# Tag Info

16

This is a common mistake, so I'd like to give an in-depth answer. Basically, what you are proposing is to rely on the ONE-WAYNESS of RSA as a ONE-WAY FUNCTION, rather than relying on its CPA or CCA security as an encryption scheme. The advantage of using RSA as a one-way function is that no padding etc is needed. Now, the first important thing to note is ...

7

This isn't really a "hard" answer, but an attempt to give some intuition or motivation. One can interpret indistinguishability as an overapproximation of the most common notions of security: Any system that is broken in a more practical way will also fail to meet indistinguishability, that is, all practically important security requirements are in fact ...

5

No. There is a difference between the type of a cipher and the construction of a cipher. If a cipher is of a specific type for which there are known IND-CPA secure constructions then that doesn't mean that an entirely different construction is secure. There are known attacks on stream ciphers, including "modern" stream ciphers such as RC4. If the generated ...

4

Katz & Lindell mention in their book "Introduction to Modern Cryptography: Principles and Protocols" an example of an IND-CPA attack from World War II. Navy cryptanalysts suspected that Japanese ciphertexts containing the fragment "AF" where referring to the Midway island. Then, they told officials at Midway to send unencrypted messages reporting they ...

2

The ideal encryption scheme $E$ would be one that, for every ciphertext $C=E(K, M)$, if the key remains secret for the adversary, the probability of identifying $M$ is negligible. Since that is not possible in practice, the second most reasonable approach is to define constraints strong enough to satisfy some definition of security. The $IND-$ notation ...

2

No. Indeed, as in the answer by Maarten, it depends on the security and strength of the stream cipher. However, even if the stream cipher is a secure pseudorandom generator (which is its proper modeling), encryption is not necessarily CPA-secure when XORing the pad with the plaintext. This is also explained in great detail in Katz-Lindell. In fact, it is ...

1

It can be proved, mathematically, that your (2), (3), and (4) are all equivalent under chosen plaintext attack. That is, if you can do any of those things then you can also do the other two! It should be obvious that (2) implies both (3) and (4): if you can decrypt a message then you know which message it is, and also you know it's not random noise. The ...

1

Yes, you can. Your construction $C=P\oplus KDF(K||N)$ is IND-CPA secure, assuming the KDF is secure, which is reasonable. This construction can be used and is sometimes used with ECIES (meaning $E_K(M)=K\oplus M$), but I'd recommend against using it. Replacing the KDF with AES is as secure as the above construction, as this mode is called CTR-mode, which is ...

1

A predictable nonce that cannot be controlled by the adversary is safe as a CFB IV (with some assumptions), as shown in the other answers. However, a nonce that can be chosen by an adversary is not safe against chosen plaintext attacks, as shown in Evaluation of Some Blockcipher Modes of Operation (page 36): Assume s = n. The adversary asks its oracle to ...

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