Diffie-Hellman

Diffie-Hellman is mainly used as key management protocol. For example, it is used for exchange keys in IPsec VPN.

It allows 2 parties, A and B, to exchange a secret key over an insecure communication channel without previous knowledge of the secret key.

Reference: Official (ISC)2 Guide to the CISSP CBK, Second Edition ((ISC)2 Press)

Advanced Encryption Standard (AES)

Advanced Encryption Standard (AES) is a symmetric key cryptography algorithm.


- Key size is either 128, 192 or 256 bits
- Block size is either 128, 192 or 256 bits


As increasing the key length and block size adds a little extra compensation, i.e. increasing the number of iteration of the algorithm, it is recommended to pick 256 bits instead of 128 bits.

Reference: Official (ISC)2 Guide to the CISSP CBK, Second Edition ((ISC)2 Press)

Data Encryption Standard (DES)

The Data Encryption Standard (DES) is a symmetric key cryptography algorithm.

- Key size is 56bits + parity check is 8bits
- Block size is 64bits

DES modes
- Stream ciphers (implemented in hardware)

     * Cipher Feedback (CFB)
        Errors in this mode of DES will propagate

     * Output Feedback (OFB)
        Susceptible to block replay attacks

- Block ciphers (implemented in software)

     *  Electronic Code Block (ECB)
         A given plaintext will always generate the same ciphertext

     * Cipher Block Chaining (CBC)
        A given plaintext will always result in different ciphertext

Double-DES and Tripe-DES

- Doubling the key from 56bits to 112bits, the strength will increase from 2^56 to 2^112
- 2DES is susceptible under Man-in-the-middle attack which its strength is as same as DES
  2DES -> -> -> DES
- 3DES can use 2 or 3 separate keys.

     * DES EEE2 (2 keys)
      Encryption with key1 >>> Encryption with key2 >>> Encryption with key3

     * DES EDE2 (2 keys)
      Encryption with key1 >>> Decryption with key2 >>> Encryption with key3

     * DES EEE3 (3 keys)
      Encryption with key1 >>> Encryption with key2 >>> Encryption with key3

     * DES EDE3 (3 keys)
      Encryption with key1 >>> Decryption with key2 >>> Encryption with key3

Reference: Official (ISC)2 Guide to the CISSP CBK, Second Edition ((ISC)2 Press)