RFC 9001 - Using TLS to Secure QUIC

4. Carrying TLS Messages

QUIC carries TLS handshake data in CRYPTO frames, each of which consists of a contiguous block of handshake data identified by an offset and length. Those frames are packaged into QUIC packets and encrypted under the current encryption level. As with TLS over TCP, once TLS handshake data has been delivered to QUIC, it is QUIC's responsibility to deliver it reliably. Each chunk of data that is produced by TLS is associated with the set of keys that TLS is currently using. If QUIC needs to retransmit that data, it MUST use the same keys even if TLS has already updated to newer keys.¶

Each encryption level corresponds to a packet number space. The packet number space that is used determines the semantics of frames. Some frames are prohibited in different packet number spaces; see Section 12.5 of [QUIC-TRANSPORT].¶

Because packets could be reordered on the wire, QUIC uses the packet type to indicate which keys were used to protect a given packet, as shown in Table 1. When packets of different types need to be sent, endpoints SHOULD use coalesced packets to send them in the same UDP datagram.¶

Table 1: Encryption Keys by Packet Type
Packet Type	Encryption Keys	PN Space
Initial	Initial secrets	Initial
0-RTT Protected	0-RTT	Application data
Handshake	Handshake	Handshake
Retry	Retry	N/A
Version Negotiation	N/A	N/A
Short Header	1-RTT	Application data

Section 17 of [QUIC-TRANSPORT] shows how packets at the various encryption levels fit into the handshake process.¶

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 0-RTT data
Rekeying (both transmit and receive)
Updating handshake state

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].¶

4.1.1. Handshake Complete

In this document, the TLS handshake is considered complete when the TLS stack has reported that the handshake is complete. This happens when the TLS stack has both sent a Finished message and verified the peer's Finished message. Verifying the peer's Finished message provides the endpoints with an assurance that previous handshake messages have not been modified. Note that the handshake does not complete at both endpoints simultaneously. Consequently, any requirement that is based on the completion of the handshake depends on the perspective of the endpoint in question.¶

4.1.2. Handshake Confirmed

In this document, the TLS handshake is considered confirmed at the server when the handshake completes. The server MUST send a HANDSHAKE_DONE frame as soon as the handshake is complete. At the client, the handshake is considered confirmed when a HANDSHAKE_DONE frame is received.¶

Additionally, a client MAY consider the handshake to be confirmed when it receives an acknowledgment for a 1-RTT packet. This can be implemented by recording the lowest packet number sent with 1-RTT keys and comparing it to the Largest Acknowledged field in any received 1-RTT ACK frame: once the latter is greater than or equal to the former, the handshake is confirmed.¶

4.1.3. Sending and Receiving Handshake Messages

In order to drive the handshake, TLS depends on being able to send and receive handshake messages. There are two basic functions on this interface: one where QUIC requests handshake messages and one where QUIC provides bytes that comprise handshake messages.¶

Before starting the handshake, QUIC provides TLS with the transport parameters (see Section 8.2) that it wishes to carry.¶

A QUIC client starts TLS by requesting TLS handshake bytes from TLS. The client acquires handshake bytes before sending its first packet. A QUIC server starts the process by providing TLS with the client's handshake bytes.¶

At any time, the TLS stack at an endpoint will have a current sending encryption level and a receiving encryption level. TLS encryption levels determine the QUIC packet type and keys that are used for protecting data.¶

Each encryption level is associated with a different sequence of bytes, which is reliably transmitted to the peer in CRYPTO frames. When TLS provides handshake bytes to be sent, they are appended to the handshake bytes for the current encryption level. The encryption level then determines the type of packet that the resulting CRYPTO frame is carried in; see Table 1.¶

Four encryption levels are used, producing keys for Initial, 0-RTT, Handshake, and 1-RTT packets. CRYPTO frames are carried in just three of these levels, omitting the 0-RTT level. These four levels correspond to three packet number spaces: Initial and Handshake encrypted packets use their own separate spaces; 0-RTT and 1-RTT packets use the application data packet number space.¶

QUIC takes the unprotected content of TLS handshake records as the content of CRYPTO frames. TLS record protection is not used by QUIC. QUIC assembles CRYPTO frames into QUIC packets, which are protected using QUIC packet protection.¶

