Using Transport Layer Security (TLS) to Secure QUIC

TLS provides two endpoints with a way to establish a means of communication over an untrusted medium (that is, the Internet) that ensures that messages they exchange cannot be observed, modified, or forged.¶

Each upper layer (handshake, alerts, and application data) is carried as a series of typed TLS records. Records are individually cryptographically protected and then transmitted over a reliable transport (typically TCP) which provides sequencing and guaranteed delivery.¶

Change Cipher Spec records cannot be sent in QUIC.¶

The TLS authenticated key exchange occurs between two entities: 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 (DH) key exchanges. PSK is the basis for 0-RTT; the latter provides perfect forward secrecy (PFS) when the DH keys are destroyed.¶

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.¶

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

• 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 it MUST NOT carry a self-contained trigger for any non-idempotent action.

A simplified TLS 1.3 handshake with 0-RTT application data is shown in Figure 1. Note that this omits the EndOfEarlyData message, which is not used in QUIC (see Section 8.3).¶

• Plaintext
• 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 is only possible 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.¶

• Sending and receiving handshake messages
• Rekeying (both transmit and receive)
• Handshake state updates

Additional functions might be needed to configure TLS.¶

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.¶

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.¶

QUIC requires that the first Initial packet from a client contain an entire cryptographic handshake message, which for TLS is the ClientHello. Though a packet larger than 1200 octets might be supported by the path, a client improves the likelihood that a packet is accepted if it ensures that the first ClientHello message is small enough to stay within this limit.¶

QUIC packet and framing add at least 36 octets of overhead to the ClientHello message. That overhead increases if the client chooses a connection ID without zero length. Overheads also do not include the token or a connection ID longer than 8 octets, both of which might be required if a server sends a Retry packet.¶

A typical TLS ClientHello can easily fit into a 1200 octet 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. Minimizing the size of these values increases the probability that they can be successfully used by a client.¶

A client is not required to fit the ClientHello that it sends in response to a HelloRetryRequest message into a single UDP datagram.¶

The TLS implementation does not need to ensure that the ClientHello is sufficiently large. QUIC PADDING frames are added to increase the size of the packet as necessary.¶

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 (see Section 4.6.2 of [TLS13]).¶

In order to be usable for 0-RTT, TLS MUST provide a NewSessionTicket message that contains the “max_early_data” extension with the value 0xffffffff; the amount of data which the client can send in 0-RTT is controlled by the “initial_max_data” transport parameter supplied by the server. A client MUST treat receipt of a NewSessionTicket that contains a “max_early_data” extension with any other value as a connection error of type PROTOCOL_VIOLATION.¶

Early data within the TLS connection MUST NOT be used. As it is for other TLS application data, a server MUST treat receiving early data on the TLS connection as a connection error of type PROTOCOL_VIOLATION.¶

A server rejects 0-RTT by rejecting 0-RTT at the TLS layer. This also prevents QUIC from sending 0-RTT data. A server will always reject 0-RTT if it sends a TLS HelloRetryRequest.¶

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 attempt to send 0-RTT again if it receives a Retry or Version Negotiation packet. These packets do not signify rejection of 0-RTT.¶

In TLS over TCP, the HelloRetryRequest feature (see Section 4.1.4 of [TLS13]) can be used to correct a client’s incorrect KeyShare extension as well as for a stateless round-trip check. From the perspective of QUIC, this just looks like additional messages carried in the Initial encryption level. Although it is in principle possible to use this feature for address verification in QUIC, QUIC implementations SHOULD instead use the Retry feature (see Section 4.4 of [QUIC-TRANSPORT]). HelloRetryRequest is still used to request key shares.¶

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

A TLS alert is turned into a QUIC connection error by converting the one-octet alert description into a QUIC error code. The alert description 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.¶

The alert level of all TLS alerts is “fatal”; a TLS stack MUST NOT generate alerts at the “warning” level.¶

After QUIC moves 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 acknowledgements 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 both received and acknowledged all CRYPTO frames for that encryption level and when all CRYPTO frames for that encryption level have been acknowledged by its peer. However, this does not guarantee that no further packets will need to be received or sent at that encryption level because a peer might not have received all the acknowledgements necessary to reach the same state.¶

