CBOR Working Group                                         M. Richardson
Internet-Draft                                  Sandelman Software Works
Intended status: Best Current Practice                     4 August 2021
Expires: 5 February 2022

            On storing CBOR encoded items on stable storage


   This document proposes an on-disk format for CBOR objects that is
   friendly to common on-disk recognition systems like the Unix file(1)

   This document is being discussed at: https://github.com/cbor-wg/cbor-

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on 5 February 2022.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements for a Magic Number . . . . . . . . . . . . . . .   4
   3.  Protocol  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  The CBOR Protocol Specific Tag  . . . . . . . . . . . . .   4
     3.2.  CBOR Tag Wrapped  . . . . . . . . . . . . . . . . . . . .   4
     3.3.  CBOR Tag Sequence . . . . . . . . . . . . . . . . . . . .   5
   4.  Advice to Protocol Developers . . . . . . . . . . . . . . . .   5
     4.1.  Is the on-wire format new?  . . . . . . . . . . . . . . .   7
     4.2.  Can many items be trivially concatenated? . . . . . . . .   7
     4.3.  Are there tags at the start?  . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  CBOR Sequence Tag . . . . . . . . . . . . . . . . . . . .   8
     6.2.  CBOR Tags for CoAP Content-Format Numbers . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  CBOR Tags for CoAP Content Formats . . . . . . . . .  10
     A.1.  Content-Format Tag Examples . . . . . . . . . . . . . . .  10
   Appendix B.  Example from Openswan  . . . . . . . . . . . . . . .  11
   Appendix C.  Changelog  . . . . . . . . . . . . . . . . . . . . .  12
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  12
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Since very early in computing, operating systems have sought ways to
   mark which files could be processed by which programs.

   For instance, the Unix file(1) command, which has existed since 1973
   [file], has been able to identify many file formats for decades.
   Many systems (Linux, MacOS, Windows) will select the correct
   application based upon the file contents, if the system can not
   determine it by other means: for instance, the classic MacOS
   maintained a resource fork that includes media type ("MIME type")
   information and therefore ideally never needs to know anything about
   the file.  Other systems do this by file extensions.

   While having a media type associated with the file is a better
   solution in general, when files become disconnected from their type
   information, such as when attempting to do forensics on a damaged
   system, then being able to identify a file type can become very

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   It is noted that in the media type registration, that a magic number
   is asked for, if available, as is a file extension.

   A challenge for the file(1) program is often that it can be confused
   by the encoding vs the content.  For instance, an Android "apk" used
   to transfer and store an application may be identified as a ZIP file.
   Both OpenOffice or MSOffice files are ZIP files of XML files.
   (Unless OpenOffice files are flat (fodp) files, in which case they
   may appear to be generic XML files.)

   As CBOR becomes a more and more common encoding for a wide variety of
   artifacts, identifying them as just "CBOR" is probably not
   sufficient.  This document provides a way to encode a magic number
   into the beginning of a CBOR format file.  Two options are presented:
   typically a CBOR Protocol author will specify one.

   A CBOR Protocol is a specification which uses CBOR as its encoding.
   Examples of CBOR Protocols currently under development include CoSWID
   [I-D.ietf-sacm-coswid], and EAT [I-D.ietf-rats-eat].  COSE itself
   [RFC8152] is considered infrastructure, however the encoding of
   public keys in CBOR as described in [I-D.ietf-cose-cbor-encoded-cert]
   would be an identified CBOR Protocol.

   A major inspiration for this document is observing the mess in ASN.1
   based systems where most files are PEM encoded, identified by the
   extension "pem", confusing public keys, private keys, certificate
   requests and S/MIME content.

   These proposals are invasive to how CBOR protocols are written to
   disk, but in both cases, the proposed envelope does not require that
   the tag be transferred on the wire.

   In addition to the on-disk identification aspects, there are some
   protocols which may benefit from having such a magic number on the
   wire if they presently using a different (legacy) encoding scheme.
   The presence of the identifiable magic sequence signals that CBOR is
   being used or a legacy scheme.

1.1.  Terminology

   The term "diagnostic notation" refers to the human-readable notation
   for CBOR data items defined in Section 8 of [RFC8949] and Appendix G
   of [RFC8610].

   The term CDDL (Concise Data Definition Language) refers to the
   language defined in [RFC8610].

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2.  Requirements for a Magic Number

   A magic number is ideally a unique fingerprint, present in the first
   4 or 8 bytes of the file, which does not change when the contents
   change, and does not depend upon the length of the file.

