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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 ...

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, ...

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 ...

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