# All Questions

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### Why perfect secrecy can be ensured when a plain message and a cipher-text based on one-time pad are correlated?

First, some well-known results are as follows. For random variables $X$ and $Y$, we have $$H(X,Y) \leq H(X) + H(Y).$$ The equality is achieved when $X$ and $Y$ are independent. Second, in one-time ...
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### Does the plaintext have to be at least as long as the key in Polyalphabetic substituion cipher

In polyalphabetic substitution cipher, if we have a key $k$ (a string of letters) of length $n$ does the plaintext $m$ (also a string of letters) have to satisfy $|m| \geq n$? Also, side question, ...
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### what is recommended x length in Diffie Hellman algorithm

According to rfc documents http://tools.ietf.org/html/rfc2412#appendix-E.2 & http://tools.ietf.org/html/rfc3526#section-3, it has defined the prime and the <...
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### How do we guarantee plaintext is coprime in RSA?

The specifications for RSA state: $P^{\phi(N)} \equiv 1 ~mod~N$ if and only if $P$ and $N$ are coprime. Here $P$ is the plaintext and $N$ is the product of two suitable primes $x_1, x_2$. My question ...
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### Duration for attacking Two-Key Triple-DES Encryption using all RAM ever built?

I am considering attacks on Two-Key Triple-DES Encryption assuming $2^{32}$ known plaintext/ciphertext pairs (that's a mere 32MiB of ciphertext) by the method devised by Paul C. van Oorschot and ...
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### Why doesn't this defeat RSA?

Apologies for the obviously ridiculous question but I need to know where I'm going wrong here. For RSA, we compute $n=pq$ for primes $p$ and $q$. We then choose an $e$ such that $gcd(e, \varphi(n))=1$...
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### How to exchange authentication keys

I am trying to create a secure Bluetooth Low Energy connection. The peripheral server (the main device other people would connect to) would need to service many clients (users) and due to hardware ...
2k views

### ASCII to same-length ASCII encryption?

I need to encrypt an ASCII string [a-zA-Z0-9:] to an ASCII [a-zA-Z0-9] string of the same length. It doesn't have to be unbreakable, it's sufficient that it won't be readable at the first sight. The ...
629 views

### Why cant Public Key Encryption be perfectly secure? [duplicate]

I would be very grateful for any help. I cant figure out why (probabilistic) public key encryption schemes can never provide perfect secrecy? Any Ideas? Excerpt: In contrast to the private-key ...
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### Outsourced Multiparty computation proof in Ideal world

I need to know in an outsourced two party computation where honst $A$ and $B$ outsource their private and secure data to a malicious server, why we need to design a simulator that interacts with an ...
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### Is there a tangible benefit in keeping the number of PBKDF iterations secret and not storing them with the ciphertext?

Suppose I have a local file and it is encrypted with a symmetric key derived from a strong passphrase, unique salt and $x$ number of PBKDF (e.g. Scrypt) iterations. I am interested in keeping this ...
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### what is the public information in Elliptic curve cryptosystems [closed]

Currently my knowledge about Elliptic curve is quite limited to the textbook and I don't know how a practical Elliptic curve cryptosystem works. I read an example about key exchange using Elliptic ...
212 views

### DH with AES - is that an acceptable security scheme over UPnP?

I'm trying to devise a good security scheme to protect comms over Wi-Fi. Right now, I have 2 devices that already communicate via UPnP but I need to secure this communication. I've come up with this: ...
330 views

### where should private and public keys be stored? [closed]

I am kind of confused whether should a newly generated key pair be stored in a keystore or in just a normal plain text file? From what I read from online source, there are serveral explainations ...
126 views

### SHA3 Keccak, absorbing data to the state

I'm working on implementation of keccak algorithm and have one problem. Tell me please, takeing an example "test": will state (before block permutations, but after padding - pad10*1) look like this: {...
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### Second generator for secp256k1 curve

I want to get a group element $h$ on the elliptic curve secp256k1. The important thing is that no one should know the discrete logarithm of $h$ with respect to $g$. That is, $h$ should be created from ...
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### Computing p and q from private key

We are given n (public modulus) where n=pq and e (encryption exponent). Then I was able to crack the private key d, using Wieners attack. So now, I have (n,e,d). My question is, is there a way to ...
108 views

### Avoiding known plaintext attacks with an additional XOR-Layer

Can a cipher that is vulnerable to an known plaintext attack be made secure by adding an additional XOR-encryption? That is: suppose I have an (w.l.o.g. symmetric) encryption algorithm E were the ...
32 views

### AES_GMAC implementation [closed]

I need to implement AES_GMAC encryption and decryption in c/c++ programming language. For the implementation, I have referred following document: http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/...
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### Feldman's VSSS: Showing the Correctness of shares that are Consistent

This is something with which I am struggling to get my head around. In Feldman's VSSS, I am aware that the Dealer broadcasts exponentials $a_{1},...,a_{t-1}$ of the secret $K$ and the coefficients ...
143 views

### how does an initialization vector work? [duplicate]

Can somebody please explain how an initialization vector works. My current understanding is that it is used to prevent 2 of the same messages looking identical after encryption, but isn't this the ...
1k views

### How do institutions like banks do RSA with big primes?

When encrypting with RSA it is often infeasible to decrypt by just doing c^d mod n, because for example when using the primes $(p,q)=(12553,1233)$, which are small ...
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### Segmenting encryption to get the same results with the same key

The given examples below are dummy and in English. They are mainly for place holding as this is a concept question rather specific example based Given AES has a block size of 128 bit (16 letters of ...
411 views

### Zero-knowledge proof of a product

I have non-negative integers $x,y,z$. I'm going to give you commitments $C(x),C(y),C(z)$ to them. Then, I would like to prove in zero knowledge that $xy=z$. I can choose the commitment scheme to ...
177 views

### Key Check Value standard pratice for asymetric cryptography?

I'm looking for a standard, or sound industry practice, for the equivalent of a Key Check Value, applied to the private key of a public key algorithm, like ECDSA. For DES or 3DES keys, practice (as ...
124 views

### Reciprocal block ciphers

I'm interested in the property of involution in block ciphers and how one would go about constructing one. Creating a cipher where $E_K(E_K(M))=M$ for any $M$ and $K$ isn't an obvious task and the use ...
3k views

### How to break AES/CBC/PKCS5 when key and IV are reused?

I'm doing a code review for a crypto solution that reuses the same key with a constant IV. I want to demonstrate that this is not the right way to do things by figuring out the key and decrypting all ...
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### Discrete logarithm over prime modulo: small input, large exponent, larger prime

I am considering using modular exponentiation as a one-way hash function. More specifically, here is the scenario. 1) Input ($m$): the input messages are small (16-bit) 2) Exponent ($e$): the ...
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### How is HMAC(message,key) more secure than Hash(key1+message+key2)

I understand how for hash functions which are vulnerable to length extension attacks (such as SHA1 and SHA2) it is safer to use a HMAC construction. What I don't understand is, how or why is ...
Is this $e(g^x,g^yH^z) = e(g^x,g^y)e(g^x,H^z)$ expression is true? where $g$ is the generator and $H \in G$
I am reading the Pinocchio paper. The paper says, in paragraph "polynomial asymptotics" of section 4.2.1, a worker, in order to include $h(s)$ into the proof, has to interpolate $p(x)$, and then ...