   Less ideal solutions have a pattern that needs to be matched, but in
   which some bytes need to be ignored.  While the Unix file(1) command
   can be told to ignore bytes, this can lead to ambiguities.

3.  Protocol

   There are two variations of this practice.  Both use CBOR Tags in a
   way that results in a deterministic first 8 to 12 bytes.

3.1.  The CBOR Protocol Specific Tag

   CBOR Protocol designers should obtain a tag for each major type of
   object that they might store on disk.  As there are more than 4
   billion available 4-byte tags, there should be little issue in
   allocating a few to each available CBOR Protocol.

   The policy is First Come First Served, so all that is required is an
   email to IANA, having filled in the small template provided in
   Section 9.2 of [RFC8949].

   This tag should be allocated by the author of the CBOR Protocol, and
   to be in the four-byte range, it should be at least 0x01000000
   (decimal 16777216) in value.

   The use of a sequence of four US-ASCII codes which are mnemonic to
   the protocol is encouraged, but not required.

   For CBOR byte strings that happen to contain a representation that is
   described by a CoAP Content-Format Number (Section 12.3 of [RFC7252],
   Subregistry Content-Formats of [IANA.core-parameters]), a tag number
   has been allocated in Section 6.2 (see Appendix A for details and

3.2.  CBOR Tag Wrapped

   This proposal starts with the Self-described CBOR tag, 55799, as
   described in Section 3.4.6 of [RFC8949].

   A second CBOR Tag is then allocated to describe the specific Protocol
   involved, as described above.

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   This proposal wraps the CBOR value as tags usually do.  Applications
   that need to send the CBOR value across a constrained link may wish
   to remove the two tags if the use is implicitly understood.  This is
   a decision of the CBOR Protocol specification.

3.3.  CBOR Tag Sequence

   This proposal makes use of CBOR Sequences as described in [RFC8742].

   This proposal consists of two tags and a constant string for a total
   of 12 bytes.

   1.  The file shall start with the Self-described CBOR Sequence tag,

   2.  The file shall continue with a CBOR tag, from the First Come
       First Served space, which uniquely identifies the CBOR Protocol.
       The use of a four-byte tag is encouraged.

   3.  The three byte CBOR byte string containing 0x42_4F_52.

   The first part identifies the file as being a CBOR Sequence, and does
   so with all the desirable properties explained in Section 3.4.6 of
   [RFC8949].  Specifically, it does not seem to conflict with any known
   file types, and it is not valid Unicode in any Unicode encoding.

   The second part identifies which CBOR Protocol is used, as described

   The third part is represented as a constant byte sequence
   0x43_42_4f_52, the ASCII characters "CBOR", which is the CBOR encoded
   data item for the three byte sequence 0x42_4f_52 ('BOR' in diagnostic
   notation).  This is the data item that is being tagged.

   The actual CBOR Protocol value then follows as the next data item(s)
   in the CBOR sequence, without a need for any further specific tag.
   The use of a CBOR Sequence allows the application to trivially remove
   the first item with the two tags.

   This means that should a file be reviewed by a human (directly in an
   editor, or in a hexdump display), it will include the ASCII
   characters "CBOR" prominently.  This value is also included simply
   because the two tags need to tag something.

4.  Advice to Protocol Developers

   This document introduces a choice between a CBOR Sequence and a
   wrapped CBOR Tag. Which should a protocol designer use?

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   In this discussion, one assumes that there is an object stored in a
   file, perhaps specified by a system operator in a configuration file.

   For example: a private key used in COSE operations, a public key/
   certificate in C509 or CBOR format, a recorded sensor reading stored
   for later transmission, or a COVID vaccination certificate that needs
   to be displayed in QRcode form.

   Both the CBOR Tag Sequence and the wrapped tag can be trivially
   removed by an application before sending the CBOR content out on the

   The CBOR Tag Sequence is a little bit easier to remove as in most
   cases, CBOR parsers will return it as a unit, and then return the
   actual CBOR item, which could be anything at all, and could include
   CBOR tags that _do_ need to be sent on wire.

   On the other hand, having the CBOR Tag Sequence in the file requires
   that all programs that expect to examine that file are able to skip
   what appears to be an empty CBOR item.  Programs which might not
   expect the CBOR Tag Sequence, but which would operate without a
   problem would include any program that expects to process CBOR
   Sequences from the file.

