SHF: The S Hexdump Format

Network Working Group                                    J. Strombergson
Request for Comments: 4194                                 InformAsic AB
Category: Standards Track                                     L. Walleij
                                                 Lunds Tekniska Hogskola
                                                            P. Faltstrom
                                                       Cisco Systems Inc
                                                            October 2005


                          The S Hexdump Format

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document specifies the S Hexdump Format (SHF), a new, XML-based
   open format for describing binary data in hexadecimal notation.  SHF
   provides the ability to describe both small and large, simple and
   complex hexadecimal data dumps in an open, modern, transport- and
   vendor-neutral format.

1.  Introduction

   In the computing, network, and embedded systems communities, several
   different types of data formats for hexadecimal data are being used.
   One of the more common formats is known as "S-records" (and several
   derivatives), which reportedly originated at the Motorola company.
   The S Hexdump Format is named in its honour.

   Typical uses of these dump formats include executable object code for
   embedded systems (i.e., "firmware"), on-chip flash memories and
   filesystems, FPGA configuration bitstreams, graphics and other
   application resources, routing tables, etc.  Unfortunately, none of
   the formats used are truly open, vendor-neutral, and/or well-defined.

   Even more problematic is the fact that none of these formats are able
   to represent the large data sizes that are getting more and more
   common.  Data dumps comprised of multiple sub-blocks with different
   Word sizes, and data sizes spanning anywhere from a few Bytes of data
   to much larger than 2^32 bits are not handled.  Also, the checksums
   included in these formats are too simplistic and for larger data
   sizes, they provide insufficient ability to accurately detect errors.
   Alternatively, the overhead needed for proper error detection is very
   large.

   Therefore, the S Hexdump format is an effort to provide a modern,
   XML-based format that is not too complex for simple tools and
   computing environments to implement, generate, parse, and use.  Yet
   the format is able to handle large data sizes and complex data
   structures, and can provide high quality error detection by
   leveraging standardized cryptographic hash functions.

   One of the simplifications introduced in the format is to disallow
   other number systems such as octal or decimal notation, and to allow
   for Word sizes of even bytes (8-bit groups) only.  This is
   intentional and was done to simplify implementations aimed for
   practical present-day applications.  Formats aimed for esoteric
   number systems or odd Word sizes may be implemented elsewhere.

   At present, the usage of the SHF format may be mainly for Internet
   transport and file storage on development machinery.  A parser for
   the XML format is presently not easily deployed in hardware devices,
   but the parsing and checksumming of the SHF data may be done by a
   workstation computer, which in turn converts the SHF tokens to an
   ordinary bitstream before the last step (e.g., of a firmware upgrade)
   commences.

   SHF is a dump format only and shall not be confused with similar
   applications, such as binary configuration formats or patches, which
   are intended to, for example, alter contents of a core memory.  Such
   applications require the possibility of modifying individual bits or
   groups of bits in the memory of a machine, and is not the intended
   usage of the mechanism described in the present document.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].

   The key word "Byte" is to be interpreted as a group of 8 bits.  The
   key word "Octet" is another name for Byte.

   The key word "Word" is to be interpreted as a group containing an
   integral number of Bytes.

   The key word "Block" is to be interpreted as an ordered sequence of
   Words, beginning at a certain address, running from lower to higher
   addresses.  A Block typically represents a sequence of Words at a
   certain address range in the memory of a computer.

   The key word "Dump" is to be interpreted as a sequence of Blocks,
   which may or may not be in a particular order.  A Dump typically
   represents some non-continuous, interesting parts of the memory of a
   computer, such that the Dump as a whole has a certain meaning, for
   example (but not limited to) a complete firmware for an embedded
   system.

   The expression "2^n" is to be interpreted as the value two (2) raised
   to the n:th power.  For example, 2^8 equals the value 256.

3.  Features and Functionality

   The SHF-format has the following features:

   o  Support for arbitrarily wide data Words

   o  Support for very large data Blocks

   o  Support for an arbitrary number of independent data Blocks

   o  Data integrity detection against errors provided by the RFC3174
      specified (see [2]) SHA-1 cryptographic signature

   o  An XML-based format

   In the embedded systems domain, 8- and 16-bit processors are still
   used in large numbers and will continue to be used for any
   foreseeable future.  Simultaneously, more and more systems are using
   64-bit and even larger Word sizes.

   SHF supports all of these systems by allowing the Word size to be
   specified.  The Word size MUST be an integer number of Bytes and at
   least one (1) Byte.

   SHF is able to represent both large and small data Blocks.  The data
   Block MUST contain at least one (1) Word.  Additionally, the data
   Block MUST NOT be larger than (2^64)-1 bits.

