Network Working Group A. Costanzo
Request for Comments: 1505 AKC Consulting
Obsoletes: 1154 D. Robinson
Computervision Corporation
R. Ullmann
August 1993
Encoding Header Field for Internet Messages
Status of this Memo
This memo defines an EXPerimental Protocol for the Internet
community. It does not specify an Internet standard. Discussion and
suggestions for improvement are requested. Please refer to the
current edition of the "IAB Official Protocol Standards" for the
standardization state and status of this protocol. Distribution of
this memo is unlimited.
IESG Note
Note that a standards-track technology already exists in this area
[11].
Abstract
This document expands upon the elective experimental Encoding header
field which permits the mailing of multi-part, multi-strUCtured
messages. It replaces RFC1154 [1].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . 3
2. The Encoding Field . . . . . . . . . . . . . . . . . 3
2.1 Format of the Encoding Field . . . . . . . . . . . 3
2.2 <count> . . . . . . . . . . . . . . . . . . . . . 4
2.3 <keyWord> . . . . . . . . . . . . . . . . . . . . 4
2.3.1 Nested Keywords . . . . . . . . . . . . . . . . 4
2.4 Comments . . . . . . . . . . . . . . . . . . . . . 4
3. Encodings . . . . . . . . . . . . . . . . . . . . . 5
3.1 Text . . . . . . . . . . . . . . . . . . . . . . . 5
3.2 Message . . . . . . . . . . . . . . . . . . . . . 6
3.3 Hex . . . . . . . . . . . . . . . . . . . . . . . 6
3.4 EVFU . . . . . . . . . . . . . . . . . . . . . . . 6
3.5 EDI-X12 and EDIFACT . . . . . . . . . . . . . . . 7
3.6 FS . . . . . . . . . . . . . . . . . . . . . . . 7
3.7 LZJU90 . . . . . . . . . . . . . . . . . . . . . . 7
3.8 LZW . . . . . . . . . . . . . . . . . . . . . . . 7
3.9 UUENCODE . . . . . . . . . . . . . . . . . . . . . 7
3.10 PEM and PEM-Clear . . . . . . . . . . . . . . . . 8
3.11 PGP . . . . . . . . . . . . . . . . . . . . . . . 8
3.12 Signature . . . . . . . . . . . . . . . . . . . 10
3.13 TAR . . . . . . . . . . . . . . . . . . . . . . 10
3.14 PostScript . . . . . . . . . . . . . . . . . . . 10
3.15 SHAR . . . . . . . . . . . . . . . . . . . . . . 10
3.16 Uniform Resource Locator . . . . . . . . . . . . 10
3.17 Registering New Keywords . . . . . . . . . . . . 11
4. FS (File System) Object Encoding . . . . . . . . . 11
4.1 Sections . . . . . . . . . . . . . . . . . . . . 12
4.1.1 Directory . . . . . . . . . . . . . . . . . . 12
4.1.2 Entry . . . . . . . . . . . . . . . . . . . . 13
4.1.3 File . . . . . . . . . . . . . . . . . . . . . 13
4.1.4 Segment . . . . . . . . . . . . . . . . . . . 13
4.1.5 Data . . . . . . . . . . . . . . . . . . . . . 14
4.2 Attributes . . . . . . . . . . . . . . . . . . . 14
4.2.1 Display . . . . . . . . . . . . . . . . . . . 14
4.2.2 Comment . . . . . . . . . . . . . . . . . . . 15
4.2.3 Type . . . . . . . . . . . . . . . . . . . . . 15
4.2.4 Created . . . . . . . . . . . . . . . . . . . 15
4.2.5 Modified . . . . . . . . . . . . . . . . . . . 15
4.2.6 Accessed . . . . . . . . . . . . . . . . . . . 15
4.2.7 Owner . . . . . . . . . . . . . . . . . . . . 15
4.2.8 Group . . . . . . . . . . . . . . . . . . . . 16
4.2.9 ACL . . . . . . . . . . . . . . . . . . . . . 16
4.2.10 Password . . . . . . . . . . . . . . . . . . . 16
4.2.11 Block . . . . . . . . . . . . . . . . . . . . 16
4.2.12 Record . . . . . . . . . . . . . . . . . . . . 17
4.2.13 Application . . . . . . . . . . . . . . . . . 17
4.3 Date Field . . . . . . . . . . . . . . . . . . . 17
4.3.1 Syntax . . . . . . . . . . . . . . . . . . . . 17
4.3.2 Semantics . . . . . . . . . . . . . . . . . . 17
5. LZJU90: Compressed Encoding . . . . . . . . . . . 18
5.1 Overview . . . . . . . . . . . . . . . . . . . . 18
5.2 Specification of the LZJU90 compression . . . . 19
5.3 The Decoder . . . . . . . . . . . . . . . . . . 21
5.3.1 An example of an Encoder . . . . . . . . . . . 27
5.3.2 Example LZJU90 Compressed Object . . . . . . . 33
6. Alphabetical Listing of Defined Encodings . . . . 34
7. Security Considerations . . . . . . . . . . . . . 34
8. References . . . . . . . . . . . . . . . . . . . . 34
9. Acknowledgements . . . . . . . . . . . . . . . . . 35
10. Authors" Addresses . . . . . . . . . . . . . . . . 36
1. Introduction
STD 11, RFC822 [2] defines an electronic mail message to consist of
two parts, the message header and the message body, separated by a
blank line.
The Encoding header field permits the message body itself to be
further broken up into parts, each part also separated from the next
by a blank line. Thus, conceptually, a message has a header part,
followed by one or more body parts, all separated by apparently blank
lines. Each body part has an encoding type. The default (no
Encoding field in the header) is a one part message body of type
"Text".
The purpose of Encoding is to be descriptive of the content of a mail
message without placing constraints on the content or requiring
additional structure to appear in the body of the message that will
interfere with other processing.
A similar message format is used in the network news facility, and
posted articles are often transferred by gateways between news and
mail. The Encoding field is perhaps even more useful in news, where
articles often are uuencoded or shar"d, and have a number of
different nested encodings of graphics images and so forth. In news
in particular, the Encoding header keeps the structural information
within the (usually concealed) article header, without affecting the
visual presentation by simple news-reading software.
