How to decrypt with AES programmatically

In this section, we are going to develop the decrypt program that can encrypt a file encrypted by the encrypt program.

Our decryption program will be similar to the encryption program and will also take three command-line arguments:

1. Input file name
2. Output file name
3. Encryption key, hex-encoded

This time, the input file is the encrypted file created by the preceding encrypt program.

Let’s make a high-level plan, similar to how we did before:

1. Read the IV from the input file.
2. Initialize decryption.
3. Decrypt chunk by chunk, reading plaintext chunks from the input file and writing the resulting plaintext chunks into the output file.
4. Read the authentication tag from the input file and set it into the cipher context.
5. Finalize decryption.

As we can see, the decryption plan is very similar to the encryption plan – initalize, process, and finalize. Let’s see how it is implemented...