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© Copyright Virtual University of Pakistan 108
Lesson 24
SYMMETRIC KEY ALGORITHMS
Cryptographic algorithms are measured in terms of key length. Following is the list of some popular
symmetric key algorithms:
DES (Data Encryption Standard) – 56 bits
IDEA (International Data Encryption Algorithm (IDEA) – 128 bits
RC2 – (block cipher) 1-2048 bits
RC4 (stream cipher) – 1-2048 bits
Rinjdael – 128-256 bits
Attacks on Symmetric Key Algorithms
Following attacks have been reported on symmetric key algorithms:
Key Search Attacks
Cryptanalysis
System-based Attacks
Key Search (Brute Force) Attacks
In this type of attack an attempt is made by the attacker to decrypt the message with every possible key.
Thus, the greater the key length, the more difficult it is to identify the key.
Cryptanalysis
Encryption algorithms can be defeated by using a combination of sophisticated mathematics and computing
power so that many encrypted messages can be deciphered without knowing the key. Such type of an attack
is called cryptanalysis.
System-Based Attacks
In it the attack is made on the cryptographic system that uses the cryptographic algorithm without actually
attacking the algorithm itself.
Public Key Algorithms
Following is the list some popular public key algorithms:
DSS – Digital Signature Standard based on DSA (Digital Standard Algorithm) –
key length is between 512-1024 bits
RSA
Elliptic Curves
Attacks on Public Key Algorithms
Key Search Attacks
The public key and its corresponding private key are linked with each other with the help of a large
composite number. These attacks attempt to derive the private key from its corresponding public key using
that number. According to an estimate 1024 bit RSA public key may be factored due to fast computers by
2020. Note that both symmetric and asymmetric algorithms are based on different techniques. In case of
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