This program uses two .java files, one for each encryption and decryption using the S_AES algorithm that uses a 16-bit plaintext and a 16-bit cipher key for encryption. An example of the terminal output (for both client and server) is included in the zip file. The class files can be run directly on a terminal using a java compiler to replicate the output from both the client and server applications respectively (that are shown in the pictures). The following files are used in this program :
The client file accepts a text input as well as a 16-bit binary key input for encryption. The text input is first converted into a binary stream and then padded with 0s to make its size divisible by 16. Each 16-bit chunk is encrypted separately (using the S_AES function) and for each 16-bit chunk, the encryption is shown step by step to maintain readability. After every 16-bit chunk is encrypted, they are concatenated to form the final ciphertext. The ciphertext, key and the padding data is sent to the server. (Note : In a real implementation, the key should be sent through a different secure channel but here we are sending it from the same channel). The client receives the plaintext back from the server after decryption to imply successful encryption and decryption. The following functions were used in the client.java file :
This function accepts two binary strings and returns the xor of those two strings.
This function performs the RotWord operation on a word (group of two nibbles in this case, word will be used to define a group of two nibbles further in this document). Essentially it just swaps the two nibbles consisting of the word.
This function performs the SubWord operation on a word (performing an s-box transformation on both nibbles in the word using a lookup table).
This function performs an s-box transformation on a nibble using a lookup table.
This function multiplies a nibble with 4 in gf(16) modulo x^4 + x + 1 using a lookup table.
This is the most important function that encrypts a 16-bit binary plaintext into a 16-bit binary ciphertext using the S-AES encryption algorithm with the help of a 16-bit secret key.
This function converts a text (UTF-8 encoded) into a binary stream.
The server file receives the ciphertext, key and padding data is separated. The ciphertext is separated into 16-bit chunks and each 16-bit chunk is decrypted separately (using the AES_Decryption function) and for each 16-bit chunk, the decryption is shown step by step. After all the chunks are decrypted, they are concatenated, the padding is removed, and it is converted back to text to reveal the original plaintext that was provided by the user at the client side. The original plaintext is then sent back to the client. The following functions were used in this file :
This function accepts two binary strings and returns the xor of those two strings.
This function performs the RotWord operation on a word (group of two nibbles in this case, word will be used to define a group of two nibbles further in this document). Essentially it just swaps the two nibbles consisting of the word.
This function performs the SubWord operation on a word (performing an s-box transformation on both nibbles in the word).
This function performs an s-box transformation on a nibble.
This function multiplies a nibble with 9 in gf(16) modulo x^4 + x + 1 using a lookup table.
This function multiplies a nibble with 9 in gf(16) modulo x^4 + x + 1 using a lookup table.
