draft-ietf-acme-tls-alpn-05.txt   draft-ietf-acme-tls-alpn-06.txt 
ACME Working Group R. Shoemaker ACME Working Group R. Shoemaker
Internet-Draft ISRG Internet-Draft ISRG
Intended status: Standards Track August 16, 2018 Intended status: Standards Track September 05, 2019
Expires: February 17, 2019 Expires: March 8, 2020
ACME TLS ALPN Challenge Extension ACME TLS ALPN Challenge Extension
draft-ietf-acme-tls-alpn-05 draft-ietf-acme-tls-alpn-06
Abstract Abstract
This document specifies a new challenge for the Automated Certificate This document specifies a new challenge for the Automated Certificate
Management Environment (ACME) protocol which allows for domain Management Environment (ACME) protocol which allows for domain
control validation using TLS. control validation using TLS.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
skipping to change at page 1, line 32 skipping to change at page 1, line 32
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This Internet-Draft will expire on February 17, 2019. This Internet-Draft will expire on March 8, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. TLS with Application Level Protocol Negotiation (TLS ALPN) 3. TLS with Application Layer Protocol Negotiation (TLS ALPN)
Challenge . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Challenge . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. acme-tls/1 Protocol Definition . . . . . . . . . . . . . 5 4. acme-tls/1 Protocol Definition . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5.1. SMI Security for PKIX Certificate Extension OID . . . . . 6 6.1. SMI Security for PKIX Certificate Extension OID . . . . . 6
5.2. ALPN Protocol ID . . . . . . . . . . . . . . . . . . . . 6 6.2. ALPN Protocol ID . . . . . . . . . . . . . . . . . . . . 6
5.3. ACME Validation Method . . . . . . . . . . . . . . . . . 6 6.3. ACME Validation Method . . . . . . . . . . . . . . . . . 6
6. Appendix: Design Rationale . . . . . . . . . . . . . . . . . 7 7. Appendix: Design Rationale . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
8. Normative References . . . . . . . . . . . . . . . . . . . . 7 9. Normative References . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction 1. Introduction
The Automatic Certificate Management Environment (ACME) The Automatic Certificate Management Environment (ACME) [RFC8555]
[I-D.ietf-acme-acme] standard specifies methods for validating standard specifies methods for validating control of domain names via
control of domain names via HTTP and DNS. Deployment experience has HTTP and DNS. Deployment experience has shown it is also useful to
shown it is also useful to be able to validate domain control using be able to validate domain control using the TLS layer alone. In
the TLS layer alone. In particular, this allows hosting providers, particular, this allows hosting providers, CDNs, and TLS-terminating
CDNs, and TLS-terminating load balancers to validate domain control load balancers to validate domain control without modifying the HTTP
without modifying the HTTP handling behavior of their backends. This handling behavior of their backends. This separation of layers can
separation of layers can improve security and usability of ACME improve security and usability of ACME validation.
validation.
Early ACME drafts specified two TLS-based challenge types: TLS-SNI-01 Early ACME drafts specified two TLS-based challenge types: TLS-SNI-01
and TLS-SNI-02. These methods were removed because they relied on and TLS-SNI-02. These methods were removed because they relied on
assumptions about the deployed base of HTTPS hosting providers that assumptions about the deployed base of HTTPS hosting providers that
proved to be incorrect. Those incorrect assumptions weakened the proved to be incorrect. Those incorrect assumptions weakened the
security of those methods and are discussed in the "Design Rationale" security of those methods and are discussed in the "Design Rationale"
appendix. appendix.
This document specifies a new TLS-based challenge type, TLS-ALPN-01. This document specifies a new TLS-based challenge type, tls-alpn-01.
This challenge requires negotiating a new application-layer protocol This challenge requires negotiating a new application-layer protocol
using the TLS Application-Layer Protocol Negotiation (ALPN) Extension using the TLS Application-Layer Protocol Negotiation (ALPN) Extension
[RFC7301]. Because no existing software implements this protocol, [RFC7301]. Because no existing software implements this protocol,
the ability to fulfill TLS-ALPN-01 challenges is effectively opt-in. the ability to fulfill tls-alpn-01 challenges is effectively opt-in.
