]> Vigil Security, LLC

Herndon, VA US housley@vigilsec.com

Security Internet-Draft The updates to RFC 5280 described in this document provide alignment with the 2008 specification for Internationalized Domain Names (IDNs) and includes support for internationalized email addresses in X.509 certificates. The update ensures that name constraints for traditional email addresses and internationalized email addresses are handled in the same manner. This document (once approved) obsoletes RFC 8399.

Introduction This document updates the Introduction in Section 1, the Name Constraints certificate extension discussion in Section 4.2.1.10, and the Processing Rules for Internationalized Names in Section 7 of RFC 5280 to provide alignment with the 2008 specification for Internationalized Domain Names (IDNs) and includes support for internationalized email addresses in X.509 certificates. An IDN in Unicode (native character) form contains at least one U-label . IDNs are carried in certificates in ACE-encoded form. That is, all U-labels within an IDN are converted to A-labels. Conversion of a U-label to an A-label is described in . The GeneralName structure supports many different name forms, including otherName for extensibility. RFC 8398 specifies the SmtpUTF8Mailbox for internationalized email addresses. Note that Internationalized Domain Names in Applications specifications published in 2003 (IDNA2003) and 2008 (IDNA2008) both refer to the Punycode algorithm for conversion .

Terminology 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 when, and only when, they appear in all capitals, as shown here.

Changes since RFC 8399 In some cases, required conversion of A-labels to U-labels in order to process name constraints for internationalized email addresses. This lead to implementation complexity and at least two security vulnerabilities. One summary of the vulnerabilities an be found in . Now, all Internationalized Domain Names (IDNs) are carried and processed as A-labels.

Updates to RFC 5280 This section provides updates to several paragraphs of . For clarity, if the entire section is not replaced, then the original text and the replacement text are shown.

Update in the Introduction (Section 1) This update provides references for IDNA2008. OLD NEW

Update in Name Constraints (Section 4.2.1.10) This update removes the ability to include constraints for a particular mailbox. This capability was not used, and removing it allows name constraints to apply to email addresses in rfc822Name and SmtpUTF8Mailbox within otherName. OLD NEW

Update in IDNs in GeneralName (Section 7.2) This update aligns with IDNA2008. Since all of Section 7.2 is replaced, the OLD text is not provided. NEW

Update in IDNs in Distinguished Names (Section 7.3) This update aligns with IDNA2008. OLD NEW

Update in Internationalized Electronic Mail Addresses (Section 7.5) This update aligns with IDNA2008 and . Since all of Section 7.5 is replaced, the OLD text is not provided. NEW

Security Considerations The Security Consideration related to IDNA2008 in are relevant to this specification. Conforming CAs SHOULD ensure that IDNs are valid according to IDNA2008, which is defined in , , , , , and the updates to these documents. Failure to use valid A-labels may yield a domain name that cannot be correctly represented in the Domain Name System (DNS). In addition, the CA/Browser Forum offers some guidance regarding internal server names in certificates . An earlier version of this specification required conversion of A-labels to U-labels in order to process name constraints for internationalized email addresses in SmtpUTF8Mailbox other names. This lead to implementation complexity and at least two security vulnerabilities. Now, all Internationalized Domain Names (IDNs) are carried and processed as A-labels.

IANA Considerations This document has no IANA actions.