QUIC CRYPTO frames only carry TLS handshake messages. TLS alerts are turned into QUIC CONNECTION_CLOSE error codes; see Section 4.8. TLS application data and other content types cannot be carried by QUIC at any encryption level; it is an error if they are received from the TLS stack.¶

When an endpoint receives a QUIC packet containing a CRYPTO frame from the network, it proceeds as follows:¶

If the packet uses the current TLS receiving encryption level, sequence the data into the input flow as usual. As with STREAM frames, the offset is used to find the proper location in the data sequence. If the result of this process is that new data is available, then it is delivered to TLS in order.
If the packet is from a previously installed encryption level, it MUST NOT contain data that extends past the end of previously received data in that flow. Implementations MUST treat any violations of this requirement as a connection error of type PROTOCOL_VIOLATION.
If the packet is from a new encryption level, it is saved for later processing by TLS. Once TLS moves to receiving from this encryption level, saved data can be provided to TLS. When TLS provides keys for a higher encryption level, if there is data from a previous encryption level that TLS has not consumed, this MUST be treated as a connection error of type PROTOCOL_VIOLATION.

Each time that TLS is provided with new data, new handshake bytes are requested from TLS. TLS might not provide any bytes if the handshake messages it has received are incomplete or it has no data to send.¶

The content of CRYPTO frames might either be processed incrementally by TLS or buffered until complete messages or flights are available. TLS is responsible for buffering handshake bytes that have arrived in order. QUIC is responsible for buffering handshake bytes that arrive out of order or for encryption levels that are not yet ready. QUIC does not provide any means of flow control for CRYPTO frames; see Section 7.5 of [QUIC-TRANSPORT].¶

Once the TLS handshake is complete, this is indicated to QUIC along with any final handshake bytes that TLS needs to send. At this stage, the transport parameters that the peer advertised during the handshake are authenticated; see Section 8.2.¶

Once the handshake is complete, TLS becomes passive. TLS can still receive data from its peer and respond in kind, but it will not need to send more data unless specifically requested -- either by an application or QUIC. One reason to send data is that the server might wish to provide additional or updated session tickets to a client.¶

When the handshake is complete, QUIC only needs to provide TLS with any data that arrives in CRYPTO streams. In the same manner that is used during the handshake, new data is requested from TLS after providing received data.¶

4.1.4. Encryption Level Changes

As keys at a given encryption level become available to TLS, TLS indicates to QUIC that reading or writing keys at that encryption level are available.¶

The availability of new keys is always a result of providing inputs to TLS. TLS only provides new keys after being initialized (by a client) or when provided with new handshake data.¶

However, a TLS implementation could perform some of its processing asynchronously. In particular, the process of validating a certificate can take some time. While waiting for TLS processing to complete, an endpoint SHOULD buffer received packets if they might be processed using keys that are not yet available. These packets can be processed once keys are provided by TLS. An endpoint SHOULD continue to respond to packets that can be processed during this time.¶

After processing inputs, TLS might produce handshake bytes, keys for new encryption levels, or both.¶

TLS provides QUIC with three items as a new encryption level becomes available:¶

A secret
An Authenticated Encryption with Associated Data (AEAD) function
A Key Derivation Function (KDF)

These values are based on the values that TLS negotiates and are used by QUIC to generate packet and header protection keys; see Section 5 and Section 5.4.¶

If 0-RTT is possible, it is ready after the client sends a TLS ClientHello message or the server receives that message. After providing a QUIC client with the first handshake bytes, the TLS stack might signal the change to 0-RTT keys. On the server, after receiving handshake bytes that contain a ClientHello message, a TLS server might signal that 0-RTT keys are available.¶

Although TLS only uses one encryption level at a time, QUIC may use more than one level. For instance, after sending its Finished message (using a CRYPTO frame at the Handshake encryption level) an endpoint can send STREAM data (in 1-RTT encryption). If the Finished message is lost, the endpoint uses the Handshake encryption level to retransmit the lost message. Reordering or loss of packets can mean that QUIC will need to handle packets at multiple encryption levels. During the handshake, this means potentially handling packets at higher and lower encryption levels than the current encryption level used by TLS.¶

In particular, server implementations need to be able to read packets at the Handshake encryption level at the same time as the 0-RTT encryption level. A client could interleave ACK frames that are protected with Handshake keys with 0-RTT data, and the server needs to process those acknowledgments in order to detect lost Handshake packets.¶