A service provider must proactively deploy new code in order to A service provider must proactively deploy new code in order to
implement TLS-ALPN-01, so we can specify stronger controls in that implement tls-alpn-01, so we can specify stronger controls in that
code, resulting in a stronger validation method. code, resulting in a stronger validation method.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
3. TLS with Application Level Protocol Negotiation (TLS ALPN) Challenge 3. TLS with Application Layer Protocol Negotiation (TLS ALPN) Challenge
The TLS with Application Level Protocol Negotiation (TLS ALPN) The TLS with Application Layer Protocol Negotiation (TLS ALPN)
validation method proves control over a domain name by requiring the validation method proves control over a domain name by requiring the
client to configure a TLS server to respond to specific connection client to configure a TLS server to respond to specific connection
attempts utilizing the ALPN extension with identifying information. attempts utilizing the ALPN extension with identifying information.
The ACME server validates control of the domain name by connecting to 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 a TLS server at one of the addresses resolved for the domain name and
verifying that a certificate with specific content is presented. verifying that a certificate with specific content is presented.
The tls-alpn-01 ACME challenge object has the following format:
type (required, string): The string "tls-alpn-01" type (required, string): The string "tls-alpn-01"
token (required, string): A random value that uniquely identifies token (required, string): A random value that uniquely identifies
the challenge. This value MUST have at least 128 bits of entropy. the challenge. This value MUST have at least 128 bits of entropy.
It MUST NOT contain any characters outside the base64url alphabet, It MUST NOT contain any characters outside the base64url alphabet
including padding characters ("="). See [RFC4086] for additional as described in [RFC4648] Section 5. Trailing'=' padding
characters MUST be stripped. See [RFC4086] for additional
information on randomness requirements. 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 The client prepares for validation by constructing a self-signed
certificate which MUST contain a acmeIdentifier extension and a certificate which MUST contain a acmeIdentifier extension and a
subjectAlternativeName extension [RFC5280]. The subjectAlternativeName extension [RFC5280]. The
subjectAlternativeName extension MUST contain a single dNSName entry subjectAlternativeName extension MUST contain a single dNSName entry
where the value is the domain name being validated. The where the value is the domain name being validated. The
acmeIdentifier extension MUST contain the SHA-256 digest [FIPS180-4] acmeIdentifier extension MUST contain the SHA-256 digest [FIPS180-4]
of the key authorization [I-D.ietf-acme-acme] for the challenge. The of the key authorization [RFC8555] for the challenge. The
acmeIdentifier extension MUST be critical so that the certificate acmeIdentifier extension MUST be critical so that the certificate
isn't inadvertently used by non-ACME software. isn't inadvertently used by non-ACME software.
The acmeIdentifier extension has the following format: The acmeIdentifier extension is identified by the id-pe-
acmeIdentifier object identifier (OID) in the id-pe arc [RFC5280]:
id-pe-acmeIdentifier OBJECT IDENTIFIER ::= { id-pe 31 } id-pe-acmeIdentifier OBJECT IDENTIFIER ::= { id-pe 31 }
Authorization ::= OCTET STRING (SIZE (32)) The extension has the following ASN.1 [X.680] format :
Authorization ::= OCTET STRING (SIZE (32))
The extnValue of the id-pe-acmeIdentifier extension is the ASN.1 DER The extnValue of the id-pe-acmeIdentifier extension is the ASN.1 DER
encoding of the Authorization structure, which contains the SHA-256 encoding [X.690] of the Authorization structure, which contains the
digest of the key authorization for the challenge. SHA-256 digest of the key authorization for the challenge.
Once this certificate has been created it MUST be provisioned such Once this certificate has been created it MUST be provisioned such
that it is returned during a TLS handshake that contains a ALPN that it is returned during a TLS handshake where the "acme-tls/1"
extension containing the value "acme-tls/1" and a SNI extension application-layer protocol has been negotiated and a Server Name
containing the domain name being validated. Indication (SNI) extension [RFC6066] has been provided containing the
domain name being validated.
A client responds with an empty object ({}) to acknowledge that the A client responds by POSTing an empty JSON object ({}) to the
challenge is ready to be validated by the server. The base64url challenge URL to acknowledge that the challenge is ready to be
encoding of the protected headers and payload is described in validated by the server. The base64url encoding of the protected
[I-D.ietf-acme-acme] Section 6.1. headers and payload is described in [RFC8555] Section 6.1.
