemeral public key is used as a "pre-message", i.e. if the handshake is using a "fallback pattern". Such a pattern 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:

• N = **N**o static key for sender
• K = Static key for sender **K**nown to recipient
• X = Static key for sender **X**mitted ("transmitted") to recipient

\pagebreak

+-------------------------+ | 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:

• N = **N**o static key for initiator
• K = Static key for initiator **K**nown to responder
• X = Static key for initiator **X**mitted ("transmitted") to responder
• I = Static key for initiator **I**mmediately transmitted to responder, despite reduced or absent identity hiding

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

• N = **N**o static key for responder
• K = Static key for responder **K**nown to responder
• X = Static key for responder **X**mitted ("transmitted") to initiator

\pagebreak

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

\pagebreak

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:

• Patterns where the initiator has pre-knowledge of the responder's static public key (i.e. patterns ending in "K") allow "zero-RTT" encryption, meaning the initiator can encrypt the first handshake payload.

• All interactive patterns allow "half-RTT" encryption of the first response payload, but the encryption only targets an initiator static public key in patterns starting with "K" or "I".

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:

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

1. 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.

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.

The confidentiality properties are:

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

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.

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.

3. 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.

4. 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.

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.

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:

0. Transmitted in clear.

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

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

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

4. 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.

5. Not transmitted, but a passive attacker can check candidates for the pair of (responder's private key, initiator's public key) and learn whether the candidate pair is correct.

6. 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.

7. 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.

8. 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:

\pagebreak

+-------------------------------+-----------------------------+ | Noise_X(s, rs): | Noise_Xnoidh(s, rs): | | <- s | <- s | | ... | ... | | -> e, es, s, ss | -> e, s, es, ss | +-------------------------------+-----------------------------+ | Noise_NX(rs): | Noise_NXnoidh(rs): |
| -> e | -> e | | <- e, ee, s, es | <- e, s, ee, es |
+-------------------------------+-----------------------------+ | Noise_XX(s, rs): | Noise_XXnoidh(s, rs): |
| -> e | -> e | | <- e, ee, s, es | <- e, s, ee, es |
| -> s, se | -> s, se |
+-------------------------------+-----------------------------+ | Noise_KX(s, rs): | Noise_KXnoidh(s, rs): |
| -> s | -> s | | ... | ... | | -> e | -> e | | <- e, ee, se, s, es | <- e, s, ee, se, es |
+-------------------------------+-----------------------------+ | Noise_IK(s, rs): | Noise_IKnoidh(s, rs): | | <- s | <- s | | ... | ... | | -> e, es, s, ss | -> e, s, es, ss |
| <- e, ee, se | <- e, ee, se |
+-------------------------------+-----------------------------+ | Noise_IX(s, rs): | Noise_IXnoidh(s, rs): | | -> e, s | -> e, s | | <- e, ee, se, s, es | <- 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 fallback handshake pattern. Using handshake re-initialization to switch from one "simple" Noise protocol to another results in a compound protocol.

Public keys that were sent in the initial message should be represented as pre-messages in the second handshake. Because an ephemeral public key was sent in the initiator's first message and used as a pre-message in the second handshake, the second's handshake pattern is a "fallback pattern", and the initiator does not need to send a new ephemeral (see Section 8.1).

Re-initializing with a fallback 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:

• Noise_XX is used for an initial full handshake if the parties haven't communicated before, after which the initiator can cache the responder's static public key.

• Noise_IK is used for a zero-RTT abbreviated handshake.

• If the responder fails to decrypt the first Noise_IK message (perhaps due to changing his static key), the responder will initiate a new fallback handshake using the Noise_XXfallback pattern which is identical to Noise_XX except re-using the ephemeral public key from the first Noise_IK message as a pre-message public key (thus is a fallback pattern).

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

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:

• If type == 0 in the initiator's first message then the initiator is performing a Noise_XX handshake (full handshake).

• If type == 1 in the initiator's first message then the initiator is performing a Noise_IK handshake (attempted abbreviated handshake).

• If type == 0 in the responder's first Noise_IK response then the responder accepted the Noise_IK message (successful abbreviated handshake).