   The SHF Dump MUST contain at least one (1) data Block.  The maximum
   number of Blocks supported is 2^64.  Each data Block in the Dump MAY
   have different Word sizes and start at different addresses.

   The checksum (or message digest) used to verify the correctness or
   data integrity of each Block is 20 Bytes (160 bits) long.  The digest
   MUST be calculated on the data actually represented by the SHF data
   Block, NOT the representation, i.e., NOT the ASCII-code.  SHA-1 is
   only able to calculate a digest for a data Block no larger than
   (2^64)-1 bits and this limits the size of each data Block in SHF to
   (2^64)-1 bits.

4.  SHF XML Specification

   The SHF format consists of an XML data structure representing a Dump.
   The Dump consists of a Dump header section and one (1) or more Block
   sections containing data.  Each Block of data is independent of any
   other Block.

   A short, symbolic example of an SHF Dump is illustrated by the
   following structure:

   <dump name="(Human readable string)" blocks="(64-bit value)">
     <block name="(Human readable string)" start_address="(64-bit
            value)" word_size="(64-bit value)" length="(64-bit value)"
            checksum="(20-Byte digest)">
        (Data)
     </block>
   </dump>

4.1.  Header Section

   The header section comprises the Dump tag, which includes the
   following attributes:

   o  name: A compulsory string of arbitrary length used by any
      interested party to identify the specific SHF Dump.

   o  blocks: An optional 64-bit hexadecimal value representing the
      number of Blocks in the specific SHF Dump.  Whenever available,
      this value should be supplied.  However, there are potential
      scenarios where the number of Blocks cannot be given beforehand.
      If the value is present, it should be verified by implementers; if
      the value is untrue, the behaviour is implementation-defined.

   After the opening Dump tag, one or more subsections of Blocks must
   follow.  Finally, the complete SHF Dump ends with a closing Dump tag.

4.2.  Block Subsection

   The Block subsection contains a Block tag and a number of data words.
   The Block tag includes the following attributes:

   o  name: A compulsory string of arbitrary length used by any
      interested party to identify the specific Block.

   o  start_address: A compulsory, 64-bit hexadecimal value representing
      the start address in Bytes for the data in the Block.

   o  word_size: A compulsory 64-bit hexadecimal value representing the
      number of Bytes (the width) of one Word of the data.

   o  length: A compulsory hexadecimal representation of an unsigned
      64-bit integer indicating the number of Words following inside the
      Block element.  If this value turns out to be untrue, the Block
      MUST be discarded.

   o  checksum: A compulsory hexadecimal representation of the 20 Byte
      SHA-1 digest of the data in the Block.

   The total size of the data in the Block (in bits) is given by the
   expression (8 * word_size * length).  The expression MUST NOT be
   larger than (2^64)-1.

   After the opening Block tag, a hexadecimal representation of the
   actual data in the Block follows.  Finally, the Block section ends
   with a closing Block tag.

5.  SHF Rules and Limits

   There are several rules and limits in SHF:

   o  All attribute values representing an actual value and the data
      MUST be in hexadecimal notation.  The only attribute excluded from
      this rule is the name attribute in the Dump and Block tags.  This
      restriction has been imposed for ease of reading the dump: a
      reader shall not be uncertain about whether a figure is in hex
      notation or not, and can always assume it is hexadecimal.

   o  All attribute values, with the exception of the checksum, MAY omit
      leading zeros.  Conversely, the checksum MUST NOT omit leading
      zeros.

   o  The data represented in a Block MUST NOT be larger than (2^64)-1
      bits.

   o  The size of a Word MUST NOT be larger than (2^64)-1 bits.  This
      implies that a Block with a Word defined to the maximum width
      cannot contain more than one Word.  An SHF consumer shall assure
      that it can handle a certain Word length before beginning to parse
      blocks of an SHF Dump.  Failure to do so may cause buffer
      overflows and endanger the stability and security of the system
      running the consuming application.

   o  The attribute values representing an actual value MUST be in
      big-endian format.  This means that the most significant
      hexadecimal digits are to be put to the left in a hexadecimal
      Word, address, or similar field.  For example, the address value
      1234 represents the address 1234 and not 3412.  While some
      computing architectures may be using little-endian Words as their
      native format, it is the responsibility of any SHF producer
      running on such an architecture to swap the attribute values to a
      big-endian format.  The reverse holds for a consumer receiving the
      big-endian SHF attributes: if the consumer is little-endian, the
      values have to be swapped around.

   o  Likewise, the words inside a Dump MUST be stored in a big-endian
      format if the word size is larger than one Byte.  Here, the same
      need for swapping Bytes around may arise, as mentioned in the
      previous paragraph.

