Using TLS to Secure QUIC

2.1. TLS Overview

TLS provides two endpoints with a way to establish a means of communication over an untrusted medium (for example, the Internet). TLS enables authentication of peers and provides confidentiality and integrity protection for messages that endpoints exchange.¶

Internally, TLS is a layered protocol, with the structure shown in Figure 1.¶

          +-------------+------------+--------------+---------+
Content   |             |            |  Application |         |
Layer     |  Handshake  |   Alerts   |     Data     |   ...   |
          |             |            |              |         |
          +-------------+------------+--------------+---------+
Record    |                                                   |
Layer     |                      Records                      |
          |                                                   |
          +---------------------------------------------------+

Each Content layer message (e.g., Handshake, Alerts, and Application Data) is carried as a series of typed TLS records by the Record layer. Records are individually cryptographically protected and then transmitted over a reliable transport (typically TCP), which provides sequencing and guaranteed delivery.¶

The TLS authenticated key exchange occurs between two endpoints: client and server. The client initiates the exchange and the server responds. If the key exchange completes successfully, both client and server will agree on a secret. TLS supports both pre-shared key (PSK) and Diffie-Hellman over either finite fields or elliptic curves ((EC)DHE) key exchanges. PSK is the basis for Early Data (0-RTT); the latter provides forward secrecy (FS) when the (EC)DHE keys are destroyed. The two modes can also be combined, to provide forward secrecy while using the PSK for authentication.¶

After completing the TLS handshake, the client will have learned and authenticated an identity for the server and the server is optionally able to learn and authenticate an identity for the client. TLS supports X.509 [RFC5280] certificate-based authentication for both server and client. When PSK key exchange is used (as in resumption), knowledge of the PSK serves to authenticate the peer.¶

The TLS key exchange is resistant to tampering by attackers and it produces shared secrets that cannot be controlled by either participating peer.¶

TLS provides two basic handshake modes of interest to QUIC:¶

• A full 1-RTT handshake, in which the client is able to send Application Data after one round trip and the server immediately responds after receiving the first handshake message from the client.¶
• A 0-RTT handshake, in which the client uses information it has previously learned about the server to send Application Data immediately. This Application Data can be replayed by an attacker so 0-RTT is not suitable for carrying instructions that might initiate any action that could cause unwanted effects if replayed.¶

A simplified TLS handshake with 0-RTT application data is shown in Figure 2.¶

    Client                                             Server

    ClientHello
   (0-RTT Application Data)  -------->
                                                  ServerHello
                                         {EncryptedExtensions}
                                                    {Finished}
                             <--------      [Application Data]
   {Finished}                -------->

   [Application Data]        <------->      [Application Data]

    () Indicates messages protected by Early Data (0-RTT) Keys
    {} Indicates messages protected using Handshake Keys
    [] Indicates messages protected using Application Data
       (1-RTT) Keys

Figure 2 omits the EndOfEarlyData message, which is not used in QUIC; see Section 8.3. Likewise, neither ChangeCipherSpec nor KeyUpdate messages are used by QUIC. ChangeCipherSpec is redundant in TLS 1.3; see Section 8.4. QUIC has its own key update mechanism; see Section 6.¶

Data is protected using a number of encryption levels:¶

• Initial Keys¶
• Early Data (0-RTT) Keys¶
• Handshake Keys¶
• Application Data (1-RTT) Keys¶

Application Data may appear only in the Early Data and Application Data levels. Handshake and Alert messages may appear in any level.¶

The 0-RTT handshake can be used if the client and server have previously communicated. In the 1-RTT handshake, the client is unable to send protected Application Data until it has received all of the Handshake messages sent by the server.¶

4.1. Interface to TLS

As shown in Figure 4, the interface from QUIC to TLS consists of four primary functions:¶

• Sending and receiving handshake messages¶
• Processing stored transport and application state from a resumed session and determining if it is valid to generate or accept early data¶
• Rekeying (both transmit and receive)¶
• Handshake state updates¶

Additional functions might be needed to configure TLS. In particular, QUIC and TLS need to agree on which is responsible for validation of peer credentials, such as certificate validation ([RFC5280]).¶

Client                                                    Server
======                                                    ======

Get Handshake
                     Initial ------------->
Install tx 0-RTT Keys
                     0-RTT - - - - - - - ->

                                              Handshake Received
                                                   Get Handshake
                     <------------- Initial
                                           Install rx 0-RTT keys
                                          Install Handshake keys
                                                   Get Handshake
                     <----------- Handshake
                                           Install tx 1-RTT keys
                     <- - - - - - - - 1-RTT

Handshake Received (Initial)
Install Handshake keys
Handshake Received (Handshake)
Get Handshake
                     Handshake ----------->
Handshake Complete
Install 1-RTT keys
                     1-RTT - - - - - - - ->

                                              Handshake Received
                                              Handshake Complete
                                             Handshake Confirmed
                                           Install rx 1-RTT keys
                     <--------------- 1-RTT
                           (HANDSHAKE_DONE)
Handshake Confirmed

4.2. TLS Version

This document describes how TLS 1.3 [TLS13] is used with QUIC.¶

In practice, the TLS handshake will negotiate a version of TLS to use. This could result in a newer version of TLS than 1.3 being negotiated if both endpoints support that version. This is acceptable provided that the features of TLS 1.3 that are used by QUIC are supported by the newer version.¶

Clients MUST NOT offer TLS versions older than 1.3. A badly configured TLS implementation could negotiate TLS 1.2 or another older version of TLS. An endpoint MUST terminate the connection if a version of TLS older than 1.3 is negotiated.¶

4.3. ClientHello Size

The first Initial packet from a client contains the start or all of its first cryptographic handshake message, which for TLS is the ClientHello. Servers might need to parse the entire ClientHello (e.g., to access extensions such as Server Name Identification (SNI) or Application Layer Protocol Negotiation (ALPN)) in order to decide whether to accept the new incoming QUIC connection. If the ClientHello spans multiple Initial packets, such servers would need to buffer the first received fragments, which could consume excessive resources if the client's address has not yet been validated. To avoid this, servers MAY use the Retry feature (see Section 8.1 of [QUIC-TRANSPORT]) to only buffer partial ClientHello messages from clients with a validated address.¶

QUIC packet and framing add at least 36 bytes of overhead to the ClientHello message. That overhead increases if the client chooses a source connection ID longer than zero bytes. Overheads also do not include the token or a destination connection ID longer than 8 bytes, both of which might be required if a server sends a Retry packet.¶

A typical TLS ClientHello can easily fit into a 1200-byte packet. However, in addition to the overheads added by QUIC, there are several variables that could cause this limit to be exceeded. Large session tickets, multiple or large key shares, and long lists of supported ciphers, signature algorithms, versions, QUIC transport parameters, and other negotiable parameters and extensions could cause this message to grow.¶

For servers, in addition to connection IDs and tokens, the size of TLS session tickets can have an effect on a client's ability to connect efficiently. Minimizing the size of these values increases the probability that clients can use them and still fit their entire ClientHello message in their first Initial packet.¶

The TLS implementation does not need to ensure that the ClientHello is large enough to meet the requirements for QUIC packets. QUIC PADDING frames are added to increase the size of the packet as necessary; see Section 14.1 of [QUIC-TRANSPORT].¶

4.4. Peer Authentication

The requirements for authentication depend on the application protocol that is in use. TLS provides server authentication and permits the server to request client authentication.¶

A client MUST authenticate the identity of the server. This typically involves verification that the identity of the server is included in a certificate and that the certificate is issued by a trusted entity (see for example [RFC2818]).¶

Note:

Where servers provide certificates for authentication, the size of the certificate chain can consume a large number of bytes. Controlling the size of certificate chains is critical to performance in QUIC as servers are limited to sending 3 bytes for every byte received prior to validating the client address; see Section 8.1 of [QUIC-TRANSPORT]. The size of a certificate chain can be managed by limiting the number of names or extensions; using keys with small public key representations, like ECDSA; or by using certificate compression [COMPRESS].¶

A server MAY request that the client authenticate during the handshake. A server MAY refuse a connection if the client is unable to authenticate when requested. The requirements for client authentication vary based on application protocol and deployment.¶

A server MUST NOT use post-handshake client authentication (as defined in Section 4.6.2 of [TLS13]), because the multiplexing offered by QUIC prevents clients from correlating the certificate request with the application-level event that triggered it (see [HTTP2-TLS13]). More specifically, servers MUST NOT send post-handshake TLS CertificateRequest messages and clients MUST treat receipt of such messages as a connection error of type PROTOCOL_VIOLATION.¶

4.5. Session Resumption

QUIC can use the session resumption feature of TLS 1.3. It does this by carrying NewSessionTicket messages in CRYPTO frames after the handshake is complete. Session resumption can be used to provide 0-RTT, and can also be used when 0-RTT is disabled.¶