2. The Encoding Field
The Encoding field consists of one or more subfields, separated by
commas. Each subfield corresponds to a part of the message, in the
order of that part"s appearance. A subfield consists of a line count
and a keyword or a series of nested keywords defining the encoding.
The line count is optional in the last subfield.
2.1 Format of the Encoding Field
The format of the Encoding field is:
[ <count> <keyword> [ <keyword> ]* , ]*
[ <count> ] <keyword> [ <keyword> ]*
where:
<count> := a decimal integer
<keyword> := a single alphanumeric token starting with an alpha
2.2 <count>
The line count is a decimal number specifying the number of text
lines in the part. Parts are separated by a blank line, which is not
included in the count of either the preceding or following part.
Blank lines consist only of CR/LF. Count may be zero, it must be
non-negative.
It is always possible to determine if the count is present because a
count always begins with a digit and a keyword always begins with a
letter.
The count is not required on the last or only part. A multi-part
message that consists of only one part is thus identical to a
single-part message.
2.3 <keyword>
Keyword defines the encoding type. The keyword is a common single-
word name for the encoding type and is not case-sensitive.
Encoding: 107 Text
2.3.1 Nested Keywords
Nested keywords are a series of keywords defining a multi-encoded
message part. The encoding keywords may either be an actual series
of encoding steps the encoder used to generate the message part or
may merely be used to more precisely identify the type of encoding
(as in the use of the keyword "Signature").
Nested keywords are parsed and generated from left to right. The
order is significant. A decoding application would process the list
from left to right, whereas, an encoder would process the Internet
message and generate the nested keywords in the reverse order of the
actual encoding process.
Encoding: 458 uuencode LZW tar (Unix binary object)
2.4 Comments
Comments enclosed in parentheses may be inserted anywhere in the
encoding field. Mail reading systems may pass the comments to their
clients. Comments must not be used by mail reading systems for
content interpretation. Other parameters defining the type of
encoding must be contained within the body portion of the Internet
message or be implied by a keyword in the encoding field.
3. Encodings
This section describes some of the defined encodings used. An
alphabetical listing is provided in Section 6.
As with the other keyword-defined parts of the header format
standard, new keywords are expected and welcomed. Several basic
principles should be followed in adding encodings. The keyword
should be the most common single word name for the encoding,
including acronyms if appropriate. The intent is that different
implementors will be likely to choose the same name for the same
encoding. Keywords should not be too general: "binary" would have
been a bad choice for the "hex" encoding.
The encoding should be as free from unnecessary idiosyncracies as
possible, except when conforming to an existing standard, in which
case there is nothing that can be done.
The encoding should, if possible, use only the 7 bit ASCII printing
characters if it is a complete transformation of a source document
(e.g., "hex" or "uuencode"). If it is essentially a text format, the
full range may be used. If there is an external standard, the
character set may already be defined. Keywords beginning with "X-"
are permanently reserved to implementation-specific use. No standard
registered encoding keyword will ever begin with "X-".
New encoding keywords which are not reserved for implementation-
specific use must be registered with the Internet Assigned Numbers
Authority (IANA). Refer to section 3.17 for additional information.
3.1 Text
This indicates that the message is in no particular encoded format,
but is to be presented to the user as-is.
The text is ISO-10646-UTF-1 [3]. As specified in STD 10, RFC821
[10], the message is expected to consist of lines of reasonable
length (less than or equal to 1000 characters).
On some older implementations of mail and news, only the 7 bit subset
of ISO-10646-UTF-1 can be used. This is identical to the ASCII 7 bit
code. On some mail transports that are not compliant with STD 10,
RFC821 [10], line length may be restricted by the service.
Text may be followed by a nested keyword to define the encoded part
further, e.g., "signature":
Encoding: 496 Text, 8 Text Signature
An automated file sending service may find this useful, for example,
to differentiate between and ignore the signature area when parsing
the body of a message for file requests.
3.2 Message
This encoding indicates that the body part is itself in the format of
an Internet message, with its own header part and body part(s). A
"message" body part"s message header may be a full Internet message
header or it may consist only of an Encoding field.
Using the message encoding on returned mail makes it practical for a
mail reading system to implement a reliable automatic resending
function, if the mailer generates it when returning contents. It is
also useful in a "copy append" MUA (mail user agent) operation.
MTAs (mail transfer agents) returning mail should generate an
Encoding header. Note that this does not require any parsing or
transformation of the returned message; the message is simply
appended un-modified; MTAs are prohibited from modifying the content
of messages.
Encoding: 7 Text (Return Reason), Message (Returned Mail)
3.3 Hex
The encoding indicates that the body part contains binary data,
encoded as 2 hexadecimal digits per byte, highest significant nibble
first.
Lines consist of an even number of hexadecimal digits. Blank lines
are not permitted. The decode process must accept lines with between
2 and 1000 characters, inclusive.
The Hex encoding is provided as a simple way of providing a method of
encoding small binary objects.
3.4 EVFU
EVFU (electronic vertical format unit) specifies that each line
begins with a one-character "channel selector". The original purpose
was to select a channel on a paper tape loop controlling the printer.
This encoding is sometimes called "FORTRAN" format. It is the
default output format of FORTRAN programs on a number of computer
systems.
The legal characters are "0" to "9", "+", "-", and space. These
correspond to the 12 rows (and absence of a punch) on a printer
control tape (used when the control unit was electromechanical).
The channels that have generally agreed definitions are:
1 advances to the first print line on the next page
0 skip a line, i.e., double-space
+ over-print the preceeding line
- skip 2 lines, i.e., triple-space
(space) print on the next line, single-space
3.5 EDI-X12 and EDIFACT
The EDI-X12 and EDIFACT keywords indicate that the message or part is
a EDI (Electronic Document Interchange) business document, formatted
according to ANSI X12 or the EDIFACT standard.
