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UNIX fun stuff - echo and dc - obfuscate/garble a string sort of


 
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# 1  
UNIX fun stuff - echo and dc - obfuscate/garble a string sort of

Hi,

Humorous UNIX Commands shows a fun way of using echo and dc to sort of obfuscate a string.

Code:
% echo '[q]sa[ln0=aln256%Pln256/snlbx]sb3135071790101768542287578439snlbxq'|dc
  GET A LIFE!

I am just wanting to know if there is a way to sort of use dc and echo to print out an obfuscated/garbled string instead kinda doing the reverse of echo | dc? Should it be dc 'GET A LIFE!' | echo, that does not seem to be the way to it Smilie

So I want to be able to get the echo'ed string instead. I am trying to see if I can use this as some way of sending out a 'secret' message or password to a script or to a user so that when I reset their password, I'll just get them to do echo '<string>' | dc instead of sending them the plaintext password.

Any advise much appreciated. Thanks.
# 2  
First: Note that:
Code:
dc <<EOF | echo
obfuscated string
EOF

produces exactly the same output as:
Code:
echo

The echo utility doesn't read arguments from standard input; it reads arguments from its argument list.

Second: If you're going to email a new password to your users, sending it to them in clear text and sending it to them with instructions saying something like:
Quote:
You can see your new password by executing the command:
echo 'obfuscated string' | dc
are equivalent. Anyone who intercepts that message will be able to run the command to produce the clear text just as easily as the user to whom the e-mail was addressed.

Third: If you send the obfuscated password and the way to turn it into plain text in separate e-mails, you have a better chance of keeping the clear text hidden. But if one e-mail message has been intercepted, there is no reason to believe that another message (containing the way to turn the obfuscated password into clear text) will not also be intercepted.

Fourth: You would be MUCH better off providing a secure method for the user to retrieve their password by distributing the new password:
  • in person (verifying employee IDs if you don't know your users),
  • providing a secure website (requiring the old password to get the new password), or
  • by a secure method of distributing hard copy of the new password (such as registered US snail mail).
This User Gave Thanks to Don Cragun For This Post:
# 3  
Hi Don,

Thanks for your reply, very much appreciated. Totally agree with what you suggested RE: delivering password security.

Just to clarify though, does that mean it is not possible to do the reverse of
Code:
echo '[q]sa[ln0=aln256%Pln256/snlbx]sb3135071790101768542287578439snlbxq'|dc

? Smilie

I am a bit curious of a way to a generate a 'magic'/garbled string of some sort. For example, how do I work out what string echo | dc would give me 'Hello World'? It does not have to be echo | dc, perhaps there is another UNIX command that is easier to use that can achieve the same thing.

I know it is trivial. I guess that is why I say UNIX fun Smilie I want to create a simple tagline of some sort to use for my scripts so I can more easily identify which ones I've written and not tampered with.

Last edited by Scrutinizer; 03-19-2015 at 06:56 AM.. Reason: CODE tags
# 4  
If you save the following script in a file named obfuscate:
Code:
#!/bin/ksh
# NAME:		obfuscate -- obfuscate text read from one or more files into a
#			     string that can be decrypted by the dc utility
#
# SYNOPSIS:	obfuscate file...
#
# OPERANDS:	file	The name of a text file containing text to be
#			obfuscated or "-" to obfuscate text read from
#			standard input.
#
# APPLICATION USAGE:
#	To decrypt the obfuscated string produced by obfuscate, use:
#		obfuscate file | read string	# Get obfuscated text
# 	Note: Do not use "read -r string" in the above command!
#		printf '%s\n' "$string"		# Show obfuscated text
#		echo "$string" | dc		# Decrypt obfuscated text
#
#	Although dc can produce artibrary length output, feeding the objuscated
#	string back into dc for decryption may be limited by {LINE_MAX} and/or
#	{ARG_MAX} restrictions.

# Initialize a to ASCII character set (skipping NUL)...
a='\1\2\3\4\5\6\7\10\11\12\13\14\15\16\17\20\21\22\23\24\25\26\27\30\31\32\33'
a="$a"'\34\35\36\37 !"#$%&'"'"'()*+,-./0123456789:;<=>?@'
a="$a"'ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~\377'

awk -v A="$a" '
function cline(inline,		i) {
	printf("256*%d+\n", index(A, "\n"))
	for(i = length(inline); i; i--) {
		printf("256*%d+\n", index(A, substr(inline, i, 1)))
	}
}
BEGIN {	print 0
}
{	line[NR] = $0
}
END {	for(i = NR; i; i--)
		cline(line[i])
	printf("[[q]sa[ln0=aln256%%Pln256/snlbx]sb]Pn[snlbxq\n]Pq\n")
}' "$@" | tee .dc_input | dc

make it executable with:
Code:
chmod +x obfuscate

and execute the command:
Code:
printf "Hello World.\nAre we there yet?\nLet's go home, now!\n" | ./obfuscate - | read string

then the command:
Code:
echo "$string"

will give you:
Code:
[q]sa[ln0=aln256%Pln256/snlbx]sb26160072918627741401952510855241017735603346265259888938898171600856988789569756293233903076568696999873394858335331444040snlbxq

and the command:
Code:
echo "$string"|dc

will give you:
Code:
Hello World.
Are we there yet?
Let's go home, now!

Is this what you're looking for? If won't work with characters that aren't in the 7-bit ASCII character set and it won't work if the text you want to obfuscate contains any NUL bytes, and on many systems it won't work if line in files you want to obfuscate are longer than LINE_MAX bytes and if the output produced by obfuscate produces a string longer than LINE_MAX, dc may not be able to decrypt it for you on some systems.

If you want to try this on a Solaris/SunOS system, change awk to /usr/xpg4/bin/awk.

The standards didn't require awk to treat a file name operand of - as a request to read standard input (although most implementations of awk already did this) until the 2013 editions of the POSIX Standards and the Single UNIX Specifications. If the implementation of awk on your system gives an diagnostic message similar to:
Code:
awk: can't open file -

you'll need a two step process (or three step, if you remove the temp file when you're done) similar to:
Code:
printf "Hello World.\nAre we there yet?\nLet's go home, now!\n" > ob$$
./obfuscate ob$$ | read string
rm -f ob$$

to make it work.

If you want to see the dc commands awk created in obfuscate for the input you fed it, the command:
Code:
cat .dc_input

after running obfuscate will show you the input dc processed for the last invocation of obfuscate. For the above commands, that would be:
Code:
0
256*10+
256*33+
256*119+
256*111+
256*110+
256*32+
256*44+
256*101+
256*109+
256*111+
256*104+
256*32+
256*111+
256*103+
256*32+
256*115+
256*39+
256*116+
256*101+
256*76+
256*10+
256*63+
256*116+
256*101+
256*121+
256*32+
256*101+
256*114+
256*101+
256*104+
256*116+
256*32+
256*101+
256*119+
256*32+
256*101+
256*114+
256*65+
256*10+
256*46+
256*100+
256*108+
256*114+
256*111+
256*87+
256*32+
256*111+
256*108+
256*108+
256*101+
256*72+
[[q]sa[ln0=aln256%Pln256/snlbx]sb]Pn[snlbxq
]Pq

Are we having fun yet? Smilie
These 4 Users Gave Thanks to Don Cragun For This Post:
 

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