Endpoints that use session resumption might need to remember some information about the current connection when creating a resumed connection. TLS requires that some information be retained; see Section 4.6.1 of [TLS13]. QUIC itself does not depend on any state being retained when resuming a connection, unless 0-RTT is also used; see Section 7.4.1 of [QUIC-TRANSPORT] and Section 4.6.1. Application protocols could depend on state that is retained between resumed connections.¶

Clients can store any state required for resumption along with the session ticket. Servers can use the session ticket to help carry state.¶

Session resumption allows servers to link activity on the original connection with the resumed connection, which might be a privacy issue for clients. Clients can choose not to enable resumption to avoid creating this correlation. Clients SHOULD NOT reuse tickets as that allows entities other than the server to correlate connections; see Section C.4 of [TLS13].¶

4.6. 0-RTT

The 0-RTT feature in QUIC allows a client to send application data before the handshake is complete. This is made possible by reusing negotiated parameters from a previous connection. To enable this, 0-RTT depends on the client remembering critical parameters and providing the server with a TLS session ticket that allows the server to recover the same information.¶

This information includes parameters that determine TLS state, as governed by [TLS13], QUIC transport parameters, the chosen application protocol, and any information the application protocol might need; see Section 4.6.3. This information determines how 0-RTT packets and their contents are formed.¶

To ensure that the same information is available to both endpoints, all information used to establish 0-RTT comes from the same connection. Endpoints cannot selectively disregard information that might alter the sending or processing of 0-RTT.¶

[TLS13] sets a limit of 7 days on the time between the original connection and any attempt to use 0-RTT. There are other constraints on 0-RTT usage, notably those caused by the potential exposure to replay attack; see Section 9.2.¶

4.7. HelloRetryRequest

The HelloRetryRequest message (see Section 4.1.4 of [TLS13]) can be used to request that a client provide new information, such as a key share, or to validate some characteristic of the client. From the perspective of QUIC, HelloRetryRequest is not differentiated from other cryptographic handshake messages that are carried in Initial packets. Although it is in principle possible to use this feature for address verification, QUIC implementations SHOULD instead use the Retry feature; see Section 8.1 of [QUIC-TRANSPORT].¶

4.8. TLS Errors

If TLS experiences an error, it generates an appropriate alert as defined in Section 6 of [TLS13].¶

A TLS alert is converted into a QUIC connection error. The AlertDescription value is added to 0x100 to produce a QUIC error code from the range reserved for CRYPTO_ERROR. The resulting value is sent in a QUIC CONNECTION_CLOSE frame of type 0x1c.¶

QUIC is only able to convey an alert level of "fatal". In TLS 1.3, the only existing uses for the "warning" level are to signal connection close; see Section 6.1 of [TLS13]. As QUIC provides alternative mechanisms for connection termination and the TLS connection is only closed if an error is encountered, a QUIC endpoint MUST treat any alert from TLS as if it were at the "fatal" level.¶

QUIC permits the use of a generic code in place of a specific error code; see Section 11 of [QUIC-TRANSPORT]. For TLS alerts, this includes replacing any alert with a generic alert, such as handshake_failure (0x128 in QUIC). Endpoints MAY use a generic error code to avoid possibly exposing confidential information.¶

4.9. Discarding Unused Keys

After QUIC has completed a move to a new encryption level, packet protection keys for previous encryption levels can be discarded. This occurs several times during the handshake, as well as when keys are updated; see Section 6.¶

Packet protection keys are not discarded immediately when new keys are available. If packets from a lower encryption level contain CRYPTO frames, frames that retransmit that data MUST be sent at the same encryption level. Similarly, an endpoint generates acknowledgments for packets at the same encryption level as the packet being acknowledged. Thus, it is possible that keys for a lower encryption level are needed for a short time after keys for a newer encryption level are available.¶

An endpoint cannot discard keys for a given encryption level unless it has received all the cryptographic handshake messages from its peer at that encryption level and its peer has done the same. Different methods for determining this are provided for Initial keys (Section 4.9.1) and Handshake keys (Section 4.9.2). These methods do not prevent packets from being received or sent at that encryption level because a peer might not have received all the acknowledgments necessary.¶

Though an endpoint might retain older keys, new data MUST be sent at the highest currently-available encryption level. Only ACK frames and retransmissions of data in CRYPTO frames are sent at a previous encryption level. These packets MAY also include PADDING frames.¶

5.1. Packet Protection Keys

QUIC derives packet protection keys in the same way that TLS derives record protection keys.¶

Each encryption level has separate secret values for protection of packets sent in each direction. These traffic secrets are derived by TLS (see Section 7.1 of [TLS13]) and are used by QUIC for all encryption levels except the Initial encryption level. The secrets for the Initial encryption level are computed based on the client's initial Destination Connection ID, as described in Section 5.2.¶

The keys used for packet protection are computed from the TLS secrets using the KDF provided by TLS. In TLS 1.3, the HKDF-Expand-Label function described in Section 7.1 of [TLS13] is used, using the hash function from the negotiated cipher suite. All uses of HKDF-Expand-Label in QUIC use a zero-length Context.¶

Note that labels, which are described using strings, are encoded as bytes using ASCII [ASCII] without quotes or any trailing NUL byte.¶

Other versions of TLS MUST provide a similar function in order to be used with QUIC.¶

The current encryption level secret and the label "quic key" are input to the KDF to produce the AEAD key; the label "quic iv" is used to derive the Initialization Vector (IV); see Section 5.3. The header protection key uses the "quic hp" label; see Section 5.4. Using these labels provides key separation between QUIC and TLS; see Section 9.6.¶

Both "quic key" and "quic hp" are used to produce keys, so the Length provided to HKDF-Expand-Label along with these labels is determined by the size of keys in the AEAD or header protection algorithm. The Length provided with "quic iv" is the minimum length of the AEAD nonce, or 8 bytes if that is larger; see [AEAD].¶

The KDF used for initial secrets is always the HKDF-Expand-Label function from TLS 1.3; see Section 5.2.¶

5.2. Initial Secrets

Initial packets apply the packet protection process, but use a secret derived from the Destination Connection ID field from the client's first Initial packet.¶

This secret is determined by using HKDF-Extract (see Section 2.2 of [HKDF]) with a salt of 0x38762cf7f55934b34d179ae6a4c80cadccbb7f0a and a IKM of the Destination Connection ID field. This produces an intermediate pseudorandom key (PRK) that is used to derive two separate secrets for sending and receiving.¶

The secret used by clients to construct Initial packets uses the PRK and the label "client in" as input to the HKDF-Expand-Label function from TLS [TLS13] to produce a 32-byte secret. Packets constructed by the server use the same process with the label "server in". The hash function for HKDF when deriving initial secrets and keys is SHA-256 [SHA].¶

This process in pseudocode is:¶

initial_salt = 0x38762cf7f55934b34d179ae6a4c80cadccbb7f0a
initial_secret = HKDF-Extract(initial_salt,
                              client_dst_connection_id)

client_initial_secret = HKDF-Expand-Label(initial_secret,
                                          "client in", "",
                                          Hash.length)
server_initial_secret = HKDF-Expand-Label(initial_secret,
                                          "server in", "",
                                          Hash.length)

The connection ID used with HKDF-Expand-Label is the Destination Connection ID in the Initial packet sent by the client. This will be a randomly-selected value unless the client creates the Initial packet after receiving a Retry packet, where the Destination Connection ID is selected by the server.¶

Future versions of QUIC SHOULD generate a new salt value, thus ensuring that the keys are different for each version of QUIC. This prevents a middlebox that recognizes only one version of QUIC from seeing or modifying the contents of packets from future versions.¶

The HKDF-Expand-Label function defined in TLS 1.3 MUST be used for Initial packets even where the TLS versions offered do not include TLS 1.3.¶

The secrets used for constructing subsequent Initial packets change when a server sends a Retry packet, to use the connection ID value selected by the server. The secrets do not change when a client changes the Destination Connection ID it uses in response to an Initial packet from the server.¶

Note:

The Destination Connection ID field could be any length up to 20 bytes, including zero length if the server sends a Retry packet with a zero-length Source Connection ID field. After a Retry, the Initial keys provide the client no assurance that the server received its packet, so the client has to rely on the exchange that included the Retry packet to validate the server address; see Section 8.1 of [QUIC-TRANSPORT].¶

Appendix A contains sample Initial packets.¶

5.3. AEAD Usage

The Authenticated Encryption with Associated Data (AEAD; see [AEAD]) function used for QUIC packet protection is the AEAD that is negotiated for use with the TLS connection. For example, if TLS is using the TLS_AES_128_GCM_SHA256 cipher suite, the AEAD_AES_128_GCM function is used.¶

