Automated Certificate Management Environment (ACME) TLS Application‑Layer Protocol Negotiation (ALPN) Challenge Extension
Internet Engineering Task Force (IETF) R.B. Shoemaker
Request for Comments: 8737 ISRG
Category: Standards Track February 2020
Automated Certificate Management Environment (ACME) TLS
Application-Layer Protocol Negotiation (ALPN) Challenge Extension
This document specifies a new challenge for the Automated Certificate
Management Environment (ACME) protocol that allows for domain control
validation using TLS.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
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Table of Contents
3. TLS with Application-Layer Protocol Negotiation (TLS ALPN)
4. acme-tls/1 Protocol Definition
5. Security Considerations
6. IANA Considerations
6.1. SMI Security for PKIX Certificate Extension OID
6.2. ALPN Protocol ID
6.3. ACME Validation Method
7. Normative References
Appendix A. Design Rationale
The Automatic Certificate Management Environment (ACME) [RFC8555]
specification describes methods for validating control of domain
names via HTTP and DNS. Deployment experience has shown it is also
useful to be able to validate domain control using the TLS layer
alone. In particular, this allows hosting providers, Content
Distribution Networks (CDNs), and TLS-terminating load balancers to
validate domain control without modifying the HTTP handling behavior
of their backends.
This document specifies a new TLS-based challenge type, tls-alpn-01.
This challenge requires negotiating a new application-layer protocol
using the TLS Application-Layer Protocol Negotiation (ALPN) Extension
[RFC7301]. Because this protocol does not build on a pre-existing
deployment base, the ability to complete tls-alpn-01 challenges
requires changes by service providers, making it explicitly an opt-in
process. Because service providers must proactively deploy new code
in order to implement tls-alpn-01, we can specify stronger controls
in that code, resulting in a stronger validation method.
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. TLS with Application-Layer Protocol Negotiation (TLS ALPN) Challenge
The TLS with Application-Layer Protocol Negotiation (TLS ALPN)
validation method proves control over a domain name by requiring the
ACME client to configure a TLS server to respond to specific
connection attempts using the ALPN extension with identifying
information. The ACME server validates control of the domain name by
connecting to a TLS server at one of the addresses resolved for the
domain name and verifying that a certificate with specific content is
The tls-alpn-01 ACME challenge object has the following format:
type (required, string): The string "tls-alpn-01"
token (required, string): A random value that uniquely identifies
the challenge. This value MUST have at least 128 bits of entropy.
It MUST NOT contain any characters outside the base64url alphabet
as described in Section 5 of [RFC4648]. Trailing '=' padding
characters MUST be stripped. See [RFC4086] for additional
information on randomness requirements.
The client prepares for validation by constructing a self-signed
certificate that MUST contain an acmeIdentifier extension and a
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