After all CRYPTO frames for a given encryption level have been sent and all expected CRYPTO frames received, and all the corresponding acknowledgments have been received or sent, an endpoint starts a timer. For 0-RTT keys, which do not carry CRYPTO frames, this timer starts when the first packets protected with 1-RTT are sent or received. To limit the effect of packet loss around a change in keys, endpoints MUST retain packet protection keys for that encryption level for at least three times the current Retransmission Timeout (RTO) interval as defined in [QUIC-RECOVERY]. Retaining keys for this interval allows packets containing CRYPTO or ACK frames at that encryption level to be sent if packets are determined to be lost or new packets require acknowledgment.¶

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.¶

Once this timer expires, an endpoint MUST NOT either accept or generate new packets using those packet protection keys. An endpoint can discard packet protection keys for that encryption level.¶

Key updates (see Section 6) can be used to update 1-RTT keys before keys from other encryption levels are discarded. In that case, packets protected with the newest packet protection keys and packets sent two updates prior will appear to use the same keys. After the handshake is complete, endpoints only need to maintain the two latest sets of packet protection keys and MAY discard older keys. Updating keys multiple times rapidly can cause packets to be effectively lost if packets are significantly delayed. Because key updates can only be performed once per round trip time, only packets that are delayed by more than a round trip will be lost as a result of changing keys; such packets will be marked as lost before this, as they leave a gap in the sequence of packet numbers.¶

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 method described in Section 7.3 of [TLS13]), except that the label for HKDF-Expand-Label uses the prefix “quic “ rather than “tls13 “. A different label provides key separation between TLS and QUIC.¶

For example, where TLS might use a label of 0x002009746c733133206b657900 to derive a key, QUIC uses 0x00200871756963206b657900.¶

The HKDF-Expand-Label function with a “quic “ label is also used to derive the initial secrets (see Section 5.2) and to derive a packet number protection key (the “pn” label, see Section 5.4).¶

The hash function for HKDF when deriving initial secrets and keys is SHA-256 [SHA].¶

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.¶

The value of initial_salt is a 20 octet sequence shown in the figure in hexadecimal notation. 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 only recognizes one version of QUIC from seeing or modifying the contents of handshake packets from future versions.¶

Note:

The Destination Connection ID is of arbitrary length, and it could be zero length if the server sends a Retry packet with a zero-length Source Connection ID field. In this case, the Initial keys provide no assurance to the client that the server received its packet; the client has to rely on the exchange that included the Retry packet for that property.

The Authentication Encryption with Associated Data (AEAD) [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, the AEAD_AES_128_GCM function is used.¶

QUIC packets are protected prior to applying packet number protection (Section 5.4). The unprotected packet number is part of the associated data (A). When removing packet protection, an endpoint first removes the protection from the packet number.¶

All QUIC packets other than Version Negotiation and Retry packets are protected with an AEAD algorithm [AEAD]. Prior to establishing a shared secret, packets are protected with AEAD_AES_128_GCM and a key derived from the destination connection ID in the client’s first Initial packet (see Section 5.2). This provides protection against off-path attackers and robustness against QUIC version unaware middleboxes, but not against on-path attackers.¶

All ciphersuites currently defined for TLS 1.3 - and therefore QUIC - have a 16-byte authentication tag and produce an output 16 bytes larger than their input.¶

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 64 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 flags octet in either the short or long header, up to and including the unprotected packet number.¶

The input plaintext, P, for the AEAD is the content of the QUIC frame following the header, 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 for example [AEBounds]). 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.¶

QUIC packet numbers are protected using a key that is derived from the current set of secrets. The key derived using the “pn” label is used to protect the packet number from casual observation. The packet number protection algorithm depends on the negotiated AEAD.¶

Packet number 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.¶

In sampling the packet ciphertext, the packet number length is assumed to be 4 octets (its maximum possible encoded length), unless there is insufficient space in the packet for sampling. The sampled ciphertext starts after allowing for a 4 octet packet number unless this would cause the sample to extend past the end of the packet. If the sample would extend past the end of the packet, the end of the packet is sampled.¶

A packet with a long header is sampled in the same way, noting that multiple QUIC packets might be included in the same UDP datagram and that each one is handled separately.¶

To ensure that this process does not sample the packet number, packet number protection algorithms MUST NOT sample more ciphertext than the minimum expansion of the corresponding AEAD.¶

Packet number protection is applied to the packet number encoded as described in Section 4.11 of [QUIC-TRANSPORT]. Since the length of the packet number is stored in the first octet of the encoded packet number, it may be necessary to progressively decrypt the packet number.¶