   As an example of where there was a problem with previous security
   systems, "PEM" format certificate files grew to be able to contain
   multiple certificates by simple concatenation.  The PKCS1 format
   could also contain a private key object followed by a one or more
   certificate objects: but only when in PEM format.  But, when in
   binary DER format, concatenation of certificates was not compatible
   with most programs.

   The use of CBOR Tag Wrapped format is easier to retrofit to an
   existing format with existing and unchangeable on-disk format.  This
   new sequence of tags are expected to be trivially ignored by an
   existing program when reading CBOR from disk, even if the program
   only supports decoding a single data item (and not a CBOR sequence).
   But, a naive program might also then transmit the additional tags
   across the network.  Removing the CBOR Tag Wrapped format requires
   knowledge of the two tags involved.  Other tags present might be
   needed.  For a representation matching a specific media-type that is
   carried in a CBOR byte string, the byte string head will already have
   to be removed for use as such a representation, so it should be easy
   to remove the enclosing tag heads as well.  This is of particular
   interest with the pre-defined tags provided by Appendix A for media-
   types with CoAP Content-Format numbers.

   Here are some considerations:

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4.1.  Is the on-wire format new?

   If the on-wire format is new, then it could be specified with the
   CBOR Tag Wrapped format if the extra eight bytes are not a problem.
   The disk format is then identical to the on-wire format.

   If the eight bytes are a problem (and they usually are if CBOR is
   being considered), then the CBOR Tag Sequence format should be
   adopted for on-disk storage.

4.2.  Can many items be trivially concatenated?

   If the programs that read the contents of the file already expect to
   process all of the items in the file (not just the first), then the
   CBOR Tag Sequence format may be easily retrofitted.

   The program involved may throw errors or warnings on the CBOR Tag
   Sequence if they have not yet been updated, but this may not be a
   problem.  If it is, then consideration should be given to CBOR Tag

   If only one item is ever expected in the file, the the use of CBOR
   Tag Sequence may present an implementation hurdle to programs that
   previously just read a single value and used it.

4.3.  Are there tags at the start?

   If the Protocol expects to use other tags values at the top-level,
   then it may be easier to explain if the CBOR Tag Sequence format is

5.  Security Considerations

   This document provides a way to identify CBOR Protocol objects.
   Clearly identifying CBOR contents on disk may have a variety of

   The most obvious is that it may allow malware to identify interesting
   objects on disk, and then corrupt them.

6.  IANA Considerations

   Section 6.1 documents the allocation that was done for a CBOR tag to
   be used in a CBOR sequence to identify the sequence (an example for
   using this tag is found in Appendix B).  Section 6.2 allocates a CBOR
   tag for each actual or potential CoAP Content-Format number (examples
   are in Appendix A).

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6.1.  CBOR Sequence Tag

   IANA has allocated tag 55800 as the CBOR Sequence tag.  This tag is
   from the First Come/First Served area.

   The value has been picked to have properties similar to the 55799 tag
   (Section 3.4.6 of [RFC8949]).

   The hexadecimal representation is: 0xd9_d9_f8.

   This is not valid UTF-8: the first 0xd9 puts the value into the
   three-byte value of UTF-8, but the 0xd9 as the second value is not a
   valid second byte for UTF-8.

   This is not valid UTF-16: the byte sequence 0xd9d9 (in either endian
   order) puts this value into the UTF-16 high-half zone, which would
   signal that this a 32-bit Unicode value.  However, the following
   16-bit big-endian value 0xf8.. is not a valid second sequence
   according to [RFC2781].  On a little-endian system, it would be
   necessary to examine the fourth byte to determine if it is valid.
   That next byte is determined by the subsequent encoding, and
   Section 3.4.6 of [RFC8949] has already determined that no valid CBOR
   encodings result in a valid UTF-16.

   Data Item:
      byte string

      indicates that the file contains CBOR Sequences

6.2.  CBOR Tags for CoAP Content-Format Numbers

   IANA is requested to allocate the tag numbers 1668546560 (0x63740000)
   to 1668612095 (0x6374FFFF) as follows:

   Data Item:
      byte string

      for each tag number NNNN, the representation of content-format
      (RFC7252) NNNN-1668546560


7.  References

7.1.  Normative References

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   [RFC8742]  Bormann, C., "Concise Binary Object Representation (CBOR)
              Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,

