PEM file format for ESNI
draft-farrell-tls-pemesni-00

Abstract

Encrypted SNI key pairs need to be configured into TLS servers, some of which can be built with different TLS libraries, so there is a benefit and little cost in documenting a file format to use for these, similar to how RFC7468 defines other PEM file formats.

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Table of Contents

• 1. Introduction
• 2. Terminology
• 3. ESNIKeys file
• 4. Security Considerations
• 5. Acknowledgements
• 6. IANA Considerations
• 7. Normative References
• Author's Address

1. Introduction

Encrypted Server Name Indication (ESNI) [I-D.ietf-tls-esni] for TLS1.3 [RFC8446] defines a confidentiality mechanism for server names in TLS. That requires publication of an ESNIKeys data structure in the DNS. An ESNIKeys structure contains the public component of a key pair that will typically be periodically (re-)generated by some key manager for a TLS server. TLS servers then need to be configured to use these key pairs, and given that various TLS servers can be built with different TLS libraries, there is a benefit in having a standard format for ESNI key pairs, just as was done with [RFC7468].

[[This idea could: a) wither on the vine, b) be published as it's own RFC, or c) end up as a PR for [I-D.ietf-tls-esni]. There is no absolute need for this to be in the RFC that defines ESNI, so (b) seems feasible if there's enough interest, hence this draft. The source for this is in https://github.com/sftcd/pemesni/ PRs are welcome there too.]]

2. Terminology

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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. ESNIKeys file

The public and private keys MUST both be PEM encoded. The file contains the catenation of the PEM encoding of the private key followed by the PEM encoding of the public key. The private key MUST be encoded as a PKCS#8 PrivateKey. The public key MUST be the base64 encoded form of the binary ESNIKeys value that is published in the DNS. The string "ESNIKEY" MUST be used in the PEM file delimiter for the public key.

There MUST only be one key pair in each file even if a server publishes multiple public keys in one ESNIKeys structure.

Figure 1 shows an example ESNI PEM File

-----BEGIN PRIVATE KEY-----
MC4CAQAwBQYDK2VuBCIEIEDyEDpfvLoFYQi4rNjAxAz7F/Dqydv5IFmcPpIyGNd8
-----END PRIVATE KEY-----
-----BEGIN ESNIKEY-----
/wG+49mkACQAHQAgB8SUB952QOphcyUR1sAvnRhY9NSSETVDuon9/CvoDVYAAhMBAQQAAAAAXYZC
TwAAAABdlBoPAAA=
-----END ESNIKEY-----
            

Figure 1: Example ESNI PEM file

4. Security Considerations

Storing cryptographic keys in files leaves them vulnerable should anyone get shell access to the TLS server machine. So: Don't let that happen:-)

5. Acknowledgements

TBD, as needed

6. IANA Considerations

There are none so this section can be deleted later.

7. Normative References

Author's Address