Before a TLS ciphersuite can be used with QUIC, a packet protection algorithm MUST be specifed for the AEAD used with that ciphersuite. This document defines algorithms for AEAD_AES_128_GCM, AEAD_AES_128_CCM, AEAD_AES_256_GCM, AEAD_AES_256_CCM (all AES AEADs are defined in [AEAD]), and AEAD_CHACHA20_POLY1305 ([CHACHA]).¶

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 - the next 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.¶

If 0-RTT keys are available (see Section 4.5), the lack of replay protection means that restrictions on their use are necessary to avoid replay attacks on the protocol.¶

A client MUST only use 0-RTT keys to protect data that is idempotent. 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 MUST NOT send ACKs with 0-RTT keys.¶

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 acknowledgements of 0-RTT packets. A client MUST NOT attempt to decrypt 0-RTT packets it receives and instead MUST discard them.¶

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.

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.¶

However, a server MUST NOT process data from incoming 1-RTT protected packets before verifying either the client Finished message or - in the case that the server has chosen to use a pre-shared key - the pre-shared key binder (see Section 4.2.11 of [TLS13]). Verifying these values provides the server with an assurance that the ClientHello has not been modified. Packets protected with 1-RTT keys MAY be stored and later decrypted and used once the handshake is complete.¶

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.¶

The QUIC version negotiation mechanism is used to negotiate the version of QUIC that is used prior to the completion of the handshake. However, this packet is not authenticated, enabling an active attacker to force a version downgrade.¶

To ensure that a QUIC version downgrade is not forced by an attacker, version information is copied into the TLS handshake, which provides integrity protection for the QUIC negotiation. This does not prevent version downgrade prior to the completion of the handshake, though it means that a downgrade causes a handshake failure.¶

TLS uses Application Layer Protocol Negotiation (ALPN) [RFC7301] to select an application protocol. The application-layer 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.¶

If the server cannot select a compatible combination of application protocol and QUIC version, it MUST abort the connection. A client MUST abort a connection if the server picks an incompatible combination of QUIC version and ALPN identifier.¶

QUIC transport parameters are carried in a TLS extension. Different versions of QUIC might define a different format for this struct.¶

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

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 carries a TransportParameters when the version of QUIC defined in [QUIC-TRANSPORT] is used.¶

The quic_transport_parameters extension is carried in the ClientHello and the EncryptedExtensions messages during the handshake.¶

While the transport parameters are technically available prior to the completion of the handshake, they cannot be fully trusted until the handshake completes, and reliance on them should be minimized. However, any tampering with the 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 if this extension is received when the transport is not QUIC.¶

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.¶

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 UDP datagrams in its first flight (see Section 4.7 of [QUIC-TRANSPORT]). Finally, because acknowledgements of Handshake packets are authenticated, a blind attacker cannot forge them. Put together, these defenses limit the level of amplification.¶

QUIC, TLS, and HTTP/2 all contain messages that have legitimate uses in some contexts, but that can be abused to cause a peer to expend processing resources without having any observable impact on the state of the connection. If processing is disproportionately large in comparison to the observable effects on bandwidth or state, then this could allow a malicious peer to exhaust processing capacity without consequence.¶

QUIC prohibits the sending of empty STREAM frames unless they are marked with the FIN bit. This prevents STREAM frames from being sent that only waste effort.¶

While there are legitimate uses for some redundant packets, implementations SHOULD track redundant packets and treat excessive volumes of any non-productive packets as indicative of an attack.¶

Packet number protection relies on the packet protection AEAD being a pseudorandom function (PRF), which is not a property that AEAD algorithms guarantee. Therefore, no strong assurances about the general security of this mechanism can be shown in the general case. The AEAD algorithms described in this document are assumed to be PRFs.¶

This construction is secure against chosen plaintext attacks (IND-CPA) [IMC].¶

Use of the same key and ciphertext sample more than once risks compromising packet number protection. Protecting two different packet numbers with the same key and ciphertext sample reveals the exclusive OR of those packet numbers. 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.¶

Note:

In some cases, inputs shorter than the full size required by the packet protection algorithm might be used.

To prevent an attacker from modifying packet numbers, values of packet numbers are transitively authenticated using packet protection; packet numbers are part of the authenticated additional data. A falsified or modified packet number can only be detected once the packet protection is removed.¶

An attacker can guess values for packet numbers and have an endpoint confirm guesses through timing side channels. 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 packet number 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.¶

• Updated to TLS 1.3 final (#1660)

• 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)

• Encrypted packet numbers.

• No significant changes.

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

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

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

No significant changes.¶

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

No significant changes.¶

• Updates to match changes in transport draft

• 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)

• 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

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