Cryptography Classification

Classification of Cryptographic Systems, Ciphers, Algorithms

1. This classification scheme has to do with the means of implementation
1.1. Classical : Pre Computer Era involvements of (e.g. before 1950)
1.2. Modern : Computer Era. Usually implies binary plaintext, key, ciphertext processing and /or encoding

2. This classification scheme has to do with the Key – Space properties
2.1. Symmetric, Private Key  : Both, Encryption and Decryption Algorithms use the same Key
2.2. Asymmetric, usually Private – Public Key pair Algorithms : Encryption and Decryption algorithms use different keys, usually related.

3. This classification scheme has to do with the way the algorithm processes plaintext, message to be encrypted.
3.1. Block Cipher : Plain text is treated as a series of fixed length sub-messages.
3.2. Stream Cipher : Used as a Block Cipher with efficiently fixed small sized sub-messages to comply with real time applications. That is a consequence of the sort time interval available to process a sub-message, e.g. A5 of GSM cellular telephony.

Note : Other classification schemes could be suggested like synchronous, asynchronous etc.