7.2.  Informative References

   [file]     Wikipedia, "file (command)", 20 January 2021,

              Mattsson, J. P., Selander, G., Raza, S., Höglund, J., and
              M. Furuhed, "CBOR Encoded X.509 Certificates (C509
              Certificates)", Work in Progress, Internet-Draft, draft-
              ietf-cose-cbor-encoded-cert-02, 12 July 2021,

              Mandyam, G., Lundblade, L., Ballesteros, M., and J.
              O'Donoghue, "The Entity Attestation Token (EAT)", Work in
              Progress, Internet-Draft, draft-ietf-rats-eat-10, 7 June
              2021, <https://www.ietf.org/archive/id/draft-ietf-rats-

              Birkholz, H., Fitzgerald-McKay, J., Schmidt, C., and D.
              Waltermire, "Concise Software Identification Tags", Work
              in Progress, Internet-Draft, draft-ietf-sacm-coswid-18, 12
              July 2021, <https://www.ietf.org/archive/id/draft-ietf-

              IANA, "Constrained RESTful Environments (CoRE)

   [RFC2781]  Hoffman, P. and F. Yergeau, "UTF-16, an encoding of ISO
              10646", RFC 2781, DOI 10.17487/RFC2781, February 2000,

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   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

Appendix A.  CBOR Tags for CoAP Content Formats

   Often, there is a need to identify a media type (or content type,
   i.e., media type optionally used with parameters) that describes a
   byte string in a CBOR data item.

   Section 5.10.3 of [RFC7252] defines the concept of a Content-Format,
   which is a short 16-bit unsigned integer that identifies a specific
   content type (media type plus optionally parameters), optionally
   together with a content encoding.

   This specification allocates CBOR tag numbers 1668546560 (0x63740000)
   to 1668612095 (0x6374FFFF) for the tagging of representations of
   specific content formats.  The tag content tagged with tag number
   NNNN (in above range) is a byte string that is to be interpreted as a
   representation of the content format NNNN-1668546560.

A.1.  Content-Format Tag Examples

   Subregistry Content-Formats of [IANA.core-parameters] defines content
   formats that can be used as examples:

   *  Content-Format 432 stands for media type application/td+json (no
      parameters).  The corresponding tag number is 1668546992 (i.e.,

      So the following CDDL snippet can be used to identify application/
      td+json representations:

      td-json = #6.1668546992(bstr)

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      Note that a byte string is used as the type of the tag content,
      because a media type representation in general can be any byte

   *  Content-Format 11050 stands for media type application/json in
      deflate encoding.

      The corresponding tag number is 1668557610 (i.e.,

      So the following CDDL snippet can be used to identify application/
      json representations compressed in deflate encoding:

      json-deflate = #6.1668557610(bstr)

      The byte string is appropriate here as the type for the tag
      content, because the compressed form is an instance of a general
      byte string.

Appendix B.  Example from Openswan

   The Openswan IPsec project has a daemon ("pluto"), and two control
   programs ("addconn", and "whack").  They communicate via a Unix-
   domain socket, over which a C-structure containing pointers to
   strings is serialized using a bespoke mechanism.  This is normally
   not a problem as the structure is compiled by the same compiler; but
   when there are upgrades it is possible for the daemon and the control
   programs to get out of sync by the bespoke serialization.  As a
   result, there are extra compensations to deal with shutting the
   daemon down.  During testing it is sometimes the case that upgrades
   are backed out.

   In addition, when doing unit testing, the easiest way to load policy
   is to use the normal policy reading process, but that is not normally
   loaded in the daemon.  Instead the IPC that is normally sent across
   the wire is compiled/serialized and placed in a file.  The above
   magic number is included in the file, and also on the IPC in order to
   distinguish the "shutdown" command CBOR operation.

   In order to reduce the problems due to serialization, the
   serialization is being changed to CBOR.  Additionally, this change
   allows the IPC to be described by CDDL, and for any language that
   encode to CBOR can be used.

   IANA has allocated the tag 1330664270, or 0x4f_50_53_4e for this
   purpose.  As a result, each file and each IPC is prefixed with:

   In diagnostic notation:

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   Or in hex:

   00000000  d9 d9 f8 da 4f 50 53 4e  43 42 4f 52         |....OPSNCBOR|

Appendix C.  Changelog


   The CBOR WG brainstormed this protocol on January 20, 2021.


   Carsten Bormann
   Universität Bremen TZI

   Email: cabo@tzi.org

   Josef 'Jeff' Sipek

   Email: jeffpc@josefsipek.net

Author's Address

   Michael Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca

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