POST /acme/authz/1234/1 POST /acme/authz/1234/1
Host: example.com Host: example.com
Content-Type: application/jose+json Content-Type: application/jose+json
{ {
"protected": base64url({ "protected": base64url({
"alg": "ES256", "alg": "ES256",
"kid": "https://example.com/acme/acct/1", "kid": "https://example.com/acme/acct/1",
"nonce": "JHb54aT_KTXBWQOzGYkt9A", "nonce": "JHb54aT_KTXBWQOzGYkt9A",
"url": "https://example.com/acme/authz/1234/1" "url": "https://example.com/acme/authz/1234/1"
}), }),
"payload": base64url({}), "payload": base64url({}),
"signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4" "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
} }
On receiving a response the server constructs and stores the key On receiving this request from a client the server constructs and
authorization from the challenge "token" value and the current client stores the key authorization from the challenge "token" value and the
account key. current client account key.
The server then verifies the client's control over the domain by The server then verifies the client's control over the domain by
verifying that the TLS server was configured as expected using the verifying that the TLS server was configured as expected using the
following steps: following steps:
1. Compute the expected SHA-256 digest of the expected key 1. The ACME server computes the expected SHA-256 digest of the key
authorization. authorization.
2. Resolve the domain name being validated and choose one of the IP 2. The ACME server resolves the domain name being validated and
addresses returned for validation (the server MAY validate chooses one of the IP addresses returned for validation (the
against multiple addresses if more than one is returned, but this server MAY validate against multiple addresses if more than one
is not required). is returned).
3. Initiate a TLS connection with the chosen IP address, this 3. The AMCE server initiates a TLS connection to the chosen IP
connection MUST use TCP port 443. The ClientHello that initiates address, this connection MUST use TCP port 443. The ACME server
the handshake MUST contain a ALPN extension with the single MUST provide a ALPN extension with the single protocol name
protocol name "acme-tls/1" and a Server Name Indication [RFC6066] "acme-tls/1" and a SNI extension containing only the domain name
extension containing the domain name being validated. being validated during the TLS handshake.
4. Verify that the ServerHello contains a ALPN extension containing 4. The ACME server verifies that during the TLS handshake the
the value "acme-tls/1" and that the certificate returned contains application-layer protocol "acme-tls/1" was successfully
a subjectAltName extension containing the dNSName being validated negotiated (and that the ALPN extension contained only the value
and no other entries and a critical acmeIdentifier extension "acme-tls/1") and that the certificate returned contains:
containing the digest computed in step 1. The comparison of
dNSNames MUST be case insensitive [RFC4343]. Note that as ACME * a subjectAltName extension containing the dNSName being
doesn't support Unicode identifiers all dNSNames MUST be encoded validated and no other entries
using the [RFC3492] rules.
* a critical acmeIdentifier extension containing the expected
SHA-256 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 [RFC3492] rules.
If all of the above steps succeed then the validation is successful, If all of the above steps succeed then the validation is successful,
otherwise it fails. Once the TLS handshake has been completed the otherwise it fails.
connection MUST be immediately closed and no further data should be
exchanged.
3.1. acme-tls/1 Protocol Definition 4. acme-tls/1 Protocol Definition
The "acme-tls/1" protocol MUST only be used for validating ACME tls- The "acme-tls/1" protocol MUST only be used for validating ACME tls-
alpn-01 challenges. The protocol consists of a TLS handshake in alpn-01 challenges. The protocol consists of a TLS handshake in
which the required validation information is transmitted. Once the which the required validation information is transmitted. Once the
handshake is completed the client MUST NOT exchange any further data handshake is completed the client MUST NOT exchange any further data
with the server and MUST immediately close the connection. with the server and MUST immediately close the connection.
4. Security Considerations 5. Security Considerations
The design of this challenges relies on some assumptions centered The design of this challenge relies on some assumptions centered
around how a server behaves during validation. around how a server behaves during validation.
The first assumption is that when a server is being used to serve 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 content for multiple DNS names from a single IP address that it
properly segregates control of those names to the users that own properly segregates control of those names to the users that own
them. This means that if User A registers Host A and User B 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 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 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 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. them to illegitimately validate control of Host A to the ACME server.