A message containing a note and 2 X12 purchase orders might have an
encoding of:
Encoding: 17 TEXT, 146 EDI-X12, 69 EDI-X12
3.6 FS
The FS (File System) keyword specifies a section consisting of
encoded file system objects. This encoding method (defined in
section 4) allows the moving of a structured set of files from one
environment to another while preserving all common elements.
3.7 LZJU90
The LZJU90 keyword specifies a section consisting of an encoded
binary or text object. The encoding (defined in section 5) provides
both compression and representation in a text format.
3.8 LZW
The LZW keyword specifies a section consisting of the data produced
by the Unix compress program.
3.9 UUENCODE
The uuencode keyword specifies a section consisting of the output of
the uuencode program supplied as part of uucp.
3.10 PEM and PEM-Clear
The PEM and PEM-Clear keywords indicate that the section is encrypted
with the methods specified in RFCs 1421-1424 [4,5,6,7] or uses the
MIC-Clear encapsulation specified therein.
A simple text object encrypted with PEM has the header:
Encoding: PEM Text
Note that while this indicates that the text resulting from the PEM
decryption is ISO-10646-UTF-1 text, the present version of PEM
further restricts this to only the 7 bit subset. A future version of
PEM may lift this restriction.
If the object resulting from the decryption starts with Internet
message header(s), the encoding is:
Encoding: PEM Message
This is useful to conceal both the encoding within and the headers
not needed to deliver the message (such as Subject:).
PEM does not provide detached signatures, but rather provides the
MIC-Clear mode to send messages with integrity checks that are not
encrypted. In this mode, the keyword PEM-Clear is used:
Encoding: PEM-Clear EDIFACT
The example being a non-encrypted EDIFACT transaction with a digital
signature. With the proper selection of PEM parameters and
environment, this can also provide non-repudiation, but it does not
provide confidentiality.
Decoders that are capable of decrypting PEM treat the two keywords in
the same way, using the contained PEM headers to distinguish the
mode. Decoders that do not understand PEM can use the PEM-Clear
keyword as a hint that it may be useful to treat the section as text,
or even continue the decode sequence after removing the PEM headers.
When Encoding is used for PEM, the RFC934 [9] encapsulation specified
in RFC1421 is not used.
3.11 PGP
The PGP keyword indicates that the section is encrypted using the
Pretty Good Privacy specification, or is a public key block, keyring,
or detached signature meaningful to the PGP program. (These objects
are distinguished by internal information.)
The keyword actually implies 3 different transforms: a compression
step, the encryption, and an ASCII encoding. These transforms are
internal to the PGP encoder/decoder. A simple text message encrypted
with PGP is specified by:
Encoding: PGP Text
An EDI transaction using ANSI X12 might be:
Encoding: 176 PGP EDI-X12
Since an evesdropper can still "see" the nested type (Text or EDI in
these examples), thus making information available to traffic
analysis which is undesirable in some applications, the sender may
prefer to use:
Encoding: PGP Message
As discussed in the description of the Message keyword, the enclosed
object may have a complete header or consist only of an Encoding:
header describing its content.
When PGP is used to transmit an encoded key or keyring, with no
object significant to the mail user agent as a result of the decoding
(e.g., text to display), the keyword is used by itself.
Another case of the PGP keyword occurs in "clear-signing" a message.
That is, sending an un-encrypted message with a digital signature
providing authentication and (in some environments) non-deniability.
Encoding: 201 Text, 8 PGP Signature, 4 Text Signature
This example indicates a 201 line message, followed by an 8 line (in
its encoded form) PGP detached signature. The processing of the PGP
section is expected (in this example) to result in a text object that
is to be treated by the receiver as a signature, possibly something
like:
[PGP signed Ariel@Process.COM Robert L Ullmann VALID/TRUSTED]
Note that the PGP signature algorithm is applied to the encoded form
of the clear-text section, not the object(s) before encoding. (Which
would be quite difficult for encodings like tar or FS). Continuing
the example, the PGP signature is then followed by a 4 line
"ordinary" signature section.
3.12 Signature
The signature keyword indicates that the section contains an Internet
message signature. An Internet message signature is an area of an
Internet message (usually located at the end) which contains a single
line or multiple lines of characters. The signature may comprise the
sender"s name or a saying the sender is fond of. It is normally
inserted automatically in all outgoing message bodies. The encoding
keyword "Signature" must always be nested and follow another keyword.
Encoding: 14 Text, 3 Text Signature
A usenet news posting program should generate an encoding showing
which is the text and which is the signature area of the posted
message.
3.13 TAR
The tar keyword specifies a section consisting of the output of the
tar program supplied as part of Unix.
3.14 PostScript
The PostScript keyword specifies a section formatted according to the
PostScript [8] computer program language definition. PostScript is a
registered trademark of Adobe Systems Inc.
3.15 SHAR
The SHAR keyword specifies a section encoded in shell archive format.
Use of shar, although supported, is not recommended.
WARNING: Because the shell archive may contain commands you may not
want executed, the decoder should not automatically execute decoded
shell archived statements. This warning also applies to any future
types that include commands to be executed by the receiver.
3.16 Uniform Resource Locator
The URL keyword indicates that the section consists of zero or more
references to resources of some type. URL provides a facility to
include by reference arbitrary external resources from various
sources in the Internet. The specification of URL is a work in
progress in the URI working group of the IETF.
3.17 Registering New Keywords
New encoding keywords which are not reserved for implementation-
specific use must be registered with the Internet Assigned Numbers
Authority (IANA). IANA acts as a central registry for these values.
IANA may reject or modify the keyword registration request if it does
not meet the criteria as specified in section 3. Keywords beginning
with "X-" are permanently reserved to implementation-specific use.
IANA will not register an encoding keyword that begins with "X-".
Registration requests should be sent via electronic mail to IANA as
follows:
To: IANA@isi.edu
Subject: Registration of a new EHF-MAIL Keyword
The mail message must specify the keyword for the encoding and
acronyms if appropriate. Documentation defining the keyword and its
proposed purpose must be included. The documentation must either
reference an external non-Internet standards document or an existing
or soon to be RFC. If applicable, the documentation should contain a
draft version of the future RFC. The draft must be submitted as a
RFCaccording to the normal procedure within a reasonable amount of
time after the keyword"s registration has been approved.
