On storing CBOR encoded items on stable storage

Document Type Active Internet-Draft (cbor WG)
Authors Michael Richardson  , Carsten Bormann 
Last updated 2021-10-21
Replaces draft-bormann-cbor-tag-coap-content-format, draft-richardson-cbor-file-magic
Stream Internet Engineering Task Force (IETF)
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Send notices to christian@amsuess.com
CBOR Working Group                                         M. Richardson
Internet-Draft                                  Sandelman Software Works
Intended status: Best Current Practice                        C. Bormann
Expires: 24 April 2022                            Universität Bremen TZI
                                                         21 October 2021

            On storing CBOR encoded items on stable storage


   This document defines an on-disk format for CBOR objects that is
   friendly to common on-disk recognition systems such as the Unix
   file(1) command.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the CBOR Working Group
   mailing list (cbor@ietf.org), which is archived at

   Source for this draft and an issue tracker can be found at
   https://github.com/cbor-wg/cbor-magic-number (https://github.com/

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 24 April 2022.

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Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Requirements for a Magic Number . . . . . . . . . . . . .   4
   2.  Protocol  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  The CBOR Protocol Specific Tag  . . . . . . . . . . . . .   4
     2.2.  Enveloping Method: CBOR Tag Wrapped . . . . . . . . . . .   5
       2.2.1.  Example . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Enveloping Method: CBOR Tag Sequence  . . . . . . . . . .   6
   3.  Advice to Protocol Developers . . . . . . . . . . . . . . . .   7
     3.1.  Is the on-wire format new?  . . . . . . . . . . . . . . .   8
     3.2.  Can many items be trivially concatenated? . . . . . . . .   8
     3.3.  Are there tags at the start?  . . . . . . . . . . . . . .   9
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  CBOR Sequence Tag . . . . . . . . . . . . . . . . . . . .   9
     5.2.  CBOR Tags for CoAP Content-Format Numbers . . . . . . . .  10
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  CBOR Tags for CoAP Content Formats . . . . . . . . .  12
     A.1.  Content-Format Tag Examples . . . . . . . . . . . . . . .  12
   Appendix B.  Example from Openswan  . . . . . . . . . . . . . . .  13
   Appendix C.  Changelog  . . . . . . . . . . . . . . . . . . . . .  13
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  14
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

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

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   For instance, the Unix file(1) command, which has existed since 1973
   [file], has been able to identify many file formats for decades based
   upon the contents of the file.

   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, starting in Mac OS, a
   resource fork was maintained that includes media type ("MIME type")
   information and therefore ideally never needs to know anything about
   the file.

   But, many other systems do this by file extensions.  Many common web
   servers derive the MIME-type information from 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

   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.
   Additionally, both OpenOffice and MSOffice files are ZIP files of XML

   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 possible method of
   enveloping data are presented: a CBOR Protocol author will specify
   one.  (A CBOR Protocol is a specification which uses CBOR as its

   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 as well.

   A major inspiration for this document is observing the mess in
   certain 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.

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   While these envelopes add information to how data conforming to CBOR
   Protocols are stored in files, there is no requirement that either
   type of envelope 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 are presently using a different (legacy) encoding
   scheme.  The presence of the identifiable magic sequence signals that
   CBOR is being used as opposed to 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].

1.2.  Requirements for a Magic Number

   A magic number is ideally a fingerprint that is unique to a CBOR
   protocol, present in the first few (small multiple of 4) 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 certain bytes, this can lead to ambiguities.

2.  Protocol

   There are two enveloping methods presented.  Which one is to be used
   is up to the CBOR Protocol author to determine.  Both use CBOR Tags
   in a way that results in a deterministic first 8 to 12 bytes.

2.1.  The CBOR Protocol Specific Tag

   In both enveloping methods, CBOR Protocol designers need to obtain a
   CBOR 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 IANA policy for 4-byte CBOR Tags 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].

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   This tag needs to be allocated by the author of the CBOR Protocol.
   In order to be in the four-byte range, and so that there are no
   leading zeros, the value needs to be in the range 0x01000000 (decimal
   16777216) to 0xFFFFFFFF (decimal 4294967295).  It is further
   suggested to avoid values that have an embedded zero byte in the four
   bytes of their binary representation (e.g., 0x12003456).

   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],
   Registry CoAP Content-Formats of [IANA.core-parameters]), a tag
   number has already been allocated in Section 5.2 (see Appendix A for
   details and examples).

2.2.  Enveloping Method: CBOR Tag Wrapped

   The CBOR Tag Wrapped method is appropriate for use with CBOR
   protocols that encode a single CBOR data item.

   It 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.

   This method 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.

