Suppose that the keystream generated by a stream cipher is predictable in the sense that if...
Suppose that the keystream generated by a stream cipher is predictable in the sense that if you are given n keystream bits, you can determine all subsequent keystream bits. Is this a practical security concern? Why or why not?
Homework Answers
Yes it is definitely a security concern,
- Let us assume there is a database that stores secret user information, like medical, payroll or even criminal records. As a security measure to protect that information, the server that handles it encrypts it using a stream cipher. For now, we’ll assume that that server is secure, and there’s no way to get it to leak the key.
- A Hacker gets a hold of all of the rows in the database. Perhaps he did it through a SQL injection attack, or maybe with a little social engineering by tricking people into things they shouldn’t be doing, like giving out secret keys, or performing certain operations. It’s usually the most effective way to break otherwise secure cryptosystems. Everything is supposed to remain secure: A Hacker only has the ciphertexts, but he doesn’t have the secret key.
- If hacker can predict the key stream generated by the Cipher, then the entire DB is in the mercy of Hacker. He can delete all the criminal records, he can modify the records. Simply he can do whatever he wants with the secret information he wants.
- Hence Predictable key stream is a security concern.
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