could you help me solving that problem ,, please don't reply with irrelevant pages from a book :(

thanks

Consider a system where a data files (F. and i denotes the file ID) is distributed over a cloud A data file is generated by an author (AU, and k denotes the author ID) and stored on a distribution server (Ds). Only authorized users (US, and denotes the user ID) previously registered on the system using their private keys (KPR) are allowed to download the data. Users' public certificates (KPU) and revocation lists (CRL) are available on a trusted Certificate Authority (CA) Authentication is done using an authentication server (AS) on a challenge response PKI based authentication. The files are to be encrypted (for each user on delivery) and the decryption key for file (i) to user (Ka) is delivered securely to the user from a data rights management server (DRMS) Using (RSA) public key cryptography whenever possible and (AES) for encrypting data for the sake of performance, and any required hashing and MAC algorithms a. Show the contents of exchanged messages between each entity and the others in the system in the case of uploading a new file on the system, downloading a file, and accessing the contents of a file. All other messages required for authentication are also required Number your messages (12, 3, ..) b. Draw a simplified diagram with all the entities as vertices and the message as a between the entities e and put on each are the message(s) number(s). c. The system is vulnerable to re-distribution of decrypted files (an authenticated user can make a new of the plain text file and post it over Facebook for example). Suggest a mechanism to prevent such a problem. d. Knowing that the file is stored on the local user storage after downloading. suggest a mode of operation that can be used to allow the player software to decrypt only the needed part of the file in memory, other parts of the file should remain encrypted on the storage. Take into consideration that access to the file content will be on the byte level and in a random access fashion (A byte from the middle of the file can be read then 4 bytes from the beginning of the file then the last 6 bytes and so on) Consider a system where a data files (F. and i denotes the file ID) is distributed over a cloud A data file is generated by an author (AU, and k denotes the author ID) and stored on a distribution server (Ds). Only authorized users (US, and denotes the user ID) previously registered on the system using their private keys (KPR) are allowed to download the data. Users' public certificates (KPU) and revocation lists (CRL) are available on a trusted Certificate Authority (CA) Authentication is done using an authentication server (AS) on a challenge response PKI based authentication. The files are to be encrypted (for each user on delivery) and the decryption key for file (i) to user (Ka) is delivered securely to the user from a data rights management server (DRMS) Using (RSA) public key cryptography whenever possible and (AES) for encrypting data for the sake of performance, and any required hashing and MAC algorithms a. Show the contents of exchanged messages between each entity and the others in the system in the case of uploading a new file on the system, downloading a file, and accessing the contents of a file. All other messages required for authentication are also required Number your messages (12, 3, ..) b. Draw a simplified diagram with all the entities as vertices and the message as a between the entities e and put on each are the message(s) number(s). c. The system is vulnerable to re-distribution of decrypted files (an authenticated user can make a new of the plain text file and post it over Facebook for example). Suggest a mechanism to prevent such a problem. d. Knowing that the file is stored on the local user storage after downloading. suggest a mode of operation that can be used to allow the player software to decrypt only the needed part of the file in memory, other parts of the file should remain encrypted on the storage. Take into consideration that access to the file content will be on the byte level and in a random access fashion (A byte from the middle of the file can be read then 4 bytes from the beginning of the file then the last 6 bytes and so on)