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ART cbor Concise Data Definition Language The Concise Data Definition Language (CDDL), as defined in RFCs 8610 and 9165, provides an easy and unambiguous way to express structures for protocol messages and data formats that are represented in Concise Binary Object Representation (CBOR) or JSON. This document updates RFC 8610 by addressing related errata reports and making other small fixes for the ABNF grammar defined for CDDL.

Introduction The Concise Data Definition Language (CDDL), as defined in and , provides an easy and unambiguous way to express structures for protocol messages and data formats that are represented in CBOR or JSON. This document updates by addressing errata reports and making other small fixes for the ABNF grammar defined for CDDL. The body of this document explains and shows motivation for the updates; the updated collected ABNF syntax in in replaces the collected ABNF syntax in .

Conventions and Definitions The terminology from applies. The grammar in is based on ABNF, which is defined in and .

Clarifications and Changes Based on Errata Reports A number of errata reports have been made regarding some details of text string and byte string literal syntax: for example, and . These are being addressed in this section, updating details of the ABNF for these literal syntaxes. Also, the changes described in need to be applied (backslashes have been lost during the RFC publication process of , garbling the text explaining backslash escaping). These changes are intended to mirror the way existing implementations have dealt with the errata reports. This document also uses the opportunity presented by the necessary cleanup of the grammar of string literals for a backward-compatible addition to the syntax for hexadecimal escapes. The latter change is not automatically forward compatible (i.e., CDDL specifications that make use of this syntax do not necessarily work with existing implementations until these are updated, which is recommended by this specification).

Updates to String Literal Grammar

Erratum ID 6527 (Text String Literals) The ABNF used in for the content of text string literals is rather permissive:

Original ABNF from RFC 8610 for Strings with Permissive ABNF for SESC (Which Did Not Allow Hex Escapes)

This allows almost any non-C0 character to be escaped by a backslash, but critically misses out on the \uXXXX and \uHHHH\uLLLL forms that JSON allows to specify characters in hex (which should apply here according to item 6 of ). (Note that CDDL imports from JSON the unwieldy \uHHHH\uLLLL syntax, which represents Unicode code points beyond U+FFFF by making them look like UTF-16 surrogate pairs; CDDL text strings do not use UTF-16 or surrogates.) Both can be solved by updating the SESC rule. This document uses the opportunity to add a popular form of directly specifying characters in strings using hexadecimal escape sequences of the form \u{hex}, where hex is the hexadecimal representation of the Unicode scalar value. The result is the new set of rules defining SESC in .

Update to String ABNF in : Allow Hex Escapes

Now that SESC is more restrictively formulated, an update to the BCHAR rule used in the ABNF syntax for byte string literals is also required:

ABNF from RFC 8610 for BCHAR

With the SESC updated as above, \' is no longer allowed in BCHAR and now needs to be explicitly included there; see .

Erratum ID 6278 (Consistent String Literals) Updating BCHAR also provides an opportunity to address , which points to an inconsistency in treating U+007F (DEL) between SCHAR and BCHAR. As U+007F is not printable, including it in a byte string literal is as confusing as for a text string literal; therefore, it should be excluded from BCHAR as it is from SCHAR. The same reasoning also applies to the C1 control characters, so the updated ABNF actually excludes the entire range from U+007F to U+009F. The same reasoning also applies to text in comments (PCHAR). For completeness, all these rules should also explicitly exclude the code points that have been set aside for UTF-16 surrogates.

Update to ABNF in : BCHAR, SCHAR, and PCHAR

(Note that, apart from addressing the inconsistencies, there is no attempt to further exclude non-printable characters from the ABNF; doing this properly would draw in complexity from the ongoing evolution of the Unicode standard that is not needed here.)

Addressing Erratum ID 6526 and Erratum ID 6543 The above changes also cover (a proposal to split off qualified byte string literals from UTF-8 byte string literals) and (lost backslashes); see for details.

Examples Demonstrating the Updated String Syntaxes The CDDL example in demonstrates various escaping techniques now available for (byte and text) strings in CDDL. Obviously, in the literals for a and x, there is no need to escape the second character, an o, as \u{6f}; this is just for demonstration. Similarly, as shown in c and z, there also is no need to escape the 🁳 or ⌘; however, escaping them may be convenient in order to limit the character repertoire of a CDDL file itself to ASCII .

Example Text and Byte String Literals with Various Escaping Techniques

In this example, the rules a to c and x to z all produce strings with byte-wise identical content: a to c are text strings and x to z are byte strings. illustrates this by showing the output generated from the start rule in , using pretty-printed hexadecimal.