QUIC can use any of the cipher suites defined in [TLS13] with the exception of TLS_AES_128_CCM_8_SHA256. A cipher suite MUST NOT be negotiated unless a header protection scheme is defined for the cipher suite. This document defines a header protection scheme for all cipher suites defined in [TLS13] aside from TLS_AES_128_CCM_8_SHA256. These cipher suites have a 16-byte authentication tag and produce an output 16 bytes larger than their input.¶

Note:

An endpoint MUST NOT reject a ClientHello that offers a cipher suite that it does not support, or it would be impossible to deploy a new cipher suite. This also applies to TLS_AES_128_CCM_8_SHA256.¶

When constructing packets, the AEAD function is applied prior to applying header protection; see Section 5.4. The unprotected packet header is part of the associated data (A). When processing packets, an endpoint first removes the header protection.¶

The key and IV for the packet are computed as described in Section 5.1. The nonce, N, is formed by combining the packet protection IV with the packet number. The 62 bits of the reconstructed QUIC packet number in network byte order are left-padded with zeros to the size of the IV. The exclusive OR of the padded packet number and the IV forms the AEAD nonce.¶

The associated data, A, for the AEAD is the contents of the QUIC header, starting from the first byte of either the short or long header, up to and including the unprotected packet number.¶

The input plaintext, P, for the AEAD is the payload of the QUIC packet, as described in [QUIC-TRANSPORT].¶

The output ciphertext, C, of the AEAD is transmitted in place of P.¶

Some AEAD functions have limits for how many packets can be encrypted under the same key and IV; see Section 6.6. This might be lower than the packet number limit. An endpoint MUST initiate a key update (Section 6) prior to exceeding any limit set for the AEAD that is in use.¶

5.4. Header Protection

Parts of QUIC packet headers, in particular the Packet Number field, are protected using a key that is derived separately from the packet protection key and IV. The key derived using the "quic hp" label is used to provide confidentiality protection for those fields that are not exposed to on-path elements.¶

This protection applies to the least-significant bits of the first byte, plus the Packet Number field. The four least-significant bits of the first byte are protected for packets with long headers; the five least significant bits of the first byte are protected for packets with short headers. For both header forms, this covers the reserved bits and the Packet Number Length field; the Key Phase bit is also protected for packets with a short header.¶

The same header protection key is used for the duration of the connection, with the value not changing after a key update (see Section 6). This allows header protection to be used to protect the key phase.¶

This process does not apply to Retry or Version Negotiation packets, which do not contain a protected payload or any of the fields that are protected by this process.¶

mask = header_protection(hp_key, sample)

pn_length = (packet[0] & 0x03) + 1
if (packet[0] & 0x80) == 0x80:
   # Long header: 4 bits masked
   packet[0] ^= mask[0] & 0x0f
else:
   # Short header: 5 bits masked
   packet[0] ^= mask[0] & 0x1f

# pn_offset is the start of the Packet Number field.
packet[pn_offset:pn_offset+pn_length] ^= mask[1:1+pn_length]

Initial Packet {
  Header Form (1) = 1,
  Fixed Bit (1) = 1,
  Long Packet Type (2) = 0,
  Reserved Bits (2),         # Protected
  Packet Number Length (2),  # Protected
  Version (32),
  DCID Len (8),
  Destination Connection ID (0..160),
  SCID Len (8),
  Source Connection ID (0..160),
  Token Length (i),
  Token (..),
  Length (i),
  Packet Number (8..32),     # Protected
  Protected Payload (0..24), # Skipped Part
  Protected Payload (128),   # Sampled Part
  Protected Payload (..)     # Remainder
}

1-RTT Packet {
  Header Form (1) = 0,
  Fixed Bit (1) = 1,
  Spin Bit (1),
  Reserved Bits (2),         # Protected
  Key Phase (1),             # Protected
  Packet Number Length (2),  # Protected
  Destination Connection ID (0..160),
  Packet Number (8..32),     # Protected
  Protected Payload (0..24), # Skipped Part
  Protected Payload (128),   # Sampled Part
  Protected Payload (..),    # Remainder
}

# pn_offset is the start of the Packet Number field.
sample_offset = pn_offset + 4

sample = packet[sample_offset..sample_offset+sample_length]

pn_offset = 1 + len(connection_id)

pn_offset = 7 + len(destination_connection_id) +
                len(source_connection_id) +
                len(payload_length)
if packet_type == Initial:
    pn_offset += len(token_length) +
                 len(token)

header_protection(hp_key, sample):
  mask = AES-ECB(hp_key, sample)

header_protection(hp_key, sample):
  counter = sample[0..3]
  nonce = sample[4..15]
  mask = ChaCha20(hp_key, counter, nonce, {0,0,0,0,0})

5.5. Receiving Protected Packets

Once an endpoint successfully receives a packet with a given packet number, it MUST discard all packets in the same packet number space with higher packet numbers if they cannot be successfully unprotected with either the same key, or - if there is a key update - a subsequent packet protection key; see Section 6. Similarly, a packet that appears to trigger a key update, but cannot be unprotected successfully MUST be discarded.¶

Failure to unprotect a packet does not necessarily indicate the existence of a protocol error in a peer or an attack. The truncated packet number encoding used in QUIC can cause packet numbers to be decoded incorrectly if they are delayed significantly.¶

5.6. Use of 0-RTT Keys

If 0-RTT keys are available (see Section 4.6.1), the lack of replay protection means that restrictions on their use are necessary to avoid replay attacks on the protocol.¶

Of the frames defined in [QUIC-TRANSPORT], the STREAM, RESET_STREAM, STOP_SENDING, and CONNECTION_CLOSE frames are potentially unsafe for use with 0-RTT as they carry application data. Application data that is received in 0-RTT could cause an application at the server to process the data multiple times rather than just once. Additional actions taken by a server as a result of processing replayed application data could have unwanted consequences. A client therefore MUST NOT use 0-RTT for application data unless specifically requested by the application that is in use.¶

An application protocol that uses QUIC MUST include a profile that defines acceptable use of 0-RTT; otherwise, 0-RTT can only be used to carry QUIC frames that do not carry application data. For example, a profile for HTTP is described in [HTTP-REPLAY] and used for HTTP/3; see Section 10.9 of [QUIC-HTTP].¶

Though replaying packets might result in additional connection attempts, the effect of processing replayed frames that do not carry application data is limited to changing the state of the affected connection. A TLS handshake cannot be successfully completed using replayed packets.¶

A client MAY wish to apply additional restrictions on what data it sends prior to the completion of the TLS handshake.¶

A client otherwise treats 0-RTT keys as equivalent to 1-RTT keys, except that it cannot send certain frames with 0-RTT keys; see Section 12.5 of [QUIC-TRANSPORT].¶

A client that receives an indication that its 0-RTT data has been accepted by a server can send 0-RTT data until it receives all of the server's handshake messages. A client SHOULD stop sending 0-RTT data if it receives an indication that 0-RTT data has been rejected.¶

A server MUST NOT use 0-RTT keys to protect packets; it uses 1-RTT keys to protect acknowledgments of 0-RTT packets. A client MUST NOT attempt to decrypt 0-RTT packets it receives and instead MUST discard them.¶

Once a client has installed 1-RTT keys, it MUST NOT send any more 0-RTT packets.¶

Note:

0-RTT data can be acknowledged by the server as it receives it, but any packets containing acknowledgments of 0-RTT data cannot have packet protection removed by the client until the TLS handshake is complete. The 1-RTT keys necessary to remove packet protection cannot be derived until the client receives all server handshake messages.¶

5.7. Receiving Out-of-Order Protected Packets

Due to reordering and loss, protected packets might be received by an endpoint before the final TLS handshake messages are received. A client will be unable to decrypt 1-RTT packets from the server, whereas a server will be able to decrypt 1-RTT packets from the client. Endpoints in either role MUST NOT decrypt 1-RTT packets from their peer prior to completing the handshake.¶

Even though 1-RTT keys are available to a server after receiving the first handshake messages from a client, it is missing assurances on the client state:¶

• The client is not authenticated, unless the server has chosen to use a pre-shared key and validated the client's pre-shared key binder; see Section 4.2.11 of [TLS13].¶
• The client has not demonstrated liveness, unless the server has validated the client's address with a Retry packet or other means; see Section 8.1 of [QUIC-TRANSPORT].¶
• Any received 0-RTT data that the server responds to might be due to a replay attack.¶

Therefore, the server's use of 1-RTT keys before the handshake is complete is limited to sending data. A server MUST NOT process incoming 1-RTT protected packets before the TLS handshake is complete. Because sending acknowledgments indicates that all frames in a packet have been processed, a server cannot send acknowledgments for 1-RTT packets until the TLS handshake is complete. Received packets protected with 1-RTT keys MAY be stored and later decrypted and used once the handshake is complete.¶

