RFC 9598

Internationalized Email Addresses in X.509 Certificates

📅 published May 2024, from ietf

RFC 9598: Internationalized Email Addresses in X.509 Certificates 💌

Okay bestie, RFC 9598 is a Standards Track doc from the IETF, dated May 2024, and it's giving main character energy 👑. Its ISSN is 2070-1721. The authors are A. Melnikov (Isode Ltd), W. Chuang (Google, Inc.), and C. Bonnell (DigiCert). It obsoletes RFC 8398 and updates RFC 5280 🫶.

Table of Contents

   1.  Introduction
   2.  Conventions Used in This Document
   3.  Name Definitions
   4.  IDNA2008
   5.  Matching of Internationalized Email Addresses in X.509
           Certificates
   6.  Name Constraints in Path Validation
   7.  Security Considerations
   8.  Differences from RFC 8398
   9.  IANA Considerations
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Appendix A.  ASN.1 Module
   Appendix B.  Example of SmtpUTF8Mailbox
   Acknowledgments
   Authors' Addresses

Abstract ✨

This doc defines a new name form for the otherName field of an X.509 Subject Alternative Name and Issuer Alternative Name extension. It lets a certificate subject be linked to an internationalized email address 💖. This document updates RFC 5280 and obsoletes RFC 8398.

Status of This Memo 📋

This is an Internet Standards Track document. It's a product of the Internet Engineering Task Force (IETF), it represents the consensus of the IETF community, it got public review, and the Internet Engineering Steering Group (IESG) approved it for publication. You can find more about Internet Standards in Section 2 of RFC 7841.

For info on the current status of this document, any errata, and how to send feedback, check https://www.rfc-editor.org/info/rfc9598 🦋.

Copyright Notice 🌈

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) as in effect on the date of publication. Please review these documents carefully, because they spell out your rights and restrictions for this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions, and they're provided without warranty as described in the Revised BSD License 🙈.

1. Introduction 🌸

Okay so [RFC5280] gives us the rfc822Name subjectAltName name type for representing email addresses as described in [RFC5321]. The syntax of rfc822Name is limited to a subset of US-ASCII characters, so it can't represent internationalized email addresses [RFC6531] 😭. This document defines a new otherName variant to represent internationalized email addresses. On top of that, this document requires all email address domains in X.509 certificates to conform to IDNA2008 [RFC5890] 💅.

This document obsoletes [RFC8398]. The main goal here is to simplify how domain labels are encoded in the domain part of internationalized email addresses. In particular, [RFC8398] says domain labels are conditionally encoded using either A-labels or U-labels. This spec makes encoding and processing simpler by requiring the A-label representation in all cases 🎀.

2. Conventions Used in This Document 📜

Here's the key-words rule, bestie: 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. Name Definitions 🦋

The GeneralName structure [RFC5280] supports lots of different name forms, including otherName for extensibility. This section specifies the SmtpUTF8Mailbox name form of otherName, so internationalized email addresses can show up in the subjectAltName of a certificate, the issuerAltName of a certificate, or anywhere else GeneralName gets used ✨.

   id-on-SmtpUTF8Mailbox OBJECT IDENTIFIER ::= { id-on 9 }

   SmtpUTF8Mailbox ::= UTF8String (SIZE (1..MAX))
   -- SmtpUTF8Mailbox conforms to Mailbox as specified
   -- in Section 3.3 of RFC 6531. Additionally, all domain
   -- labels included in the SmtpUTF8Mailbox value are
   -- encoded as LDH labels. In particular, domain labels
   -- are not encoded as U-labels and instead are encoded
   -- using their A-label representation.

When the subjectAltName (or issuerAltName) extension has an internationalized email address with a non-ASCII Local-part, the address MUST be stored in the SmtpUTF8Mailbox name form of otherName 🧚.

SmtpUTF8Mailbox is a tweaked version of the internationalized Mailbox from Section 3.3 of [RFC6531], which itself came from Mailbox in Section 4.1.2 of [RFC5321]. [RFC6531] lays out ABNF rules for Mailbox whose parts got modified for internationalization: Local-part, Dot-string, Quoted-string, QcontentSMTP, Domain, and Atom. Local-part was updated to also support UTF8-non-ascii, which is described in Section 3.1 of [RFC6532]. Also, domain got extended to support U-labels, as defined in [RFC5890] 💜.

