Allow RFC 4648 content encodings

RFC 4648 is a more modern standard for encodings, and addresses
ambiguities and application-specific limits of the "base64"
encoding from RFC 2045.  It also adds several commonly used
variations, such as "base64url", that are relevant to modern systems.

RFC 2045 values are still allowed, although for "base64", RFC 4648
now takes precedence.

Author: handrews

jsonschema-validation.xml

@@ -10,6 +10,7 @@
 <!ENTITY RFC3987 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3987.xml">
 <!ENTITY RFC4291 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.4291.xml">
 <!ENTITY RFC4329 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.4329.xml">
+<!ENTITY RFC4648 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.4648.xml">
 <!ENTITY RFC5322 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.5322.xml">
 <!ENTITY RFC5890 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.5890.xml">
 <!ENTITY RFC5891 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.5891.xml">
@@ -330,7 +331,7 @@
                     </t>
                 </section>
 
-                <section title="pattern">
+                <section title="pattern" anchor="pattern">
                     <t>
                         The value of this keyword MUST be a string. This string SHOULD be a
                         valid regular expression, according to the ECMA 262 regular expression
@@ -759,8 +760,16 @@
                     If the instance value is a string, this property defines that the string
                     SHOULD be interpreted as binary data and decoded using the encoding
                     named by this property.
-                    <xref target="RFC2045">RFC 2045, Sec 6.1</xref> lists the possible
-                    values for this property.
+                </t>
+
+                <t>
+                    Possible values for this property are listed in
+                    <xref target="RFC2045">RFC 2045, Sec 6.1</xref> and
+                    <xref target="RFC4648">RFC 4548</xref>.  For "base64", which is defined
+                    in both RFCs, the definition in RFC 4648, which removes line length
+                    limitations, SHOULD be used, as various other specifications have
+                    mandated different lengths.  Note that line lengths within a string
+                    can be constrained using the <xref target="pattern">"pattern"</xref> keyword.
                 </t>
 
                 <t>
@@ -991,6 +1000,7 @@
             &RFC3986;
             &RFC3987;
             &RFC4291;
+            &RFC4648;
             &RFC5322;
             &RFC5890;
             &RFC5891;