[Computer Security] 2. Cryptographic Tools

2. Cryptographic Tools 

2-1.  Confidentiality with Symmetric Encryption

- Symmetric(= Conventional, Single-key) Encryption

Used for secret communication in various fields

Plaintext	input to encryption algorithm, the original message or data
Secret key	input to encryption algorithm, the exact substitutions and transformations depend on the key
Encryption Algorithm	performing various substitutions and transformations on the plaintext
Ciphertext	scrambled message produced as output
Decryption Algorithm	taking the ciphertext and the secret key and produces the original plaintext

 

There are two requirements:

1. The opponent should be unable to decrypt ciphertext or discover the key → need a strong encryption algorithm

2. The sender and receiver must have copies of  secret key in a secure fashion and keep them secure 

 

There are two general approaches to attacking a symmetric encryption scheme:

1. cryptanalysis: to attempt to deduce a specific plaintext or the key being used

2. brute-force attack: to try every possible key on a piece of ciphertext

 

 

- Symmetric Block Encryption Algorithm

Block cipher: to process the plaintext input in fixe-size blocks and produces a block of ciphertext of equal size for each plaintext block(DES, triple DES, AES...)

DES (Data Encryption Standard)	56-bit key length makes it vulnerable to a brute-force attack
Triple DES	to repeat DES 3 times using either 2 or 3 unique keys with its 168-bit key length, it overcomes the vulnerability to brute-force attack of DES relatively sluggishing and small block size(64-bit)
AES (Advanced Encryption Standard)	128-bit block size and 256-bit key size -> improving efficiency and security strength

 

ECB(Electronic Codebook) mode:

▷ Each block of plaintext is encrypted using the same key

▷ For lengthy data, it may not be secure because of being exploited regularities to ease the task of decryption → "modes of operation" overcome this

 

 

- Stream Ciphers

▷ Keystream(the output of the generator) is combined one byte at a time with the plaintext stream using the bitwise XOR operation

▷ Stream ciphers are always faster and use far less code than do block ciphers(but block ciphers can reuse the key)

 

 

 

2-2. Message Authentication and Hash Functions

- Message Authentication and Encryption

Message authentication: for data integrity, process to verify that received or stored messages are authentic

 

Symmetric encryption alone is not a suitable tool for data authentication(reordering is a threat)

→ It is possible to combine encrypted messages and authentication tag

 

Message Authentication Code:

▷ Assuming that A and B share a secret key $K_{AB}$, and both calculate the message authentication code as a complex function -> $MAC_{m} = F(K_{AB}, M)$

▷ Authentication algorithm is similar to encryption, but one difference is that it need not be reversible

 

 

One-way Hash Function:  

 

 

 

 

 

 

Textbook used: www.amazon.com/Computer-Security-Principles-Practice-4th/dp/0134794109/ref=sr_1_1?dchild=1&keywords=computer+security+principles+and+practice+4th+edition&qid=1620241308&sr=8-1

Amazon.com