This document refines the internationalized Mailbox ABNF rules as described in [RFC6531] and calls the result SmtpUTF8Mailbox. In SmtpUTF8Mailbox, labels with non-ASCII characters MUST be stored in A-label (rather than U-label) form [RFC5890]. This restriction cuts complexity for implementations of the certification path validation algorithm in Section 6 of [RFC5280]. In SmtpUTF8Mailbox, domain labels that only use ASCII characters (meaning neither A- nor U-labels) SHALL use NR-LDH restrictions as specified in Section 2.3.1 of [RFC5890]. NR-LDH stands for "Non-Reserved Letters Digits Hyphen," and it's the set of LDH labels that don't have "--" characters in the third and forth character positions, which excludes "tagged domain names" like A-labels. To make octet-for-octet comparisons of SmtpUTF8Mailbox values easier, all NR-LDH and A-label labels that make up the domain part SHALL only be encoded with lowercase letters 🌈. Just like the treatment of rfc822Name in [RFC5280], SmtpUTF8Mailbox is an envelope Mailbox. It has no phrase (like a common name) before it, no comment (text in parentheses) after it, and it isn't surrounded by "<" and ">" characters.

Because of the name constraint compatibility reasons in Section 6, a SmtpUTF8Mailbox subjectAltName MUST NOT be used unless the Local-part of the email address has non-ASCII characters. When the Local-part is ASCII, an rfc822Name subjectAltName MUST be used instead of SmtpUTF8Mailbox. That keeps legacy software that only supports rfc822Name happy 🥹. Table 1 below sums up how to use rfc822Name and SmtpUTF8Mailbox.

                   +=================+=================+
                   | Local-part char | subjectAltName  |
                   +=================+=================+
                   | ASCII-only      | rfc822Name      |
                   +-----------------+-----------------+
                   | non-ASCII       | SmtpUTF8Mailbox |
                   +-----------------+-----------------+

                     Table 1: Email Address Formatting

SmtpUTF8Mailbox is encoded as UTF8String. The UTF8String encoding MUST NOT contain a Byte Order Mark (BOM) [RFC3629], so values stay consistent across implementations, especially for comparison 🫧.

Non-ASCII Local-part values may also include ASCII characters.

4. IDNA2008 🧚

To make comparing email addresses easier, all email address domains in X.509 certificates MUST conform to IDNA2008 [RFC5890] (and avoid any "mappings" mentioned in that document). Using non-conforming email address domains opens the door to conversion errors between alternate forms. This applies to SmtpUTF8Mailbox and rfc822Name in subjectAltName, issuerAltName, and anywhere else these show up 🌸.

5. Matching of Internationalized Email Addresses in X.509 Certificates 💜

Equivalence comparisons with SmtpUTF8Mailbox have two steps: a domain part step and a Local-part step. The comparison form for Local-parts is always UTF-8. The comparison form for domain parts is always the LDH label encoding ([RFC5890]) of the relevant domain labels. Comparing LDH labels in domain parts makes things simpler for the certification path validation algorithm in Section 6 of [RFC5280], because there's no need to convert domain labels back to Unicode 🦋.

Comparing two SmtpUTF8Mailboxes is straightforward, with no setup work needed. They're considered equivalent if there's an exact octet-for-octet match 💖.

Comparing an SmtpUTF8Mailbox with an rfc822Name always fails 😤. SmtpUTF8Mailbox values SHALL contain a Local-part with one or more non-ASCII characters, while rfc822Names only include ASCII characters (Local-part included). So an SmtpUTF8Mailbox and an rfc822Name will never match.

Comparing SmtpUTF8Mailbox values with internationalized email addresses from other sources (like received email messages, user input, and so on) needs extra setup steps for the domain part and the Local-part. To prep the address you're comparing against the SmtpUTF8Mailbox value, first remove any phrases, comments, and "<" or ">" characters ✨.

For the setup of the domain part, the following conversions SHALL be performed:

Convert all labels that make up the domain part and include non-ASCII characters to A-labels, if they aren't already in that form. a. Detect all U-labels present within the domain part using Section 5.1 of [RFC5891]. b. Transform all detected U-labels (Unicode) to A-labels (ASCII) as specified in Section 5.5 of [RFC5891].
Convert all uppercase letters found within the NR-LDH and A-label labels that make up the domain part to lowercase letters.

For the setup of the Local-part, the Local-part MUST be verified to conform to the requirements of [RFC6530] and [RFC6531], including being a string in UTF-8 form. Most importantly, the Local-part MUST NOT be transformed in any way, like through case folding or normalization of any kind 🙅‍♀️. The Local-part of an internationalized email address is already UTF-8. Once setup is done, the two are compared octet for octet again.

