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CODEGROUP(1) CODEGROUP(1)
NAME
codegroup - encode / decode binary file as five letter
codegroups
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
complete and correct. Files being decoded may contain
other information before and after the codegroups, allow-
ing in-the-clear annotations to be included.
codegroup makes no attempt, on its own, to prevent your
message 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 uuen-
code 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:
base64:
H4sICFJ9MzYAA2EudGFyAOxba3faSNKer+lf0SezO3YmgLnY2I6TyQIGgwOGBTtOYjuJEMJo
DJJGF1+ys//9rarulpqLHRi/mdk9G84JIKGuqq579eNkNn745q9sNru9tcXhs5gtFPAzm83l
xad88WyxmNssbhe3sps8m8ttZ/M/8K1vL9oPP0RBaPggypU1vrad+59zosj0HqAj9xF//pe8
WsaVNbTH1rfkAfoobm7ea//cZn4rtv/mNtq/kM9t/cCz31Io9foftz9nnW77oMdfcdMdWJe+
uuencode:
begin 644 data.bin
M'XL("&7._R\ VUO;V\ /9U+FN2XSF3G6H5OA1(?HOB<=/<7__X7TN<PJ[L&
M=?-&1;I+) B8 0;P?_Z'?WY_-=7Q"T_JSZ_6)X9?&"$\OU9[N'\A[A%^L^6=
M?^M[OOV+:9=UM9J^] MAS_ ;X0O]U];(Z?<WWE9_\^[/]ZMM\OO[CG'^2M\M
M_G(+,US/LWKZE1#C^YO?D_;O#G[7][2R^+0>XJ^&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, and passes the ``telephone test''
of being readable without any modifiers such as ``capi-
tal'' and ``lower-case''. Avoiding punctuation marks and
lower case letters makes the output of codegroup much eas-
ier 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
containing 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 exist-
ing 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 cho-
sen as a test sample. Compression with gzip compacted the
file to 15062 bytes. It was then encrypted for transmis-
sion to a single recipient with pgp, which resulted in a
15233 byte file. (Even though pgp has its own compres-
sion, smaller files usually result from initial compres-
sion with gzip. In this case, pgp alone would have pro-
duced 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 orig-
inal 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 ``-'',
codegroup reads from standard input; if no outfile is
given, or outfile is a single ``-'', output is sent to
standard output. 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 code-
group, or send an archive created with tar or zip.
SEE ALSO
base64(1), gzip(1), pgp(1), splits(1), tar(1), uuen-
code(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
example, to a nonexistent input file or no codegroups
found in the input.
AUTHOR
John Walker
http://www.fourmilab.ch/
This software is in the public domain. Permission to use,
copy, modify, and distribute this software and its docu-
mentation 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.
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