QUIC also needs access to keys that might not ordinarily be available to a TLS implementation. For instance, a client might need to acknowledge Handshake packets before it is ready to send CRYPTO frames at that encryption level. TLS therefore needs to provide keys to QUIC before it might produce them for its own use.¶

4.1.5. TLS Interface Summary

Figure 5 summarizes the exchange between QUIC and TLS for both client and server. Solid arrows indicate packets that carry handshake data; dashed arrows show where application data can be sent. Each arrow is tagged with the encryption level used for that transmission.¶

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

Figure 5: Interaction Summary between QUIC and TLS

Figure 5 shows the multiple packets that form a single "flight" of messages being processed individually, to show what incoming messages trigger different actions. This shows multiple "Get Handshake" invocations to retrieve handshake messages at different encryption levels. New handshake messages are requested after incoming packets have been processed.¶

Figure 5 shows one possible structure for a simple handshake exchange. The exact process varies based on the structure of endpoint implementations and the order in which packets arrive. Implementations could use a different number of operations or execute them in other orders.¶

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 version of TLS newer 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 field 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 QUIC's requirements for datagrams that carry Initial packets; see Section 14.1 of [QUIC-TRANSPORT]. QUIC implementations use PADDING frames or packet coalescing to ensure that datagrams are large enough.¶

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]).¶

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 Appendix 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 seven 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.¶

4.6.1. Enabling 0-RTT

The TLS early_data extension in the NewSessionTicket message is defined to convey (in the max_early_data_size parameter) the amount of TLS 0-RTT data the server is willing to accept. QUIC does not use TLS early data. QUIC uses 0-RTT packets to carry early data. Accordingly, the max_early_data_size parameter is repurposed to hold a sentinel value 0xffffffff to indicate that the server is willing to accept QUIC 0-RTT data. To indicate that the server does not accept 0-RTT data, the early_data extension is omitted from the NewSessionTicket. The amount of data that the client can send in QUIC 0-RTT is controlled by the initial_max_data transport parameter supplied by the server.¶

Servers MUST NOT send the early_data extension with a max_early_data_size field set to any value other than 0xffffffff. A client MUST treat receipt of a NewSessionTicket that contains an early_data extension with any other value as a connection error of type PROTOCOL_VIOLATION.¶

A client that wishes to send 0-RTT packets uses the early_data extension in the ClientHello message of a subsequent handshake; see Section 4.2.10 of [TLS13]. It then sends application data in 0-RTT packets.¶

A client that attempts 0-RTT might also provide an address validation token if the server has sent a NEW_TOKEN frame; see Section 8.1 of [QUIC-TRANSPORT].¶

4.6.2. Accepting and Rejecting 0-RTT

A server accepts 0-RTT by sending an early_data extension in the EncryptedExtensions; see Section 4.2.10 of [TLS13]. The server then processes and acknowledges the 0-RTT packets that it receives.¶

A server rejects 0-RTT by sending the EncryptedExtensions without an early_data extension. A server will always reject 0-RTT if it sends a TLS HelloRetryRequest. When rejecting 0-RTT, a server MUST NOT process any 0-RTT packets, even if it could. When 0-RTT was rejected, a client SHOULD treat receipt of an acknowledgment for a 0-RTT packet as a connection error of type PROTOCOL_VIOLATION, if it is able to detect the condition.¶

When 0-RTT is rejected, all connection characteristics that the client assumed might be incorrect. This includes the choice of application protocol, transport parameters, and any application configuration. The client therefore MUST reset the state of all streams, including application state bound to those streams.¶

A client MAY reattempt 0-RTT if it receives a Retry or Version Negotiation packet. These packets do not signify rejection of 0-RTT.¶

4.6.3. Validating 0-RTT Configuration

When a server receives a ClientHello with the early_data extension, it has to decide whether to accept or reject 0-RTT data from the client. Some of this decision is made by the TLS stack (e.g., checking that the cipher suite being resumed was included in the ClientHello; see Section 4.2.10 of [TLS13]). Even when the TLS stack has no reason to reject 0-RTT data, the QUIC stack or the application protocol using QUIC might reject 0-RTT data because the configuration of the transport or application associated with the resumed session is not compatible with the server's current configuration.¶

