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n is the product of two large primes, so by definition n is always a large number. The question wants you to write a program that decrypts the cipehertext, which by definition means that the program must return the plaintext.


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The general class of algorithms are called secret-strengthening protocols. The idea is that Alice has a weak secret, p, and by conducting a short handshake with Bob, who knows a strong secret, N, Alice derives a combined secret H(p,N) which is a strong secret. Secure Remote Password is an example of such a secret-strengthening protocol combined with a ...


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If you use PBKDF2 then you are using a salt, because that's how PBKDF2 is defined: it is a function which takes as input a password and a salt. So there is already a salt somewhere. Or else Bob is using a "fixed salt", i.e. not a salt at all, and we can say that Bob is not really running PBKDF2. Salts need not be secret. Their virtue is in being unique (as ...


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Since you generate the user-specific keys, you can also keep a copy of these keys somewhere (somewhere safe, preferably) and use them when needed. Alternatively, you can generate the keys with a cryptographic derivation system which uses a "superkey" and the user's identity. For instance, consider the following: The superkey is K. A user is identified ...


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The only way I can think of is to use asymmetric keys. Generate an asymmetric key pair KS on the server. Retain the private key from this pair and hand out the public key to every single user. Generate an asymmetric key pair KC for every single user. Let the user retain the private keys from their pairs and send the public keys to the server. Every time ...



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