Internationalization Updates to RFC 5280

  * Enhanced support for internationalized names is specified in
    Section 7, with rules for encoding and comparing
    Internationalized Domain Names, Internationalized Resource
    Identifiers (IRIs), and distinguished names.  These rules are
    aligned with comparison rules established in current RFCs,
    including [RFC3490], [RFC3987], and [RFC4518].

  * Enhanced support for internationalized names is specified in
    Section 7, with rules for encoding and comparing
    Internationalized Domain Names, Internationalized Resource
    Identifiers (IRIs), and distinguished names.  These rules are
    aligned with comparison rules established in current RFCs,
    including [RFC3987], [RFC4518], [RFC5890], and [RFC5891].

   A name constraint for Internet mail addresses MAY specify a
   particular mailbox, all addresses at a particular host, or all
   mailboxes in a domain.  To indicate a particular mailbox, the
   constraint is the complete mail address.  For example,
   "[email protected]" indicates the root mailbox on the host
   "example.com".  To indicate all Internet mail addresses on a
   particular host, the constraint is specified as the host name.  For
   example, the constraint "example.com" is satisfied by any mail
   address at the host "example.com".  To specify any address within a
   domain, the constraint is specified with a leading period (as with
   URIs).  For example, ".example.com" indicates all the Internet mail
   addresses in the domain "example.com", but not Internet mail
   addresses on the host "example.com".

   A name constraint for Internet mail addresses MAY specify all
   addresses at a particular host or all mailboxes in a domain.  To
   indicate all Internet mail addresses on a particular host, the
   constraint is specified as the host name.  For example, the
   constraint "example.com" is satisfied by any mail address at the
   host "example.com".  To specify any address within a domain, the
   constraint is specified with a leading period (as with URIs).  For
   example, ".example.com" indicates all the Internet mail addresses
   in the domain "example.com" but not Internet mail addresses on
   the host "example.com".

   Internationalized Domain Names (IDNs) may be included in certificates
   and CRLs in the subjectAltName and issuerAltName extensions, name
   constraints extension, authority information access extension,
   subject information access extension, CRL distribution points
   extension, and issuing distribution point extension.  Each of these
   extensions uses the GeneralName type; one choice in GeneralName is
   the dNSName field, which is defined as type IA5String.

   IA5String is limited to the set of ASCII characters.  To accommodate
   IDNs, U-labels are converted to A-labels.  The A-label is the
   encoding of the U-label according to the Punycode algorithm [RFC3492]
   with the ACE prefix "xn--" added at the beginning of the string.

   When comparing DNS names for equality, conforming implementations
   MUST perform a case-insensitive exact match on the entire DNS name.
   When evaluating name constraints, conforming implementations MUST
   perform a case-insensitive exact match on a label-by-label basis.  As
   noted in Section 4.2.1.10, any DNS name that may be constructed by
   adding labels to the left-hand side of the domain name given as the
   constraint is considered to fall within the indicated subtree.

   Implementations that have a user interface SHOULD convert IDNs to
   Unicode for display.  Specifically, conforming implementations
   convert A-labels to U-labels for display purposes.

   Implementation consideration: There are increased memory requirements
   for IDNs.  An IDN ACE label will begin with the four additional
   characters "xn--", and an IDN can require as many as five ASCII
   characters to specify a single international character.

   Domain Names may also be represented as distinguished names using
   domain components in the subject field, the issuer field, the
   subjectAltName extension, or the issuerAltName extension.  As with
   the dNSName in the GeneralName type, the value of this attribute is
   defined as an IA5String.  Each domainComponent attribute represents a
   single label.  To represent a label from an IDN in the distinguished
   name, the implementation MUST perform the "ToASCII" label conversion
   specified in Section 4.1 of RFC 3490.  The label SHALL be considered
   a "stored string".  That is, the AllowUnassigned flag SHALL NOT be
   set.

   Domain names may also be represented as distinguished names using
   domain components in the subject field, the issuer field, the
   subjectAltName extension, or the issuerAltName extension.  As with
   the dNSName in the GeneralName type, the value of this attribute is
   defined as an IA5String.  Each domainComponent attribute represents a
   single label.  To represent a label from an IDN in the distinguished
   name, the implementation MUST convert all U-labels to A-labels.

   Electronic Mail addresses may be included in certificates and CRLs in
   the subjectAltName and issuerAltName extensions, name constraints
   extension, authority information access extension, subject
   information access extension, issuing distribution point extension,
   or CRL distribution points extension.  Each of these extensions uses
   the GeneralName construct.  If the email address includes an IDN but
   the local-part of the email address can be represented in ASCII, then
   the email address is placed in the rfc822Name choice of GeneralName,
   which is defined as type IA5String.  If the local-part of the
   internationalized email address cannot be represented in ASCII, then
   the internationalized email address is placed in the otherName choice
   of GeneralName using the conventions in RFC 8398 [RFC8398].

   When the host-part contains an IDN, conforming implementations MUST
   convert all U-labels to A-labels.

   7.5.1.  Local-Part Contains Only ASCII Characters

   Two email addresses are considered to match if:

      1) The local-part of each name is an exact match, AND

      2) The host-part of each name matches using a case-insensitive
         ASCII comparison.

   Implementations that have a user interface SHOULD convert the
   host-part of internationalized email addresses specified in these
   extensions to Unicode before display.  Specifically, conforming
   implementations convert A-labels to U-labels for display purposes.

   7.5.2.  Local-Part Contains Non-ASCII Characters

   When the local-part contains non-ASCII characters, conforming
   implementations MUST place the internationalized email address in the
   SmtpUTF8Mailbox within the otherName choice of GeneralName as
   specified in Section 3 of RFC 8398 [RFC8398].  Note that the UTF8
   encoding of the internationalized email address MUST NOT contain a
   Byte-Order-Mark (BOM) [RFC3629] to aid comparison.  The email address
   local-part within the SmtpUTF8Mailbox MUST conform to the
   requirements of [RFC6530] and [RFC6531].

   Two email addresses are considered to match if:

      1) The local-part of each name is an exact match, AND

      2) The host-part of each name matches using a case-insensitive
         ASCII comparison.

   Implementations that have a user interface SHOULD convert the
   host-part of internationalized email addresses specified in these
   extensions to Unicode before display.  Specifically, conforming
   implementations convert A-labels to U-labels for display purposes.