Extensions to Automatic Certificate Management Environment for end user S/MIME certificates
draft-ietf-acme-email-smime-08

Abstract

This document specifies identifiers and challenges required to enable the Automated Certificate Management Environment (ACME) to issue certificates for use by email users that want to use S/MIME.

Status of This Memo

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Copyright Notice

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Table of Contents

• 1. Introduction
• 2. Conventions Used in This Document
• 3. Use of ACME for issuing end user S/MIME certificates
• 3.1. ACME challenge email
• 3.2. ACME response email
• 4. Internationalization Considerations
• 5. IANA Considerations
• 5.1. ACME Identifier Type
• 5.2. ACME Challenge Type
• 6. Security Considerations
• 7. Normative References
• Appendix A. Acknowledgements
• Author's Address

1. Introduction

ACME [RFC8555] is a mechanism for automating certificate management on the Internet. It enables administrative entities to prove effective control over resources like domain names, and automates the process of generating and issuing certificates.

This document describes an extension to ACME for use by S/MIME. Section 3 defines extensions for issuing end user S/MIME [RFC8550] certificates.

2. Conventions Used in This Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

3. Use of ACME for issuing end user S/MIME certificates

ACME [RFC8555] defines "dns" Identifier Type that is used to verify that a particular entity has control over a domain or specific service associated with the domain. In order to be able to issue end-user S/MIME certificates, ACME needs a new Identifier Type that proves ownership of an email address.

This document defines a new Identifier Type "email" which corresponds to an (all ASCII) email address [RFC5321] or Internationalized Email addresses [RFC6531]. (When Internationalized Email addresses are used, both U-labels and A-labels [RFC5890] are allowed in the domain part.) This can be used with S/MIME or other similar service that requires possession of a certificate tied to an email address.

Any identifier of type "email" in a newOrder request MUST NOT have a wildcard ("*") character in its value.

A new challenge type "email-reply-00" is used with "email" Identifier Type, which provides proof that an ACME client has control over an email address:

1. ACME server generates a "challenge" email message with the subject "ACME: <token-part1>", where <token-part1> is the base64url encoded [RFC4648] first part of the token, which contains at least 64 bit of entropy. The challenge email message structure is described in more details in Section 3.1. The second part of the token (token-part2, which also contains at least 64 bit of entropy) is returned over HTTPS [RFC2818] to the ACME client.
2. ACME client concatenates "token-part1" and "token-part2" to create "token", calculates key-authz (as per Section 8.1 of [RFC8555]), then includes the base64url encoded SHA-256 digest [FIPS180-4] of the key authorization in the body of a response email message containing a single text/plain MIME body part [RFC2045]. The response email message structure is described in more details in Section 3.2

For an identifier of type "email", CSR MUST contain the request email address in an extensionRequest attribute [RFC2985] requesting a subjectAltName extension.

3.1. ACME challenge email

A "challenge" email message MUST have the following structure:

1. The message Subject header field has the following syntax: "ACME: <token-part1>", where the prefix "ACME:" is followed by folding white space (FWS, see [RFC5322]) and then by <token-part1> is the base64url encoded first part of the ACME token that MUST be at least 64 bit long after decoding. Due to recommended 78 octet line length limit in [RFC5322], the subject line can be folded, so whitespaces (if any) within the <token-part1> MUST be ignored. [RFC2231] encoding of subject MUST be supported, but when used, only "UTF-8" and "US-ASCII" charsets MUST be used (i.e. other charsets MUST NOT be used).
2. The To header field MUST be the email address of the entity that requested S/MIME certificate to be generated.
3. The message MAY contain Reply-To header field.
4. The message MUST include the "Auto-Submitted: auto-generated" header field [RFC3834]. The "Auto-Submitted" header field SHOULD include "type=acme" parameter. It MAY include other optional parameters as allowed by syntax of Auto-Submitted header field.
5. The message MAY contain Reply-To header field.
6. In order to prove authenticity of a challenge message, it MUST be either DKIM [RFC6376] signed or S/MIME [RFC8551] signed. If DKIM signing is used, the resulting DKIM-Signature header field MUST contain the "h=" tag that includes at least "From", "Sender", "Reply-To", "To", "CC", "Subject", "Date", "In-Reply-To", "References", "Message-ID", "Content-Type" and "Content-Transfer-Encoding" header fields. The message MUST also pass DMARC validation [RFC7489], which implies DKIM and SPF validation [RFC7208].
7. The body of the challenge message is not used for automated processing, so it can be any media type. (However there are extra requirements on S/MIME signing, if used. See below.) Typically it is text/plain or text/html containing human readable explanation of the purpose of the message. If S/MIME signing is used to prove authenticity of the challenge message, then multipart/signed or "application/pkcs7-mime; smime-type=signed-data;" media type should be used. Either way, it MUST use S/MIME header protection.

Example ACME "challenge" email (note that DKIM related header fields are not included for simplicity).

  
  Auto-Submitted: auto-generated; type=acme
  Date: Sat, 1 Sep 2018 10:08:55 +0100
  Message-ID: <A2299BB.FF7788@example.org>
  From: acme-generator@example.org
  To: alexey@example.com
  Subject: ACME: <base64url-encoded-token-with-64-bits-of-entropy>
  Content-Type: text/plain
  MIME-Version: 1.0
   
  This is an automatically generated ACME challenge for email address
  "alexey@example.com". If you haven't requested an S/MIME
  certificate generation for this email address, be very afraid.
  If you did request it, your email client might be able to process
  this request automatically, or you might have to paste the first
  token part into an external program.

