07-21-2007, 04:47 PM
| | Re: cipher question
On Sat, 21 Jul 2007, in the Usenet newsgroup comp.security.misc, in article
<firstname.lastname@example.org .com>, Eman On wrote:
>I was wondering if there was a cypher that could encrypt something and
>keep the encrypted content the same length or shorter.
1. Encrypt something to the same length: Any number of algorithms that
have been developed over the centuries. For a crude example, read the
man page on that wonky browser you're using, and discover "rot-13"
which is a simple Caesar substitution. From the command line, this
can be implemented by
[compton ~]$ echo 'This is to be encrypted' | tr 'M-ZA-Lm-za-l' 'A-Za-z'
Hvwg wg hc ps sbqfmdhsr
While any six-year-old can break this encryption (in seconds once they
know the trick), a Vigenere or Jeffersonian code disk would be more
secure (might need an 8 or 9 year old to crack it), and the ultimate
scheme using a one-time keypad is literally unbreakable. There are a
lot of schemes in-between. Pay your money - take your pick.
2. Encrypt something to a shorter length: Two techniques should leap
out at you - data compression first, then any encryption - or changing
character sets using something like 'mimencode'
[compton ~]$ whatis compress zip gzip bzip2 mimencode
compress (1) - compress and expand data (version 4.1)
zip (1) - package and compress (archive) files
gzip (1) - compress or expand files
bzip2 (1) - a block-sorting file compressor, v0.9.0
mimencode (1) - Translate to and from mail-oriented encoding formats
-rw-rw-r-- 1 gferg ldp 77918 Nov 18 2004 Disk-Encryption-HOWTO
-rw-rw-r-- 1 gferg ldp 22890 Nov 18 2004 Disk-Encryption-HOWTO.gz
Then of course, you _could_ look at PGP...
>"This is unencrypted data" (length of 24)
That's rather a short example - but you should also look at code books
(telegraphic codes disappeared with the proliferation of the telephone,
but codes like the Bentley, or ABC were designed to minimize data
length). Go to your public library, and find a copy of the book "The
Code-Breakers" by David Kahn (originally 1967, by Macmillan, about 1200
pages, but there is at least several editions - each expanding over the
earlier, as well as a slightly condensed paper-pack edition) for quite
extensive material on the subject. Some codebooks condensed entire
phrases to a single 5 character codeword - hence the name 'code
condenser' - but you should read the book to see some of the pitfalls.
One of the biggest is creating your own scheme when you don't understand
>Is there something like this?
Well, you _are_ posting from a search engine - did you bother to think
about using the d4mn thing for it's intended purpose, or are you
expecting others will do your homework for you? Heck, you might even
discover the Usenet newsgroup 'sci.crypt' among others:
[compton ~]$ zgrep crypt big.8.list.07.15.07.gz | grep -v Moderated
comp.security.pgp.test Testing signing, encryption and decryption using PGP.
sci.crypt Different methods of data en/decryption.
sci.crypt.random-numbers Generating cryptographic strength randomness.
talk.politics.crypto The relation between cryptography and government.
but I STRONGLY advise you to read at least a months worth of postings
before you post a question like this - they tend not to tolerate mindless
questions that can be answered by using a search engine first.