Acknowledgements Thanks to David Benjamin and Wei Chuang for identifying the issue and a solution. Thanks to Takahiro Nemoto, John Klensin, and Mike Ounsworth for their careful review and thoughtful comments.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Punycode is a simple and efficient transfer encoding syntax designed for use with Internationalized Domain Names in Applications (IDNA). It uniquely and reversibly transforms a Unicode string into an ASCII string. ASCII characters in the Unicode string are represented literally, and non-ASCII characters are represented by ASCII characters that are allowed in host name labels (letters, digits, and hyphens). This document defines a general algorithm called Bootstring that allows a string of basic code points to uniquely represent any string of code points drawn from a larger set. Punycode is an instance of Bootstring that uses particular parameter values specified by this document, appropriate for IDNA. [STANDARDS-TRACK] ISO/IEC 10646-1 defines a large character set called the Universal Character Set (UCS) which encompasses most of the world's writing systems. The originally proposed encodings of the UCS, however, were not compatible with many current applications and protocols, and this has led to the development of UTF-8, the object of this memo. UTF-8 has the characteristic of preserving the full US-ASCII range, providing compatibility with file systems, parsers and other software that rely on US-ASCII values but are transparent to other values. This memo obsoletes and replaces RFC 2279. This document defines a new protocol element, the Internationalized Resource Identifier (IRI), as a complement of the Uniform Resource Identifier (URI). An IRI is a sequence of characters from the Universal Character Set (Unicode/ISO 10646). A mapping from IRIs to URIs is defined, which means that IRIs can be used instead of URIs, where appropriate, to identify resources. The approach of defining a new protocol element was chosen instead of extending or changing the definition of URIs. This was done in order to allow a clear distinction and to avoid incompatibilities with existing software. Guidelines are provided for the use and deployment of IRIs in various protocols, formats, and software components that currently deal with URIs. The previous Lightweight Directory Access Protocol (LDAP) technical specifications did not precisely define how character string matching is to be performed. This led to a number of usability and interoperability problems. This document defines string preparation algorithms for character-based matching rules defined for use in LDAP. [STANDARDS-TRACK] This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] This document is one of a collection that, together, describe the protocol and usage context for a revision of Internationalized Domain Names for Applications (IDNA), superseding the earlier version. It describes the document collection and provides definitions and other material that are common to the set. [STANDARDS-TRACK] This document is the revised protocol definition for Internationalized Domain Names (IDNs). The rationale for changes, the relationship to the older specification, and important terminology are provided in other documents. This document specifies the protocol mechanism, called Internationalized Domain Names in Applications (IDNA), for registering and looking up IDNs in a way that does not require changes to the DNS itself. IDNA is only meant for processing domain names, not free text. [STANDARDS-TRACK] This document specifies rules for deciding whether a code point, considered in isolation or in context, is a candidate for inclusion in an Internationalized Domain Name (IDN). It is part of the specification of Internationalizing Domain Names in Applications 2008 (IDNA2008). [STANDARDS-TRACK] The use of right-to-left scripts in Internationalized Domain Names (IDNs) has presented several challenges. This memo provides a new Bidi rule for Internationalized Domain Names for Applications (IDNA) labels, based on the encountered problems with some scripts and some shortcomings in the 2003 IDNA Bidi criterion. [STANDARDS-TRACK] Full use of electronic mail throughout the world requires that (subject to other constraints) people be able to use close variations on their own names (written correctly in their own languages and scripts) as mailbox names in email addresses. This document introduces a series of specifications that define mechanisms and protocol extensions needed to fully support internationalized email addresses. These changes include an SMTP extension and extension of email header syntax to accommodate UTF-8 data. The document set also includes discussion of key assumptions and issues in deploying fully internationalized email. This document is a replacement for RFC 4952; it reflects additional issues identified since that document was published. [STANDARDS-TRACK] This document specifies an SMTP extension for transport and delivery of email messages with internationalized email addresses or header information. [STANDARDS-TRACK] RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document defines a new name form for inclusion in the otherName field of an X.509 Subject Alternative Name and Issuer Alternative Name extension that allows a certificate subject to be associated with an internationalized email address. This document updates RFC 5280. Informative References Until now, there has been no standard method for domain names to use characters outside the ASCII repertoire. This document defines internationalized domain names (IDNs) and a mechanism called Internationalizing Domain Names in Applications (IDNA) for handling them in a standard fashion. IDNs use characters drawn from a large repertoire (Unicode), but IDNA allows the non-ASCII characters to be represented using only the ASCII characters already allowed in so-called host names today. This backward-compatible representation is required in existing protocols like DNS, so that IDNs can be introduced with no changes to the existing infrastructure. IDNA is only meant for processing domain names, not free text. [STANDARDS-TRACK] Several years have passed since the original protocol for Internationalized Domain Names (IDNs) was completed and deployed. During that time, a number of issues have arisen, including the need to update the system to deal with newer versions of Unicode. Some of these issues require tuning of the existing protocols and the tables on which they depend. This document provides an overview of a revised system and provides explanatory material for its components. This document is not an Internet Standards Track specification; it is published for informational purposes. The updates to RFC 5280 described in this document provide alignment with the 2008 specification for Internationalized Domain Names (IDNs) and add support for internationalized email addresses in X.509 certificates. CA/Browser Forum Datadog Security Labs