Client-server application that allows a client process to send encrypted data to a server process.

The server decrypts the received data and writes it to a file. Communication between client and server takes place via sockets.

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Client Specifications

The client application takes the following input parameters:

• the name of a file (text or binary; for testing, a text file is recommended)
• a 64-bit key K (representable as an unsigned long long int)
• an integer p that defines the desired degree of parallelism
• the IP address and port number of the server

The client reads the input file, extracts its contents as a byte sequence F, and determines its length L. It splits the content into n blocks Bi of 64 bits each:

F = B1B2 … Bn

If the last block Bn is shorter than 64 bits, it is padded with '\0's.

Each block Bi is encrypted by performing a bitwise XOR with the key K, resulting in an encrypted block Ci = Bi XOR K.

The complete encrypted message C is the concatenation of all blocks:

C = C1C2…Cn

The client sends this encrypted message to the server, along with the original file length L and the key K.

Message format:

[C1C2…Cn, L, K]

After sending the data, the client waits for an acknowledgment message from the server before terminating.

Features:

• Uses up to p threads to perform parallel encryption
• Handles server-side connection closures
• Ignores interruptions from signals such as SIGINT, SIGALRM, SIGUSR1, SIGUSR2, and SIGTERM during encryption and sending
• Avoids race conditions when using shared data structures between threads

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Server specifications

The server application accepts the following input parameters:

• an integer p for the desired parallelism level during decryption
• a string s to use as a prefix for output filenames
• an integer l representing the maximum number of concurrent connections that can be handled by the server

The server listens on a specific port and accepts connection requests from multiple client processes.

Once a client connects, the server receives an encrypted message containing:

• the ciphertext C
• the original length L
• the encryption key K

The ciphertext C is split into n 64-bit blocks. Using up to p threads (where p ≤ n), the server decrypts each block via XOR:

Bi = Ci XOR K

Decrypted blocks are stored in a shared buffer. Once the full message is reconstructed, it is saved to a file using the prefix s for the filename.

The server sends an acknowledgment to the client after decryption is complete, and before writing to disk. It then closes the connection.

Features:

• Threads performing decryption or file I/O are immune to signal interruptions (SIGINT, SIGALRM, SIGUSR1, SIGUSR2, SIGTERM)
• Decrypted data is stored in a dynamically allocated list of blocks
• Synchronization with semaphores ensures the absence of race conditions during shared buffer access

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General Requirements

• Both client and server accept parameters via command-line arguments or environment variables

• Both components include error handling for:

  • Invalid input
  • System call failures
  • I/O errors

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Example: Encryption Process

Plaintext input:

"il mio nome e’ legenda"

Binary representation:

01001001 01101100 00100000 01101101 01101001 01101111 
00100000 01101110 01101111 01101101 01100101 00100000 
01100101 00100111 00100000 01101100 01100101 01100111 
01100101 01101110 01100100 01100001

Length:

L = 22 bytes = 176 bits

Key:

K = "emiliano"

Binary representation:

01100101 01101101 01101001 01101100 01101001 01100001 01101110 01101111

Block division:

B1 = 01001001 01101100 00100000 01101101 01101001 01101111 00100000 01101110
B2 = 01101111 01101101 01100101 00100000 01100101 00100111 00100000 01101100
B3 = 01100101 01100111 01100101 01101110 01100100 01100001 00000000 00000000

Encrypted blocks (XOR with key):

C1 = B1 XOR K =
01001001 01101100 00100000 01101101 01101001 01101111 00100000 01101110
XOR
01100101 01101101 01101001 01101100 01101001 01100001 01101110 01101111
=
00101100 00000001 01001001 00000001 00000000 00001110 01001110 00000001

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This project is an example of low-level system programming with multi-threading and network socket communication, focusing on concurrency, synchronization, and signal safety.