ACME Working Group R. Shoemaker
Internet-Draft ISRG
Intended status: Standards Track August 16, 2018
Expires: February 17, 2019
ACME TLS ALPN Challenge Extension
draft-ietf-acme-tls-alpn-05
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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. TLS with Application Level Protocol Negotiation (TLS ALPN)
Challenge . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. acme-tls/1 Protocol Definition . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5.1. SMI Security for PKIX Certificate Extension OID . . . . . 6
5.2. ALPN Protocol ID . . . . . . . . . . . . . . . . . . . . 6
5.3. ACME Validation Method . . . . . . . . . . . . . . . . . 6
6. Appendix: Design Rationale . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
8. Normative References . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
The Automatic Certificate Management Environment (ACME)
[I-D.ietf-acme-acme] standard specifies 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,
CDNs, and TLS-terminating load balancers to validate domain control
without modifying the HTTP handling behavior of their backends. This
separation of layers can improve security and usability of ACME
validation.
Early ACME drafts specified two TLS-based challenge types: TLS-SNI-01
and TLS-SNI-02. These methods were removed because they relied on
assumptions about the deployed base of HTTPS hosting providers that
proved to be incorrect. Those incorrect assumptions weakened the
security of those methods and are discussed in the "Design Rationale"
appendix.
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 no existing software implements this protocol,
the ability to fulfill TLS-ALPN-01 challenges is effectively opt-in.
A service provider must proactively deploy new code in order to
implement TLS-ALPN-01, so we can specify stronger controls in that
code, resulting in a stronger validation method.
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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. 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 to respond to specific connection
attempts utilizing 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 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 ("="). See [RFC4086] for additional
information on randomness requirements.
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 acmeIdentifier 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
acmeIdentifier extension MUST contain the SHA-256 digest [FIPS180-4]
of the key authorization [I-D.ietf-acme-acme] for the challenge. The
acmeIdentifier extension MUST be critical so that the certificate
isn't inadvertently used by non-ACME software.
The acmeIdentifier extension has the following format:
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id-pe-acmeIdentifier OBJECT IDENTIFIER ::= { id-pe 31 }
Authorization ::= OCTET STRING (SIZE (32))
The extnValue of the id-pe-acmeIdentifier extension is the ASN.1 DER
encoding of the Authorization structure, which contains the SHA-256
digest of the key authorization for the challenge.
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.
A client responds with an empty object ({}) to acknowledge that the
challenge is ready to be validated by the server. The base64url
encoding of the protected headers and payload is described in
[I-D.ietf-acme-acme] Section 6.1.
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({}),
"signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
}
On receiving a response the server constructs and stores the key
authorization from the challenge "token" value and the current client
account key.
The server then verifies the client's control over the domain by
verifying that the TLS server was configured as expected using the
following steps:
1. Compute the expected SHA-256 digest of the expected key
authorization.
2. Resolve the domain name being validated and choose one of the IP
addresses returned for validation (the server MAY validate
against multiple addresses if more than one is returned, but this
is not required).
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3. Initiate a TLS connection with the chosen IP address, this
connection MUST use TCP port 443. The ClientHello that initiates
the handshake MUST contain a ALPN extension with the single
protocol name "acme-tls/1" and a Server Name Indication [RFC6066]
extension containing the domain name being validated.
4. 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 acmeIdentifier 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 TLS handshake has been completed the
connection MUST 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 completed 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 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 to be served by User B or Host B to be served by
User A. 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 violate [RFC7301] by
blindly agreeing to use the "acme-tls/1" protocol without actually
understanding it.
To further mitigate the risk of users claiming domain names used by
other users on the same infrastructure hosting providers, CDNs, and
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other service providers should not allow users to provide their own
certificates for the TLS ALPN validation process. If providers wish
to implement TLS ALPN validation they SHOULD only generate
certificates used for validation themselves and not expose this
functionality to users.
5. IANA Considerations
[[RFC Editor: please replace XXXX below by the RFC number.]]
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
add the following entry:
+---------+----------------------+------------+
| Decimal | Description | References |
+---------+----------------------+------------+
| 31 | id-pe-acmeIdentifier | RFC XXXX |
+---------+----------------------+------------+
5.2. ALPN Protocol ID
Within the Transport Layer Security (TLS) Extensions registry, the
"Application-Layer Protocol Negotiation (ALPN) Protocol IDs" table is
to be updated to add the following entry:
+------------+------------------------------------------+-----------+
| Protocol | Identification Sequence | Reference |
+------------+------------------------------------------+-----------+
| ACME-TLS/1 | 0x61 0x63 0x6d 0x65 0x2d 0x74 0x6c 0x73 | RFC XXXX |
| | 0x2f 0x31 ("acme-tls/1") | |
+------------+------------------------------------------+-----------+
5.3. ACME Validation Method
The "ACME Validation Methods" registry is to be updated to include
the following entry:
+-------------+-----------------+-----------+
| Label | Identifier Type | Reference |
+-------------+-----------------+-----------+
| tls-alpn-01 | dns | RFC XXXX |
+-------------+-----------------+-----------+
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6. Appendix: Design Rationale
The TLS ALPN challenge exists to replace the TLS SNI challenge
defined in the early ACME drafts. This challenge was convenient for
service providers 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 as it allowed validation purely within the TLS layer.
A security issue was discovered in the TLS SNI challenge by Frans
Rosen which allowed users of various service providers to
illegitimately validate control of the DNS names of other users of
the provider. When the TLS SNI challenge was designed it was assumed
that a user would only be able to respond to TLS traffic via SNI for
domain names they controlled (i.e. if User A registered Host A and
User B registered Host B with a service provider that User A wouldn't
be able to respond to 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 respond to SNI traffic
for the SNI names used by the TLS SNI challenge validation process.
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
that User A would be able to answer, proving control of Host B.
7. Acknowledgements
The author would like to thank all those whom have provided design
insights and editorial review of this document, including Richard
Barnes, Ryan Hurst, Adam Langley, Ryan Sleevi, Jacob Hoffman-Andrews,
Daniel McCarney, Marcin Walas, and Martin Thomson and especially
Frans Rosen who discovered the vulnerability in the TLS SNI method
which necessitated the writing of this specication.
8. 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-14 (work in progress),
August 2018.
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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>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
Author's Address
Roland Bracewell Shoemaker
Internet Security Research Group
Email: roland@letsencrypt.org
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