# Tagged Questions

The attacker can specify his own plain text and encrypt or sign it.

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### IND-CPA security scheme [duplicate]

Possible Duplicate: CPA Secure Chosen plaintext scheme The example for IND-CPA secure schemes given is generally: for a random r, $Enc_k(m) =(r|| E_k(r)$ $XOR$ $m)$ where $E$ is a $PRF$ ...
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### Why is an Encrypt-and-MAC scheme with deterministic MAC not IND-CPA secure?

I'm preparing myself to exam, but I have a lot of troubles with rigorous proofs. It's the task from two-years ago exam. At the bottom I reminded one definition. Let $(Gen_E,Enc,Dec)$ be an IND-CPA-...
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### How to construct a variable length IND-CPA cipher from a fixed length one?

Suppose we can construct a secure (IND-CPA) encryption scheme for fixed length messages. I was wondering if there is a natural way to extend this construction for messages of variable length such that ...
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### Why is CBC with predictable IV considered insecure against chosen-plaintext attack?

I just learned that using CBC encryption with an IV which is predictable is not secure. From what I understand, using certain plain texts, and then guessing the IV that it uses, the attacker can ...
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### Figuring out key in hill cipher (chosen-plaintext attack)

I have been wondering what approach to take in order to figure out what key was used to encrypt a message using the hill cipher. I know it is possible to obtain it even if it were just a known-...
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### Is using a predictable IV with CFB mode safe or not?

While writing this answer, I noted that NIST SP 800-38A says that (emphasis mine): "For the CBC and CFB modes, the IVs must be unpredictable. In particular, for any given plaintext, it must not be ...
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### What is the difference between known-plaintext attack and chosen-plaintext attack?

I am very confused between the concept of known-plaintext attack and chosen-plaintext attack. It seems to me that these two are the same thing, but it definitely is not. Can anyone explain to me how ...
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### Practical necessity of semantic security under chosen plain text attack (CPA) in CBC mode

I was not able to understand why we practically need a CPA security in Cipher Block Chaining. (which insist on having a random IV), let say if the encryption is not CPA secure i.e , the adversary can ...
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### How does a chosen plaintext attack on RSA work?

How can one run a chosen plaintext attack on RSA? If I can send some plaintexts and get the ciphertexts, how can I find a relation between them which helps me to crack another ciphertext?
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### CPA distinguisher for matrix multiplication in GF(256) with randomized padding

What is the best CPA distinguisher for the function $F_k:\{0,1\}^{8n}\to\{0,1\}^{16n}$ described below? Let $E_k$ be a $2n\times2n$ matrix with elements in $GF(2^8)$, selected by generating bit ...
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### Message space in security definitions

What is the message space in the following example and how does a message space relate to a security definition? I mean, what difference does it make to such a definition if your message space is \$\{...