Note:

TLS implementations might provide all 1-RTT secrets prior to handshake completion. Even where QUIC implementations have 1-RTT read keys, those keys are not to be used prior to completing the handshake.¶

The requirement for the server to wait for the client Finished message creates a dependency on that message being delivered. A client can avoid the potential for head-of-line blocking that this implies by sending its 1-RTT packets coalesced with a Handshake packet containing a copy of the CRYPTO frame that carries the Finished message, until one of the Handshake packets is acknowledged. This enables immediate server processing for those packets.¶

A server could receive packets protected with 0-RTT keys prior to receiving a TLS ClientHello. The server MAY retain these packets for later decryption in anticipation of receiving a ClientHello.¶

A client generally receives 1-RTT keys at the same time as the handshake completes. Even if it has 1-RTT secrets, a client MUST NOT process incoming 1-RTT protected packets before the TLS handshake is complete.¶

5.8. Retry Packet Integrity

Retry packets (see the Retry Packet section of [QUIC-TRANSPORT]) carry a Retry Integrity Tag that provides two properties: it allows discarding packets that have accidentally been corrupted by the network; only an entity that observes an Initial packet can send a valid Retry packet.¶

The Retry Integrity Tag is a 128-bit field that is computed as the output of AEAD_AES_128_GCM ([AEAD]) used with the following inputs:¶

• The secret key, K, is 128 bits equal to 0xbe0c690b9f66575a1d766b54e368c84e.¶
• The nonce, N, is 96 bits equal to 0x461599d35d632bf2239825bb.¶
• The plaintext, P, is empty.¶
• The associated data, A, is the contents of the Retry Pseudo-Packet, as illustrated in Figure 8:¶

The secret key and the nonce are values derived by calling HKDF-Expand-Label using 0xd9c9943e6101fd200021506bcc02814c73030f25c79d71ce876eca876e6fca8e as the secret, with labels being "quic key" and "quic iv" (Section 5.1).¶

Retry Pseudo-Packet {
  ODCID Length (8),
  Original Destination Connection ID (0..160),
  Header Form (1) = 1,
  Fixed Bit (1) = 1,
  Long Packet Type (2) = 3,
  Unused (4),
  Version (32),
  DCID Len (8),
  Destination Connection ID (0..160),
  SCID Len (8),
  Source Connection ID (0..160),
  Retry Token (..),
}

The Retry Pseudo-Packet is not sent over the wire. It is computed by taking the transmitted Retry packet, removing the Retry Integrity Tag and prepending the two following fields:¶

ODCID Length:

The ODCID Length field contains the length in bytes of the Original Destination Connection ID field that follows it, encoded as an 8-bit unsigned integer.¶

Original Destination Connection ID:

The Original Destination Connection ID contains the value of the Destination Connection ID from the Initial packet that this Retry is in response to. The length of this field is given in ODCID Length. The presence of this field ensures that a valid Retry packet can only be sent by an entity that observes the Initial packet.¶

6.1. Initiating a Key Update

Endpoints maintain separate read and write secrets for packet protection. An endpoint initiates a key update by updating its packet protection write secret and using that to protect new packets. The endpoint creates a new write secret from the existing write secret as performed in Section 7.2 of [TLS13]. This uses the KDF function provided by TLS with a label of "quic ku". The corresponding key and IV are created from that secret as defined in Section 5.1. The header protection key is not updated.¶

For example, to update write keys with TLS 1.3, HKDF-Expand-Label is used as:¶

secret_<n+1> = HKDF-Expand-Label(secret_<n>, "quic ku",
                                 "", Hash.length)

The endpoint toggles the value of the Key Phase bit and uses the updated key and IV to protect all subsequent packets.¶

An endpoint MUST NOT initiate a key update prior to having confirmed the handshake (Section 4.1.2). An endpoint MUST NOT initiate a subsequent key update unless it has received an acknowledgment for a packet that was sent protected with keys from the current key phase. This ensures that keys are available to both peers before another key update can be initiated. This can be implemented by tracking the lowest packet number sent with each key phase, and the highest acknowledged packet number in the 1-RTT space: once the latter is higher than or equal to the former, another key update can be initiated.¶

Note:

Keys of packets other than the 1-RTT packets are never updated; their keys are derived solely from the TLS handshake state.¶

The endpoint that initiates a key update also updates the keys that it uses for receiving packets. These keys will be needed to process packets the peer sends after updating.¶

An endpoint MUST retain old keys until it has successfully unprotected a packet sent using the new keys. An endpoint SHOULD retain old keys for some time after unprotecting a packet sent using the new keys. Discarding old keys too early can cause delayed packets to be discarded. Discarding packets will be interpreted as packet loss by the peer and could adversely affect performance.¶

6.2. Responding to a Key Update

A peer is permitted to initiate a key update after receiving an acknowledgment of a packet in the current key phase. An endpoint detects a key update when processing a packet with a key phase that differs from the value used to protect the last packet it sent. To process this packet, the endpoint uses the next packet protection key and IV. See Section 6.3 for considerations about generating these keys.¶

If a packet is successfully processed using the next key and IV, then the peer has initiated a key update. The endpoint MUST update its send keys to the corresponding key phase in response, as described in Section 6.1. Sending keys MUST be updated before sending an acknowledgment for the packet that was received with updated keys. By acknowledging the packet that triggered the key update in a packet protected with the updated keys, the endpoint signals that the key update is complete.¶

An endpoint can defer sending the packet or acknowledgment according to its normal packet sending behaviour; it is not necessary to immediately generate a packet in response to a key update. The next packet sent by the endpoint will use the updated keys. The next packet that contains an acknowledgment will cause the key update to be completed. If an endpoint detects a second update before it has sent any packets with updated keys containing an acknowledgment for the packet that initiated the key update, it indicates that its peer has updated keys twice without awaiting confirmation. An endpoint MAY treat such consecutive key updates as a connection error of type KEY_UPDATE_ERROR.¶

An endpoint that receives an acknowledgment that is carried in a packet protected with old keys where any acknowledged packet was protected with newer keys MAY treat that as a connection error of type KEY_UPDATE_ERROR. This indicates that a peer has received and acknowledged a packet that initiates a key update, but has not updated keys in response.¶

6.3. Timing of Receive Key Generation

Endpoints responding to an apparent key update MUST NOT generate a timing side-channel signal that might indicate that the Key Phase bit was invalid (see Section 9.4). Endpoints can use dummy packet protection keys in place of discarded keys when key updates are not yet permitted. Using dummy keys will generate no variation in the timing signal produced by attempting to remove packet protection, and results in all packets with an invalid Key Phase bit being rejected.¶

The process of creating new packet protection keys for receiving packets could reveal that a key update has occurred. An endpoint MAY generate new keys as part of packet processing, but this creates a timing signal that could be used by an attacker to learn when key updates happen and thus leak the value of the Key Phase bit.¶

Endpoints are generally expected to have current and next receive packet protection keys available. For a short period after a key update completes, up to the PTO, endpoints MAY defer generation of the next set of receive packet protection keys. This allows endpoints to retain only two sets of receive keys; see Section 6.5.¶

Once generated, the next set of packet protection keys SHOULD be retained, even if the packet that was received was subsequently discarded. Packets containing apparent key updates are easy to forge and - while the process of key update does not require significant effort - triggering this process could be used by an attacker for DoS.¶

For this reason, endpoints MUST be able to retain two sets of packet protection keys for receiving packets: the current and the next. Retaining the previous keys in addition to these might improve performance, but this is not essential.¶

6.4. Sending with Updated Keys

An endpoint never sends packets that are protected with old keys. Only the current keys are used. Keys used for protecting packets can be discarded immediately after switching to newer keys.¶

Packets with higher packet numbers MUST be protected with either the same or newer packet protection keys than packets with lower packet numbers. An endpoint that successfully removes protection with old keys when newer keys were used for packets with lower packet numbers MUST treat this as a connection error of type KEY_UPDATE_ERROR.¶

6.5. Receiving with Different Keys

For receiving packets during a key update, packets protected with older keys might arrive if they were delayed by the network. Retaining old packet protection keys allows these packets to be successfully processed.¶

As packets protected with keys from the next key phase use the same Key Phase value as those protected with keys from the previous key phase, it is necessary to distinguish between the two, if packets protected with old keys are to be processed. This can be done using packet numbers. A recovered packet number that is lower than any packet number from the current key phase uses the previous packet protection keys; a recovered packet number that is higher than any packet number from the current key phase requires the use of the next packet protection keys.¶

Some care is necessary to ensure that any process for selecting between previous, current, and next packet protection keys does not expose a timing side channel that might reveal which keys were used to remove packet protection. See Section 9.5 for more information.¶