4. FS (File System) Object Encoding
The file system encoding provides a standard, transportable encoding
of file system objects from many different operating systems. The
intent is to allow the moving of a structured set of files from one
environment to another while preserving common elements. At the same
time, files can be moved within a single environment while preserving
all attributes.
The representations consist of a series of nested sections, with
attributes defined at the appropriate levels. Each section begins
with an open bracket "[" followed by a directive keyword and ends
with a close bracket "]". Attributes are lines, beginning with a
keyword. Lines which begin with a LWSP (linear white space)
character are continuation lines.
Any string-type directive or attribute may be a simple string not
starting with a quotation mark ( " ) and not containing special
characters (e.g. newline) or LWSP (space and tab). The string name
begins with the first non-LWSP character on the line following the
attribute or directive keyword and ends with the last non-LWSP
character.
Otherwise, the character string name is enclosed in quotes. The
string itself contains characters in ISO-10646-UTF-1 but is quoted
and escaped at octet level (as elsewhere in RFC822 [2]). The strings
begin and end with a quotation mark ( " ). Octets equal to quote in
the string are escaped, as are octets equal to the escape characters
(" and ). The escaped octets may be part of a UTF multi-octet
character. Octets that are not printable are escaped with nnn octal
representation. When an escape () occurs at the end of a line, the
escape, the end of the line, and the first character of the next
line, which must be one of the LWSP characters, are removed
(ignored).
[ file Simple-File.Name
[ file " Long file name starting with spaces and having a couple
[sic] of nasties in it like this newline12near the end."
Note that in the above example, there is one space (not two) between
"couple" and "[sic]". The encoder may choose to use the nnn sequence
for any character that might cause trouble. Refer to section 5.1 for
line length recommendations.
4.1 Sections
A section starts with an open bracket, followed by a keyword that
defines the type of section.
The section keywords are:
directory
entry
file
segment
data
The encoding may start with either a file, directory or entry. A
directory section may contain zero or more file, entry, and directory
sections. A file section contains a data section or zero or more
segment sections. A segment section contains a data section or zero
or more segment sections.
4.1.1 Directory
This indicates the start of a directory. There is one parameter, the
entry name of the directory:
[ directory foo
...
]
4.1.2 Entry
The entry keyword represents an entry in a directory that is not a
file or a sub-directory. Examples of entries are soft links in Unix,
or access categories in Primos. A Primos access category might look
like this:
[ entry SYS.ACAT
type ACAT
created 27 Jan 1987 15:31:04.00
acl SYADMIN:* ARIEL:DALURWX $REST:
]
4.1.3 File
The file keyword is followed by the entry name of the file. The
section then continues with attributes, possibly segments, and then
data.
[ file MY.FILE
created 27 Feb 1987 12:10:20.07
modified 27 Mar 1987 16:17:03.02
type DAM
[ data LZJU90
* LZJU90
...
]]
4.1.4 Segment
This is used to define segments of a file. It should only be used
when encoding files that are actually segmented. The optional
parameter is the number or name of the segment.
When encoding Macintosh files, the two forks of the file are treated
as segments:
[ file A.MAC.FILE
display "A Mac File"
type MAC
comment "I created this myself"
...
[ segment resource
[ data ...
...
]]
[ segment data
[ data ...
...
]]]
4.1.5 Data
The data section contains the encoded data of the file. The encoding
method is defined in section 5. The data section must be last within
the containing section.
4.2 Attributes
Attributes may occur within file, entry, directory, and segment
sections. Attributes must occur before sub-sections.
The attribute directives are:
display
type
created
modified
accessed
owner
group
acl
password
block
record
application
4.2.1 Display
This indicates the display name of the object. Some systems, such as
the Macintosh, use a different form of the name for matching or
uniqueness.
4.2.2 Comment
This contains an arbitrary comment on the object. The Macintosh
stores this attribute with the file.
4.2.3 Type
The type of an object is usually of interest only to the operating
system that the object was created on.
Types are:
ACAT access category (Primos)
CAM contiguous access method (Primos)
DAM direct access method (Primos)
FIXED fixed length records (VMS)
FLAT `flat file", sequence of bytes (Unix, DOS, default)
ISAM indexed-sequential access method (VMS)
LINK soft link (Unix)
MAC Macintosh file
SAM sequential access method (Primos)
SEGSAM segmented direct access method (Primos)
SEGDAM segmented sequential access method (Primos)
TEXT lines of ISO-10646-UTF-1 text ending with CR/LF
VAR variable length records (VMS)
4.2.4 Created
Indicates the creation date of the file. Dates are in the format
defined in section 4.3.
4.2.5 Modified
Indicates the date and time the file was last modified or closed
after being open for write.
4.2.6 Accessed
Indicates the date and time the file was last accessed on the
original file system.
4.2.7 Owner
The owner directive gives the name or numerical ID of the owner or
creator of the file.
4.2.8 Group
The group directive gives the name(s) or numerical IDs of the group
or groups to which the file belongs.
4.2.9 ACL
This directive specifies the access control list attribute of an
object (the ACL attribute may occur more than once within an object).
The list consist of a series of pairs of IDs and access codes in the
format:
user-ID:access-list
There are four reserved IDs:
$OWNER the owner or creator
$GROUP a member of the group or groups
$SYSTEM a system administrator
$REST everyone else
The access list is zero or more single letters:
A add (create file)
D delete
L list (read directory)
P change protection
R read
U use
W write
X execute
* all possible access
4.2.10 Password
The password attribute gives the access password for this object.
Since the content of the object follows (being the raison d"etre of
the encoding), the appearance of the password in plain text is not
considered a security problem. If the password is actually set by
the decoder on a created object, the security (or lack) is the
responsibility of the application domain controlling the decoder as
is true of ACL and other protections.
4.2.11 Block
The block attribute gives the block size of the file as a decimal
number of bytes.
