Page 123 - DCAP309_INFORMATION_SECURITY_AND_PRIVACY

Unit 8: Cryptography and Encryption

Block ciphers can operate in one of several modes; the following four are the most important: Notes
1. Electronic Codebook (ECB) mode is the simplest, most obvious application: the shared
key is used to encrypt the plaintext block to form a ciphertext block. Two identical plaintext
blocks, then, will always generate the same ciphertext block. Although this is the most
common mode of block ciphers, it is susceptible to a variety of brute-force attacks.

2. Cipher Block Chaining (CBC) mode adds a feedback mechanism to the encryption scheme.
In CBC, the plaintext is exclusively-ORed (XORed) with the previous ciphertext block
prior to encryption. In this mode, two identical blocks of plaintext never encrypt to the
same ciphertext.
3. Cipher Feedback (CFB) mode is a block cipher implementation as a self-synchronizing
stream cipher. CFB mode allows data to be encrypted in units smaller than the block size,
which might be useful in some applications such as encrypting interactive terminal input.
If we were using 1-byte CFB mode.

Example: Each incoming character is placed into a shift register the same size as the block,
encrypted, and the block transmitted. At the receiving side, the ciphertext is
decrypted and the extra bits in the block (i.e., everything above and beyond the one byte) are
discarded.
4. Output Feedback (OFB) mode is a block cipher implementation conceptually similar to a
synchronous stream cipher. OFB prevents the same plaintext block from generating the
same ciphertext block by using an internal feedback mechanism that is independent of
both the plaintext and ciphertext bitstreams.

Task Explain the function of Stream ciphers.
8.5.2 Public Key Cryptography

Public-key cryptography has been said to be the most significant new development in
cryptography in the last 300-400 years. Modern PKC was first described publicly by Stanford
University professor Martin Hellman and graduate student Whitfield Diffie in 1976. Their paper
described a two-key crypto system in which two parties could engage in a secure communication
over a non-secure communications channel without having to share a secret key.
Public-key cryptography, also known as asymmetric cryptography, is a form of cryptography
in which a user has a pair of cryptographic keys — a public key and a private key.

The keys are related mathematically, but the private key cannot be practically derived from the
public key. A message encrypted with the public key can be decrypted only with the
corresponding private key.
The two main branches of public key cryptography are:
1. Public Key Encryption: A message encrypted with a recipient’s public key cannot be
decrypted by anyone except the recipient possessing the corresponding private key. This
is used to ensure confidentiality.
2. Digital Signatures: A message signed with a sender’s private key can be verified by
anyone who has access to the sender’s public key, thereby proving that the sender signed
it and that the message has not been tampered with. This is used to ensure authenticity.

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