To summarize non-normatively, the comparison steps, setup included, are:

If the domain contains U-labels, transform them to A-labels.
If any NR-LDH or A-label domain label in the domain part contains uppercase letters, lowercase them.
Compare strings octet for octet for equivalence.

This specification expressly does not define any wildcard characters, so SmtpUTF8Mailbox comparison implementations MUST NOT interpret any characters as wildcards 🚫. Instead, to specify multiple email addresses through SmtpUTF8Mailbox, the certificate MUST use multiple subjectAltNames or issuerAltNames to explicitly carry each additional email address 🫶.

6. Name Constraints in Path Validation ✨

6.1. Hot Take: rfc822Name Constraints Get a Glow-Up 💅

okay so this section updates Section 4.2.1.10 of [RFC5280] to extend rfc822Name name constraints to SmtpUTF8Mailbox subjectAltNames 🦋 basically, SmtpUTF8Mailbox-aware path validators now run the name constraint comparison on the subject distinguished name AND both forms of subject alternative names (rfc822Name and SmtpUTF8Mailbox) 🫶 it's giving consistency, bestie!

Both rfc822Name and SmtpUTF8Mailbox subjectAltNames live in the same email address namespace 🌸 so here's the drama: legacy Certification Authorities (CAs) that are constrained to issue certificates for a specific set of domains would have no matching UTF-8 constraints 😭 so [RFC9549] updates, modifies, and extends the rfc822Name name constraints from [RFC5280] to cover SmtpUTF8Mailbox subjectAltNames too ✨ this makes sure that introducing SmtpUTF8Mailbox does not break any existing name constraints. iconic 👑

Now the lowkey important part 🙈 it's not valid to put non-ASCII UTF-8 characters in the Local-part of an rfc822Name name constraint, and name constraints that include a Local-part are rarely, if ever, used in practice. so [RFC9549] allows the forms that cover all addresses at a host, or all mailboxes in a domain, and deprecates rfc822Name name constraints that pin down one particular mailbox. in other words, rfc822Name constraints with a Local-part SHOULD NOT be used. 💀

6.2. The Comparison Glow-Up 🧚

Constraint comparison with an SmtpUTF8Mailbox subjectAltName starts with the setup steps from Section 5 🎀 setup converts the inputs (a subject distinguished name, an rfc822Name, or an SmtpUTF8Mailbox subjectAltName on one side, and an rfc822Name name constraint on the other) into constraint comparison form. for both the name constraint and the subject, all A-labels and NR-LDH labels get lowercased 🍓 then you strip the Local-part and the "@" separator from each rfc822Name and SmtpUTF8Mailbox, which leaves just the domain part ✨

after setup, you follow the comparison steps from Section 4.2.1.10 of [RFC5280], like this:

if the resulting name constraint domain starts with a "." character, then for the name constraint to match, a suffix of the resulting subject alternative name domain MUST match the name constraint (including the leading ".") octet for octet. 🔥
if the resulting name constraint domain does NOT start with a "." character, then for the name constraint to match, the entire resulting subject alternative name domain MUST match the name constraint octet for octet. 💖

6.3. Who Can Issue What 👑

Certificate Authorities that want to issue CA certificates with email address name constraints MUST use rfc822Name subjectAltNames only 🫶 those names MUST be IDNA2008-conformant with no mappings, and non-ASCII domains get encoded in A-labels only 🌈

Here's the visual, which is non-normative (so it's just here to vibe with you) 🦋 example (1) shows a permitted rfc822Name ASCII-only host name constraint and the matching rfc822Name and SmtpUTF8Mailbox email addresses. example (2) shows a permitted rfc822Name host name constraint with an A-label and its matching addresses. also, an email address with an ASCII-only Local-part is encoded as rfc822Name even if the domain has Unicode in it 🙈 not me being picky, just the rules 💅

   +-------------------------------------------------------------------+
   |  Root CA Cert                                                     |
   +-------------------------------------------------------------------+
                                     |
                                     v
   +-------------------------------------------------------------------+
   |  Intermediate CA Cert                                             |
   |      Permitted                                                    |
   |        rfc822Name: elementary.school.example.com (1)              |
   |                                                                   |
   |        rfc822Name: xn--pss25c.example.com (2)                     |
   |                                                                   |
   +-------------------------------------------------------------------+
                                     |
                                     v
   +-------------------------------------------------------------------+
   |  Entity Cert (w/explicitly permitted subjects)                    |
   |    SubjectAltName Extension                                       |
   |      rfc822Name: student@elementary.school.example.com (1)        |
   |      SmtpUTF8Mailbox: u+5B66u+751F@elementary.school.example.com  |
   |        (1)                                                        |
   |                                                                   |
   |      rfc822Name: student@xn--pss25c.example.com (2)               |
   |      SmtpUTF8Mailbox: u+533Bu+751F@xn--pss25c.example.com (2)     |
   |                                                                   |
   +-------------------------------------------------------------------+

