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CODEGROUP(1) USER COMMANDS CODEGROUP(1)
NAME
codegroup - encode / decode binary file as five letter code-
groups
SYNOPSIS
codegroup -d|-e [ -u ] [ infile [ outfile ] ]
DESCRIPTION
For decades, spies have written their encoded messages in
groups of five letters.
codegroup encodes any binary file into this form, allowing
it to be transmitted through any medium, and decodes files
containing codegroups into the original input. Encoded
files contain a 16-bit cyclical redundancy check (CRC) and
file size to verify, when decoded, that the message is com-
plete and correct. Files being decoded may contain other
information before and after the codegroups, allowing in-
the-clear annotations to be included.
codegroup makes no attempt, on its own, to prevent your mes-
sage from being read. Cryptographic security should be
delegated to a package intended for that purpose, such as
pgp. codegroup can then be applied to the encrypted binary
output, transforming it into easily transmitted text. Text
created by codegroup uses only upper case ASCII letters and
spaces. Unlike files encoded with uuencode or pgp's ``ASCII
armour'' facility, the output of codegroup can be easily
(albeit tediously) read over the telephone, broadcast on
shortwave radio to agents in the field, or sent by telegram,
telex, or Morse code.
To illustrate the difference, here are the first few lines
of a binary file encoded by:
uuencode:
begin 644 data.bin
M'XL("&7._R\ VUO;V\ /9U+FN2XSF3G6H5OA1(?HOB<=/<7__X7TNXJ^&PI/\[?7-7U]KU=]SSWQ?
pgp:
-----BEGIN PGP MESSAGE-----
Version: 2.6.2i
hIwCCb8iTku3pBUBA/9oSDlfk/On9bwjmTnB98Eejr6agkPSi3n6hd8JkAtJd33f
kzFq18Jo0xzRUWZ7Di6Jq/FXpeI1yztVDqispbcYOP0aDv4JZOSF1kRsmJ9xK9Bo
Cv4a967IXPkkRsjIAkx0B39dYxCzf8kHUn4THmyV/b2qLUZ0cc+mr8hxFfFpuYSM
codegroup:
ZZZZZ YBPIL AIAIG FMOPP CPAAA DGNGP GPGPA ADNJN ELJKO ELIMO
GEOHF KIFGP IFBCB PKCPI YJMHE PHBHP PPOBH NCOHD AKLLL AGHFP
DEGEF LKELC EAIJI ABAGP AHPPO IHHPH OHPDF YNFPB ALEPO KMPKP
NGCHI GFPBI CBDML PFGHL LIHPC BOOBB HOLDO FJNHP OLHLL OPNIL
Only codegroup conforms to the telegraphic convention of all
upper case letters. Avoiding punctuation marks and lower
case letters makes the output of codegroup much easier to
transmit over a voice or traditional telegraphic link.
OPTIONS
-decode Decodes the input, previously created by code-
group, to recover the original input file, and
verifies it to detect truncation or corruption
of the contents.
-encode Encodes the input into an output text file con-
taining five letter code groups (default).
-usage Print how-to-call information.
All options may be abbreviated to a single letter.
APPLICATION NOTES
Encoding a binary file as ASCII characters inevitably
increases its size. When used in conjunction with existing
compression and encryption tools, the resulting growth in
file size is usually acceptable. For example, a random
extract of electronic mail 32768 bytes in length was chosen
as a test sample. Compression with gzip compacted the file
to 15062 bytes. It was then encrypted for transmission to a
single recipient with pgp, which resulted in a 15233 byte
file. (Even though pgp has its own compression, smaller
files usually result from initial compression with gzip. In
this case, pgp alone would have produced a file of 15420
bytes.)
codegroup transforms the encrypted file into a 37296 byte
text file. Thus, due to compression, the code groups for
the encrypted file are only a little larger than the origi-
nal cleartext.
Restricting the character set and including spaces between
groups results in substantially larger output files than
those produced by uuencode and pgp. Files encoded with
codegroup are about 2.5 times the size of the input file,
while uuencode and pgp expand the file only about 35%.
codegroup is thus preferable only for applications where its
limited character set is an advantage.
FILES
If no infile is specified or infile is a single ``-'', code-
group reads from standard input; if no outfile is given, or
outfile is a single ``-'', output is sent to standard out-
put. The input and output are processed strictly serially;
consequently codegroup may be used in pipelines.
BUGS
When a CRC error is detected, no indication is given of the
location in the file where the error(s) occurred. When
sending large files, you may want to break them into pieces
with the splits utility (available from the Web page cited
below) so, in case of error, only the erroneous pieces have
to be re-sent.
It might be nice to embed the original file name and modes
in the encoded output, but this opens the door to all kinds
of system-dependent problems. You can always include this
information as text before the first codegroup, or send an
archive created with tar or zip.
SEE ALSO
gzip(1), pgp(1), splits(1), tar(1), uuencode(1), zip(1)
EXIT STATUS
codegroup returns status 0 if processing was completed
without errors, 1 if errors were detected in decoding a file
which indicate the output is incorrect or incomplete, and 2
if processing could not be performed at all due, for exam-
ple, to a nonexistent input file or no codegroups found in
the input.
AUTHOR
John Walker
kelvin@fourmilab.ch
This software is in the public domain. Permission to use,
copy, modify, and distribute this software and its documen-
tation for any purpose and without fee is hereby granted,
without any conditions or restrictions. This software is
provided ``as is'' without express or implied warranty.