Alternatively, endpoints can retain only two sets of packet protection keys, swapping previous for next after enough time has passed to allow for reordering in the network. In this case, the Key Phase bit alone can be used to select keys.¶

An endpoint MAY allow a period of approximately the Probe Timeout (PTO; see [QUIC-RECOVERY]) after promoting the next set of receive keys to be current before it creates the subsequent set of packet protection keys. These updated keys MAY replace the previous keys at that time. With the caveat that PTO is a subjective measure - that is, a peer could have a different view of the RTT - this time is expected to be long enough that any reordered packets would be declared lost by a peer even if they were acknowledged and short enough to allow a peer to initiate further key updates.¶

Endpoints need to allow for the possibility that a peer might not be able to decrypt packets that initiate a key update during the period when the peer retains old keys. Endpoints SHOULD wait three times the PTO before initiating a key update after receiving an acknowledgment that confirms that the previous key update was received. Failing to allow sufficient time could lead to packets being discarded.¶

An endpoint SHOULD retain old read keys for no more than three times the PTO after having received a packet protected using the new keys. After this period, old read keys and their corresponding secrets SHOULD be discarded.¶

6.6. Limits on AEAD Usage

This document sets usage limits for AEAD algorithms to ensure that overuse does not give an adversary a disproportionate advantage in attacking the confidentiality and integrity of communications when using QUIC.¶

The usage limits defined in TLS 1.3 exist for protection against attacks on confidentiality and apply to successful applications of AEAD protection. The integrity protections in authenticated encryption also depend on limiting the number of attempts to forge packets. TLS achieves this by closing connections after any record fails an authentication check. In comparison, QUIC ignores any packet that cannot be authenticated, allowing multiple forgery attempts.¶

QUIC accounts for AEAD confidentiality and integrity limits separately. The confidentiality limit applies to the number of packets encrypted with a given key. The integrity limit applies to the number of packets decrypted within a given connection. Details on enforcing these limits for each AEAD algorithm follow below.¶

Endpoints MUST count the number of encrypted packets for each set of keys. If the total number of encrypted packets with the same key exceeds the confidentiality limit for the selected AEAD, the endpoint MUST stop using those keys. Endpoints MUST initiate a key update before sending more protected packets than the confidentiality limit for the selected AEAD permits. If a key update is not possible or integrity limits are reached, the endpoint MUST stop using the connection and only send stateless resets in response to receiving packets. It is RECOMMENDED that endpoints immediately close the connection with a connection error of type AEAD_LIMIT_REACHED before reaching a state where key updates are not possible.¶

For AEAD_AES_128_GCM and AEAD_AES_256_GCM, the confidentiality limit is 2^23 encrypted packets; see Appendix B.1. For AEAD_CHACHA20_POLY1305, the confidentiality limit is greater than the number of possible packets (2^62) and so can be disregarded. For AEAD_AES_128_CCM, the confidentiality limit is 2^21.5 encrypted packets; see Appendix B.2. Applying a limit reduces the probability that an attacker can distinguish the AEAD in use from a random permutation; see [AEBounds], [ROBUST], and [GCM-MU].¶

In addition to counting packets sent, endpoints MUST count the number of received packets that fail authentication during the lifetime of a connection. If the total number of received packets that fail authentication within the connection, across all keys, exceeds the integrity limit for the selected AEAD, the endpoint MUST immediately close the connection with a connection error of type AEAD_LIMIT_REACHED and not process any more packets.¶

For AEAD_AES_128_GCM and AEAD_AES_256_GCM, the integrity limit is 2^52 invalid packets; see Appendix B.1. For AEAD_CHACHA20_POLY1305, the integrity limit is 2^36 invalid packets; see [AEBounds]. For AEAD_AES_128_CCM, the integrity limit is 2^21.5 invalid packets; see Appendix B.2. Applying this limit reduces the probability that an attacker can successfully forge a packet; see [AEBounds], [ROBUST], and [GCM-MU].¶

Endpoints that limit the size of packets MAY use higher confidentiality and integrity limits; see Appendix B for details.¶

Future analyses and specifications MAY relax confidentiality or integrity limits for an AEAD.¶

Any TLS cipher suite that is specified for use with QUIC MUST define limits on the use of the associated AEAD function that preserves margins for confidentiality and integrity. That is, limits MUST be specified for the number of packets that can be authenticated and for the number of packets that can fail authentication. Providing a reference to any analysis upon which values are based - and any assumptions used in that analysis - allows limits to be adapted to varying usage conditions.¶

6.7. Key Update Error Code

The KEY_UPDATE_ERROR error code (0xe) is used to signal errors related to key updates.¶

8.1. Protocol Negotiation

QUIC requires that the cryptographic handshake provide authenticated protocol negotiation. TLS uses Application Layer Protocol Negotiation ([ALPN]) to select an application protocol. Unless another mechanism is used for agreeing on an application protocol, endpoints MUST use ALPN for this purpose.¶

When using ALPN, endpoints MUST immediately close a connection (see Section 10.2 of [QUIC-TRANSPORT]) with a no_application_protocol TLS alert (QUIC error code 0x178; see Section 4.8) if an application protocol is not negotiated. While [ALPN] only specifies that servers use this alert, QUIC clients MUST use error 0x178 to terminate a connection when ALPN negotiation fails.¶

An application protocol MAY restrict the QUIC versions that it can operate over. Servers MUST select an application protocol compatible with the QUIC version that the client has selected. The server MUST treat the inability to select a compatible application protocol as a connection error of type 0x178 (no_application_protocol). Similarly, a client MUST treat the selection of an incompatible application protocol by a server as a connection error of type 0x178.¶

8.2. QUIC Transport Parameters Extension

QUIC transport parameters are carried in a TLS extension. Different versions of QUIC might define a different method for negotiating transport configuration.¶

Including transport parameters in the TLS handshake provides integrity protection for these values.¶

   enum {
      quic_transport_parameters(0x39), (65535)
   } ExtensionType;

The extension_data field of the quic_transport_parameters extension contains a value that is defined by the version of QUIC that is in use.¶

The quic_transport_parameters extension is carried in the ClientHello and the EncryptedExtensions messages during the handshake. Endpoints MUST send the quic_transport_parameters extension; endpoints that receive ClientHello or EncryptedExtensions messages without the quic_transport_parameters extension MUST close the connection with an error of type 0x16d (equivalent to a fatal TLS missing_extension alert, see Section 4.8).¶

Transport parameters become available prior to the completion of the handshake. A server might use these values earlier than handshake completion. However, the value of transport parameters is not authenticated until the handshake completes, so any use of these parameters cannot depend on their authenticity. Any tampering with transport parameters will cause the handshake to fail.¶

Endpoints MUST NOT send this extension in a TLS connection that does not use QUIC (such as the use of TLS with TCP defined in [TLS13]). A fatal unsupported_extension alert MUST be sent by an implementation that supports this extension if the extension is received when the transport is not QUIC.¶

Negotiating the quic_transport_parameters extension causes the EndOfEarlyData to be removed; see Section 8.3.¶

8.3. Removing the EndOfEarlyData Message

The TLS EndOfEarlyData message is not used with QUIC. QUIC does not rely on this message to mark the end of 0-RTT data or to signal the change to Handshake keys.¶

Clients MUST NOT send the EndOfEarlyData message. A server MUST treat receipt of a CRYPTO frame in a 0-RTT packet as a connection error of type PROTOCOL_VIOLATION.¶

As a result, EndOfEarlyData does not appear in the TLS handshake transcript.¶

8.4. Prohibit TLS Middlebox Compatibility Mode

Appendix D.4 of [TLS13] describes an alteration to the TLS 1.3 handshake as a workaround for bugs in some middleboxes. The TLS 1.3 middlebox compatibility mode involves setting the legacy_session_id field to a 32-byte value in the ClientHello and ServerHello, then sending a change_cipher_spec record. Both field and record carry no semantic content and are ignored.¶

This mode has no use in QUIC as it only applies to middleboxes that interfere with TLS over TCP. QUIC also provides no means to carry a change_cipher_spec record. A client MUST NOT request the use of the TLS 1.3 compatibility mode. A server SHOULD treat the receipt of a TLS ClientHello with a non-empty legacy_session_id field as a connection error of type PROTOCOL_VIOLATION.¶

9.1. Session Linkability

Use of TLS session tickets allows servers and possibly other entities to correlate connections made by the same client; see Section 4.5 for details.¶

9.2. Replay Attacks with 0-RTT

As described in Section 8 of [TLS13], use of TLS early data comes with an exposure to replay attack. The use of 0-RTT in QUIC is similarly vulnerable to replay attack.¶

Endpoints MUST implement and use the replay protections described in [TLS13], however it is recognized that these protections are imperfect. Therefore, additional consideration of the risk of replay is needed.¶