        Figure 1: Name Constraints with SmtpUTF8Name and rfc822Name

7. Security Considerations 🔐

Using SmtpUTF8Mailbox for certificate subjectAltName (and issuerAltName) brings along many of the same security considerations described in Section 8 of [RFC5280] 😬 but it adds a brand new issue by allowing non-ASCII characters in the email address Local-part. as mentioned in Section 4.4 of [RFC5890] and in Section 4 of [RFC6532], Unicode means there's a risk of visually similar and identical characters being used to deceive the recipient 💀 the former document lists some ways to mitigate these attacks. for more background on Unicode security issues, see [WEBER] 📚

also, a single string of Unicode user-perceived characters can be encoded in multiple ways 🌀 various Unicode normalization forms exist, but [RFC6531] does not require any of them for encoding the Local-part. so it may be possible to encode a Local-part value in several ways 🧩 to protect against attacks where the mail system and the Certification Authority use different encodings for SmtpUTF8Mailbox values, this specification requires an octet-for-octet comparison of the Local-part 🛡️ heads up though: requiring binary comparison may cause interoperability problems when the mail system uses one encoding and the Certification Authority uses another 😭

8. Differences from RFC 8398 🦋

This document obsoletes [RFC8398] 💅 there are three major changes in this spec:

in all cases, domain labels in mail addresses SHALL be encoded as LDH labels. in particular, domain names SHALL NOT be encoded using U-Labels, so use A-Labels instead 🍓
to go with the first change, the mail address matching algorithm from Section 5 of [RFC8398] has been changed to only accept domain labels encoded in their A-label form ✨
the procedure for processing rfc822Name name constraints from Section 6 of [RFC8398] has also been changed to only accept domain labels encoded in their A-label form 🌸

9. IANA Considerations 🧚

IANA has updated the reference for the id-mod-lamps-eai-addresses-2016 module in the "SMI Security for PKIX Module Identifier" (1.3.6.1.5.5.7.0) registry so it points to this document instead of [RFC8398] 🎀

IANA has also updated the reference for the SmtpUTF8Mailbox otherName in the "SMI Security for PKIX Other Name Forms" (1.3.6.1.5.5.7.8) registry so it points to this document instead of [RFC8398] 💜

10. References 📚

10.1. Normative References

[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>.

[RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
           10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
           2003, <https://www.rfc-editor.org/info/rfc3629>.

[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>.

[RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
           DOI 10.17487/RFC5321, October 2008,
           <https://www.rfc-editor.org/info/rfc5321>.

[RFC5890]  Klensin, J., "Internationalized Domain Names for
           Applications (IDNA): Definitions and Document Framework",
           RFC 5890, DOI 10.17487/RFC5890, August 2010,
           <https://www.rfc-editor.org/info/rfc5890>.

[RFC5891]  Klensin, J., "Internationalized Domain Names in
           Applications (IDNA): Protocol", RFC 5891,
           DOI 10.17487/RFC5891, August 2010,
           <https://www.rfc-editor.org/info/rfc5891>.

[RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
           Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
           February 2012, <https://www.rfc-editor.org/info/rfc6530>.

[RFC6531]  Yao, J. and W. Mao, "SMTP Extension for Internationalized
           Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,
           <https://www.rfc-editor.org/info/rfc6531>.

[RFC6532]  Yang, A., Steele, S., and N. Freed, "Internationalized
           Email Headers", RFC 6532, DOI 10.17487/RFC6532, February
           2012, <https://www.rfc-editor.org/info/rfc6532>.

[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>.

[RFC8398]  Melnikov, A., Ed. and W. Chuang, Ed., "Internationalized
           Email Addresses in X.509 Certificates", RFC 8398,
           DOI 10.17487/RFC8398, May 2018,
           <https://www.rfc-editor.org/info/rfc8398>.

[RFC9549]  Housley, R., "Internationalization Updates to RFC 5280",
           RFC 9549, DOI 10.17487/RFC9549, March 2024,
           <https://www.rfc-editor.org/info/rfc9549>.

10.2. Informative References

[RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
           Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
           DOI 10.17487/RFC5912, June 2010,
           <https://www.rfc-editor.org/info/rfc5912>.

