Symmetric Encryption

Why is security needed in networks?

Nearly all networks are shared, and all traffic can be accessed and modified by users.

What is a Passive attack?

Eavesdropping (sniffing) on data.

What is an Active attack?

An attempt to alter data being transmitted, such as spoofing or hijacking.

What is the purpose of encryption in data security?

To protect data so that its contents cannot be read by intruders.

List the services provided by an encryption-based protocol.

Authentication, Integrity, Confidentiality, Non-Repudiation.

What are the two types of encryption?

Secret-Key (Symmetric) and Public-Key (Asymmetric) encryption.

What is ciphertext?

Encrypted message that is sent over an insecure channel.

Explain the concept of Substitution Ciphers.

Substitution Ciphers replace one symbol with another in plaintext.

How does a Shift Cipher (Additive Cipher) work?

It encrypts by shifting characters a fixed number of places in the alphabet.

What defines a Transposition Cipher?

It changes the location of symbols without substituting them.

What is the key size limitation of DES?

The key size is 64 bits, but only 56 bits are actually used.

Why has DES been considered insecure since 1999?

Its key size can be cracked in approximately 10 hours with modern computing.

What is the key size of AES and its block size?

AES has variable key sizes of 128, 192, or 256 bits and operates on a 128-bit block size.

What is unique about the decryption process in AES compared to DES?

The decryption algorithm for AES is not identical to the encryption algorithm.

What is IDEA and its key size?

The IDEA algorithm is a block cipher that operates on a 64-bit plaintext block with a key size of 128 bits.

What is a Key Distribution Centre (KDC)?

It is required to distribute keys in symmetric key cryptography.

What is the risk of disclosing a symmetric key?

If revealed, communications can be compromised.

What does the notation {M}Kab indicate in symmetric encryption?

It indicates that message M is encrypted with the shared secret key Kab.