QUIC requires additional transport state to be associated with a 0-RTT session ticket. One common way to implement this is using stateless session tickets and storing this state in the session ticket. Application protocols that use QUIC might have similar requirements regarding associating or storing state. This associated state is used for deciding whether 0-RTT data must be rejected. For example, HTTP/3 settings [QUIC-HTTP] determine how 0-RTT data from the client is interpreted. Other applications using QUIC could have different requirements for determining whether to accept or reject 0-RTT data.¶

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 0x0100 to produce a QUIC error code from the range reserved for CRYPTO_ERROR; see Section 20.1 of [QUIC-TRANSPORT]. 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 (0x0128 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.¶

4.9.1. Discarding Initial Keys

Packets protected with Initial secrets (Section 5.2) are not authenticated, meaning that an attacker could spoof packets with the intent to disrupt a connection. To limit these attacks, Initial packet protection keys are discarded more aggressively than other keys.¶

The successful use of Handshake packets indicates that no more Initial packets need to be exchanged, as these keys can only be produced after receiving all CRYPTO frames from Initial packets. Thus, a client MUST discard Initial keys when it first sends a Handshake packet and a server MUST discard Initial keys when it first successfully processes a Handshake packet. Endpoints MUST NOT send Initial packets after this point.¶

This results in abandoning loss recovery state for the Initial encryption level and ignoring any outstanding Initial packets.¶

4.9.2. Discarding Handshake Keys

An endpoint MUST discard its Handshake keys when the TLS handshake is confirmed (Section 4.1.2).¶

4.9.3. Discarding 0-RTT Keys

0-RTT and 1-RTT packets share the same packet number space, and clients do not send 0-RTT packets after sending a 1-RTT packet (Section 5.6).¶

Therefore, a client SHOULD discard 0-RTT keys as soon as it installs 1-RTT keys as they have no use after that moment.¶

Additionally, a server MAY discard 0-RTT keys as soon as it receives a 1-RTT packet. However, due to packet reordering, a 0-RTT packet could arrive after a 1-RTT packet. Servers MAY temporarily retain 0-RTT keys to allow decrypting reordered packets without requiring their contents to be retransmitted with 1-RTT keys. After receiving a 1-RTT packet, servers MUST discard 0-RTT keys within a short time; the RECOMMENDED time period is three times the Probe Timeout (PTO, see [QUIC-RECOVERY]). A server MAY discard 0-RTT keys earlier if it determines that it has received all 0-RTT packets, which can be done by keeping track of missing packet numbers.¶

5. Packet Protection

As with TLS over TCP, QUIC protects packets with keys derived from the TLS handshake, using the AEAD algorithm [AEAD] negotiated by TLS.¶

QUIC packets have varying protections depending on their type:¶

Version Negotiation packets have no cryptographic protection.
Retry packets use AEAD_AES_128_GCM to provide protection against accidental modification and to limit the entities that can produce a valid Retry; see Section 5.8.
Initial packets use AEAD_AES_128_GCM with keys derived from the Destination Connection ID field of the first Initial packet sent by the client; see Section 5.2.
All other packets have strong cryptographic protections for confidentiality and integrity, using keys and algorithms negotiated by TLS.

This section describes how packet protection is applied to Handshake packets, 0-RTT packets, and 1-RTT packets. The same packet protection process is applied to Initial packets. However, as it is trivial to determine the keys used for Initial packets, these packets are not considered to have confidentiality or integrity protection. Retry packets use a fixed key and so similarly lack confidentiality and integrity protection.¶

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 with 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 the input keying material (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.¶

Appendix A contains sample Initial packets.¶

5.3. AEAD Usage

The Authenticated Encryption with Associated Data (AEAD) function (see [AEAD]) 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.¶

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.¶

5.4.1. Header Protection Application

Header protection is applied after packet protection is applied (see Section 5.3). The ciphertext of the packet is sampled and used as input to an encryption algorithm. The algorithm used depends on the negotiated AEAD.¶

The output of this algorithm is a 5-byte mask that is applied to the protected header fields using exclusive OR. The least significant bits of the first byte of the packet are masked by the least significant bits of the first mask byte, and the packet number is masked with the remaining bytes. Any unused bytes of mask that might result from a shorter packet number encoding are unused.¶

Figure 6 shows a sample algorithm for applying header protection. Removing header protection only differs in the order in which the packet number length (pn_length) is determined (here "^" is used to represent exclusive OR).¶