• If type == 1 in the responder's first Noise_IK response then the responder failed to authenticate the initiator's Noise_IK message and is performing a Noise_XXfallback handshake, using the initiator's ephemeral public key as a pre-message (fallback handshake).

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:

• The first three messages can have their payloads padded with random bytes to a constant size, regardless of which handshake is executed.

• Instead of a type byte, the responder can use trial decryption to differentiate between an initial message using Noise_XX or Noise_IK.

• Instead of a type byte, an initiator who sent a Noise_IK initial message can use trial decryption to differentiate between a response using Noise_IK or Noise_XXfallback.

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 [@elligator] 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

• GENERATE_KEYPAIR(): Returns a new Curve25519 key pair.

• DH(keypair, public_key): Executes the Curve25519 DH function (aka "X25519" in [@rfc7748]. The null public key value is all zeros, which will always produce an output of all zeros. Other invalid public key values will also produce an output of all zeros.

• DHLEN = 32

10.2. The 448 DH functions

• GENERATE_KEYPAIR(): Returns a new Curve448 key pair.

• DH(keypair, public_key): Executes the Curve448 DH function (aka "X448" in [@rfc7748]. The null public key value is all zeros, which will always produce an output of all zeros. Other invalid public key values will also produce an output of all zeros.

• DHLEN = 56

10.3. The ChaChaPoly cipher functions

• ENCRYPT(k, n, ad, plaintext) / DECRYPT(k, n, ad, ciphertext): AEAD_CHACHA20_POLY1305 from [@rfc7539]. The 96-bit nonce is formed by encoding 32 bits of zeros followed by little-endian encoding of n. (Earlier implementations of ChaCha20 used a 64-bit nonce; with these implementations it's compatible to encode n directly into the ChaCha20 nonce without the 32-bit zero prefix).

10.4. The AESGCM cipher functions

• ENCRYPT(k, n, ad, plaintext) / DECRYPT(k, n, ad, ciphertext): AES256 with GCM from [@nistgcm] with a 128-bit tag appended to the ciphertext. The 96-bit nonce is formed by encoding 32 bits of zeros followed by big-endian encoding of n.

10.5. The SHA256 hash function

• HASH(input): SHA-256 from [@nistsha2].
• HASHLEN = 32
• BLOCKLEN = 64

10.6. The SHA512 hash function

• HASH(input): SHA-512 from [@nistsha2].
• HASHLEN = 64
• BLOCKLEN = 128

10.7. The BLAKE2s hash function

• HASH(input): BLAKE2s from [@rfc7693] with digest length 32.
• HASHLEN = 32
• BLOCKLEN = 64

10.8. The BLAKE2b hash function

• HASH(input): BLAKE2b from [@rfc7693] with digest length 64.
• HASHLEN = 64
• BLOCKLEN = 128

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:

• Noise_XX_25519_AESGCM_SHA256
• Noise_N_25519_ChaChaPoly_BLAKE2s
• Noise_XXfallback_448_AESGCM_SHA512
• Noise_IK_448_ChaChaPoly_BLAKE2b

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

• NoisePSK_XX_25519_AESGCM_SHA256
• NoisePSK_N_25519_ChaChaPoly_BLAKE2s
• NoisePSK_XXfallback_448_AESGCM_SHA512
• NoisePSK_IK_448_ChaChaPoly_BLAKE2b

12. Application responsibilities ================================

An application built on Noise must consider several issues:

• Choosing crypto functions: The 25519 DH functions are recommended for typical uses, though the 448 DH functions might offer some extra security in case a cryptanalytic attack is developed against elliptic curve cryptography. The 448 DH functions should be used with a 512-bit hash like SHA512 or BLAKE2b. The 25519 DH functions may be used with a 256-bit hash like SHA256 or BLAKE2s, though a 512-bit hash might offer some extra security in case a cryptanalytic attack is developed against the smaller hash functions.

• Extensibility: Applications are recommended to use an extensible data format for the payloads of all messages (e.g. JSON, Protocol Buffers). This ensures that fields can be added in the future which are ignored by older implementations.

• Padding: Applications are recommended to use a data format for the payloads of all encrypted messages that allows padding. This allows implementations to avoid leaking information about message sizes. Using an extensible data format, per the previous bullet, will typically suffice.