[WEBER]    Weber, C., "Unraveling Unicode: A Bag of Tricks for Bug
           Hunting", July 2009, <https://www.lookout.net/files/
           Chris_Weber_Character%20Transformations%20v1.7_IUC33.pdf>.

Appendix A. ASN.1 Module ✨

okay bestie, this is the bit where the spec gets super precise 🤓💅 The following ASN.1 module normatively specifies the SmtpUTF8Mailbox structure. So this is the official, must-match definition, no cap 🫶 It uses the ASN.1 definitions from [RFC5912] with the 2002 ASN.1 notation used in that document. And yep, [RFC5912] updates normative documents using older ASN.1 notation 🦋

LAMPS-EaiAddresses-2016
{ iso(1) identified-organization(3) dod(6)
  internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
  id-mod-lamps-eai-addresses-2016(92) }

DEFINITIONS IMPLICIT TAGS ::=
BEGIN

IMPORTS
OTHER-NAME
FROM PKIX1Implicit-2009
  { iso(1) identified-organization(3) dod(6) internet(1) security(5)
  mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-implicit-02(59) }

id-pkix
FROM PKIX1Explicit-2009
  { iso(1) identified-organization(3) dod(6) internet(1) security(5)
  mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-explicit-02(51) } ;

--
-- otherName carries additional name types for subjectAltName,
-- issuerAltName, and other uses of GeneralNames.
--

id-on OBJECT IDENTIFIER ::= { id-pkix 8 }

SmtpUtf8OtherNames OTHER-NAME ::= { on-SmtpUTF8Mailbox, ... }

on-SmtpUTF8Mailbox OTHER-NAME ::= {
    SmtpUTF8Mailbox IDENTIFIED BY id-on-SmtpUTF8Mailbox
}

id-on-SmtpUTF8Mailbox OBJECT IDENTIFIER ::= { id-on 9 }

SmtpUTF8Mailbox ::= UTF8String (SIZE (1..MAX))
-- SmtpUTF8Mailbox conforms to Mailbox as specified
-- in Section 3.3 of RFC 6531. Additionally, all domain
-- labels included in the SmtpUTF8Mailbox value are
-- encoded as LDH Labels. In particular, domain labels
-- are not encoded as U-Labels and instead are encoded
-- using their A-label representation.

END

Appendix B. Example of SmtpUTF8Mailbox 🎀

okay so this part is totally non-normative (aka just an example, not a rule 🙈) and it shows how you'd use SmtpUTF8Mailbox as an otherName in GeneralName to encode the email address "u+533Bu+751F@xn--pss25c.example.com" 💌

The hexadecimal DER encoding of the block is:

a02b0608 2b060105 05070809 a01f0c1d e58cbbe7 949f4078 6e2d2d70
73733235 632e6578 616d706c 652e636f 6d

The text decoding is:

0  43: [0] {
2   8:   OBJECT IDENTIFIER '1 3 6 1 5 5 7 8 9'
12  31:   [0] {
14  29:     UTF8String 'u+533Bu+751F@xn--pss25c.example.com'
      :     }
      :   }

The example was encoded using Google's "der-ascii" program, and the text decoding above is output from Peter Gutmann's "dumpasn1" program 🧚✨

Acknowledgments 🫶

The authors thank David Benjamin for providing the motivation for this document 🌸 Additionally, the authors thank Éric Vyncke, John Levine, Peter van Dijk, Rich Salz, Russ Housley, and Tim Hollebeek for their reviews and feedback, which meaningfully improved the document 👑

The authors also recognize and appreciate the following individuals for their contributions to [RFC8398] 💜

Thank you to Magnus Nystrom for motivating this document. Thanks to Russ Housley, Nicolas Lidzborski, Laetitia Baudoin, Ryan Sleevi, Sean Leonard, Sean Turner, John Levine, and Patrik Falstrom for their feedback. Also special thanks to John Klensin for his valuable input on internationalization, Unicode, and ABNF formatting; to Jim Schaad for his help with the ASN.1 example and his helpful feedback; and especially to Viktor Dukhovni for helping us with name constraints and his many detailed document reviews. 🥹

Authors' Addresses 🦋

Alexey Melnikov Isode Ltd 14 Castle Mews Hampton, Middlesex TW12 2NP United Kingdom Email: Alexey.Melnikov@isode.com

Wei Chuang Google, Inc. 1600 Amphitheater Parkway Mountain View, CA United States of America Email: weihaw@google.com

Corey Bonnell DigiCert Pittsburgh, PA United States of America Email: corey.bonnell@digicert.com

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