Internet DRAFT - draft-shoemaker-acme-tls-alpn
draft-shoemaker-acme-tls-alpn
Network Working Group R. Shoemaker
Internet-Draft ISRG
Intended status: Standards Track February 22, 2018
Expires: August 26, 2018
ACME TLS ALPN Challenge Extension
draft-shoemaker-acme-tls-alpn-00
Abstract
This document specifies a new challenge for the Automated Certificate
Management Environment (ACME) protocol which allows for domain
control validation using TLS.
Status of This Memo
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This Internet-Draft will expire on August 26, 2018.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. TLS with Application Level Protocol Negotiation (TLS ALPN)
Challenge . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3.1. acme-tls/1 Protocol Definition . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5.1. SMI Security for PKIX Certificate Extension OID . . . . . 5
5.2. ACME Validation Method . . . . . . . . . . . . . . . . . 5
6. Appendix: Design Rationale . . . . . . . . . . . . . . . . . 6
7. Normative References . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
The Automatic Certificate Management Environment (ACME)
[I-D.ietf-acme-acme] specification doesn't specify a TLS layer
validation method which limits the points at which validation can be
performed. This document extends the ACME specification to include a
TLS based validation method that uses the Application Level Protocol
Negotiation extension.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
[RFC2119].
3. TLS with Application Level Protocol Negotiation (TLS ALPN) Challenge
The TLS with Application Level Protocol Negotiation (TLS ALPN)
validation method proves control over a domain name by requiring the
client to configure a TLS server referenced by the DNS A and/or AAAA
Resource Records for the domain name to respond to specific
connection attempts utilizing the ALPN extension [RFC7301]. The
server validates control of the domain name by connecting to the TLS
server and verifying a certificate with specific content is
presented.
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,
including padding characters ("=").
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GET /acme/authz/1234/1 HTTP/1.1
Host: example.com
HTTP/1.1 200 OK
{
"type": "tls-alpn-01",
"url": "https://example.com/acme/authz/1234/1",
"status": "pending",
"token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
}
The client prepares for validation by constructing a self-signed
certificate which MUST contain a acmeValidation-v1 extension and a
subjectAlternativeName extension [RFC5280]. The
subjectAlternativeName extension MUST contain a single dNSName entry
where the value is the domain name being validated. The
acmeValidation-v1 extension MUST contain the SHA-256 digest
[FIPS180-4] of the key authorization [I-D.ietf-acme-acme] for the
challenge. The acmeValidation extension MUST be critical so that the
certificate isn't inadvertently used to make trust decisions.
id-pe-acmeIdentifier OBJECT IDENTIFIER ::= { id-pe 30 }
id-pe-acmeIdentifier-v1 OBJECT IDENTIFIER ::= { id-pe-acmeIdentifier 1 }
acmeValidation-v1 ::= OCTET STRING (SIZE (32))
Once this certificate has been created it MUST be provisioned such
that it is returned during a TLS handshake that contains a ALPN
extension containing the value "acme-tls/1" and a SNI extension
containing the domain name being validated.
When ready the client acknowledges this by sending a POST message
containing the key authorization, as defined in [I-D.ietf-acme-acme]
section 8.1, to the challenge URL.
keyAuthorization (required, string): The key authorization for this
challenge. This value MUST match the token from the challenge and
the client's account key.
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POST /acme/authz/1234/1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/1",
"nonce": "JHb54aT_KTXBWQOzGYkt9A",
"url": "https://example.com/acme/authz/1234/1"
}),
"payload": base64url({
"keyAuthorization": "evaGxfADs...62jcerQ"
}),
"signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
}
On receiving this the server MUST verify that the key authorization
in the request matches the "token" value in the challenge and the
client's account key. If they do not match then the server MUST
return a HTTP error in response to the POST request in which the
client sent the challenge.
The server then verifies the client's control over the domain by
verifying that the TLS server was configured as expected using these
steps:
1. Compute the expected SHA-256 [FIPS180-4] digest of the expected
key authorization.
2. Initiate a TLS connection with the domain name being validated,
this connection MUST be sent to TCP port 443. The ClientHello
that initiates the handshake MUST contain a ALPN extension with
the value "acme-tls/1" and a Server Name Indication [RFC6066]
extension containing the domain name being validated.