   Whether or not to remove the tags for specific further processing is
   a decision made by the CBOR Protocol specification.

2.2.1.  Example

   To construct an example without registering a new tag, we use the
   technique described in Appendix A to translate the Content-Format
   number registered for application/senml+cbor, the number 112, into
   the tag 1668546560+112 = 1668546672.

   With this tag, the SenML-CBOR pack [{0: "current", 6: 3, 2: 1.5}]
   would be enveloped as (in diagnostic notation):

   55799(1668546672([{0: "current", 6: 3, 2: 1.5}]))

   Or in hex:

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   d9 d9f7                       # tag(55799)
      da 63740070                # tag(1668546672)
         81                      # array(1)
            a3                   # map(3)
               00                # unsigned(0)
               67                # text(7)
                  63757272656e74 # "current"
               06                # unsigned(6)
               03                # unsigned(3)
               02                # unsigned(2)
               f9 3e00           # primitive(15872)

   In other words, the unique fingerprint for application/senml+cbor is
   composed of the 8 bytes d9d9f7da63740070 hex, after which the
   unadorned CBOR data (81... for the SenML data) is appended.

2.3.  Enveloping Method: CBOR Tag Sequence

   The CBOR Taq Sequence method is appropriate for use with CBOR
   Sequences as described in [RFC8742].

   This method prepends a data item to the sequence to be tagged that
   consists of two tags nested around 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.
       As with the previous method, the use of a four-byte tag is
       encouraged that encodes without zero bytes.

   3.  The encoded 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 appear to conflict with any
   known file types, and it is not valid Unicode in any Unicode

   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.

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   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.

   Should this 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.

3.  Advice to Protocol Developers

   This document introduces a choice between wrapping a single CBOR data
   item into a (pair of) identifying CBOR tags, or prepending an
   identifying encoded CBOR data item (which in turn contains a pair of
   identifying CBOR tags) to a CBOR Sequence (which might be single data

   Which should a protocol designer use?

   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 QR code 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 a CBOR item with two tags nested around a three-
   byte byte string.. 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

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   certificate objects: but only when in PEM format.  Annoyingly, when
   in binary DER format (which like CBOR is self-delimiting),
   concatenation of certificates was not compatible with most programs
   as they did not expect to read more than one item in the file.

   The use of CBOR Tag Wrapped format is easier to retrofit to an
   existing format with existing and unchangeable on-disk format for a
   single CBOR data item.  This new sequence of tags is expected to be
   trivially ignored by many existing programs 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 in the form of survey questions:

3.1.  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 on the wire (and they often are if
   CBOR is being considered), then the CBOR Tag Sequence format should
   be adopted for on-disk storage.

3.2.  Can many items be trivially concatenated?

   If the programs that read the contents of the file already expect to
   process all of the CBOR data 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 use of CBOR Tag
   Sequence may present an implementation hurdle to programs that
   previously just read a single data item and used it.

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3.3.  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

4.  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 exfiltrate or corrupt them.

5.  IANA Considerations

   Section 5.1 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 5.2 allocates a CBOR
   tag for each actual or potential CoAP Content-Format number (examples
   are in Appendix A).

5.1.  CBOR Sequence Tag

   IANA has allocated tag 55800 as the tag for the CBOR Tag Sequence
   Enveloping Method.

   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 of the encoded tag head is:

   This is not valid UTF-8: the first 0xd9 introduces a three-byte
   sequence in UTF-8, but the 0xd9 as the second value is not a valid
   second byte for UTF-8.

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   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 valid UTF-16.

   Data Item:
      tagged byte string

      indicates that the file contains CBOR Sequences

5.2.  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 NNNNNNNN, the representation of content-format
      (RFC7252) NNNNNNNN-1668546560


6.  References

6.1.  Normative References

   [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,

6.2.  Informative References

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

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              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-19, 20
              October 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,

   [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>.

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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
   NNNNNNNN (in above range) is a byte string that is to be interpreted
   as a representation of the content format NNNNNNNN-1668546560.

A.1.  Content-Format Tag Examples

   Registry 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)

      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)

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      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 a CBOR
   TAG Sequence.

   In diagnostic notation:


   Or in hex:

   D9 D9F8         # tag(55800)
      DA 4F50534E  # tag(1330664270)
         43        # bytes(3)
            424F52 # "BOR"

Appendix C.  Changelog

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   The CBOR WG brainstormed this protocol on January 20, 2021 via a
   number of productive email exchanges on the mailing list.


   Josef 'Jeff' Sipek

   Email: jeffpc@josefsipek.net

Authors' Addresses

   Michael Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca

   Carsten Bormann
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen

   Phone: +49-421-218-63921
   Email: cabo@tzi.org

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