Page 553 - StudyBook.pdf

Basis of Cryptography • Chapter 9 537

El Gamal

The El Gamal algorithm is essentially an updated and extended version of the orig-
inal Diffie-Hellman algorithm based on discrete logarithms.The security of the
algorithm is roughly on par with that of the RSA algorithm. El Gamal has a few
drawbacks, mainly its larger output and random input requirement. Encrypted El
Gamal ciphertext is much longer than the original plaintext input, so it should not
be used in places where bandwidth is a limiting factor, such as over slow wide area
network (WAN) links.The El Gamal algorithm also requires a suitable source of
randomness to function properly. It is worth noting that the Digital Signature
Algorithm (DSA) was based on the El Gamal algorithm. DSA is a complementary
protocol to RSA that is widely used in the OpenSSH implementation of the
Secure Shell (SSH) protocol.

RSA

Shortly after the appearance of the Diffie-Hellman algorithm, Ron Rivest,Adi
Shamir, and Leonard Adleman proposed another public key encryption system.
Their proposal is now known as the RSA algorithm, named for the last initials of
the researchers.

TEST DAY TIP

Depending on the literature you read, public-key cryptography and
public-key encryption systems are terms that are used interchangeably.

The RSA algorithm shares many similarities with the Diffie-Hellman algorithm
in that RSA is also based on multiplying and factoring large integers. However,
RSA is significantly faster than Diffie-Hellman, leading to a split in the asymmetric
cryptography field that refers to Diffie-Hellman and similar algorithms as Public
Key Distribution Systems (PKDS), and RSA and similar algorithms as Public Key
Encryption (PKE). PKDS systems are used as session-key exchange mechanisms,
while PKE systems are considered fast enough to encrypt small messages. However,
PKE systems like RSA are not considered fast enough to encrypt large amounts of
data such as entire file systems or high-speed communications lines.

www.syngress.com