4.2.12 Record
The record attribute gives the record size of the file as a decimal
number of bytes.
4.2.13 Application
This specifies the application that the file was created with or
belongs to. This is of particular interest for Macintosh files.
4.3 Date Field
Various attributes have a date and time subsequent to and associated
with them.
4.3.1 Syntax
The syntax of the date field is a combination of date, time, and
timezone:
DD Mon YYYY HH:MM:SS.FFFFFF [+-]HHMMSS
Date := DD Mon YYYY 1 or 2 Digits " " 3 Alpha " " 4 Digits
DD := Day e.g. "08", " 8", "8"
Mon := Month "Jan" "Feb" "Mar" "Apr"
"May" "Jun" "Jul" "Aug"
"Sep" "Oct" "Nov" "Dec"
YYYY := Year
Time := HH:MM:SS.FFFFFF 2 Digits ":" 2 Digits [ ":" 2 Digits
["." 1 to 6 Digits ] ]
e.g. 00:00:00, 23:59:59.999999
HH := Hours 00 to 23
MM := Minutes 00 to 59
SS := Seconds 00 to 60 (60 only during a leap second)
FFFFF:= Fraction
Zone := [+-]HHMMSS "+" "-" 2 Digits [ 2 Digits
[ 2 Digits ] ]
HH := Local Hour Offset
MM := Local Minutes Offset
SS := Local Seconds Offset
4.3.2 Semantics
The date information is that which the file system has stored in
regard to the file system object. Date information is stored
differently and with varying degrees of precision by different
computer file systems. An encoder must include as much date
information as it has available concerning the file system object. A
decoder which receives an object encoded with a date field containing
greater precision than its own must disregard the excessive
information. Zone is Co-ordinated Universal Time "UTC" (formerly
called "Greenwich Mean Time"). The field specifies the time zone of
the file system object as an offset from Universal Time. It is
expressed as a signed [+-] two, four or six digit number.
A file that was created April 15, 1993 at 8:05 p.m. in Roselle Park,
New Jersey, U.S.A. might have a date field which looks like:
15 Apr 1993 20:05:22.12 -0500
5. LZJU90: Compressed Encoding
LZJU90 is an encoding for a binary or text object to be sent in an
Internet mail message. The encoding provides both compression and
representation in a text format that will successfully survive
transmission through the many different mailers and gateways that
comprise the Internet and connected mail networks.
5.1 Overview
The encoding first compresses the binary object, using a modified
LZ77 algorithm, called LZJU90. It then encodes each 6 bits of the
output of the compression as a text character, using a character set
chosen to survive any translations between codes, such as ASCII to
EBCDIC. The 64 six-bit strings 000000 through 111111 are represented
by the characters "+", "-", "0" to "9", "A" to "Z", and "a" to "z".
The output text begins with a line identifying the encoding. This is
for visual reference only, the "Encoding:" field in the header
identifies the section to the user program. It also names the object
that was encoded, usually by a file name.
The format of this line is:
* LZJU90 <name>
where <name> is optional. For example:
* LZJU90 vmunix
This is followed by the compressed and encoded data, broken into
lines where convenient. It is recommended that lines be broken every
78 characters to survive mailers than incorrectly restrict line
length. The decoder must accept lines with 1 to 1000 characters on
each line. After this, there is one final line that gives the number
of bytes in the original data and a CRC of the original data. This
should match the byte count and CRC found during decompression.
This line has the format:
* <count> <CRC>
where <count> is a decimal number, and CRC is 8 hexadecimal digits.
For example:
* 4128076 5AC2D50E
The count used in the Encoding: field in the message header is the
total number of lines, including the start and end lines that begin
with *. A complete example is given in section 5.3.2.
5.2 Specification of the LZJU90 compression
The Lempel-Ziv-Storer-Szymanski model of mixing pointers and literal
characters is used in the compression algorithm. Repeat occurrences
of strings of octets are replaced by pointers to the earlier
occurrence.
The data compression is defined by the decoding algorithm. Any
encoder that emits symbols which cause the decoder to produce the
original input is defined to be valid.
There are many possible strategies for the maximal-string matching
that the encoder does, section 5.3.1 gives the code for one such
algorithm. Regardless of which algorithm is used, and what tradeoffs
are made between compression ratio and execution speed or space, the
result can always be decoded by the simple decoder.
The compressed data consists of a mixture of unencoded literal
characters and copy pointers which point to an earlier occurrence of
the string to be encoded.
Compressed data contains two types of codewords:
LITERAL pass the literal directly to the uncompressed output.
COPY length, offset
go back offset characters in the output and copy length
characters forward to the current position.
To distinguish between codewords, the copy length is used. A copy
length of zero indicates that the following codeword is a literal
codeword. A copy length greater than zero indicates that the
following codeword is a copy codeword.
To improve copy length encoding, a threshold value of 2 has been
suBTracted from the original copy length for copy codewords, because
the minimum copy length is 3 in this compression scheme.
The maximum offset value is set at 32255. Larger offsets offer
extremely low improvements in compression (less than 1 percent,
typically).
No special encoding is done on the LITERAL characters. However,
unary encoding is used for the copy length and copy offset values to
improve compression. A start-step-stop unary code is used.
A (start, step, stop) unary code of the integers is defined as
follows: The Nth codeword has N ones followed by a zero followed by
a field of size START + (N * STEP). If the field width is equal to
STOP then the preceding zero can be omitted. The integers are laid
out sequentially through these codewords. For example, (0, 1, 4)
would look like:
Codeword Range
0 0
10x 1-2
110xx 3-6
1110xxx 7-14
1111xxxx 15-30
Following are the actual values used for copy length and copy offset:
The copy length is encoded with a (0, 1, 7) code leading to a maximum
copy length of 256 by including the THRESHOLD value of 2.
Codeword Range
0 0
10x 3-4
110xx 5-8
1110xxx 9-16
11110xxxx 17-32
111110xxxxx 33-64
1111110xxxxxx 65-128
1111111xxxxxxx 129-256
The copy offset is encoded with a (9, 1, 14) code leading to a
maximum copy offset of 32255. Offset 0 is reserved as an end of
compressed data flag.