6.  SHF DTD

   The contents of the element named "block" and the attributes
   "blocks", "address", "word_size" and "checksum" should only contain
   the characters that are valid hexbyte sequences.  These are:

    whitespace ::= (#x20 | #x9 | #xC | #xD | #xA)
    hexdigit   ::= [0-9A-Fa-f]
    hexbytes   ::= whitespace* hexdigit (hexdigit|whitespace)*

   A parser reading in an SHF file should silently ignore any other
   characters that (by mistake) appear in any of these elements or
   attributes.  These alien characters should be treated as if they did
   not exist.  Also note that "whitespace" has no semantic meaning; it
   is only valid for the reason of improving the human readability of
   the Dump.  Whitespace may be altogether removed and the hexbyte
   sequences concatenated if desired.  Notice that the fact that word
   size is to be given in a number of bytes implies that the number of
   hexadecimal digits inside a block need to be even.  Malformed blocks
   should be ignored by implementations.

   <!--
     DTD for the S Hexdump Format, as of 2003-10-10
     Linus Walleij, Joachim Strombergson, Patrik Faltstrom 2003

     Refer to this DTD as:

     <!ENTITY % SHF PUBLIC "-//IETF//DTD SHF//EN"
                "http://ietf.org/dtd/shf.dtd">
          %SHF;
   -->
   <?xml version="1.0" encoding="UTF-8"?>

   <!ELEMENT dump (block)+>
   <!ATTLIST dump
          name          CDATA    #REQUIRED
          blocks        CDATA    #IMPLIED>

   <!ELEMENT block (#PCDATA)>
   <!ATTLIST block
          name          CDATA    #REQUIRED
          address       CDATA    #REQUIRED
          word_size     CDATA    #REQUIRED
          length        CDATA    #REQUIRED
          checksum      CDATA    #REQUIRED>

7.  SHF Examples

   This section contains three different SHF examples, illustrating the
   usage of SHF and the attributes in SHF.

   The first example is a simple SHF Dump with a single Block of data:

   <?xml version="1.0" encoding="UTF-8"?>
   <dump name="Simple SHF example" blocks="01">
     <block name="Important message in hex format" address="0400"
       word_size="01" length="1f"
       checksum="5601b6acad7da5c7b92036786250b053f05852c3">
         41 6c 6c 20 79 6f 75 72 20 62 61 73 65 20 61 72
         65 20 62 65 6c 6f 6e 67 20 74 6f 20 75 73 0a
     </block>
   </dump>
   The second example is a program in 6502 machine code residing at
   memory address 0x1000, which calculates the 13 first Fibonacci
   numbers and stores them at 0x1101-0x110d:

   <?xml version="1.0" encoding="UTF-8"?>
   <dump name="6502 Fibonacci" blocks="02">
     <block name="Code" address="1000" word_size="01" length="2a"
       checksum="5cab5bf8ee299af1ad17e8093d941914eb5930c7">
         a9 01 85 20 85 21 20 1e 10 20 1e 10 18 a5 21 aa
         65 20 86 20 85 21 20 1e 10 c9 c8 90 ef 60 ae 00
         11 a5 21 9d 00 11 ee 00 11 60
     </block>
     <block name="Mem" address="1100" word_size="01" length="e"
       checksum="c8c2001c42b0226a5d9f7c2f24bd47393166487a">
         01 00 00 00 00 00 00 00 00 00 00 00 00 00
     </block>
   </dump>

   The final example contains a Block of 40-bit wide data:

<?xml version="1.0" encoding="UTF-8"?>
<dump name="Example of an SHF dump with wide data words" blocks="00001">
  <block name="SMIL memory dump" address="000" word_size="5"
        length="1A" checksum="ff2033489aff0e4e4f0cd7901afc985f7a213c97">
      00100 00200 00000 00090 00000 00036 00300 00400
      00852 00250 00230 00858 00500 00600 014DC 00058
      002A8 000B8 00700 00800 000B0 00192 00100 00000
      00900 00A00 00000 0000A 40000 00000 00B00 00C00
      00000 00000 00000 00001 00D00 00E00 00000 00100
      0CCCC CCCCD 00F00 01000 00000 00010 80000 00000
      00100 00790 00000 00234
  </block>
</dump>

8.  SHF Security Considerations

   The SHF format is a format for representing hexadecimal data that one
   wants to transfer, manage, or transform.  The format itself does not
   guarantee that the represented data is not falsely represented,
   malicious, or otherwise dangerous.

   The data integrity of the SHF file as a whole is to be provided, if
   needed, by means external to the SHF file, such as the generic
   signing mechanism described by RFC 3275 [3].