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]

Figure 6: Header Protection Pseudocode

Specific header protection functions are defined based on the selected cipher suite; see Section 5.4.3 and Section 5.4.4.¶

Figure 7 shows an example long header packet (Initial) and a short header packet (1-RTT). Figure 7 shows the fields in each header that are covered by header protection and the portion of the protected packet payload that is sampled.¶

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
}

Figure 7: Header Protection and Ciphertext Sample

Before a TLS cipher suite can be used with QUIC, a header protection algorithm MUST be specified for the AEAD used with that cipher suite. This document defines algorithms for AEAD_AES_128_GCM, AEAD_AES_128_CCM, AEAD_AES_256_GCM (all these AES AEADs are defined in [AEAD]), and AEAD_CHACHA20_POLY1305 (defined in [CHACHA]). Prior to TLS selecting a cipher suite, AES header protection is used (Section 5.4.3), matching the AEAD_AES_128_GCM packet protection.¶

5.4.2. Header Protection Sample

The header protection algorithm uses both the header protection key and a sample of the ciphertext from the packet Payload field.¶

The same number of bytes are always sampled, but an allowance needs to be made for the removal of protection by a receiving endpoint, which will not know the length of the Packet Number field. The sample of ciphertext is taken starting from an offset of 4 bytes after the start of the Packet Number field. That is, in sampling packet ciphertext for header protection, the Packet Number field is assumed to be 4 bytes long (its maximum possible encoded length).¶

An endpoint MUST discard packets that are not long enough to contain a complete sample.¶

To ensure that sufficient data is available for sampling, packets are padded so that the combined lengths of the encoded packet number and protected payload is at least 4 bytes longer than the sample required for header protection. The cipher suites defined in [TLS13] -- other than TLS_AES_128_CCM_8_SHA256, for which a header protection scheme is not defined in this document -- have 16-byte expansions and 16-byte header protection samples. This results in needing at least 3 bytes of frames in the unprotected payload if the packet number is encoded on a single byte, or 2 bytes of frames for a 2-byte packet number encoding.¶

The sampled ciphertext can be determined by the following pseudocode:¶

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

sample = packet[sample_offset..sample_offset+sample_length]

Where the packet number offset of a short header packet can be calculated as:¶

pn_offset = 1 + len(connection_id)

And the packet number offset of a long header packet can be calculated as:¶

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)

For example, for a packet with a short header, an 8-byte connection ID, and protected with AEAD_AES_128_GCM, the sample takes bytes 13 to 28 inclusive (using zero-based indexing).¶

Multiple QUIC packets might be included in the same UDP datagram. Each packet is handled separately.¶

5.4.3. AES-Based Header Protection

This section defines the packet protection algorithm for AEAD_AES_128_GCM, AEAD_AES_128_CCM, and AEAD_AES_256_GCM. AEAD_AES_128_GCM and AEAD_AES_128_CCM use 128-bit AES in Electronic Codebook (ECB) mode. AEAD_AES_256_GCM uses 256-bit AES in ECB mode. AES is defined in [AES].¶

This algorithm samples 16 bytes from the packet ciphertext. This value is used as the input to AES-ECB. In pseudocode, the header protection function is defined as:¶

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

5.4.4. ChaCha20-Based Header Protection

When AEAD_CHACHA20_POLY1305 is in use, header protection uses the raw ChaCha20 function as defined in Section 2.4 of [CHACHA]. This uses a 256-bit key and 16 bytes sampled from the packet protection output.¶

The first 4 bytes of the sampled ciphertext are the block counter. A ChaCha20 implementation could take a 32-bit integer in place of a byte sequence, in which case, the byte sequence is interpreted as a little-endian value.¶

The remaining 12 bytes are used as the nonce. A ChaCha20 implementation might take an array of three 32-bit integers in place of a byte sequence, in which case, the nonce bytes are interpreted as a sequence of 32-bit little-endian integers.¶

The encryption mask is produced by invoking ChaCha20 to protect 5 zero bytes. In pseudocode, the header protection function is defined as:¶

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.¶

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.¶

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 Section 17.2.5 of [QUIC-TRANSPORT]) carry a Retry Integrity Tag that provides two properties: it allows the discarding of packets that have accidentally been corrupted by the network, and 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 (..),
}

Figure 8: Retry Pseudo-Packet

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.