Codeword Range
0xxxxxxxxx 0-511
10xxxxxxxxxx 512-1535
110xxxxxxxxxxx 1536-3583
1110xxxxxxxxxxxx 3485-7679
11110xxxxxxxxxxxxx 7680-15871
11111xxxxxxxxxxxxxx 15872-32255
The 0 has been chosen to signal the start of the field for ease of
encoding. (The bit generator can simply encode one more bit than is
significant in the binary representation of the excess.)
The stop values are useful in the encoding to prevent out of range
values for the lengths and offsets, as well as shortening some codes
by one bit.
The worst case compression using this scheme is a 1/8 increase in
size of the encoded data. (One zero bit followed by 8 character
bits). After the character encoding, the worst case ratio is 3/2 to
the original data.
The minimum copy length of 3 has been chosen because the worst case
copy length and offset is 3 bits (3) and 19 bits (32255) for a total
of 22 bits to encode a 3 character string (24 bits).
5.3 The Decoder
As mentioned previously, the compression is defined by the decoder.
Any encoder that produced output that is correctly decoded is by
definition correct.
The following is an implementation of the decoder, written more for
clarity and as much portability as possible, rather than for maximum
speed.
When optimized for a specific environment, it will run significantly
faster.
/* LZJU 90 Decoding program */
/* Written By Robert Jung and Robert Ullmann, 1990 and 1991. */
/* This code is NOT COPYRIGHT, not protected. It is in the true
Public Domain. */
#include <stdio.h>
#include <string.h>
typedef unsigned char uchar;
typedef unsigned int uint;
#define N 32255
#define THRESHOLD 3
#define STRTP 9
#define STEPP 1
#define STOPP 14
#define STRTL 0
#define STEPL 1
#define STOPL 7
static FILE *in;
static FILE *out;
static int getbuf;
static int getlen;
static long in_count;
static long out_count;
static long crc;
static long crctable[256];
static uchar xxcodes[] =
"+-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ
abcdefghijklmnopqrstuvwxyz";
static uchar ddcodes[256];
static uchar text[N];
#define CRCPOLY 0xEDB88320
#define CRC_MASK 0xFFFFFFFF
#define UPDATE_CRC(crc, c)
crc = crctable[((uchar)(crc) ^ (uchar)(c)) & 0xFF]
^ (crc >> 8)
#define START_RECD "* LZJU90"
void MakeCrctable() /* Initialize CRC-32 table */
{
uint i, j;
long r;
for (i = 0; i <= 255; i++) {
r = i;
for (j = 8; j > 0; j--) {
if (r & 1)
r = (r >> 1) ^ CRCPOLY;
else
r >>= 1;
}
crctable[i] = r;
}
}
int GetXX() /* Get xxcode and translate */
{
int c;
do {
if ((c = fgetc(in)) == EOF)
c = 0;
} while (c == "n");
in_count++;
return ddcodes[c];
}
int GetBit() /* Get one bit from input buffer */
{
int c;
while (getlen <= 0) {
c = GetXX();
getbuf = c << (10-getlen);
getlen += 6;
}
c = (getbuf & 0x8000) != 0;
getbuf <<= 1;
getbuf &= 0xFFFF;
getlen--;
return(c);
}
int GetBits(int len) /* Get len bits */
{
int c;
while (getlen <= 10) {
c = GetXX();
getbuf = c << (10-getlen);
getlen += 6;
}
if (getlen < len) {
c = (uint)getbuf >> (16-len);
getbuf = GetXX();
c = getbuf >> (6+getlen-len);
getbuf <<= (10+len-getlen);
getbuf &= 0xFFFF;
getlen -= len - 6;
}
else {
c = (uint)getbuf >> (16-len);
getbuf <<= len;
getbuf &= 0xFFFF;
getlen -= len;
}
return(c);
}
int DecodePosition() /* Decode offset position pointer */
{
int c;
int width;
int plus;
int pwr;
plus = 0;
pwr = 1 << STRTP;
for (width = STRTP; width < STOPP; width += STEPP) {
c = GetBit();
if (c == 0)
break;
plus += pwr;
pwr <<= 1;
}
if (width != 0)
c = GetBits(width);
c += plus;
return(c);
}
int DecodeLength() /* Decode code length */
{
int c;
int width;
int plus;
int pwr;
plus = 0;
pwr = 1 << STRTL;
for (width = STRTL; width < STOPL; width += STEPL) {
c = GetBit();
if (c == 0)
break;
plus += pwr;
pwr <<= 1;
}
if (width != 0)
c = GetBits(width);
c += plus;
return(c);
}
void InitCodes() /* Initialize decode table */
{
int i;
for (i = 0; i < 256; i++) ddcodes[i] = 0;
for (i = 0; i < 64; i++) ddcodes[xxcodes[i]] = i;
return;
}
main(int ac, char **av) /* main program */
{
int r;
int j, k;
int c;
int pos;
char buf[80];
char name[3];
long num, bytes;
if (ac < 3) {
fprintf(stderr, "usage: judecode in outn");
return(1);
}
in = fopen(av[1], "r");
if (!in){
fprintf(stderr, "Can"t open %sn", av[1]);
return(1);
}
out = fopen(av[2], "wb");
if (!out) {
fprintf(stderr, "Can"t open %sn", av[2]);
fclose(in);
return(1);
}
while (1) {
if (fgets(buf, sizeof(buf), in) == NULL) {
fprintf(stderr, "Unexpected EOFn");
return(1);
}
if (strncmp(buf, START_RECD, strlen(START_RECD)) == 0)
break;
}
in_count = 0;
out_count = 0;
getbuf = 0;
getlen = 0;
InitCodes();
MakeCrctable();
crc = CRC_MASK;
r = 0;
while (feof(in) == 0) {
c = DecodeLength();
if (c == 0) {
c = GetBits(8);
UPDATE_CRC(crc, c);
out_count++;
text[r] = c;
fputc(c, out);
if (++r >= N)
r = 0;
}
else {
pos = DecodePosition();
if (pos == 0)
break;
pos--;
j = c + THRESHOLD - 1;
pos = r - pos - 1;
if (pos < 0)
pos += N;
for (k = 0; k < j; k++) {
c = text[pos];
text[r] = c;
UPDATE_CRC(crc, c);
out_count++;
fputc(c, out);
if (++r >= N)
r = 0;
if (++pos >= N)
pos = 0;
}
}
}
fgetc(in); /* skip newline */
if (fscanf(in, "* %ld %lX", &bytes, &num) != 2) {
fprintf(stderr, "CRC record not foundn");
return(1);
}
else if (crc != num) {
fprintf(stderr,
"CRC error, expected %lX, found %lXn",
crc, num);
return(1);
}
else if (bytes != out_count) {
fprintf(stderr,
"File size error, expected %lu, found %lun",
bytes, out_count);
return(1);
}
else
fprintf(stderr,
"File decoded to %lu bytes correctlyn",
out_count);
fclose(in);
fclose(out);
return(0);
}
5.3.1 An example of an Encoder
Many algorithms are possible for the encoder, with different
tradeoffs between speed, size, and complexity. The following is a
simple example program which is fairly efficient; more sophisticated
implementations will run much faster, and in some cases produce
somewhat better compression.
