I'm new to Cryptography so please free to edit my question for clarity and add the appropriate tag.
I'm trying to research if the following is possible. Suppose
- John has a message that he encrypts to share with more than one recipients.
- The message is different, i.e. it's unique to the recipients (though the recipients don't know that).
- The recipients must each be given a unique password so as not be able to decrypt each others message.
Questions (sorry for more than one question, but they're related):
- What approach should I take? Links to references or further reading will be appreciated
- Is the one encryption with different decryption the wrong approach to achieve goal 3? What approaches would you suggest?
The objective are two fold:
- allow a teacher to easily (hence 1 ciphertext) encrypt "answers to exam questions" (text and binary data e.g. images) to distribute to selected students. Reason for not allowing anyone outside of the selected students is to prevent students who haven't taken the exam to know the answer thus allowing reuse of the exam questions.
- allow students to see teacher's comments and feedback specific to that students' attempted answers. Since students answered differently, the students need not share the same password and are discouraged from using the same password. This also helps teacher track which students leaked answers to future students.
Update: I've bought Bruce Schneier's book to learn up on terminology and concepts so I won't be at a loss with terms like ciphertext.