draft-ietf-acme-email-tls-00.txt   draft-ietf-acme-email-tls-01.txt 
Network Working Group A. Melnikov Network Working Group A. Melnikov
Internet-Draft Isode Ltd Internet-Draft Isode Ltd
Intended status: Informational June 18, 2017 Intended status: Informational October 28, 2017
Expires: December 20, 2017 Expires: May 1, 2018
Extensions to Automatic Certificate Management Environment for email TLS Extensions to Automatic Certificate Management Environment for email TLS
draft-ietf-acme-email-tls-00 draft-ietf-acme-email-tls-01
Abstract Abstract
This document specifies identifiers and challenges required to enable This document specifies identifiers and challenges required to enable
the Automated Certificate Management Environment (ACME) to issue the Automated Certificate Management Environment (ACME) to issue
certificates for use by TLS email services. certificates for use by TLS email services.
Status of This Memo Status of This Memo
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provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 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. Conventions Used in This Document . . . . . . . . . . . . . . 2 2. Conventions Used in This Document . . . . . . . . . . . . . . 2
3. Use of ACME for use by TLS-protected SMTP and IMAP services . 2 3. Use of ACME for use by TLS-protected SMTP and IMAP services . 2
3.1. "service" JWS header parameter . . . . . . . . . . . . . 3 3.1. "service" JWS header parameter . . . . . . . . . . . . . 3
3.2. "port" JWS header parameter . . . . . . . . . . . . . . . 3 3.2. "port" JWS header parameter . . . . . . . . . . . . . . . 3
3.3. TLS with Server Name Indication (SNI) challenge for email 3.3. DNS challenge for email services . . . . . . . . . . . . 4
services . . . . . . . . . . . . . . . . . . . . . . . . 4 3.4. CAPABILITY challenge for email services . . . . . . . . . 4
3.4. DNS challenge for email services . . . . . . . . . . . . 4 4. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.5. CAPABILITY challenge for email services . . . . . . . . . 5 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
4. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 6 6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Normative References . . . . . . . . . . . . . . . . . . . . 6 7. Normative References . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction 1. Introduction
[I-D.ietf-acme-acme] is a mechanism for automating certificate [I-D.ietf-acme-acme] is a mechanism for automating certificate
management on the Internet. It enables administrative entities to management on the Internet. It enables administrative entities to
prove effective control over resources like domain names, and prove effective control over resources like domain names, and
automates the process of generating and issuing certificates. automates the process of generating and issuing certificates.
This document describes extensions to ACME for use by email services. This document describes extensions to ACME for use by email services.
Section 3 defines extensions for how email services (such as SMTP, Section 3 defines extensions for how email services (such as SMTP,
IMAP) can get certificates for use with TLS. IMAP) can get certificates for use with TLS.
2. Conventions Used in This Document 2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. Use of ACME for use by TLS-protected SMTP and IMAP services 3. Use of ACME for use by TLS-protected SMTP and IMAP services
SMTP [RFC5321] (including SMTP submission) and IMAP [RFC3501] servers SMTP [RFC5321] (including SMTP Submission [RFC6409]) and IMAP
use TLS to provide server identity authentication, data [RFC3501] servers use TLS [RFC5246] to provide server identity
confidentiality and integrity services. Such TLS protected email authentication, data confidentiality and integrity services. Such
services either use STARTTLS command or run on a separate TLS- TLS protected email services either use STARTTLS command or run on a
protected port. separate TLS-protected port .
[I-D.ietf-acme-acme] defines several challenge types that can be [I-D.ietf-acme-acme] defines several challenge types that can be
extended for use by email services. This document also defines some extended for use by email services. This document also defines some
new challenge types specific to SMTP and IMAP. new challenge types specific to SMTP and IMAP.
In order to use these challenges JWS [RFC7515] object used by In order to use these challenges JWS [RFC7515] object used by
[I-D.ietf-acme-acme] is extended. The following extra requirements [I-D.ietf-acme-acme] is extended. The following extra requirements
are in addition to requirements on JWS objects sent in ACME defined are in addition to requirements on JWS objects sent in ACME defined
in Section 6.2 of [I-D.ietf-acme-acme]: in Section 6.2 of [I-D.ietf-acme-acme]:
1. "service" JWS header parameter MUST be included. See Section 3.1 1. "service" JWS header parameter MUST be included. See Section 3.1
for more details. for more details.
2. "port" JWS header parameter MUST (SHOULD?) be included. See 2. "port" JWS header parameter MUST (SHOULD?) be included. See
Section 3.2 for more details. Section 3.2 for more details.