QUIC is not vulnerable to replay attack, except via the application protocol information it might carry. The management of QUIC protocol state based on the frame types defined in [QUIC-TRANSPORT] is not vulnerable to replay. Processing of QUIC frames is idempotent and cannot result in invalid connection states if frames are replayed, reordered or lost. QUIC connections do not produce effects that last beyond the lifetime of the connection, except for those produced by the application protocol that QUIC serves.¶

Note:

TLS session tickets and address validation tokens are used to carry QUIC configuration information between connections. Specifically, to enable a server to efficiently recover state that is used in connection establishment and address validation. These MUST NOT be used to communicate application semantics between endpoints; clients MUST treat them as opaque values. The potential for reuse of these tokens means that they require stronger protections against replay.¶

A server that accepts 0-RTT on a connection incurs a higher cost than accepting a connection without 0-RTT. This includes higher processing and computation costs. Servers need to consider the probability of replay and all associated costs when accepting 0-RTT.¶

Ultimately, the responsibility for managing the risks of replay attacks with 0-RTT lies with an application protocol. An application protocol that uses QUIC MUST describe how the protocol uses 0-RTT and the measures that are employed to protect against replay attack. An analysis of replay risk needs to consider all QUIC protocol features that carry application semantics.¶

Disabling 0-RTT entirely is the most effective defense against replay attack.¶

QUIC extensions MUST describe how replay attacks affect their operation, or prohibit their use in 0-RTT. Application protocols MUST either prohibit the use of extensions that carry application semantics in 0-RTT or provide replay mitigation strategies.¶

9.3. Packet Reflection Attack Mitigation

A small ClientHello that results in a large block of handshake messages from a server can be used in packet reflection attacks to amplify the traffic generated by an attacker.¶

QUIC includes three defenses against this attack. First, the packet containing a ClientHello MUST be padded to a minimum size. Second, if responding to an unverified source address, the server is forbidden to send more than three times as many bytes as the number of bytes it has received (see Section 8.1 of [QUIC-TRANSPORT]). Finally, because acknowledgments of Handshake packets are authenticated, a blind attacker cannot forge them. Put together, these defenses limit the level of amplification.¶

9.4. Header Protection Analysis

[NAN] analyzes authenticated encryption algorithms that provide nonce privacy, referred to as "Hide Nonce" (HN) transforms. The general header protection construction in this document is one of those algorithms (HN1). Header protection is applied after the packet protection AEAD, sampling a set of bytes (sample) from the AEAD output and encrypting the header field using a pseudorandom function (PRF) as follows:¶

protected_field = field XOR PRF(hp_key, sample)

The header protection variants in this document use a pseudorandom permutation (PRP) in place of a generic PRF. However, since all PRPs are also PRFs [IMC], these variants do not deviate from the HN1 construction.¶

As hp_key is distinct from the packet protection key, it follows that header protection achieves AE2 security as defined in [NAN] and therefore guarantees privacy of field, the protected packet header. Future header protection variants based on this construction MUST use a PRF to ensure equivalent security guarantees.¶

Use of the same key and ciphertext sample more than once risks compromising header protection. Protecting two different headers with the same key and ciphertext sample reveals the exclusive OR of the protected fields. Assuming that the AEAD acts as a PRF, if L bits are sampled, the odds of two ciphertext samples being identical approach 2^(-L/2), that is, the birthday bound. For the algorithms described in this document, that probability is one in 2^64.¶

To prevent an attacker from modifying packet headers, the header is transitively authenticated using packet protection; the entire packet header is part of the authenticated additional data. Protected fields that are falsified or modified can only be detected once the packet protection is removed.¶

9.5. Header Protection Timing Side-Channels

An attacker could guess values for packet numbers or Key Phase and have an endpoint confirm guesses through timing side channels. Similarly, guesses for the packet number length can be tried and exposed. If the recipient of a packet discards packets with duplicate packet numbers without attempting to remove packet protection they could reveal through timing side-channels that the packet number matches a received packet. For authentication to be free from side-channels, the entire process of header protection removal, packet number recovery, and packet protection removal MUST be applied together without timing and other side-channels.¶

For the sending of packets, construction and protection of packet payloads and packet numbers MUST be free from side-channels that would reveal the packet number or its encoded size.¶

During a key update, the time taken to generate new keys could reveal through timing side-channels that a key update has occurred. Alternatively, where an attacker injects packets this side-channel could reveal the value of the Key Phase on injected packets. After receiving a key update, an endpoint SHOULD generate and save the next set of receive packet protection keys, as described in Section 6.3. By generating new keys before a key update is received, receipt of packets will not create timing signals that leak the value of the Key Phase.¶

This depends on not doing this key generation during packet processing and it can require that endpoints maintain three sets of packet protection keys for receiving: for the previous key phase, for the current key phase, and for the next key phase. Endpoints can instead choose to defer generation of the next receive packet protection keys until they discard old keys so that only two sets of receive keys need to be retained at any point in time.¶

9.6. Key Diversity

In using TLS, the central key schedule of TLS is used. As a result of the TLS handshake messages being integrated into the calculation of secrets, the inclusion of the QUIC transport parameters extension ensures that handshake and 1-RTT keys are not the same as those that might be produced by a server running TLS over TCP. To avoid the possibility of cross-protocol key synchronization, additional measures are provided to improve key separation.¶

The QUIC packet protection keys and IVs are derived using a different label than the equivalent keys in TLS.¶

To preserve this separation, a new version of QUIC SHOULD define new labels for key derivation for packet protection key and IV, plus the header protection keys. This version of QUIC uses the string "quic". Other versions can use a version-specific label in place of that string.¶

The initial secrets use a key that is specific to the negotiated QUIC version. New QUIC versions SHOULD define a new salt value used in calculating initial secrets.¶

9.7. Randomness

QUIC depends on endpoints being able to generate secure random numbers, both directly for protocol values such as the connection ID, and transitively via TLS. See [RFC4086] for guidance on secure random number generation.¶

A.1. Keys

The labels generated during the execution of the HKDF-Expand-Label function (that is, HkdfLabel.label) and part of the value given to the HKDF-Expand function in order to produce its output are:¶

client in:

00200f746c73313320636c69656e7420696e00¶

server in:

00200f746c7331332073657276657220696e00¶

quic key:

00100e746c7331332071756963206b657900¶

quic iv:

000c0d746c733133207175696320697600¶

quic hp:

00100d746c733133207175696320687000¶

The initial secret is common:¶

initial_secret = HKDF-Extract(initial_salt, cid)
    = 7db5df06e7a69e432496adedb0085192
      3595221596ae2ae9fb8115c1e9ed0a44

The secrets for protecting client packets are:¶

client_initial_secret
    = HKDF-Expand-Label(initial_secret, "client in", "", 32)
    = c00cf151ca5be075ed0ebfb5c80323c4
      2d6b7db67881289af4008f1f6c357aea

key = HKDF-Expand-Label(client_initial_secret, "quic key", "", 16)
    = 1f369613dd76d5467730efcbe3b1a22d

iv  = HKDF-Expand-Label(client_initial_secret, "quic iv", "", 12)
    = fa044b2f42a3fd3b46fb255c

hp  = HKDF-Expand-Label(client_initial_secret, "quic hp", "", 16)
    = 9f50449e04a0e810283a1e9933adedd2

The secrets for protecting server packets are:¶

server_initial_secret
    = HKDF-Expand-Label(initial_secret, "server in", "", 32)
    = 3c199828fd139efd216c155ad844cc81
      fb82fa8d7446fa7d78be803acdda951b

key = HKDF-Expand-Label(server_initial_secret, "quic key", "", 16)
    = cf3a5331653c364c88f0f379b6067e37

iv  = HKDF-Expand-Label(server_initial_secret, "quic iv", "", 12)
    = 0ac1493ca1905853b0bba03e

hp  = HKDF-Expand-Label(server_initial_secret, "quic hp", "", 16)
    = c206b8d9b9f0f37644430b490eeaa314

A.2. Client Initial

The client sends an Initial packet. The unprotected payload of this packet contains the following CRYPTO frame, plus enough PADDING frames to make a 1162-byte payload:¶

060040f1010000ed0303ebf8fa56f129 39b9584a3896472ec40bb863cfd3e868
04fe3a47f06a2b69484c000004130113 02010000c000000010000e00000b6578
616d706c652e636f6dff01000100000a 00080006001d00170018001000070005
04616c706e0005000501000000000033 00260024001d00209370b2c9caa47fba
baf4559fedba753de171fa71f50f1ce1 5d43e994ec74d748002b000302030400
0d0010000e0403050306030203080408 050806002d00020101001c0002400100
3900320408ffffffffffffffff050480 00ffff07048000ffff08011001048000
75300901100f088394c8f03e51570806 048000ffff