• Session termination: Applications must consider that a sequence of Noise transport messages could be truncated by an attacker. Applications should include explicit length fields or termination signals inside of transport payloads to signal the end of an interactive session, or the end of a one-way stream of transport messages.

• Length fields: Applications must handle any framing or additional length fields for Noise messages, considering that a Noise message may be up to 65535 bytes in length. If an explicit length field is needed, applications are recommended to add a 16-bit big-endian length field prior to each message.

• Type fields: Applications are recommended to include a single-byte type field prior to each Noise handshake message (and prior to the length field, if one is included). A recommended idiom is for the value zero to indicate no change from the current Noise protocol, and for applications to reject messages with an unknown value. This allows future protocol versions to specify handshake re-initialization or any other compatibility-breaking change (protocol extensions that don't break compatibility can be handled within Noise payloads).

\pagebreak

13. Security considerations ===========================

This section collects various security considerations:

• Session termination: Preventing attackers from truncating a stream of transport messages is an application responsibility. See previous section.

• Rollback: If parties decide on a Noise protocol based on some previous negotiation that is not included as prologue, then a rollback attack might be possible. This is a particular risk with handshake re-initialization, and requires careful attention if a Noise handshake is preceded by communication between the parties.

• Misusing public keys as secrets: It might be tempting to use a pattern with a pre-message public key and assume that a successful handshake implies the other party's knowledge of the public key. Unfortunately, this is not the case, since setting public keys to invalid values might cause predictable DH output. For example, a Noise_NK_25519 initiator might send an invalid ephemeral public key to cause a known DH output of all zeros, despite not knowing the responder's static public key. If the parties want to authenticate with a shared secret, it must be passed in as a PSK.

• Channel binding: Depending on the DH functions, it might be possible for a malicious party to engage in multiple sessions that derive the same shared secret key by setting public keys to invalid values that cause predictable DH output (as in previous bullet). This is why a higher-level protocol should use the handshake hash (h) for a unique channel binding, instead of ck, as explained in Section 9.4.

• Incrementing nonces: Reusing a nonce value for n with the same key k for encryption would be catastrophic. Implementations must carefully follow the rules for nonces. Nonces are not allowed to wrap back to zero due to integer overflow, and the maximum nonce value is reserved for future use. This means parties are not allowed to send more than 2^64^-1 transport messages.

• Fresh ephemerals: Every party in a Noise protocol should send a new ephemeral public key and perform a DH with it prior to sending any encrypted data. Otherwise replay of a handshake message could trigger catastrophic key reuse. This is one rationale behind the patterns in Section 8, and the validity rules in Section 8.1. It's also the reason why one-way handshakes only allow transport messages from the sender, not the recipient.

• Protocol names: The protocol name used with Initialize() must uniquely identify the combination of handshake pattern and crypto functions for every key it's used with (whether ephemeral key pair, static key pair, or PSK). If the same secret key was reused with the same protocol name but a different set of cryptographic operations then bad interactions could occur.

• Pre-shared symmetric keys: Pre-shared symmetric keys must be secret values with 256 bits of entropy.

• Data volumes: The AESGCM cipher functions suffer a gradual reduction in security as the volume of data encrypted under a single key increases. Due to this, parties should not send more than 2^56^ bytes (roughly 72 petabytes) encrypted by a single key. If sending such large volumes of data is a possibility, different cipher functions should be chosen.

• Hash collisions: If an attacker can find hash collisions on prologue data or the handshake hash, they may be able to perform "transcript collision" attacks that trick the parties into having different views of handshake data. It is important to use Noise with collision-resistant hash functions, and replace the hash function at any sign of weakness.

• Implementation fingerprinting: If this protocol is used in settings with anonymous parties, care should be taken that implementations behave identically in all cases. This may require mandating exact behavior for handling of invalid DH public keys.

14. Rationale =============

This section collects various design rationale:

Nonces are 64 bits in length because:

• Some ciphers only have 64 bit nonces (e.g. Salsa20).
• 64 bit nonces were used in the initial specification and implementations of ChaCha20, so Noise nonces can be used with these implementations.
• 64 bits makes it easy for the entire nonce to be treated as an integer and incremented.
• 96 bits nonces (e.g. in RFC 7539) are a confusing size where it's unclear if random nonces are acceptable.

