Details of the Encryption Scheme

The design of a cryptosystem is based on the following two cryptography algorithms −

• Public Algorithms − With this option, all the details of the algorithm are in the public domain, known to everyone.
• Proprietary algorithms − The details of the algorithm are only known by the system designers and users.

In case of proprietary algorithms, security is ensured through obscurity. Private algorithms may not be the strongest algorithms as they are developed in-house and may not be extensively investigated for weakness.

Secondly, they allow communication among closed group only. Hence they are not suitable for modern communication where people communicate with large number of known or unknown entities. Also, according to Kerckhoff’s principle, the algorithm is preferred to be public with strength of encryption lying in the key.

Thus, the first assumption about security environment is that the encryption algorithm is known to the attacker.

Availability of Ciphertext

We know that once the plaintext is encrypted into ciphertext, it is put on unsecure public channel (say email) for transmission. Thus, the attacker can obviously assume that it has access to the ciphertext generated by the cryptosystem.

Availability of Plaintext and Ciphertext

This assumption is not as obvious as other. However, there may be situations where an attacker can have access to plaintext and corresponding ciphertext. Some such possible circumstances are −

• The attacker influences the sender to convert plaintext of his choice and obtains the ciphertext.
• The receiver may divulge the plaintext to the attacker inadvertently. The attacker has access to corresponding ciphertext gathered from open channel.
• In a public-key cryptosystem, the encryption key is in open domain and is known to any potential attacker. Using this key, he can generate pairs of corresponding plaintexts and ciphertexts.