# Tag Info

10

After reading the paper How to Break XML Encryption (thanks to Krzysztof for the link), here are my two cents. This attack relies on the fact that a CBC-ciphertext C = (IV, C1, ... Cd) can be decomposed into pairs of (IV, C1), (C1, C2), (C2, C3), ... (C(d-1), Cd), each of which is also a valid CBC ciphertext for the same key, relating to the corresponding ...

9

Neither. It means that an attacker can decrypt all messages that have been encrypted using this standard. The attack is a padding oracle attack. That means that, if the attacker has a ciphertext they want to decrypt, they can send several variations of the ciphertext to the server. By analyzing the server's responses (e.g., error messages returned), it ...

8

Proving $\gcd(e, e_2) = 1$ is easy; all you need to do is rely on the property $\gcd(e, e_2) = \gcd(e, e-e_2)$ Now $e$ and $e_2$ differs in a single bit (because Peter flipped one bit in $e$ to form $e_2$), and hence $|e - e_2|$ is a power of two (the sign of which depends if Peter flipped a zero bit or a one bit), and hence has $2$ as its only prime ...

7

There is no such thing, but there is something called a chosen-ciphertext attack. In this kind of attack, the attacker is assumed to have a way to trick someone who knows the secret key into decrypting arbitrary message blocks and tell him the result. The attacker can choose some arbitrary nonsense as an "encrypted message" and ask to see the (usually) ...

6

Definitions / Introduction We define (this is solely for our example): $enc()$ and $dec()$ as the encryption and decryption function, with a constant key, any block cipher will do $\oplus$ is the XOR operation $n$ the amount of plain text blocks. the length of a block in bytes is $16$ (i.e. 128 bit) $m_1$ through $m_n$ the plain text blocks $c_0$ through ...

5

In general, CTR mode is not secure against chosen-ciphertext attacks. (The same goes for the other classic block cipher modes of operation too; to get security against chosen-ciphertext attacks, you need authenticated encryption.) In your stated attack scenario, the attacker can obviously use the decryption oracle to decrypt any ciphertexts they intercept, ...

5

In step 2, the adversary outputs two messages. One of these will be selected at random for encryption. You can think of the adversary sending these messages to a "challenger" that also has oracle access (or is the oracle itself). It doesn't really matter who is running the challenge since the challenger doesn't have any "intelligence." All the challenger ...

4

The idea of IND-CCA2 (indistinguishable under an adaptive chosen-ciphertext attack) is that the attacker has no chance to distinguish the ciphertexts of two given plaintext messages, even if it can feed the decryption machine other ciphertexts for decryption. In the second part of the experiment, the adversary has to chose two messages for the challenge (of ...

4

Another variant (besides the ones explained by Henning) which I also would call chosen-ciphertext attack, is one where the attacker doesn't get the whole plaintext corresponding to its chosen ciphertext, but only a result like "valid" or "not valid", i.e. he has a validation oracle, with some useful definition of "valid". The goal is to decrypt some message ...

3

Your third question asked how this attack relates to pre-existing applications of CBC. Before being used to break XML encryption, it was used on certain protocols run over TLS/SSL. See my answer here. For other protocols like IPSEC, WTLS, and SSH, the original paper proposing the attack has a good discussion of its applicability (in Section 5).

2

I am assuming that $n$ is small enough that the counter never rolls over and repeats, and that the IV is chosen randomly from the space of all possible IVs. The length of the plaintext is leaked, but that is leaked by the ciphertext anyways. The plaintext for a given ciphertext is leaked as the attacker can feed that in to the oracle. AFAIK, however, ...

2

The standard approach is to break this problem into two pieces: What information is unavoidably leaked, merely by computing the desired function? In your case, the goal is to compute $\sum_i x_i$. This sum unavoidably leaks a little bit of information about the $x_i$'s. For instance, as you correctly state, if we somehow know that all $x_i$'s are ...

2

All the details have been published in the paper titled "IEEE Symposium on Security and Privacy - Cryptography in the Web: The Case of Cryptographic Design Flaws in ASP.NET" http://netifera.com/research/poet/ieee-aspnetcrypto.pdf

1

The security claim on page 5 of the Linear Cramer-Shoup paper is that their modified scheme is CCA secure, which is weaker than the IND-CCA2 security of the original DDH based Cramer-Shoup scheme. However, from the outline of the security proof, it seems the author actually means the LCS scheme is CCA2 secure. Also note the first sentence on page 6: ...

1

This is tricky and I don't know that there is a generic way to take care of all domain/auxiliary information. The way we typically do proofs in multi-party computations is by defining an ideal world and show that the information generated in the ideal world (usually the encrypted inputs and the outputs) could be used to simulate the real world protocol ...

1

[EDIT: I am assuming a public key system here] IND-CCA2 summary: get encryption key. make decryption queries on chosen ciphertexts. obtain challenge ciphertext on $m_L$, $m_R$, two equal-length messages chosen by you, the attacker. The challenger is suppsed to randomply pick one, encrypt it and return the ciphertext. make decryption queries as in 2. ...

Only top voted, non community-wiki answers of a minimum length are eligible