The recommended hash function families are SHA2 and BLAKE2 because:

• SHA2 is widely available.
• SHA2 is often used alongside AES.
• BLAKE2 is fast and similar to ChaCha20.

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

• SHA-256 and BLAKE2s have sufficient collision-resistance at the 128-bit security level.
• SHA-256 and BLAKE2s require less RAM, and less calculation when processing smaller inputs (due to smaller block size), than their larger brethren (SHA-512 and BLAKE2b).
• SHA-256 and BLAKE2s are faster on 32-bit processors than their larger brethren.

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

• 256 bits is a conservative length for cipher keys when considering cryptanalytic safety margins, time/memory tradeoffs, multi-key attacks, and quantum attacks.
• Pre-shared key length is fixed to simplify testing and implementation, and to deter users from mistakenly using low-entropy passwords as pre-shared keys.

The authentication data in a ciphertext is 128 bits because:

• Some algorithms (e.g. GCM) lose more security than an ideal MAC when truncated.
• Noise may be used in a wide variety of contexts, including where attackers can receive rapid feedback on whether MAC guesses are correct.
• A single fixed length is simpler than supporting variable-length tags.

The GCM security limit is 2^56^ bytes because:

• This is 2^52^ AES blocks (each block is 16 bytes). The limit is based on the risk of birthday collisions being used to rule out plaintext guesses. The probability an attacker could rule out a random guess on a 2^56^ byte plaintext is less than 1 in 1 million (roughly (2^52^ * 2^52^) / 2^128^).

Big-endian length fields are recommended because:

• Length fields are likely to be handled by parsing code where big-endian "network byte order" is traditional.
• Some ciphers use big-endian internally (e.g. GCM, SHA2).
• While it's true that Curve25519, Curve448, and ChaCha20/Poly1305 use little-endian, these will likely be handled by specialized libraries, so there's not a strong argument for aligning with them.

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

• ChaCha20 uses a little-endian block counter internally.
• AES-GCM uses a big-endian block counter internally.
• It makes sense to use consistent endianness in the cipher code.

The MixKey() design uses HKDF because:

• HKDF applies multiple layers of hashing between each MixKey() input. This "extra" hashing might mitigate the impact of hash function weakness.
• HKDF is well-known and is used in similar ways in other protocols (e.g. Signal, IPsec).
• HKDF and HMAC are widely-used constructions. If some weakness is found in a hash function, cryptanalysts will likely analyze that weakness in the context of HKDF and HMAC.

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

• MixHash() is more efficient than MixKey().
• MixHash() avoids any IPR concerns regarding mixing identity data into session keys (see KEA+).
• MixHash() produces a non-secret h value that might be useful to higher-level protocols, e.g. for channel-binding.

The h value hashes handshake ciphertext instead of plaintext because:

• This ensures h is a non-secret value that can be used for channel-binding or other purposes without leaking secret information.
• This provides stronger guarantees against ciphertext malleability.

Session termination is left to the application because:

• Providing a termination signal in Noise doesn't help the application much, since the application still has to use the signal correctly.
• For an application with its own termination signal, having a second termination signal in Noise is likely to be confusing rather than helpful.

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

• One-time ephemeral public keys make explicit nonces unnecessary.
• Explicit nonces allow reuse of ephemeral public keys. However reusing ephemerals (with periodic replacement) is more complicated, requires a secure time source, is less secure in case of ephemeral compromise, and only provides a small optimization, since key generation can be done for a fraction of the cost of a DH operation.
• Explicit nonces increase message size.
• Explicit nonces make it easier to "backdoor" crypto implementations, e.g. by modifying the RNG so that key recovery data is leaked through the nonce fields.

15. IPR ========

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

\pagebreak

16. Acknowledgements =====================

Noise is inspired by:

• The NaCl and CurveCP protocols from Dan Bernstein et al [@nacl; @curvecp].
• The SIGMA and HOMQV protocols from Hugo Krawczyk [@sigma; @homqv].
• The Ntor protocol from Ian Goldberg et al [@ntor].
• The analysis of OTR by Mario Di Raimondo et al [@otr].
• The analysis by Caroline Kudla and Kenny Paterson of "Protocol 4" by Simon Blake-Wilson et al [@kudla2005; @blakewilson1997].

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 ================