Cryptography Class by Casey W. O'Brien
MPICT Summer Faculty Development Week
The plaintext contains repeated text.
When encrypted in ECB mode, the ciphpertext shows repeated blocks,
which is a flaw--some information about the plaintext has
been preserved, which can be used to attack the cipher.
In CBC mode, the repeated plaintext does not
lead to repeated ciphertext.
Unfortunately, with carefully chosen plaintext
this system can be broken with the
BEAST
attack.
3DES
The usual way 3DES is implemented is:
- Encrypt with Key 1
- Run the decryption algorithm with Key 2
- Encrypt with Key 1
You could just encrypt three times, and that would work. But this
implementation makes it easy to build a hardware
device designed for 3DES which is also backwards-compatible
with DES--just run it with K1=K2.
Ref:
Why do we use encrypt-decrypt-encrypt (EDE) in 3DES, rather than encrypting three times?
Here's CrypTool performing AES with a key of:
AB CD 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Notice that patterns in the input do not lead to obvious
patterns in the output:
RC4 with CrypTools
RC4 is a symmetric algorithm with key sizes from
1 to 256 bytes (8 to 4096 bits).
Here's Cryptool encrypting with a 8-bit key of AA:
Here is CrypTool Cracking RC4 with a brute force attack.
Here are the results: the low-entropy result is the
correct plaintext.
PGP Encryption
We are using PGP v. 8.0.3 (the last freeware version)
on Windows XP because it won't run on Win 7.
This "Secure Viewer" sounds interesting, using a special
font to avoid radio emissions from a CRT monitor leaking
your key. But when I tried it, it did not work.
ECB Mode Demo
Here's a demo from
Wikipedia
that shows why ECB mode is evil--images don't even
become scrambled enough to rended the image
unrecognizable.
I used CrypTools to prepare these three images:
Original Image
Encrypted with DES in ECB Mode
As you can see, the image is not well obscured--you
can still see the apple.
Encrypted with DES in CBC Mode
This is much better--there is no trace of the
original image visible.
Complete steps to do this project are
here.
Encrypted with 3DES in ECB Mode
3DES uses more rounds of encryption, and two keys,
to scramble the data more, but in ECB mode it
still doesn't obscure the image.
Encrypted with 3DES in CBC Mode
This is much better--there is no trace of the
original image visible.
Encrypted with AES (key = 0)
Encrypted with XOR (key = AA)
Encrypted with XOR (key = 0123456789ABCDEF)
Encrypted with RSA (key type RSA-512)
I was surprised to see patterns here, but I found
a comment saying it suffers from a
problem similar to ECB, unless "padding" or "armoring"
is used.
Encrypted with Twofish (key = 0)
Encrypted with Serpent (key = 0)
Encrypted with RC6 (key = 0)
Encrypted with MARS (key = 0)
Last modified 1:22 pm Thursday, June 28, 2012