| This dissertation is about developing a secure e-mail system named “Secure E-mail Systems” or SEMS that allow users to encrypt e-mails. The application uses 128-bit key block encryption with the key that changes in every block. The encryption algorithm is designed to remain secure even when the keys that encrypt a few blocks are known. It provides message integrity check, timestamp, attachment encryption, authentication and acknowledgment, all with minimal overhead. An address book is implemented to store keys with a protocol that can update the key using normal encryption routine. It also provides stream encryption using 127-bit key stream ciphers when faster encryptions are required.; In this dissertation, an encryption technique is developed to reduce the number of plaintext-ciphertext pairs produced by the encryption using the same key. This is done by changing the key after an encryption of every message block and will create only one plaintext-ciphertext pair encrypted from the same key. It explains how to create a key generator function that does not compromise the security even when some certain additional information is available to the attacker.; This dissertation also introduces three authentication and acknowledgement protocols using secret key that do not require the notary to keep a record for each transaction and the content of the message is not revealed to the notary. The protocols limit the number of encryption operation of the notary to 14 regardless of the size of the message. A slight modification to the protocol used in the application reduces the probability of success in cheating to 1/(2.40 × 1037), regardless of the computation resources the cheater has.; The stream cipher used in the application is the modified Zeng-Rao stream ciphers and is proven to generate a period of (2m−1 ).(2n−1) − 2 m+n−4 m ≠ n, where m and n are the size of the LFSRs used in the stream ciphers. Key streams are found to have large linear complexity and the linear complexity profiles closely follow the line y = x/2. Sequences generated from this stream ciphers are tested with Golomb's Randomness Postulates and are found to have very good randomness statistics. |