This example also shows that the encoder need not use the entire
window available. Not using the full window costs a small amount of
compression, but can greatly increase the speed of some algorithms.
/* LZJU 90 Encoding program */
/* Written By Robert Jung and Robert Ullmann, 1990 and 1991. */
/* This code is NOT COPYRIGHT, not protected. It is in the true
Public Domain. */
#include <stdio.h>
typedef unsigned char uchar;
typedef unsigned int uint;
#define N 24000 /* Size of window buffer */
#define F 256 /* Size of look-ahead buffer */
#define THRESHOLD 3
#define K 16384 /* Size of hash table */
#define STRTP 9
#define STEPP 1
#define STOPP 14
#define STRTL 0
#define STEPL 1
#define STOPL 7
#define CHARSLINE 78
static FILE *in;
static FILE *out;
static int putlen;
static int putbuf;
static int char_ct;
static long in_count;
static long out_count;
static long crc;
static long crctable[256];
static uchar xxcodes[] =
"+-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ
abcdefghijklmnopqrstuvwxyz";
uchar window_text[N + F + 1];
/* text contains window, plus 1st F of window again
(for comparisons) */
uint hash_table[K];
/* table of pointers into the text */
#define CRCPOLY 0xEDB88320
#define CRC_MASK 0xFFFFFFFF
#define UPDATE_CRC(crc, c)
crc = crctable[((uchar)(crc) ^ (uchar)(c)) & 0xFF]
^ (crc >> 8)
void MakeCrctable() /* Initialize CRC-32 table */
{
uint i, j;
long r;
for (i = 0; i <= 255; i++) {
r = i;
for (j = 8; j > 0; j--) {
if (r & 1)
r = (r >> 1) ^ CRCPOLY;
else
r >>= 1;
}
crctable[i] = r;
}
}
void PutXX(int c) /* Translate and put xxcode */
{
c = xxcodes[c & 0x3F];
if (++char_ct > CHARSLINE) {
char_ct = 1;
fputc("n", out);
}
fputc(c, out);
out_count++;
}
void PutBits(int c, int len) /* Put rightmost "len" bits of "c" */
{
c <<= 16 - len;
c &= 0xFFFF;
putbuf = (uint) c >> putlen;
c <<= 16 - putlen;
c &= 0xFFFF;
putlen += len;
while (putlen >= 6) {
PutXX(putbuf >> 10);
putlen -= 6;
putbuf <<= 6;
putbuf &= 0xFFFF;
putbuf = (uint) c >> 10;
c = 0;
}
}
void EncodePosition(int ch) /* Encode offset position pointer */
{
int width;
int prefix;
int pwr;
pwr = 1 << STRTP;
for (width = STRTP; ch >= pwr; width += STEPP, pwr <<= 1)
ch -= pwr;
if ((prefix = width - STRTP) != 0)
PutBits(0xffff, prefix);
if (width < STOPP)
width++;
/* else if (width > STOPP)
abort(); do nothing */
PutBits(ch, width);
}
void EncodeLength(int ch) /* Encode code length */
{
int width;
int prefix;
int pwr;
pwr = 1 << STRTL;
for (width = STRTL; ch >= pwr; width += STEPL, pwr <<= 1)
ch -= pwr;
if ((prefix = width - STRTL) != 0)
PutBits(0xffff, prefix);
if (width < STOPL)
width++;
/* else if (width > STOPL)
abort(); do nothing */
PutBits(ch, width);
}
main(int ac, char **av) /* main program */
{
uint r, s, i, c;
uchar *p, *rp;
int match_position;
int match_length;
int len;
uint hash, h;
if (ac < 3) {
fprintf(stderr, "usage: juencode in outn");
return(1);
}
in = fopen(av[1], "rb");
if (!in) {
fprintf(stderr, "Can"t open %sn", av[1]);
return(1);
}
out = fopen(av[2], "w");
if (!out) {
fprintf(stderr, "Can"t open %sn", av[2]);
fclose(in);
return(1);
}
char_ct = 0;
in_count = 0;
out_count = 0;
putbuf = 0;
putlen = 0;
hash = 0;
MakeCrctable();
crc = CRC_MASK;
fprintf(out, "* LZJU90 %sn", av[1]);
/* The hash table inititialization is somewhat arbitrary */
for (i = 0; i < K; i++) hash_table[i] = i % N;
r = 0;
s = 0;
/* Fill lookahead buffer */
for (len = 0; len < F && (c = fgetc(in)) != EOF; len++) {
UPDATE_CRC(crc, c);
in_count++;
window_text[s++] = c;
}
while (len > 0) {
/* look for match in window at hash position */
h = ((((window_text[r] << 5) ^ window_text[r+1])
<< 5) ^ window_text[r+2]);
p = window_text + hash_table[h % K];
rp = window_text + r;
for (i = 0, match_length = 0; i < F; i++) {
if (*p++ != *rp++) break;
match_length++;
}
match_position = r - hash_table[h % K];
if (match_position <= 0) match_position += N;
if (match_position > N - F - 2) match_length = 0;
if (match_position > in_count - len - 2)
match_length = 0; /* ! :-) */
if (match_length > len)
match_length = len;
if (match_length < THRESHOLD) {
EncodeLength(0);
PutBits(window_text[r], 8);
match_length = 1;
}
else {
EncodeLength(match_length - THRESHOLD + 1);
EncodePosition(match_position);
}
for (i = 0; i < match_length &&
(c = fgetc(in)) != EOF; i++) {
UPDATE_CRC(crc, c);
in_count++;
window_text[s] = c;
if (s < F - 1)
window_text
[s + N] = c;
if (++s > N - 1) s = 0;
hash = ((hash << 5) ^ window_text[r]);
if (r > 1) hash_table[hash % K] = r - 2;
if (++r > N - 1) r = 0;
}
while (i++ < match_length) {
if (++s > N - 1) s = 0;
hash = ((hash << 5) ^ window_text[r]);
if (r > 1) hash_table[hash % K] = r - 2;
if (++r > N - 1 ) r = 0;
len--;
}
}
/* end compression indicator */
EncodeLength(1);
EncodePosition(0);
PutBits(0, 7);
fprintf(out, "n* %lu %08lXn", in_count, crc);
fprintf(stderr, "Encoded %lu bytes to %lu symbolsn",
in_count, out_count);
fclose(in);
fclose(out);
return(0);
}
5.3.2 Example LZJU90 Compressed Object
The following is an example of an LZJU90 compressed object. Using
this as source for the program in section 5.3 will reveal what it is.
