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Full Discussion: OpenSSL
Special Forums Cybersecurity OpenSSL Post 302955621 by Ultrix on Saturday 19th of September 2015 03:10:15 PM
Old 09-19-2015
OpenSSL

I just started playing around with Unix's OpenSSL utility. I can't seem to get the hang of it, and the man page isn't helping much. I wanted to experiment with file encryption, so I created a dummy text file with one line of text and tried to encrypt it using DES. I used the following command:

Code:
openssl des -in Encrypt.txt -out Encrypted.txt

As expected, this produced an encrypted file whose contents were illegible. Then I tried to decrypt the file. I figured since most of these encryption algorithms are based on XORing the key with the message, I would be able to apply the same command to get the original plaintext. So I used a variation of the same command.

Code:
openssl des -in Encrypted.txt -out Decrypted.txt

I opened the file Decrypted.txt and found that it was still illegible. I hadn't gotten the original plaintext back. "Okay," I thought, "I guess I don't know much about how to use DES, since it's a block cypher. Maybe I'll try a stream cypher, since I'm more familiar with how they work, and I know for a fact that for a stream cypher, encryption and decryption use the same algorithm."

I used the following commands:

Code:
openssl rc4 -in Encrypt.txt -out Encrypted.txt
openssl rc4 -in Encrypted.txt -out Decrypted.txt

Still doesn't work.

Then I noticed something strange when looking at the files. The encryption algorithm had added a human-readable prefix to the encrypted file, reading "Salted__". So it looks like what happened was the encryption algorithm added that prefix, when meant that when the same algorithm was applied again, it had a different starting point, so when it started decrypting the original message, after XORing the first eight bytes of the key with the prefix, it would be XORing a completely different part of the key with the same characters, resulting in a gibberish result.

So my question is, what is the proper way to encrypt and decrypt files using OpenSSL? Can anyone point me to any good tutorials? Because, as I said, the man page isn't providing much help.
 

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DES(3)							User Contributed Perl Documentation						    DES(3)

NAME
Crypt::DES - Perl DES encryption module SYNOPSIS
use Crypt::DES; DESCRIPTION
The module implements the Crypt::CBC interface, which has the following methods blocksize =item keysize =item encrypt =item decrypt FUNCTIONS
blocksize Returns the size (in bytes) of the block cipher. keysize Returns the size (in bytes) of the key. Optimal size is 8 bytes. new my $cipher = new Crypt::DES $key; This creates a new Crypt::DES BlockCipher object, using $key, where $key is a key of "keysize()" bytes. encrypt my $cipher = new Crypt::DES $key; my $ciphertext = $cipher->encrypt($plaintext); This function encrypts $plaintext and returns the $ciphertext where $plaintext and $ciphertext should be of "blocksize()" bytes. decrypt my $cipher = new Crypt::DES $key; my $plaintext = $cipher->decrypt($ciphertext); This function decrypts $ciphertext and returns the $plaintext where $plaintext and $ciphertext should be of "blocksize()" bytes. EXAMPLE
my $key = pack("H16", "0123456789ABCDEF"); my $cipher = new Crypt::DES $key; my $ciphertext = $cipher->encrypt("plaintex"); # NB - 8 bytes print unpack("H16", $ciphertext), " "; NOTES
Do note that DES only uses 8 byte keys and only works on 8 byte data blocks. If you're intending to encrypt larger blocks or entire files, please use Crypt::CBC in conjunction with this module. See the Crypt::CBC documentation for proper syntax and use. Also note that the DES algorithm is, by today's standard, weak encryption. Crypt::Blowfish is highly recommended if you're interested in using strong encryption and a faster algorithm. SEE ALSO
Crypt::Blowfish Crypt::IDEA Bruce Schneier, Applied Cryptography, 1995, Second Edition, published by John Wiley & Sons, Inc. COPYRIGHT
The implementation of the DES algorithm was developed by, and is copyright of, Eric Young (eay@mincom.oz.au). Other parts of the perl extension and module are copyright of Systemics Ltd ( http://www.systemics.com/ ). Cross-platform work and packaging for single algorithm distribution is copyright of W3Works, LLC. MAINTAINER
This single-algorithm package and cross-platform code is maintained by Dave Paris <amused@pobox.com>. perl v5.16.3 2005-12-08 DES(3)
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