The second assumption is that a server will not violate [RFC7301] by The second assumption is that a server will not violate [RFC7301] by
blindly agreeing to use the "acme-tls/1" protocol without actually blindly agreeing to use the "acme-tls/1" protocol without actually
understanding it. understanding it.
To further mitigate the risk of users claiming domain names used by To further mitigate the risk of users claiming domain names used by
other users on the same infrastructure hosting providers, CDNs, and other users on the same infrastructure hosting providers, CDNs, and
other service providers should not allow users to provide their own other service providers SHOULD NOT allow users to provide their own
certificates for the TLS ALPN validation process. If providers wish certificates for the TLS ALPN validation process. If providers wish
to implement TLS ALPN validation they SHOULD only generate to implement TLS ALPN validation they SHOULD only generate
certificates used for validation themselves and not expose this certificates used for validation themselves and not expose this
functionality to users. functionality to users.
5. IANA Considerations The extensions to the ACME protocol described in this document build
upon the Security Considerations and threat model defined in
[RFC8555] Section 10.1.
6. IANA Considerations
[[RFC Editor: please replace XXXX below by the RFC number.]] [[RFC Editor: please replace XXXX below by the RFC number.]]
5.1. SMI Security for PKIX Certificate Extension OID 6.1. SMI Security for PKIX Certificate Extension OID
Within the SMI-numbers registry, the "SMI Security for PKIX 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 Certificate Extension (1.3.6.1.5.5.7.1)" table is to be updated to
add the following entry: add the following entry:
+---------+----------------------+------------+ +---------+----------------------+------------+
| Decimal | Description | References | | Decimal | Description | References |
+---------+----------------------+------------+ +---------+----------------------+------------+
| 31 | id-pe-acmeIdentifier | RFC XXXX | | 31 | id-pe-acmeIdentifier | RFC XXXX |
+---------+----------------------+------------+ +---------+----------------------+------------+
5.2. ALPN Protocol ID 6.2. ALPN Protocol ID
Within the Transport Layer Security (TLS) Extensions registry, the Within the Transport Layer Security (TLS) Extensions registry, the
"Application-Layer Protocol Negotiation (ALPN) Protocol IDs" table is "Application-Layer Protocol Negotiation (ALPN) Protocol IDs" table is
to be updated to add the following entry: to be updated to add the following entry:
+------------+------------------------------------------+-----------+ +------------+------------------------------------------+-----------+
| Protocol | Identification Sequence | Reference | | Protocol | Identification Sequence | Reference |
+------------+------------------------------------------+-----------+ +------------+------------------------------------------+-----------+
| ACME-TLS/1 | 0x61 0x63 0x6d 0x65 0x2d 0x74 0x6c 0x73 | RFC XXXX | | ACME-TLS/1 | 0x61 0x63 0x6d 0x65 0x2d 0x74 0x6c 0x73 | RFC XXXX |
| | 0x2f 0x31 ("acme-tls/1") | | | | 0x2f 0x31 ("acme-tls/1") | |
+------------+------------------------------------------+-----------+ +------------+------------------------------------------+-----------+
5.3. ACME Validation Method 6.3. ACME Validation Method
The "ACME Validation Methods" registry is to be updated to include The "ACME Validation Methods" registry is to be updated to include
the following entry: the following entry:
+-------------+-----------------+-----------+ +-------------+-----------------+-----------+
| Label | Identifier Type | Reference | | Label | Identifier Type | Reference |
+-------------+-----------------+-----------+ +-------------+-----------------+-----------+
| tls-alpn-01 | dns | RFC XXXX | | tls-alpn-01 | dns | RFC XXXX |
+-------------+-----------------+-----------+ +-------------+-----------------+-----------+
6. Appendix: Design Rationale 7. Appendix: Design Rationale
The TLS ALPN challenge exists to replace the TLS SNI challenge The TLS ALPN challenge exists to replace the TLS SNI challenge
defined in the early ACME drafts. This challenge was convenient for defined in the early ACME drafts. This challenge was convenient for
service providers who were either operating large TLS layer load service providers who were either operating large TLS layer load
balancing systems at which they wanted to perform validation or balancing systems at which they wanted to perform validation or
running servers fronting large numbers of DNS names from a single running servers fronting large numbers of DNS names from a single
host as it allowed validation purely within the TLS layer. host as it allowed validation purely within the TLS layer.