Generated CBOR from CDDL Example (Pretty-Printed Hexadecimal)

Small Enabling Grammar Changes Each subsection that follows specifies a small change to the grammar that is intended to enable certain kinds of specifications. These changes are backward compatible (i.e., CDDL files that comply with continue to match the updated grammar) but not necessarily forward compatible (i.e., CDDL specifications that make use of these changes cannot necessarily be processed by existing implementations of ).

Empty Data Models requires a CDDL file to have at least one rule.

ABNF from RFC 8610 for Top-Level Rule cddl

This makes sense when the file has to stand alone, as a CDDL data model needs to have at least one rule to provide an entry point (i.e., a start rule). With CDDL modules , CDDL files can also include directives, and these might be the source of all the rules that ultimately make up the module created by the file. Any other rule content in the file has to be available for directive processing, making the requirement for at least one rule cumbersome. Therefore, the present update extends the grammar as in and turns the existence of at least one rule into a semantic constraint, to be fulfilled after processing of all directives.

Update to Top-Level ABNF in Appendices B and C of RFC 8610

Non-Literal Tag Numbers and Simple Values The existing ABNF syntax for expressing tags in CDDL is as follows:

Original ABNF from RFC 8610 for Tag Syntax

This means tag numbers can only be given as literal numbers (uints). Some specifications operate on ranges of tag numbers; for example, has a range of tag numbers 1668546817 (0x63740101) to 1668612095 (0x6374FFFF) to tag specific content formats. This cannot currently be expressed in CDDL. Similar considerations apply to simple values (#7.xx). This update extends the syntax to the following:

Update to Tag and Simple Value ABNF in Appendices B and C of RFC 8610 ") ]]>

For #6, the head-number stands for the tag number. For #7, the head-number stands for the simple value if it is in the ranges 0..23 or 32..255 (as per Section of RFC 8949 , the simple values 24..31 are not used). For 24..31, the head-number stands for the "additional information", e.g., #7.25 or #7.<25> is a float16, etc. (All ranges mentioned here are inclusive.) So the above range can be expressed in a CDDL fragment such as: = #6.(content) ct-tag-number = 1668546817..1668612095 ; or use 0x63740101..0x6374FFFF ]]>

Security Considerations The grammar fixes and updates in this document are not believed to create additional security considerations. The security considerations in apply. Specifically, the potential for confusion is increased in an environment that uses a combination of CDDL tools, some of which have been updated and some of which have not, in particular based on . Attackers may want to exploit such potential confusion by crafting CDDL models that are interpreted differently by different parts of a system. There will be a period of transition from the details that the grammar in handled in a less well-defined way, to the updated grammar defined in the present document. This transition might offer one (but not the only) type of opportunity for the kind of attack that relies on differences between implementations. Implementations that make use of CDDL models operationally already need to ascertain the provenance (and thus authenticity and integrity) and applicability of models they employ. At the time of writing, it is expected that the models will generally be processed by a software developer, within a software development environment. Therefore, developers are advised to treat CDDL models with the same care as any other source code.

IANA Considerations This document has no IANA actions.

References Normative References Informative References RFC Errata RFC 8610 RFC Errata RFC 8610 RFC Errata RFC 8610 RFC Errata RFC 8610 RFC Errata RFC 8610 The Unicode Consortium

Updated Collected ABNF for CDDL This appendix is normative. It provides the full ABNF from as updated by the present document.

