Homework 1: Applied Cryptography

Hard deadline: Wednesday, February 11, 11:59 pm.
Soft deadline: Monday, February 9, 11:59 pm

Reminders from the course syllabus: You must work alone on homework assignments. You may not collaborate with anyone else on the homeworks. Your solution must represent your own, individual work. Discussing course concepts in general terms with other students is okay, but discussing solutions to homework problems is not. Submissions between the soft and hard deadlines automatically receive a 25% penalty.

You should first read the course notes on applied cryptography before starting this homework.

Warning: This homework has two parts. You are currently looking at Part I. You need to decrypt Part II then do all the problems in it. Part II is much longer than Part I, so you should start early.

Part I

Write a Java program to decrypt Part II of this homework. Part II is encrypted with hybrid encryption.

Specification:

• Your program's main method must be located in the default package and in a class named HybridDecrypt.
• We will compile your program from the command-line as follows:

  javac HybridDecrypt.java

  All the code for your program should be located in that one file.

• We will grade your program by invoking it from the command-line as follows:

  java HybridDecrypt pubkey privkey sesskey iv ciphertext plaintext

  where pubkey, privkey, sesskey, iv, and ciphertext are command-line arguments specifying the names of input files containing a public key, private key, session key, IV, and ciphertext; and an output file for the plaintext; as described below:
  • pubkey: The RSA public key that was used to encrypt the session key. The file contains the raw bytes of an X.509 encoding of the key.
  • privkey: The 3072-bit RSA private key corresponding to pubkey. The file contains the raw bytes of a PKCS8 encoding of the key.
  • sesskey: The 128-bit AES session key encrypted with RSA and OAEPwithSHA-256andMGF1 padding. The file contains the raw bytes of the key.
  • iv: The initialization vector (IV) for the ciphertext's CBC encryption. The file contains the raw bytes of the IV.
  • ciphertext: The plaintext encrypted with the AES session key in CBC mode with PKCS5 padding.
  • plaintext: Your program should output the decrypted ciphertext into this file.

Use your program to decrypt the ciphertext found in this archive: hw1.II.zip. The ciphertext file is named hw1.II.html.enc. You'll also find files containing the public key, private key, session key, and IV that were used to produce the ciphertext.

Hint: Reference the Java crypto library documentation as needed:

• Java Cryptography Architecture: Reference Guide
• Java Cryptography Architecture: Oracle Providers Documentation
• Java Cryptography Architecture: Standard Algorithm Name Documentation