]> Zscaler

yrosomakho@zscaler.com

University of Applied Sciences Bonn-Rhein-Sieg

Hannes.Tschofenig@gmx.net

Security Transport Layer Security encrypted client hello sslkeylog This document specifies an extension to the SSLKEYLOGFILE format to support the logging of information about Encrypted Client Hello (ECH) related secrets. Two new labels are introduced, namely ECH_SECRET and ECH_CONFIG, which log the Hybrid Public Key Encryption (HPKE)-derived shared secret and the ECHConfig used for the ECH, respectively. This extension aims to facilitate debugging of TLS connections employing ECH. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Transport Layer Security Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction Debugging protocols with TLS can be difficult due to encrypted communications. Analyzing these messages in diagnostic and debug tools requires inspecting the encrypted content. Various TLS implementations have informally adopted a file format to log the secret values generated by the TLS key schedule, aiding in this analysis. In many implementations, the file that the secrets are logged to is specified in an environment variable named "SSLKEYLOGFILE". standardizes this format. With the introduction of additional secrets are derived during the handshake to encrypt the ClientHello message using Hybrid Public Key Encryption (HPKE) . This document extends the SSLKEYLOGFILE format to also offer support for the ECH extension to enable debugging of ECH-enabled connections. The proposed extension can also be used with all protocols that support ECH, including TLS 1.3 , DTLS 1.3 and QUIC .

Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

SSLKEYLOGFILE Labels for ECH This document defines two new labels for SSLKEYLOGFILE format: ECH_SECRET and ECH_CONFIG. The client SHOULD log the labels if it offered ECH regardless of server acceptance. The server MAY log the labels only if it successfully decrypted and accepted ECH offered by the client. The 32-byte random value from the Outer ClientHello message is used as the client_random value for these log records. The client MUST NOT log the labels for connections that use the GREASE ECH extension (see Section 6.2 of ).

ECH_SECRET This label corresponds to the KEM shared secret derived during the HPKE key schedule process. Length of the secret is defined by the KEM negotiated for use with ECH.

ECH_CONFIG This label is used to log the ECHConfig used for construction of the ECH extension. Note that the value is logged in hexadecimal representation, similarly to other entries in the SSLKEYLOGFILE.

Client_random for other TLS 1.3 SSLKEYLOGFILE Entries The SSLKEYLOGFILE uses the random value from the ClientHello message as a "connection identifier". This creates ambiguity since the TLS handshake with ECH contains two different random values, one in the Outer ClientHello structure and the second one in the Inner ClientHello. The SSLKEYLOGFILE entries corresponding to TLS 1.3 secrets for connections that successfully negotiated ECH MUST use the random from the Inner ClientHello structure. In all other cases the random value from the Outer ClientHello structure MUST be used.

Security Considerations The applicability statement of also applies to this document: if unauthorized entities gain access to the logged secrets then the core guarantees that TLS provides are completely undermined. This specification extends the SSLKEYLOGFILE specification and therefore introduces the following threats:

• Access to the ECH_SECRET record in the SSLKEYLOGFILE allows the attacker to decrypt the ECH extension and thereby reveal the content of the ClientHello message, including the payload of the Server Name Indication (SNI) extension.
• Access to the HPKE-established shared secret introduces a potential attack surface against the HPKE library since access to this keying material is not ncessarily available otherwise.

Implementers MUST take measures to prevent unauthorized access to the SSLKEYLOGFILE text file. According to SSLKEYLOGFILE specification , this extension is intended for use in systems where TLS only protects test data. While the access this information provides to TLS connections can be useful for diagnosing problems during development, this mechanism MUST NOT be used in a production environment.

IANA Considerations This document has no IANA actions.

References Normative References Mozilla A format that supports the logging information about the secrets used in a TLS connection is described. Recording secrets to a file in SSLKEYLOGFILE format allows diagnostic and logging tools that use this file to decrypt messages exchanged by TLS endpoints. Independent Fastly Cryptography Consulting LLC Cloudflare This document describes a mechanism in Transport Layer Security (TLS) for encrypting a ClientHello message under a server public key. Discussion Venues This note is to be removed before publishing as an RFC. Source for this draft and an issue tracker can be found at https://github.com/tlswg/draft-ietf-tls-esni (https://github.com/tlswg/draft-ietf-tls-esni). In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References This document describes a scheme for hybrid public key encryption (HPKE). This scheme provides a variant of public key encryption of arbitrary-sized plaintexts for a recipient public key. It also includes three authenticated variants, including one that authenticates possession of a pre-shared key and two optional ones that authenticate possession of a key encapsulation mechanism (KEM) private key. HPKE works for any combination of an asymmetric KEM, key derivation function (KDF), and authenticated encryption with additional data (AEAD) encryption function. Some authenticated variants may not be supported by all KEMs. We provide instantiations of the scheme using widely used and efficient primitives, such as Elliptic Curve Diffie-Hellman (ECDH) key agreement, HMAC-based key derivation function (HKDF), and SHA2. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF. This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. This document specifies version 1.3 of the Datagram Transport Layer Security (DTLS) protocol. DTLS 1.3 allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. The DTLS 1.3 protocol is based on the Transport Layer Security (TLS) 1.3 protocol and provides equivalent security guarantees with the exception of order protection / non-replayability. Datagram semantics of the underlying transport are preserved by the DTLS protocol. This document obsoletes RFC 6347. This document defines the core of the QUIC transport protocol. QUIC provides applications with flow-controlled streams for structured communication, low-latency connection establishment, and network path migration. QUIC includes security measures that ensure confidentiality, integrity, and availability in a range of deployment circumstances. Accompanying documents describe the integration of TLS for key negotiation, loss detection, and an exemplary congestion control algorithm. This document describes how Transport Layer Security (TLS) is used to secure QUIC.

Acknowledgments We would like to thank Stephen Farrell, Martin Thomson and Peter Wu for their review comments.