# Tagged Questions

A cipher which uses a different encryption key every time, as long as the message. The key is XOR'ed with the message to render the cipher text which can then be XOR'ed with the same key to get the plain text.

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### If you have a file and XOR every bit with a random bit, can you extract any information?

Say you have a file that is not random, and you XOR every bit with a random bit (not pseudo, but really random). Can someone who sees only the result extract any information from it? Obviously, it ...
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### Authentication using a one-time pad

I am building a small embedded system of two nodes that communicate wirelessly. The microcontrollers I'm using are very limited: they only have 256 bytes of RAM. I would like to be able to ...
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### Composing hashes and/or MACs

Suppose I have n hash/MAC functions, of which n-1 are extremely insecure, leaving only one that is in fact cryptographically secure. If I use all of them on a message and the XOR them together the ...
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### Why does a one-time-pad key have to be at least as long as a message? [closed]

I am studying one time pad and find it hard to understand. What happens if the key is one bit or 100 bits shorter than the message?
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### Deciphering a key from XOR encrypted cypher using boolean logic

Assume there's an unencrypted message A, and an encrypted message B. You know that message B was encrypted using a simple XOR method of A with a private key K, resulting in message B. Thus, B = A ⊕ K ...
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### One time pad (OTP) perfect secrecy with different key space

Let say $K_{0} = \left \{ 0,1\right \}^n$ $K_{1} = K=\left \{ 0,1\right \}^n$ \ $0^n$ $[b\leftarrow \left \{0,1 \right\}, k \leftarrow K_{b}:b=1|k \neq 0^n]$ --- (1) Key is chosen using ...
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### How vulnerable is the one-time-pad (OTP) encryption, if the OTP is used twice, with a random substitution scheme

So I was wondering, I read up about OTP encrption and can see how using an OTP encryption is in itself unbreakable. However, I looked at how OTP is broken if the OTP-key is used on more than one ...
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### Can one construct OTPs without using XOR?

The typical version of the one-time-pad (OTP) uses XOR to combine a key pad and a message. ($c=m\oplus k$) Now let's assume some other scenarios which have the practical application of blinding. Do ...
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### How to decrypt unusual Many Time Pad

How does one decrypt the many time pad: ($R$ is a random string, $C$ is ciphertext, $P$ is plaintext) $R_0 \oplus R_1 = C_0$ $R_2 \oplus R_1 = C_1$ $P_0 \oplus R_1 = C_2$ assuming that $R_n$ are ...
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### Can you exchange a shared key without any hardness assumptions?

Imagine that P=NP, and one way functions don't exist. Can two people end up with a random shared key of arbitrary length, if every exchange is public? They have true RNGs, and know who they are ...
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### Perfect secrecy of a crypto system

Suppose we have the following crypto system: $P = C = K = \{0, 1, . . . , n − 1\}$, $E_k(x) = (x + k) \bmod n$ and $D_k(y) = (y − k) \bmod n$. Prove that the crytosystem has perfect secrecy. Perfect ...
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### Building a pad for OTP on-the-fly with Diffie-Hellman

Through repeated DH attempts, could Alice and Bob build a large random key for use as a one-time pad? My intuition is that this protocol would be as hard as breaking DH (discrete logarithm).
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### Is multiplicative blinding less secure than additive?

It's easy to see that additive blinding (e.g., $x+r$ for secret x and random r) is perfectly secure in a finite field (this is a one-time-pad) and statistically secure for $r$ uniformly distributed in ...
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### Decrypt a message which is encrypted using XOR? [duplicate]

This is a puzzle asked in a contest. Given that encryption , decryption happens as per following rule/code: ...
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### To prove $r_2$ is a uniformly at random value in $Z_n$, where $r_2=r_1 . m$

$m$ is arbitrary value in $Z_n$, where n is RSA modulo. Then we do: $r_2=r_1 . m (modn)$, where $r_1$ is a random value such that $r_1\in Z^*_n$. ** Question(1): is $r_2$ a uniformly at random ...
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### Perfect secrecy over Stirling numbers

For all $c_0\leftarrow m_0 \oplus k$ there exists a $k'$ such that $c_1 \leftarrow m_1 \oplus k'$, where $m_0 \neq m_1$ and $c_0 = c_1$. Assuming a truly random $k$, the first assignment is a ...
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### problem with “one time pad” [closed]

I am a french student and I read the post on the forums (How does one attack a two-time pad (i.e. one time pad with key reuse)?). I need your help if possible. I 'm 11 encrypted messages , all ...
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### zendo data size restrictions

zendo is a new mobile phone app which encrypts messages using a one-time pad: http://techcrunch.com/2015/03/24/one-time-pads-ride-again/ users have to meet in person and exchange a key via a qr ...
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### OTP same message encrypted twice (with different keys)

Lets say we have a message $m$ that we encrypt with two keys $k_1, k_2$ such that $k_1 \ne k_2$. This will produce two ciphertexts $c_1, c_2$ such that $c_1 = m \oplus k_1, c_2 = m \oplus k_2$. The ...
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### Many time pad attack (XOR) [duplicate]

The question is quite similar to Many time pad attack and I was trying to rely on the top answer, but still am a bit confused, so any explanation and help will be much appreciated. Assume I have a ...
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### Is One Time Pad something like this? [closed]

Alphabet: [A-Z] = [0-25] Plaintext = HELLO = 7 4 11 11 14 // QUESTION: do this key must be as long as plaintext? KEY: [QWERT] KEY = 16 23 4 17 19 Plaintext + Key = 23 27 15 28 33 Plaintext + Key ...
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### Can shuffling plaintext produce a useable key?

If I 'shuffle' the characters in a long text, is there some number of shuffles that will produce apparent randomness approaching a cryptographically secure random number generator? A chapter in a ...