9.  IANA Considerations

   This section contains the registration information for the MIME type
   to SHF.  The media type has been chosen to comply with the guidelines
   in [4].

   o  Registration: application/shf+xml
   o  MIME media type name: application
   o  MIME subtype name: shf+xml
   o  Required parameters: charset

   Required parameters: charset

   This parameter must exist and must be set to "UTF-8".  No other
   character sets are allowed for transporting SHF data.  The character
   set designator MUST be uppercase.

   Encoding considerations:

   This media type may contain binary content; accordingly, when used
   over a transport that does not permit binary transfer, an appropriate
   encoding must be applied.

   Security considerations:

   A hex Dump in itself has no other security considerations than what
   applies for any other XML file.  However, the included binary data
   may in decoded form contain any executable code for a target
   platform.  If additional security is desired, additional transport
   security solutions may be applied.  For target code contained in a
   hex Dump, developers may want to include certificates, checksums, and
   the like in hexdump form for the target platform.  Such uses are
   outside the scope of this document and a matter of implementation.

   Interoperability considerations:

   n/a

   Published specification:

   This media type is a proper subset of the XML 1.0 specification [5].
   One restriction is made: no entity references other than the five
   predefined general entities references ("&amp;", "&lt;", "&gt;",
   "&apos;", and "&quot;") and numeric entity references may be present.
   Neither the "XML" declaration (e.g., <?xml version="1.0" ?>) nor the
   "DOCTYPE" declaration (e.g., <!DOCTYPE ...>) need be present.  (XML
   fragments are allowed.)  All other XML 1.0 instructions (e.g., CDATA
   blocks, processing instructions, and so on) are allowed.

   Applications that use this media type: any program or individual
   wishing to make use of this XML 1.0 subset for hexdump exchange.

   Additional information:

   o  Magic number: There is no single initial Byte sequence that is
      always present for SHF files
   o  File extension: shf
   o  Macintosh File Type code: none

   Intended usage: COMMON.

   Author/Change controller: this MIME transport type is controlled by
   the IETF.

10.  Extensions

   The attributes of elements in the SHF XML format may be extended when
   need arises.  For example, certain applications will want to
   represent executable code as an SHF Dump, and may then need an
   execution start address to be associated with certain Dump Blocks, so
   that the address can be configured as a starting point for the CPU
   part of any processor code present in the Block, as opposed to the
   raw data, which is already given a start address by way of the
   "address" attribute.  This can be done by extending the Block tag
   with a "start_address" attribute.

   Another possible scenario is when a dump is applied to a computer
   system with several independent address spaces, such as a system with
   two CPUs, each with independent memories.  In this case, a user may
   want to add an "address_space" attribute.

   As long as such new attributes are added, with no attributes being
   removed or redefined, the resulting Dump shall be considered a valid
   SHF Dump and transported using the application/xml+shf transport
   type.  Parsers unaware of the modified namespace shall silently
   ignore any such extended attributes, or simply duplicate them from
   input to output when processing an SHF file as a filter.  The
   management of such extended attributes is a matter of convention
   between different classes of users and not a matter of the IETF.

11.  Additional Information

   Contact for further information: c.f., the "Authors' Addresses"
   section of this memo.

   Acknowledgements: The SMIL memory Dump was kindly provided by Sten
   Henriksson at Lund University.  Proofreading and good feedback on the
   SHF document was generously provided by Peter Lindgren, Tony Hansen,
   Larry Masinter, and Clive D.W. Feather.  We also want to thank the
   Applications area workgroup for their help during development.

12.  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  Eastlake, 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
        (SHA1)", BCP 14, RFC 3174, September 2001.

   [3]  Eastlake, 3rd, D., Joseph, J., and D. David, "(Extensible Markup
        Language) XML-Signature Syntax and Processing", BCP 14,
        RFC 3275, March 2002.

   [4]  Makoto, M., Simon, S., and D. Dan, "(Extensible Markup Language)
        XML Media Types", RFC 3023, January 2001.

   [5]  Bray, Tim, Paoli, Jean, Sperberg-McQueen, C. M. and Maler, Eve,
        Yergeau, Francois, "Extensible Markup Language (XML) 1.0 (Third
        Edition)", http://www.w3.org/TR/REC-xml.

Authors' Addresses

   Joachim Strombergson
   InformAsic AB
   Hugo Grauers gata 5a
   Gothenburg  411 33
   SE

   Phone: +46 31 68 54 90
   EMail: Joachim.Strombergson@InformAsic.com
   URI:   http://www.InformAsic.com/


   Linus Walleij
   Lunds Tekniska Hogskola
   Master Olofs Vag 24
   Lund  224 66
   SE

   Phone: +46 703 193678
   EMail: triad@df.lth.se


   Patrik Faltstrom
   Cisco Systems Inc
   Ledasa
   273 71 Lovestad
   Sweden

   EMail: paf@cisco.com
   URI:   http://www.cisco.com

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