Figure 1

3.2. ACME response email

A "response" email message MUST have the following structure:

1. The message Subject header field has the following syntax: "<Reply-prefix> ACME: <token-part1>", where <Reply-prefix> is typically the reply prefix "Re: " and the string "ACME:" is followed by folding white space (FWS, see [RFC5322]) and then by <token-part1>. <token-part1> is the base64url encoded first part of the ACME token (as received in the ACME challenge) that MUST be at least 64 bit long after decoding. Due to recommended 78 octet line length limit in [RFC5322], the subject line can be folded, so whitespaces (if any) within the <token-part1> MUST be ignored. [RFC2231] encoding of subject MUST be supported, but when used, only "UTF-8" and "US-ASCII" charsets MUST be used (i.e. other charsets MUST NOT be used).
2. The From: header field contains the email address of the user that is requesting S/MIME certificate issuance.
3. The To: header field of the response contains the value from the Reply-To: header field from the challenge message (if set) or from the From: header field of the challenge message otherwise.
4. The Cc: header field is ignored if present in the "response" email message.
5. The In-Reply-To: header field SHOULD be set to the Message-ID header field of the challenge message according to rules in Section 3.6.4 of [RFC5322].
6. Media type of the "response" email message is either text/plain or multipart/alternative containing text/plain as one of the alternatives. The text/plain body part (whether or not it is inside multipart/alternative) MUST contain a block of lines starting with the line "-----BEGIN ACME RESPONSE-----", followed by one or more line containing base64url encoded SHA-256 digest [FIPS180-4] of the key authorization, calculated based on token-part1 (received over email) and token-part2 (received over HTTPS). (Note that due to historic line length limitations in email, line endings (CRLFs) can be freely inserted in the middle of the encoded digest, so they MUST be ignored when processing it.). The final line of the encoded digest is followed by the line containing "-----END ACME RESPONSE-----". Any text before and after this block is ignored. For example such text might explain what to do with it for ACME-unaware clients.
7. There is no need to use any Content-Transfer-Encoding other than 7bit for the text/plain body part, however use of Quoted-Printable or base64 is not prohibited in a "response" email message.
8. In order to prove authenticity of a response message, it MUST be DKIM [RFC6376] signed. The resulting DKIM-Signature header field MUST contain the "h=" tag that includes at least "From", "Sender", "Reply-To", "To", "CC", "Subject", "Date", "In-Reply-To", "References", "Message-ID", "Content-Type" and "Content-Transfer-Encoding" header fields.

Example ACME "response" email (note that DKIM related header fields are not included for simplicity).

  
   Date: Sat, 1 Sep 2018 11:12:00 +0100
   Message-ID: <111-22222-3333333@example.com>
   From: alexey@example.com
   To: acme-generator@example.org
   Subject: Re: ACME: <base64url-encoded-token-with-enough-entropy>
   Content-Type: text/plain
   MIME-Version: 1.0
   
   -----BEGIN ACME RESPONSE-----
   LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowy
   jxAjEuX0.9jg46WB3rR_AHD-EBXdN7cBkH1WOu0tA3M9
   fm21mqTI
   -----END ACME RESPONSE-----

Figure 2

4. Internationalization Considerations

[RFC8616] updated/clarified use of DKIM/SPF/DMARC with Internationalized Email addresses [RFC6531]. Please consult RFC 8616 in regards to any changes that need to be implemented.

Use of non ASCII characters in left hand sides of Internationalized Email addresses requires putting Internationalized Email Addresses in X.509 Certificates [RFC8398].

5. IANA Considerations

5.1. ACME Identifier Type

IANA is requested to register a new Identifier type in the "ACME Identifier Types" registry defined in Section 9.7.7 of [RFC8555] with Label "email" and a Reference to [RFCXXXX], [RFC5321] and [RFC6531]. The new Identifier Type corresponds to an (all ASCII) email address [RFC5321] or Internationalized Email addresses [RFC6531].

5.2. ACME Challenge Type

IANA is also requested to register a new entry in the "ACME Validation Methods" registry defined in Section 9.7.8 of [RFC8555]. This entry is as follows:

6. Security Considerations

Please see Security Considerations of [RFC8555] for general security considerations related to use of ACME. This challenge/response protocol demonstrates that an entity that controls the private key (corresponding to the public key in the certificate) also controls the named email account. Any claims about the correctness or fitness-for-purpose of the email address must be otherwise assured.

Security of "email-reply-00" challenge type depends on security of email system. A third party that can read and reply to user's email messages (by possessing a user's password or a secret derived from it that can give read and reply access, such as "password equivalent" information; or by being given permissions to act on user's behalf using email delegation feature common in some email systems) can request S/MIME certificates and is indistinguishable from the email account owner.

Email system in its turn depends on DNS. A third party that can manipulate DNS MX records for a domain might be able to redirect email and can get (at least temporary) read and reply access to it. Similar considerations apply to SPF and DMARC TXT records in DNS. Use of DNSSEC by email system administrators is recommended to avoid easy spoofing of DNS records affecting email system.

7. Normative References

Appendix A. Acknowledgements

Thank you to Andreas Schulze, Gerd v. Egidy, James A Baker, Ben Schwartz and Michael Jenkins for suggestions, comments and corrections on this document.

Author's Address