Encoding: 7 LZJU90 Text
* LZJU90 example
8-mBtWA7WBVZ3dEBtnCNdU2WkE4owW+l4kkaApW+o4Ir0k33Ao4IE4kk
bYtk1XY618NnCQl+OHQ61d+J8FZBVVCVdClZ2-LUI0v+I4EraItasHbG
VVg7c8tdk2lCBtr3U86FZANVCdnAcUCNcAcbCMUCdicx0+u4wEETHcRM
7tZ2-6Btr268-Eh3cUAlmBth2-IUo3As42laIE2Ao4Yq4G-cHHT-wCEU
6tjBtnAci-I++
* 190 081E2601
6. Alphabetical Listing of Defined Encodings
Keyword Description Section Reference(s)
_______ ___________ _______ ____________
EDIFACT EDIFACT format 3.5
EDI-X12 EDI X12 format 3.5 ANSI X12
EVFU FORTRAN format 3.4
FS File System format 3.6, 4
Hex Hex binary format 3.3
LZJU90 LZJU90 format 3.7, 5
LZW LZW format 3.8
Message Encapsulated Message 3.2 STD 11, RFC822
PEM, PEM-Clear Privacy Enhanced Mail 3.10 RFC1421-1424
PGP Pretty Good Privacy 3.11
Postscript Postscript format 3.14 [8]
Shar Shell Archive format 3.15
Signature Signature 3.12
Tar Tar format 3.13
Text Text 3.1 IS 10646
uuencode uuencode format 3.9
URL external URL-reference 3.16
7. Security Considerations
Security of content and the receiving (decoding) system is discussed
in sections 3.10, 3.11, 3.15, and 4.2.10. The considerations
mentioned also apply to other encodings and attributes with similar
functions.
8. References
[1] Robinson, D. and R. Ullmann, "Encoding Header Field for Internet
Messages", RFC1154, Prime Computer, Inc., April 1990.
[2] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC822, University of Delaware, August 1982.
[3] International Organization for Standardization, Information
Technology -- Universal Coded Character Set (UCS). ISO/IEC
10646-1:1993, June 1993.
[4] Linn, J., "Privacy Enhancement for Internet Electronic Mail: Part
I: Message Encryption and Authentication Procedures" RFC1421,
IAB IRTF PSRG, IETF PEM WG, February 1993.
[5] Kent, S., "Privacy Enhancement for Internet Electronic Mail: Part
II: Certificate-Based Key Management", RFC1422, IAB IRTF PSRG,
IETF PEM, BBN, February 1993.
[6] Balenson, D., "Privacy Enhancement for Internet Electronic Mail:
Part III: Algorithms, Modes, and Identifiers", RFC1423, IAB IRTF
PSRG, IETF PEM WG, TIS, February 1993.
[7] Kaliski, B., "Privacy Enhancement for Internet Electronic Mail:
Part IV: Key Certification and Related Services", RFC1424, RSR
Laboratories, February 1993.
[8] Adobe Systems Inc., PostScript Language Reference Manual. 2nd
Edition, 2nd Printing, January 1991.
[9] Rose, M. and E. Steffererud, "Proposed Standard for Message
Encapsulation", RFC934, Delaware and NMA, January 1985.
[10] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC821,
USC/Information Sciences Institute, August 1982.
[11] Borenstein, N., and N. Freed, "MIME (Multipurpose Internet Mail
Extensions): Mechanisms for Specifying and Describing the Format
of Internet Message Bodies", RFC1341, Bellcore, Innosoft, June
1992.
[12] Borenstein, N., and M. Linimon, "Extension of MIME Content-Types
to a New Medium", RFC1437, 1 April 1993.
9. Acknowledgements
The authors would like to thank Robert Jung for his contributions to
this work, in particular the public domain sample code for LZJU90.
10. Authors" Addresses
Albert K. Costanzo
AKC Consulting Inc.
P.O. Box 4031
Roselle Park, NJ 07204-0531
Phone: +1 908 298 9000
Email: AL@AKC.COM
David Robinson
Computervision Corporation
100 Crosby Drive
Bedford, MA 01730
Phone: +1 617 275 1800 x2774
Email: DRB@Relay.CV.COM
Robert Ullmann
Phone: +1 617 247 7959
Email: ariel@world.std.com