Let's try to enhance the security of D.S. to do this you can for example use a variant called D.S. x.
The idea is to keep the algorithm while fixing some major problems.
Making kingsize longer means that the number of operations required to test all possible keys will rise
exponentially.
The Morath operation of the DE ESX algorithm is as follows.
The first block of input is Exel armed with the first 64 bit key.
The result is encrypted using a 56 bit key.
The result is then X ored with the third key.
That also has 64 bits.
In theory having three keys.
Their summary length is 184 bits the size however does not contribute directly to the ciphertext security
cryptographic analysis shows that the practical security the key is entropy boosted from 64 to more
or less 88 bits.
This is a large step forward
ESX is considerably more secure than D.S. the algorithm is relatively fast which is why it was implemented
in the EFI system.
A file encryption system used in earlier Windows systems
and other variant of D.S. is yes.
Triple D s.
The name says it all.
The D.S.O. algorithm is applied in the solution three times to each data block.
The mechanism is as follows.
First a block is encrypted like in D.
Yes it is then decrypted using a different key and finally the result is encrypted using yet another
key as you remember.
Encryption is symmetric and data encryption standard this operation doesn't really tighten the security.
The second phase of the operation does not affect or improve the overall security of the ciphertext.
The main downside of using Triple D Yes is the Scifres low speed.
It's possibly the slowest symmetric algorithm in use today.
All other surfers are much faster and most of them are more secure.
The practical security of trippled Yes is about one hundred bits of strength.
There are also other symmetric ciphers.
Many of them belong to the RC run Rivest Code family.
If you're guessing that the ciphers were created by Ron Rivest turns out you're right.
The first one most showcases are C-2 this special purpose cipher was developed in 1987 for the software
company Lotus.
It was kept secret for some time at first until a researcher once again proved the principles we discussed
earlier are true and published the ciphers mode of operation on a usenet group.
RC to supposedly strong security was debunked the implemented block size with 64 bits it's too short.
The cipher is also susceptible to chosen plaintext attacks that we talked about earlier.
Let's now comment on the attack categories.
The first basic type of cryptanalysis attack relies on an attacker having access to the ciphertext only
this type of attack is completely passive an attacker has access to the ciphertext it must crack it.
The second class of attacks known as plaintext attacks involves the assumption that an attacker has
access both to the plaintext and to the ciphertext.
This is actually not infrequent at all.
If you encrypt something in a network an attacker can in most cases rightly assume that some information
is heavily patterned
an e-mail message very often starts off with high or when you encrypt data using SSL and IP protocol
header containing your PC's IP address will be at the start.
In most cases an attacker has in fact access to at least part of the plaintext and the ciphertext.
The third attack category plaintext injection involves a manipulation of the plain text in order to
observe resulting ciphertext.
An attacker may send a crafted message to you without raising suspicions to make you encrypt it and
send it back.
This happens quite often as well.
It's crucial for the system to be immune against the two letter attack models.
All adequate algorithms are now secured against the ciphertext only cryptanalysis attack
the true security of the RC algorithm is estimated at thirty four bits.
This means that it can be cracked instantly
another version of the algorithm.
RC 5 was developed in 1994 for RSA
The algorithm is very flexible many variables used in the scheme can be altered.
You can for example specify a round number block size in Keeling's.
There can be from one to two hundred and fifty five rounds and block size can be from 32 to 128 bits.
Keys can even be up to two kilobytes.
The mechanism used in RC 5 is depicted in the slide
will now discuss the algorithms mode of operation briefly
first.
A key has to be divided into subkey.
The number of subcase must be equal to the selected number of rounds.
Then as usual split the message block into two even pieces.
The two halves are combined with the first two subcase the round keys.
Then perform the exo are and transpose bits bits reordered in the two halves.
RC 5 in its variant called RC 6 were entered into a contest to choose the D.S. algorithm's replacements
Sipher the did not come close to winning both algorithms are assessed negatively and because of this
they are not considered particularly secure.