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v An encryption system in which the sender and receiver of a message
share a single, common key that is used to encrypt and decrypt the message.
v Symmetric-key systems are simpler and faster, but their main
drawback is that the two parties must somehow exchange the key in a secure way.
Public-key encryption avoids this problem because the public key can be
distributed in a non-secure way, and the private key is never transmitted.
v Symmetric-key cryptography is sometimes called secret-key
cryptography. The most popular symmetric-key system is the Data
Encryption Standard (DES).
v The disadvantage of symmetric cryptography is that it presumes two
parties have agreed on a key and been able to exchange that key in a secure
manner prior to communication.
Secret key cryptography addresses the privacy requirement.
Example
: Data Encryption Standard (DES)
Secret
Key Authentication
r is some random number sent by sender
Ek(r) is encryption of random number r
r’ is some random number sent by receiver
Ek(r’) is encryption of random number r
Before two
parties can send information securely, they must first exchange a secret key. How
can the two parties exchange a key secretly before they can communicate in
secret? Even if the sender and receiver found a channel that they believed to
be secure, in the past there has been no way to test the secrecy of each key.
Quantum cryptography solves this problem. It allows the sender and receiver to
test and guarantee the secrecy of each individual key.
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