# Tag Info

8

When encrypting something with RSA, using PKCS#1 v1.5, the data that is to be encrypted is first padded, then the padded value is converted into an integer, and the RSA modular exponentiation (with the public exponent) is applied. Upon decryption, the modular exponentiation (with the private exponent) is applied, and then the padding is removed. The core of ...

7

These aren't "attacks" in and of themselves, they are simply a way to classify attacks depending on how many assumptions they make. For instance, if an attack requires plaintext-ciphertext pairs to recover the key, but they don't have to be any particular pairs, that attack is categorized as a known-plaintext attack. However if another attack required the ...

5

I do not remember if we checked this explicitly, but my guess is that in the chosen-plaintext setting the biclique attack would still be faster than the exhaustive search, maybe by the factor of 2 compared with 4 in the chosen-ciphertext setting. However, both results are pretty far from declaring AES broken in any sense. Such small gain over exhaustive ...

3

Okay. So first up, let's eliminate encrypt-then-sign. Why is this a problem? The idea behind a signature is to prove that a message came from me even in the presence of malicious actors. If a malicious actor changes the ciphertext under the signature, clearly this invalidates the signature as per expectations, however, that is only one possible attack ...

2

The initial notion of semantic security from Goldwasser and Micali has been shown to be euqivalent to what we call today indistinguishability under chosen plaintext attacks (IND-CPA). Yes that's only security against a passive adversary and actually the weakest reasonable security notion that we use today. The authors of the second paper you link seem to ...

2

The answer to your question is contained in the Authenticity bound (Theorem 5.1). This is because Authenticity implies non-malleability (see e.g. http://eprint.iacr.org/2011/092.pdf). Note that only one term in the bound refers to the length of the tag (referred to by the variable $\tau$): \mathbf{Adv}_{OCB}^{auth}[\mathrm{Perm}(n), \tau] (A) \leq ...

2

No. RSA-OAEP is indistinguishable under adaptive chosen cipher text attacks (and even non malleable under adaptive chosen cipher text attacks), but it is not an instance of authenticated encryption. - The sender who encrypts the message might even be anonymous to the recipient who decrypts the message. More generally, in a successful Chosen Cipher text ...

2

No, CCA does not imply authenticated encryption. CCA tries to recover the secret using chosen ciphertexts. A well designed block cipher should in itself already process the property that the key cannot be retrieved. If used with a properly implemented block mode of operation, this property should hold. Using authenticated encryption a CCA attack should not ...

2

The best you can get for homomorphic encryption schemes is non-adaptive chosen ciphertext security (IND-CCA1 security), see e.g. here for a quite up to date characterization. As you rightly observe homomorphic encryption schemes are malleable by definition and cannot provide adaptive security against chosen ciphertext attacks (be IND-CCA2 secure). Since ...

1

Well, one obvious way he can decrypt ANY cipher block is just give it to his Decryption machine; that machine will give him the matching plaintext block, which is precisely what he is looking for. Now, normally when we give an attacker a decryption oracle, and give him a challenge "decrypt this specific message", we put a limitation on the oracle that it ...

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