For example, if the client were to respond to the "tls-sni-email-00" For example, if the client were to respond to the "dns-email-00"
challenge, it would send the following request: challenge, it would send the following request:
POST /acme/authz/asdf/0 HTTP/1.1 POST /acme/authz/asdf/0 HTTP/1.1
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": "Q_s3MWoqT05TrdkM2MTDcw", "nonce": "Q_s3MWoqT05TrdkM2MTDcw",
"url": "https://example.com/acme/authz/asdf/0", "url": "https://example.com/acme/authz/asdf/0",
"service": "smtp", "service": "smtp",
"port": 25 "port": 25
}), }),
"payload": base64url({ "payload": base64url({
"type": "tls-sni-email-00", "type": "dns-email-00",
"keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggQiE" "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggQiE"
}), }),
"signature": "7cbg5JO1Gf5YLjjF...SpkUfcdPai9uVYYU" "signature": "7cbg5JO1Gf5YLjjF...SpkUfcdPai9uVYYU"
} }
Figure 1 Figure 1
3.1. "service" JWS header parameter 3.1. "service" JWS header parameter
The "service" JWS header parameter specifies the service for which The "service" JWS header parameter specifies the service for which
TLS server certificate should be issued. Valid values come from TLS server certificate should be issued. Valid values come from
"Service Names and Transport Protocol Port Numbers" IANA registry "Service Names and Transport Protocol Port Numbers" IANA registry
<https://www.iana.org/assignments/service-names-port-numbers/service- <https://www.iana.org/assignments/service-names-port-numbers/service-
names-port-numbers.xhtml>. ACME server MAY include SRV-ID [RFC6125] names-port-numbers.xhtml>. ACME server MAY include SRV-ID [RFC6125]
subjectAltNames in issued certificates. subjectAltNames in issued certificates.
[[This parameter might have applicability beyond email services.]]
3.2. "port" JWS header parameter 3.2. "port" JWS header parameter
The "port" JWS header parameter specifies the TCP port number where The "port" JWS header parameter specifies the TCP port number where
the corresponding service is running. the corresponding service is running.
[[This parameter might have applicability beyond email services.]] [[This parameter might have applicability beyond email services.]]
3.3. TLS with Server Name Indication (SNI) challenge for email services 3.3. DNS challenge for email services
"tls-sni-email-00" is very similar to "tls-sni-01" defined in
Section 8.3 of [I-D.ietf-acme-acme].
The difference between processing of "tls-sni-email-00" and "tls-sni-
01" are listed below:
1. SAN A MUST be constructed as follows: compute the SHA-256 digest
[FIPS180-4] of the challenge token and encode it in lowercase
hexadecimal form. The dNSName is "<x>.<y>.<token>.acme.invalid",
where <x> is the first half of the hexadecimal representation and
<y> is the second half, and <token> was generated by the ACME
server. SAN B MUST be constructed as follows: compute the
SHA-256 digest of the key authorization and encode it in
lowercase hexadecimal form. The dNSName is
"<x>.<y>.<ka>.acme.invalid" where <x> is the first half of the
hexadecimal representation and <x> is the second half, and <ka>
is the key authorization. [[OPEN ISSUE: Should service name and
port number be incorporated into SAN A and B?]]
2. When verifying the client's control of the domain/service, ACME
server connects to port as specified in "port" JWS header
parameter (Section 3.2), instead of port 443. When connecting to
ports 25, 143 and 587, ACME server needs to use STARTTLS command.
When connecting to ports 465 or 993, ACME server initiate TLS
negotiation immediately upon connection to the corresponding
ports. In all cases ACME server presents SAN A in the SNI field,
constructed as specified above.
3.4. DNS challenge for email services
"dns-email-00" is very similar to "dns-01" defined in Section 8.4 of "dns-email-00" is very similar to "dns-01" defined in Section 8.4 of
[I-D.ietf-acme-acme]. [I-D.ietf-acme-acme].
The difference between processing of "dns-email-00" and "dns-01" are The difference between processing of "dns-email-00" and "dns-01" are
listed below: listed below:
1. The TXT record used to validate this challenge is 1. The TXT record used to validate this challenge is
_<port>._<service>_acme-challenge.<domain>. For example, for _<port>._<service>_acme-challenge.<domain>. For example, for
domain "example.com" and IMAP service running on port 993, the domain "example.com" and IMAP service running on port 993, the
TXT record name is _993._imaps._acme-challenge.example.com. For TXT record name is _993._imaps._acme-challenge.example.com. For
domain "example.net" and IMAP service running on port 143, the domain "example.net" and IMAP service running on port 143, the
TXT record name is _143._imap._acme-challenge.example.next. TXT record name is _143._imap._acme-challenge.example.next.
2. [[OPEN ISSUE: Should service name and port number be incorporated 2. [[OPEN ISSUE: Should service name and port number be incorporated
into the hash?]] into the hash?]]
