Extensions to Automatic Certificate Management Environment for end-user S/MIME certificates

ACME defines a "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 or Internationalized Email addresses . (When Internationalized Email addresses are used, both U-labels and A-labels 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: An end-user initiates issuance of an S/MIME certificate for one of her email addresses. This might be done using email client UI (and might use some HTTP API underneath), by visiting a Certificate Authority web page or by sending an email to a well known Certificate Authority's email address. This document doesn't prescribe how exactly S/MIME certificate issuance is initiated. The ACME server (run by the Certificate Authority or their authorized third party) generates a "challenge" email message with the subject "ACME: <token-part1>", where <token-part1> is the base64url encoded first part of the token, which contains at least 64 bits of entropy. (ACME server MUST generate token afresh for each S/MIME issuance request.) The challenge email message structure is described in more details in . The second part of the token (token-part2, which also contains at least 64 bits of entropy) is returned over HTTPS to the ACME client. The ACME client concatenates "token-part1" and "token-part2" to create "token", calculates keyAuthorization (as per Section 8.1 of ), then includes the base64url encoded SHA-256 digest of the key authorization in the body of a response email message containing a single text/plain MIME body part . The response email message structure is described in more details in . For an identifier of type "email", the PKCS#10 Certificate Signing Request (CSR) MUST contain the requested email address in an extensionRequest attribute requesting a subjectAltName extension.

A "challenge" email message MUST have the following structure: The message Subject header field has the following syntax: "ACME: <token-part1>", where the prefix "ACME:" is followed by folding white space (FWS, see ) and then by <token-part1>, which is the base64url encoded first part of the ACME token that MUST be at least 64 bits long after decoding. Due to the recommended 78-octet line length limit in , the subject line can be folded, so whitespaces (if any) within the <token-part1> MUST be ignored. encoding of the message Subject header field MUST be supported, but when used, only "UTF-8" and "US-ASCII" charsets MUST be used (i.e. other charsets MUST NOT be used). The To header field MUST be the email address of the entity that requested the S/MIME certificate to be generated. The message MAY contain a Reply-To header field. The message MUST include the "Auto-Submitted: auto-generated" header field . The "Auto-Submitted" header field SHOULD include the "type=acme" parameter. It MAY include other optional parameters as allowed by the syntax of the Auto-Submitted header field. In order to prove authenticity of a challenge message, it MUST be either DKIM signed or S/MIME 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 , which implies DKIM and SPF validation . 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 a human-readable explanation of the purpose of the message. If S/MIME signing is used to prove authenticity of the challenge message, then the multipart/signed or "application/pkcs7-mime; smime-type=signed-data;" media type should be used. Either way, it MUST use S/MIME header protection.

An example ACME "challenge" email (note that DKIM related header fields are not included for simplicity). From: acme-generator@example.org To: alexey@example.com Subject: ACME: 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. ]]>

A valid "response" email message MUST have the following structure: 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 preceded and followed by folding white space (FWS, see ) 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 bits long after decoding. Due to recommended 78 octet line length limit in , the subject line can be folded, so whitespaces (if any) within the <token-part1> MUST be ignored. encoding of the Subject header field MUST be supported, but when used, only "UTF-8" and "US-ASCII" charsets MUST be used (i.e. other charsets MUST NOT be used). When parsing subjects, ACME servers must decode encoding (if any) and then they can ignore any prefix before the "ACME:" label. The From: header field contains the email address of the user that is requesting S/MIME certificate issuance. 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. The Cc: header field is ignored if present in the "response" email message. 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 . List-* header fields MUST be absent (i.e., the reply can't come from a mailing list) The 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 the base64url-encoded SHA-256 digest of the key authorization, calculated from concatenated token-part1 (received over email) and token-part2 (received over HTTPS). See the 3rd bullet point in for more details. (Note that due to historical 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 a 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. 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. In order to prove authenticity of a response message, it MUST be DKIM 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. The message MUST also pass DMARC validation , which implies DKIM and SPF validation .

Example ACME "response" email (note that DKIM related header fields are not included for simplicity). From: alexey@example.com To: acme-generator@example.org Subject: Re: ACME: Content-Type: text/plain MIME-Version: 1.0 -----BEGIN ACME RESPONSE----- LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowy jxAjEuX0.9jg46WB3rR_AHD-EBXdN7cBkH1WOu0tA3M9 fm21mqTI -----END ACME RESPONSE----- ]]>

Please see Security Considerations of 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. I.e. ACME server is only vouching that the requested email address seem to belong to the entity that requested the certificate. The security of the "email-reply-00" challenge type depends on the security of the 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 a user's behalf using email delegation feature common in some email systems) can request S/MIME certificates using the protocol specified in this document and is indistinguishable from the email account owner. This has several possible implications: an entity that compromised an email account would be able to request S/MIME certificates using the protocol specified in this document and such entity couldn't be distinguished from the legitimate email account owner (unless some external sources of information are consulted); for email addresses with legitimate shared access/control by multiple users, any such user would be able to request S/MIME certificates using the protocol specified in this document and such requests can't be attributed to a specific user without consulting external systems (such as IMAP/SMTP access logs); protocol specified in this document is not suitable for use with email addresses associated with mailing lists . An 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.