A security issue was discovered in the TLS SNI challenge by Frans A security issue was discovered in the TLS SNI challenge by Frans
Rosen which allowed users of various service providers to Rosen which allowed users of various service providers to
illegitimately validate control of the DNS names of other users of illegitimately validate control of the DNS names of other users of
the provider. When the TLS SNI challenge was designed it was assumed 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 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 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 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 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 be a security property provided by a number of large service
providers. Because of this users were able to respond to SNI traffic providers. Because of this users were able to respond to SNI traffic
for the SNI names used by the TLS SNI challenge validation process. 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 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 respectively User A would be able to claim the SNI name for Host B
validation for Host B and when the validation connection was made and when the validation connection was made that User A would be able
that User A would be able to answer, proving control of Host B. to answer, proving 'control' of Host B. As the SNI name used was a
subdomain of the domain name being validated, rather than the domain
name itself, it was likely to not already be registered with the
service provider for traffic routing, making it much easier for a
hijack to occur.
7. Acknowledgements 8. Acknowledgements
The author would like to thank all those whom have provided design The author would like to thank all those whom have provided design
insights and editorial review of this document, including Richard insights and editorial review of this document, including Richard
Barnes, Ryan Hurst, Adam Langley, Ryan Sleevi, Jacob Hoffman-Andrews, Barnes, Ryan Hurst, Adam Langley, Ryan Sleevi, Jacob Hoffman-Andrews,
Daniel McCarney, Marcin Walas, and Martin Thomson and especially Daniel McCarney, Marcin Walas, Martin Thomson and especially Frans
Frans Rosen who discovered the vulnerability in the TLS SNI method Rosen who discovered the vulnerability in the TLS SNI method which
which necessitated the writing of this specication. necessitated the writing of this specification.
8. Normative References 9. Normative References
[FIPS180-4] [FIPS180-4]
Department of Commerce, National., "NIST FIPS 180-4, Department of Commerce, National., "NIST FIPS 180-4,
Secure Hash Standard", March 2012, Secure Hash Standard", March 2012,
<http://csrc.nist.gov/publications/fips/fips180-4/ <http://csrc.nist.gov/publications/fips/fips180-4/
fips-180-4.pdf>. 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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003, (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
<https://www.rfc-editor.org/info/rfc3492>. <https://www.rfc-editor.org/info/rfc3492>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086, "Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005, DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>. <https://www.rfc-editor.org/info/rfc4086>.
[RFC4343] Eastlake 3rd, D., "Domain Name System (DNS) Case [RFC4343] Eastlake 3rd, D., "Domain Name System (DNS) Case
Insensitivity Clarification", RFC 4343, Insensitivity Clarification", RFC 4343,
DOI 10.17487/RFC4343, January 2006, DOI 10.17487/RFC4343, January 2006,
<https://www.rfc-editor.org/info/rfc4343>. <https://www.rfc-editor.org/info/rfc4343>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>. <https://www.rfc-editor.org/info/rfc5280>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066, Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011, DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>. <https://www.rfc-editor.org/info/rfc6066>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan, [RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol "Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>. July 2014, <https://www.rfc-editor.org/info/rfc7301>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
<https://www.rfc-editor.org/info/rfc8555>.
[X.680] International Telecommunication Union, ., "Information
technology -- Abstract Syntax Notation One (ASN.1):
Specification of basic notation", 2015,
<https://www.itu.int/ITU-T/studygroups/com17/languages/
X.680-0207.pdf>.
[X.690] International Telecommunication Union, ., "Information
Technology -- ASN.1 encoding rules: Specification of Basic
Encoding Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER)", 2015,
<https://www.itu.int/ITU-T/studygroups/com17/languages/
X.690-0207.pdf>.
Author's Address Author's Address
Roland Bracewell Shoemaker Roland Bracewell Shoemaker
Internet Security Research Group Internet Security Research Group
Email: roland@letsencrypt.org Email: roland@letsencrypt.org
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