ABNF for CDDL as Updated " genericarg = "<" S type1 S *("," S type1 S ) ">" type = type1 *(S "/" S type1) type1 = type2 [S (rangeop / ctlop) S type2] ; space may be needed before the operator if type2 ends in a name type2 = value / typename [genericarg] / "(" S type S ")" / "{" S group S "}" / "[" S group S "]" / "~" S typename [genericarg] / "&" S "(" S group S ")" / "&" S groupname [genericarg] / "#" "6" ["." head-number] "(" S type S ")" / "#" "7" ["." head-number] / "#" DIGIT ["." uint] ; major/ai / "#" ; any head-number = uint / ("<" type ">") rangeop = "..." / ".." ctlop = "." id group = grpchoice *(S "//" S grpchoice) grpchoice = *(grpent optcom) grpent = [occur S] [memberkey S] type / [occur S] groupname [genericarg] ; preempted by above / [occur S] "(" S group S ")" memberkey = type1 S ["^" S] "=>" / bareword S ":" / value S ":" bareword = id optcom = S ["," S] occur = [uint] "*" [uint] / "+" / "?" uint = DIGIT1 *DIGIT / "0x" 1*HEXDIG / "0b" 1*BINDIG / "0" value = number / text / bytes int = ["-"] uint ; This is a float if it has fraction or exponent; int otherwise number = hexfloat / (int ["." fraction] ["e" exponent ]) hexfloat = ["-"] "0x" 1*HEXDIG ["." 1*HEXDIG] "p" exponent fraction = 1*DIGIT exponent = ["+"/"-"] 1*DIGIT text = %x22 *SCHAR %x22 SCHAR = %x20-21 / %x23-5B / %x5D-7E / NONASCII / SESC SESC = "\" ( %x22 / "/" / "\" / ; \" \/ \\ %x62 / %x66 / %x6E / %x72 / %x74 / ; \b \f \n \r \t (%x75 hexchar) ) ; \uXXXX hexchar = "{" (1*"0" [ hexscalar ] / hexscalar) "}" / non-surrogate / (high-surrogate "\" %x75 low-surrogate) non-surrogate = ((DIGIT / "A"/"B"/"C" / "E"/"F") 3HEXDIG) / ("D" %x30-37 2HEXDIG ) high-surrogate = "D" ("8"/"9"/"A"/"B") 2HEXDIG low-surrogate = "D" ("C"/"D"/"E"/"F") 2HEXDIG hexscalar = "10" 4HEXDIG / HEXDIG1 4HEXDIG / non-surrogate / 1*3HEXDIG bytes = [bsqual] %x27 *BCHAR %x27 BCHAR = %x20-26 / %x28-5B / %x5D-7E / NONASCII / SESC / "\'" / CRLF bsqual = "h" / "b64" id = EALPHA *(*("-" / ".") (EALPHA / DIGIT)) ALPHA = %x41-5A / %x61-7A EALPHA = ALPHA / "@" / "_" / "$" DIGIT = %x30-39 DIGIT1 = %x31-39 HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F" HEXDIG1 = DIGIT1 / "A" / "B" / "C" / "D" / "E" / "F" BINDIG = %x30-31 S = *WS WS = SP / NL SP = %x20 NL = COMMENT / CRLF COMMENT = ";" *PCHAR CRLF PCHAR = %x20-7E / NONASCII NONASCII = %xA0-D7FF / %xE000-10FFFD CRLF = %x0A / %x0D.0A ]]>

Details about Covering Erratum ID 6543 This appendix is informative. notes that the ABNF used in for the content of byte string literals lumps together byte strings notated as text with byte strings notated in base16 (hex) or base64 (but see also updated BCHAR rule in ):

Original ABNF from RFC 8610 for BCHAR

Change Proposed by Erratum ID 6543 Erratum ID 6543 proposes handling the two cases in separate ABNF rules (where, with an updated SESC, BCHAR obviously needs to be updated as above):

Proposal from Erratum ID 6543 to Split the Byte String Rules

This potentially causes a subtle change, which is hidden in the WS rule:

ABNF Definition of WS from RFC 8610

This allows any non-C0 character in a comment, so this fragment becomes possible: The current text is not unambiguously saying whether the three apostrophes need to be escaped with a \ or not, as in: ... which would be supported by the existing ABNF in .

No Further Change Needed after Updating String Literal Grammar This document takes the simpler approach of leaving the processing of the content of the byte string literal to a semantic step after processing the syntax of the bytes and BCHAR rules, as updated by Figures and in (updates prompted by the combination of and ). Therefore, the rules in (as updated by ) are applied to the result of this processing where bsqual is given as h or b64. Note that this approach also works well with the use of byte strings in . It does require some care when copying-and-pasting into CDDL models from ABNF that contains single quotes (which may also hide as apostrophes in comments); these need to be escaped or possibly replaced by %x27. Finally, the approach taken lends support to extending bsqual in CDDL similar to the way this is done for CBOR diagnostic notation in . (Note that, at the time of writing, the processing of string literals is quite similar for both CDDL and Extended Diagnostic Notation (EDN), except that CDDL has end-of-line comments that are ";" based and EDN has two comment syntaxes: one in-line "/" based and one end-of-line "#" based.)

Acknowledgments Many thanks go to the submitters of the errata reports addressed in this document. In one of the ensuing discussions, proposed defining an ABNF rule "NONASCII", of which we have included the essence. Special thanks to the reviewers , (Shepherd Review and further guidance), (AD Review and further guidance), and (detailed IESG review).