Standards Track J. Strombergson
Internet-Draft InformAsic AB
Expires: September 23, 2005 L. Walleij
Lunds Tekniska Hogskola
P. Faltstrom
Cisco Systems Inc
March 22, 2005
The S Hexdump Format
draft-strombergson-shf-06.txt
Status of this Memo
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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
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complex hexadecimal data dumps in an open, modern, transport and
vendor neutral format.
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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 data sizes that are getting more and more common. Data
dumps comprised of multiple sub-blocks with different Word sizes,
data sizes spanning anywhere from a few Bytes of data to data sizes
much larger than 2^32 bits are not handled. Also, the checksums
included in these formats are too simplistic and for larger data
sizes provides insufficient ability to accurately detect errors.
Alternatively, the overhead needed for proper error detection is very
large.
The S Hexdump format therefore 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)
commence.
SHF is a dump format only and shall not be confused with similar
applications, such as binary configuration formats or patches, which
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are intended to e.g. alter contents of a core memory. Such
applications require the possibility to modify 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.
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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-continous, 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.
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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 forseeable
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.
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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 a 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. There are however 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.
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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.
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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 for 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 can
not 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, so that e.g. 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 The words inside a Dump MUST likewise 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.
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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>
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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 a 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
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</block>
</dump>
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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].
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9. IANA Considerations
This section contains the registration information for the MIME type
to SHF.
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 the XML 1.0 specification
[WWWXML]. One restriction is made: no entity references other than
the five predefined general entities references ("&", "<", ">", "'",
and """) and numeric entity references may be present. Neither the
"XML" declaration (e.g., ) nor the "DOCTYPE" declaration (e.g., )
need be present. (XML fragments are allowed.) All other XML 1.0
instructions (e.g., CDATA blocks, processing instructions, and so on)
are allowed.
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Applications which 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.
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10. Extensions
The attributes of elements in the SHF XML format may be extended when
need arise. For example, certain applications will want to represent
executable code as a SHF Dump and may then need a 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 CPU:s 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, transported using the application/xml+shf transport type,
and 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.
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11. Additional information
Contact for further information: c.f., the "Author's Address" section
of this memo.
Acknowledgments: The SMIL memory Dump was kindly provided by Sten
Henriksson at Lund University. Proofreading and good feedback on the
SHF draft 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. 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", BCP 14, RFC 3023, January 2001.
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
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Patrik Faltstrom
Cisco Systems Inc
Ledasa
273 71 Lovestad
Sweden
Email: paf@cisco.com
URI: http://www.cisco.com
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