CISSP考试指南笔记：3.15 对称系统的类型
CISSP考试指南笔记：3.15 对称系统的类型
Data Encryption Standard
DEA is the algorithm that fulfills DES, which is really just a standard.
DES is a symmetric block encryption algorithm. It uses a 64-bit key: 56 bits make up the true key, and 8 bits are used for parity.
DES Modes
Electronic Code Book (ECB) | ECB mode operates like a code book. This mode is the fastest and easiest, so we use it to encrypt small amounts of data, such as PINs, challenge-response values in authentication processes, and encrypting keys. Some important characteristics of ECB mode encryption are as follows: • Operations can be run in parallel, which decreases processing time. • Errors are contained. • It is only usable for the encryption of short messages. • It cannot carry out preprocessing functions before receiving plaintext. |
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Cipher Block Chaining (CBC) | Cipher Block Chaining (CBC) mode does not reveal a pattern because each block of text, the key, and the value based on the previous block are processed in the algorithm and applied to the next block of text |
Cipher Feedback (CFB) | Cipher Feedback (CFB) mode is really a combination of a block cipher and a stream cipher. |
Output Feedback (OFB) | Output Feedback (OFB) is a mode that a block cipher can work in when it needs to emulate a stream because it encrypts small amounts of data at a time, but it has a smaller chance of creating and extending errors throughout the full encryption process. |
Counter (CTR) mode | Counter (CTR) mode is very similar to OFB mode, but instead of using a randomly unique IV value to generate the keystream values, this mode uses an IV counter that increments for each plaintext block that needs to be encrypted. The unique counter ensures that each block is XORed with a unique keystream value. |
Triple-DES
3DES can work in different modes, and the mode chosen dictates the number of keys used and what functions are carried out:
- DES-EEE3 Uses three different keys for encryption, and the data is encrypted, encrypted, encrypted.
- DES-EDE3 Uses three different keys for encryption, and the data is encrypted, decrypted, encrypted.
- DES-EEE2 The same as DES-EEE3, but uses only two keys, and the first and third encryption processes use the same key.
- DES-EDE2 The same as DES-EDE3, but uses only two keys, and the first and third encryption processes use the same key.
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