The unprotected header indicates a length of 1182 bytes: the 4-byte packet number, 1162 bytes of frames, and the 16-byte authentication tag. The header includes the connection ID and a packet number of 2:¶

c300000001088394c8f03e5157080000449e00000002

Protecting the payload produces output that is sampled for header protection. Because the header uses a 4-byte packet number encoding, the first 16 bytes of the protected payload is sampled, then applied to the header:¶

sample = d1b1c98dd7689fb8ec11d242b123dc9b

mask = AES-ECB(hp, sample)[0..4]
     = 437b9aec36

header[0] ^= mask[0] & 0x0f
     = c0
header[18..21] ^= mask[1..4]
     = 7b9aec34
header = c000000001088394c8f03e5157080000449e7b9aec34

The resulting protected packet is:¶

c000000001088394c8f03e5157080000 449e7b9aec34d1b1c98dd7689fb8ec11
d242b123dc9bd8bab936b47d92ec356c 0bab7df5976d27cd449f63300099f399
1c260ec4c60d17b31f8429157bb35a12 82a643a8d2262cad67500cadb8e7378c
8eb7539ec4d4905fed1bee1fc8aafba1 7c750e2c7ace01e6005f80fcb7df6212
30c83711b39343fa028cea7f7fb5ff89 eac2308249a02252155e2347b63d58c5
457afd84d05dfffdb20392844ae81215 4682e9cf012f9021a6f0be17ddd0c208
4dce25ff9b06cde535d0f920a2db1bf3 62c23e596d11a4f5a6cf3948838a3aec
4e15daf8500a6ef69ec4e3feb6b1d98e 610ac8b7ec3faf6ad760b7bad1db4ba3
485e8a94dc250ae3fdb41ed15fb6a8e5 eba0fc3dd60bc8e30c5c4287e53805db
059ae0648db2f64264ed5e39be2e20d8 2df566da8dd5998ccabdae053060ae6c
7b4378e846d29f37ed7b4ea9ec5d82e7 961b7f25a9323851f681d582363aa5f8
9937f5a67258bf63ad6f1a0b1d96dbd4 faddfcefc5266ba6611722395c906556
be52afe3f565636ad1b17d508b73d874 3eeb524be22b3dcbc2c7468d54119c74
68449a13d8e3b95811a198f3491de3e7 fe942b330407abf82a4ed7c1b311663a
c69890f4157015853d91e923037c227a 33cdd5ec281ca3f79c44546b9d90ca00
f064c99e3dd97911d39fe9c5d0b23a22 9a234cb36186c4819e8b9c5927726632
291d6a418211cc2962e20fe47feb3edf 330f2c603a9d48c0fcb5699dbfe58964
25c5bac4aee82e57a85aaf4e2513e4f0 5796b07ba2ee47d80506f8d2c25e50fd
14de71e6c418559302f939b0e1abd576 f279c4b2e0feb85c1f28ff18f58891ff
ef132eef2fa09346aee33c28eb130ff2 8f5b766953334113211996d20011a198
e3fc433f9f2541010ae17c1bf202580f 6047472fb36857fe843b19f5984009dd
c324044e847a4f4a0ab34f719595de37 252d6235365e9b84392b061085349d73
203a4a13e96f5432ec0fd4a1ee65accd d5e3904df54c1da510b0ff20dcc0c77f
cb2c0e0eb605cb0504db87632cf3d8b4 dae6e705769d1de354270123cb11450e
fc60ac47683d7b8d0f811365565fd98c 4c8eb936bcab8d069fc33bd801b03ade
a2e1fbc5aa463d08ca19896d2bf59a07 1b851e6c239052172f296bfb5e724047
90a2181014f3b94a4e97d117b4381303 68cc39dbb2d198065ae3986547926cd2
162f40a29f0c3c8745c0f50fba3852e5 66d44575c29d39a03f0cda721984b6f4
40591f355e12d439ff150aab7613499d bd49adabc8676eef023b15b65bfc5ca0
6948109f23f350db82123535eb8a7433 bdabcb909271a6ecbcb58b936a88cd4e
8f2e6ff5800175f113253d8fa9ca8885 c2f552e657dc603f252e1a8e308f76f0
be79e2fb8f5d5fbbe2e30ecadd220723 c8c0aea8078cdfcb3868263ff8f09400
54da48781893a7e49ad5aff4af300cd8 04a6b6279ab3ff3afb64491c85194aab
760d58a606654f9f4400e8b38591356f bf6425aca26dc85244259ff2b19c41b9
f96f3ca9ec1dde434da7d2d392b905dd f3d1f9af93d1af5950bd493f5aa731b4
056df31bd267b6b90a079831aaf579be 0a39013137aac6d404f518cfd4684064
7e78bfe706ca4cf5e9c5453e9f7cfd2b 8b4c8d169a44e55c88d4a9a7f9474241
e221af44860018ab0856972e194cd934

A.3. Server Initial

The server sends the following payload in response, including an ACK frame, a CRYPTO frame, and no PADDING frames:¶

02000000000600405a020000560303ee fce7f7b37ba1d1632e96677825ddf739
88cfc79825df566dc5430b9a045a1200 130100002e00330024001d00209d3c94
0d89690b84d08a60993c144eca684d10 81287c834d5311bcf32bb9da1a002b00
020304

The header from the server includes a new connection ID and a 2-byte packet number encoding for a packet number of 1:¶

c1000000010008f067a5502a4262b50040750001

As a result, after protection, the header protection sample is taken starting from the third protected byte:¶

sample = 2cd0991cd25b0aac406a5816b6394100
mask   = 2ec0d8356a
header = cf000000010008f067a5502a4262b5004075c0d9

The final protected packet is then:¶

cf000000010008f067a5502a4262b500 4075c0d95a482cd0991cd25b0aac406a
5816b6394100f37a1c69797554780bb3 8cc5a99f5ede4cf73c3ec2493a1839b3
dbcba3f6ea46c5b7684df3548e7ddeb9 c3bf9c73cc3f3bded74b562bfb19fb84
022f8ef4cdd93795d77d06edbb7aaf2f 58891850abbdca3d20398c276456cbc4
2158407dd074ee

A.4. Retry

This shows a Retry packet that might be sent in response to the Initial packet in Appendix A.2. The integrity check includes the client-chosen connection ID value of 0x8394c8f03e515708, but that value is not included in the final Retry packet:¶

ff000000010008f067a5502a4262b574 6f6b656e04a265ba2eff4d829058fb3f
0f2496ba

A.5. ChaCha20-Poly1305 Short Header Packet

This example shows some of the steps required to protect a packet with a short header. This example uses AEAD_CHACHA20_POLY1305.¶

In this example, TLS produces an application write secret from which a server uses HKDF-Expand-Label to produce four values: a key, an IV, a header protection key, and the secret that will be used after keys are updated (this last value is not used further in this example).¶

secret
    = 9ac312a7f877468ebe69422748ad00a1
      5443f18203a07d6060f688f30f21632b

key = HKDF-Expand-Label(secret, "quic key", "", 32)
    = c6d98ff3441c3fe1b2182094f69caa2e
      d4b716b65488960a7a984979fb23e1c8

iv  = HKDF-Expand-Label(secret, "quic iv", "", 12)
    = e0459b3474bdd0e44a41c144

hp  = HKDF-Expand-Label(secret, "quic hp", "", 32)
    = 25a282b9e82f06f21f488917a4fc8f1b
      73573685608597d0efcb076b0ab7a7a4

ku  = HKDF-Expand-Label(secret, "quic ku", "", 32)
    = 1223504755036d556342ee9361d25342
      1a826c9ecdf3c7148684b36b714881f9

The following shows the steps involved in protecting a minimal packet with an empty Destination Connection ID. This packet contains a single PING frame (that is, a payload of just 0x01) and has a packet number of 654360564. In this example, using a packet number of length 3 (that is, 49140 is encoded) avoids having to pad the payload of the packet; PADDING frames would be needed if the packet number is encoded on fewer bytes.¶

pn                 = 654360564 (decimal)
nonce              = e0459b3474bdd0e46d417eb0
unprotected header = 4200bff4
payload plaintext  = 01
payload ciphertext = 655e5cd55c41f69080575d7999c25a5bfb

The resulting ciphertext is the minimum size possible. One byte is skipped to produce the sample for header protection.¶

sample = 5e5cd55c41f69080575d7999c25a5bfb
mask   = aefefe7d03
header = 4cfe4189

The protected packet is the smallest possible packet size of 21 bytes.¶

packet = 4cfe4189655e5cd55c41f69080575d7999c25a5bfb

B.1. Analysis of AEAD_AES_128_GCM and AEAD_AES_256_GCM Usage Limits

[GCM-MU] specify concrete bounds for AEAD_AES_128_GCM and AEAD_AES_256_GCM as used in TLS 1.3 and QUIC. This section documents this analysis using several simplifying assumptions:¶