3.5. CAPABILITY challenge for email services 3.4. CAPABILITY challenge for email services
For "capability-smtp-00" challenge, ACME client (== SMTP server) For "capability-smtp-00" challenge, ACME client (== SMTP server)
constructs a key authorization from the "token" value provided in the constructs a key authorization from the "token" value provided in the
challenge and the client's account key. The client then computes the challenge and the client's account key. The client then computes the
SHA-256 digest [FIPS180-4] of the key authorization. SMTP server SHA-256 digest [FIPS180-4] of the key authorization. SMTP server
than returns the base64url encoding of this digest as a value of the than returns the base64url encoding of this digest as a value of the
"ACME" EHLO capability. For example: "ACME" EHLO capability. For example:
250-smtp.example.com 250-smtp.example.com
250-SIZE 250-SIZE
skipping to change at page 5, line 32 skipping to change at page 4, line 47
250-HELP 250-HELP
250-DSN 250-DSN
250-CHUNKING 250-CHUNKING
250-AUTH SCRAM-SHA-1 250-AUTH SCRAM-SHA-1
250-AUTH=SCRAM-SHA-1 250-AUTH=SCRAM-SHA-1
250-STARTTLS 250-STARTTLS
250-ACME gfj9Xq...Rg85nM 250-ACME gfj9Xq...Rg85nM
250-MT-PRIORITY 250-MT-PRIORITY
250 ENHANCEDSTATUSCODES 250 ENHANCEDSTATUSCODES
Note that for this document only ACME capability is relevant. Note that in the above example only presence of the ACME and possibly
STARTTLS capabilities is relevant as far as this document is
concerned.
Figure 2 Figure 2
Similarly, "capability-imap-00" challenge, ACME client (== IMAP Similarly, "capability-imap-00" challenge, ACME client (== IMAP
server) constructs a key authorization from the "token" value server) constructs a key authorization from the "token" value
provided in the challenge and the client's account key. The client provided in the challenge and the client's account key. The client
then computes the SHA-256 digest [FIPS180-4] of the key then computes the SHA-256 digest [FIPS180-4] of the key
authorization. SMTP server than returns the base64url encoding of authorization. SMTP server than returns the base64url encoding of
this digest as a value of the "ACME" capability: this digest as a value of the "ACME" capability:
* OK [CAPABILITY IMAP4rev1 LOGINDISABLED LITERAL+ ENABLE STARTTLS ACME=gfj9Xq...Rg85nM] Example IMAP4rev1 server ready * OK [CAPABILITY IMAP4rev1 LOGINDISABLED LITERAL+ ENABLE STARTTLS ACME=gfj9Xq...Rg85nM] Example IMAP4rev1 server ready
or or
* CAPABILITY IMAP4rev1 LOGINDISABLED LITERAL+ ENABLE STARTTLS ACME=gfj9Xq...Rg85nM * CAPABILITY IMAP4rev1 LOGINDISABLED LITERAL+ ENABLE STARTTLS ACME=gfj9Xq...Rg85nM
Note that for this document only ACME capability is relevant. Note that in the above example only presence of the ACME and possibly
STARTTLS capabilities is relevant as far as this document is
concerned.
Figure 3 Figure 3
4. Open Issues 4. Open Issues
[[This section should be empty before publication]] [[This section should be empty before publication]]
1. Add support for LMTP? 1. Should the same certificate be allowed to be used on both IMAP
(143) and IMAPS (993) ports?
2. One possible alternative for issuing TLS certificates for email 2. Add support for LMTP?
3. One possible alternative for issuing TLS certificates for email
services is to define a new Identifier Type that specifies services is to define a new Identifier Type that specifies
service@domain. The current version of the document just reuses service@domain. The current version of the document just reuses
"dns". "dns".
5. IANA Considerations 5. IANA Considerations
IANA is requested to register the following ACME challenge types that IANA is requested to register the following ACME challenge types that
are used with Identifier Type "dns": "tls-sni-email", "dns-email", are used with Identifier Type "dns": "dns-email", "capability-smtp"
"capability-smtp" and "capability-imap". The reference for all of and "capability-imap". The reference for all of them is this
them is this document. document.
6. Security Considerations 6. Security Considerations
TBD. TBD.
7. Normative References 7. Normative References
[I-D.ietf-acme-acme] [I-D.ietf-acme-acme]
Barnes, R., Hoffman-Andrews, J., and J. Kasten, "Automatic Barnes, R., Hoffman-Andrews, J., and J. Kasten, "Automatic
Certificate Management Environment (ACME)", draft-ietf- Certificate Management Environment (ACME)", draft-ietf-
acme-acme-06 (work in progress), March 2017. acme-acme-06 (work in progress), March 2017.
[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,
<http://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<http://www.rfc-editor.org/info/rfc3501>. <https://www.rfc-editor.org/info/rfc3501>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008, DOI 10.17487/RFC5321, October 2008,
<http://www.rfc-editor.org/info/rfc5321>. <https://www.rfc-editor.org/info/rfc5321>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509 within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer (PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <http://www.rfc-editor.org/info/rfc6125>. 2011, <https://www.rfc-editor.org/info/rfc6125>.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
<https://www.rfc-editor.org/info/rfc6409>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web [RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <http://www.rfc-editor.org/info/rfc7515>. 2015, <https://www.rfc-editor.org/info/rfc7515>.
Author's Address Author's Address
Alexey Melnikov Alexey Melnikov
Isode Ltd Isode Ltd
14 Castle Mews 14 Castle Mews
Hampton, Middlesex TW12 2NP Hampton, Middlesex TW12 2NP
UK UK
EMail: Alexey.Melnikov@isode.com EMail: Alexey.Melnikov@isode.com
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