3. Verify that the ServerHello contains a ALPN extension containing
the value "acme-tls/1" and that the certificate returned contains
a subjectAltName extension containing the dNSName being validated
and no other entries and a critical acmeValidation extension
containing the digest computed in step 1. The comparison of
dNSNames MUST be case insensitive [RFC4343]. Note that as ACME
doesn't support Unicode identifiers all dNSNames MUST be encoded
using the [RFC3492] rules.
If all of the above steps succeed then the validation is successful,
otherwise it fails. Once the handshake has been completed the
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connection should be immediately closed and no further data should be
exchanged.
3.1. acme-tls/1 Protocol Definition
The "acme-tls/1" protocol MUST only be used for validating ACME tls-
alpn-01 challenges. The protocol consists of a TLS handshake in
which the required validation information is transmitted. Once the
handshake is complete the client MUST not exchange any further data
with the server and MUST immediately close the connection.
4. Security Considerations
The design of this challenges relies on some assumptions centered
around how a server behaves during validation.
The first assumption is that when a server is being used to serve
content for multiple DNS names from a single IP address that it
properly segregates control of those names to the users on the server
that own them. This means that if User A registers Host A and User B
registers Host B the server should not allow a TLS request using a
SNI value for Host A that only User A should be able to serve that
request. If the server allows User B to serve this request it allows
them to illegitimately validate control of Host A to the ACME server.
The second assumption is that a server will not blindly agree to use
the acme-tls/1 protocol without actually knowing about the protocol
itself, which is a violation of [RFC7301].
5. IANA Considerations
5.1. SMI Security for PKIX Certificate Extension OID
Within the SMI-numbers registry, the "SMI Security for PKIX
Certificate Extension (1.3.6.1.5.5.7.1)" table is to be updated to
include the following entry:
+---------+----------------------+------------+
| Decimal | Description | References |
+---------+----------------------+------------+
| 30 | id-pe-acmeIdentifier | RFC XXXX |
+---------+----------------------+------------+
5.2. ACME Validation Method
The "ACME Validation Methods" registry is to be updated to include
the following entry:
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+-------------+-----------------+-----------+
| Label | Identifier Type | Reference |
+-------------+-----------------+-----------+
| tls-alpn-01 | dns | RFC XXXX |
+-------------+-----------------+-----------+
6. Appendix: Design Rationale
The TLS ALPN challenge exists to replace the TLS SNI challenge
defined in the original ACME document. This challenge allowed
validation of domain control purely within the TLS layer which
provided convenience for server operators who were either operating
large TLS layer load balancing systems at which they wanted to
perform validation or running servers fronting large numbers of DNS
names from a single host.
A security issue was discovered in the TLS SNI challenge which
allowed users of certain service providers to illegitimately validate
control of the DNS names of other users, as long as those users were
also using those service providers. When the TLS SNI challenge was
designed it was assumed that a user would only be able to claim TLS
traffic via SNI for domain names they controlled (i.e. if User A
registered Host A with a service provider they wouldn't be able to
claim SNI traffic for Host B). This turns out not to be a security
property provided by a number of large service providers. Because of
this users were able to claim SNI traffic for the non-valid SNI names
the TLS SNI challenge used to signal what was being validated to the
server. This meant that if User A and User B had registered Host A
and Host B respectively User A would be able to claim the SNI name
for a validation for Host B and when the validation connection was
made to the shared IP address that User A would be able to answer,
proving control.
7. Normative References
[FIPS180-4]
Department of Commerce, National., "NIST FIPS 180-4,
Secure Hash Standard", March 2012,
<http://csrc.nist.gov/publications/fips/fips180-4/
fips-180-4.pdf>.
[I-D.ietf-acme-acme]
Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", draft-ietf-acme-acme-09 (work in progress),
December 2017.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
<https://www.rfc-editor.org/info/rfc3492>.
[RFC4343] Eastlake 3rd, D., "Domain Name System (DNS) Case
Insensitivity Clarification", RFC 4343,
DOI 10.17487/RFC4343, January 2006,
<https://www.rfc-editor.org/info/rfc4343>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.
Author's Address
Roland Bracewell Shoemaker
Internet Security Research Group
Email: roland@letsencrypt.org
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