• The number of ciphertext blocks an attacker uses in forgery attempts is bounded by v * l, the number of forgery attempts and the size of each packet (in blocks).¶
• The amount of offline work done by an attacker does not dominate other factors in the analysis.¶

The bounds in [GCM-MU] are tighter and more complete than those used in [AEBounds], which allows for larger limits than those described in [TLS13].¶

2 * (q * l)^2 / 2^n

q <= 2^35 / l

(1 / 2^(8 * n)) + ((2 * v) / 2^(2 * n))
        + ((2 * o * v) / 2^(k + n)) + (n * (v + (v * l)) / 2^k)

v <= 2^64 / l

v <= 2^192 / l

B.2. Analysis of AEAD_AES_128_CCM Usage Limits

TLS [TLS13] and [AEBounds] do not specify limits on usage for AEAD_AES_128_CCM. However, any AEAD that is used with QUIC requires limits on use that ensure that both confidentiality and integrity are preserved. This section documents that analysis.¶

[CCM-ANALYSIS] is used as the basis of this analysis. The results of that analysis are used to derive usage limits that are based on those chosen in [TLS13].¶

For confidentiality, Theorem 2 in [CCM-ANALYSIS] establishes that an attacker gains a distinguishing advantage over an ideal pseudorandom permutation (PRP) of no more than:¶

(2l * q)^2 / 2^n

The integrity limit in Theorem 1 in [CCM-ANALYSIS] provides an attacker a strictly higher advantage for the same number of messages. As the targets for the confidentiality advantage and the integrity advantage are the same, only Theorem 1 needs to be considered.¶

Theorem 1 establishes that an attacker gains an advantage over an ideal PRP of no more than:¶

v / 2^t + (2l * (v + q))^2 / 2^n

As t and n are both 128, the first term is negligible relative to the second, so that term can be removed without a significant effect on the result.¶

This produces a relation that combines both encryption and decryption attempts with the same limit as that produced by the theorem for confidentiality alone. For a target advantage of 2^-57, this results in:¶

v + q <= 2^34.5 / l

By setting q = v, values for both confidentiality and integrity limits can be produced. Endpoints that limit packets to 2^11 bytes therefore have both confidentiality and integrity limits of 2^26.5 packets. Endpoints that do not restrict packet size have a limit of 2^21.5.¶

C.1. Since draft-ietf-quic-tls-32

• Added final values for Initial key derivation, Retry authentication, and TLS extension type for the QUIC Transport Parameters extension (#4431) (#4431)¶
• Corrected rules for handling of 0-RTT (#4393, #4394)¶

C.2. Since draft-ietf-quic-tls-31

• Packet protection limits are based on maximum-sized packets; improved analysis (#3701, #4175)¶

C.3. Since draft-ietf-quic-tls-30

• Add a new error code for AEAD_LIMIT_REACHED code to avoid conflict (#4087, #4088)¶

C.4. Since draft-ietf-quic-tls-29

• Updated limits on packet protection (#3788, #3789)¶
• Allow for packet processing to continue while waiting for TLS to provide keys (#3821, #3874)¶

C.5. Since draft-ietf-quic-tls-28

• Defined limits on the number of packets that can be protected with a single key and limits on the number of packets that can fail authentication (#3619, #3620)¶
• Update Initial salt, Retry keys, and samples (#3711)¶

C.6. Since draft-ietf-quic-tls-27

• Allowed CONNECTION_CLOSE in any packet number space, with restrictions on use of the application-specific variant (#3430, #3435, #3440)¶
• Prohibit the use of the compatibility mode from TLS 1.3 (#3594, #3595)¶

C.7. Since draft-ietf-quic-tls-26

• No changes¶

C.8. Since draft-ietf-quic-tls-25

• No changes¶

C.9. Since draft-ietf-quic-tls-24

• Rewrite key updates (#3050)¶

  • Allow but don't recommend deferring key updates (#2792, #3263)¶
  • More completely define received behavior (#2791)¶
  • Define the label used with HKDF-Expand-Label (#3054)¶

C.10. Since draft-ietf-quic-tls-23

• Key update text update (#3050):¶

  • Recommend constant-time key replacement (#2792)¶
  • Provide explicit labels for key update key derivation (#3054)¶
• Allow first Initial from a client to span multiple packets (#2928, #3045)¶
• PING can be sent at any encryption level (#3034, #3035)¶

C.11. Since draft-ietf-quic-tls-22

• Update the salt used for Initial secrets (#2887, #2980)¶

C.12. Since draft-ietf-quic-tls-21

• No changes¶

C.13. Since draft-ietf-quic-tls-20

• Mandate the use of the QUIC transport parameters extension (#2528, #2560)¶
• Define handshake completion and confirmation; define clearer rules when it encryption keys should be discarded (#2214, #2267, #2673)¶

C.14. Since draft-ietf-quic-tls-18

• Increased the set of permissible frames in 0-RTT (#2344, #2355)¶
• Transport parameter extension is mandatory (#2528, #2560)¶

C.15. Since draft-ietf-quic-tls-17

• Endpoints discard initial keys as soon as handshake keys are available (#1951, #2045)¶
• Use of ALPN or equivalent is mandatory (#2263, #2284)¶

C.16. Since draft-ietf-quic-tls-14

• Update the salt used for Initial secrets (#1970)¶
• Clarify that TLS_AES_128_CCM_8_SHA256 isn't supported (#2019)¶

• Change header protection¶

  • Sample from a fixed offset (#1575, #2030)¶
  • Cover part of the first byte, including the key phase (#1322, #2006)¶

• TLS provides an AEAD and KDF function (#2046)¶

  • Clarify that the TLS KDF is used with TLS (#1997)¶
  • Change the labels for calculation of QUIC keys (#1845, #1971, #1991)¶
• Initial keys are discarded once Handshake keys are available (#1951, #2045)¶

C.17. Since draft-ietf-quic-tls-13

• Updated to TLS 1.3 final (#1660)¶

C.18. Since draft-ietf-quic-tls-12

• Changes to integration of the TLS handshake (#829, #1018, #1094, #1165, #1190, #1233, #1242, #1252, #1450)¶

  • The cryptographic handshake uses CRYPTO frames, not stream 0¶
  • QUIC packet protection is used in place of TLS record protection¶
  • Separate QUIC packet number spaces are used for the handshake¶
  • Changed Retry to be independent of the cryptographic handshake¶
  • Limit the use of HelloRetryRequest to address TLS needs (like key shares)¶
• Changed codepoint of TLS extension (#1395, #1402)¶

C.19. Since draft-ietf-quic-tls-11

• Encrypted packet numbers.¶

C.20. Since draft-ietf-quic-tls-10

• No significant changes.¶

C.21. Since draft-ietf-quic-tls-09

• Cleaned up key schedule and updated the salt used for handshake packet protection (#1077)¶

C.22. Since draft-ietf-quic-tls-08

• Specify value for max_early_data_size to enable 0-RTT (#942)¶
• Update key derivation function (#1003, #1004)¶

C.23. Since draft-ietf-quic-tls-07

• Handshake errors can be reported with CONNECTION_CLOSE (#608, #891)¶

C.24. Since draft-ietf-quic-tls-05

No significant changes.¶

C.25. Since draft-ietf-quic-tls-04

• Update labels used in HKDF-Expand-Label to match TLS 1.3 (#642)¶

C.26. Since draft-ietf-quic-tls-03

No significant changes.¶

C.27. Since draft-ietf-quic-tls-02

• Updates to match changes in transport draft¶

C.28. Since draft-ietf-quic-tls-01

• Use TLS alerts to signal TLS errors (#272, #374)¶
• Require ClientHello to fit in a single packet (#338)¶
• The second client handshake flight is now sent in the clear (#262, #337)¶
• The QUIC header is included as AEAD Associated Data (#226, #243, #302)¶
• Add interface necessary for client address validation (#275)¶
• Define peer authentication (#140)¶
• Require at least TLS 1.3 (#138)¶
• Define transport parameters as a TLS extension (#122)¶
• Define handling for protected packets before the handshake completes (#39)¶
• Decouple QUIC version and ALPN (#12)¶

C.29. Since draft-ietf-quic-tls-00

• Changed bit used to signal key phase¶
• Updated key phase markings during the handshake¶
• Added TLS interface requirements section¶
• Moved to use of TLS exporters for key derivation¶
• Moved TLS error code definitions into this document¶

C.30. Since draft-thomson-quic-tls-01

• Adopted as base for draft-ietf-quic-tls¶
• Updated authors/editors list¶
• Added status note¶