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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I'm trying to solve a question about one time pads but I'm not sure if my assumptions are correct. I am to assume that I have a one time pad with perfect secrecy. This one time pad is used to ...
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### Reusing one-time-pad with random plaintext

I understand that by definition one-time-pad keys cannot be reused but I was thinking about the case where it is used to send random data and couldn't find anything on the subject so here is my idea: ...
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### Many Time Attack [closed]

How i can XOR these two cipher texts encrypted by using same key First ...
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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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If I had two ciphertexts $$c_1 = m_1 \oplus y_1 \text{ and } c_2 = m_2 \oplus y_2$$ will it be possible to obtain $$z_1= (m_1 \lor m_2) \oplus y_1 \oplus y_2 \text{ and } z_2= (m_1 \land m_2) \... 2answers 6k views ### Proving the semantic security of the One Time pad Currently hearing a lecture on cryptography, and the professor gave us the definition of semantic security, which is roughly the following (formally not quite complete, but you get the idea): Given ... 1answer 533 views ### 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). 1answer 725 views ### What do you call one time pad where pseudo-random numbers are used? What is the encryption method called when pseudo-random numbers are used instead of true random numbers? 0answers 97 views ### 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 ... 1answer 1k views ### 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 ... 3answers 2k views ### If a cipher has key length shorter than plaintext, then it is not perfectly secure I am trying to verify the statement above. So far I only know that a One-Time-Pad is the only “perfectly secure” cipher. It has a key length which is exactly the same as the plaintext. I think the ... 1answer 123 views ### 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 (... 2answers 443 views ### Reusing message in One Time Pad One of the main rules of the OTP is, that a key should never ever be reused. But if we use some commutative operation (XOR for example) for generating the cipher text, then I don't see any difference ... 1answer 479 views ### Using modulus in one time pad? I would like to know what is the real purpose of using a modulus in the equivalent of a Vernam cipher: Data d, large number M. Assume the key length is \lambda bits. Then M could be 2^\... 2answers 691 views ### Can we use numbers as a pad in the Vernam cipher - why or why not? I was playing with the Vernam cipher on some online converter. But when I tried to encrypt my message string with numbers, it remained unchanged. Moreover, it was ignoring numbers and was encrypting ... 1answer 2k views ### Isn't a simple Vernam cipher as secure as known symmetric key algorithms? I have just read little about the Vernam cipher and the problems of implementing it. But, in practice, would not the key, if generated by a commonly used programming language's ... 1answer 2k views ### Entropy of the key Suppose a 1000-bit key used in the one-time pad is not randomly and uniformly generated. Suppose that the values of the first 5 bits are 0, and the other 995 bits are randomly ... 1answer 216 views ### Compression function to guarantee randomness of one time pads? First of all, I would like to note that I am not building my own crypto. I am simply curious and would like to learn. What I am wondering is this: if we are worried about randomness of something ... 2answers 108 views ### One Time Pad for large changing files i wondered what the security implications are if I do the following. If I had a large file encrypted with a OTP and want to change only a few bytes in the plaintext. what security vulnerabilities do I ... 1answer 2k views ### One-time pad, zero key and Shannon I'm supposed to prove that OTP without the zero key k=0^n is not perfectly secret anymore. I understand that it's not because an attacker learns something by looking at the plaintext and ciphertext. ... 3answers 15k views ### How to calculate Modulo using a standard calculator for a one time pad encryption I wrote a simple python script to preform one time pad encryption I would like to be able to decrypt the ciphertext by hand. In my script I use mod 26. This is how I was shown to calculate a mod ... 3answers 4k views ### One time pad: why is it useless in practice? The symmetric cryptosystem one-time pad (OTP) seems to be very beautiful since it is perfectly secret according to Shannon. Many books, however, point out the main drawback: one must create a secret ... 1answer 96 views ### 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 ... 1answer 1k views ### For a one-time pad, which MAC method is information-theoretically secure? In the the main post about MAC methods it mentions a few methods: Authenticate And Encrypt: The sender computes a MAC of the plaintext, encrypts the plaintext, and then appends the MAC to the ... 3answers 363 views ### Would a symmetric cipher with a keylength a big as the data length be information theoretically secure? One-Time-Pad is information theoretically secure as long as the random number stream is evenly long or longer than the data stream it encrypts, for a "deciphered" message could have been any message ... 1answer 364 views ### Why does a one-time-pad key have to be at least as long as a message? 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? 1answer 160 views ### One-time pad mistake in the transmission? I have a question which arised by analyzing the one-time pad cipher. I encrypt a binary sequence a_1,a_2,..,a_N using a one-time pad with key sequence k_1,k_2,k_3,..., so I send a_1+k_1, a_2+k_2,\... 0answers 644 views ### One-Time-Pad with key-reuse: Faster way of decrypting? I know there are already of few questions about this and I'm working with the advices that were given but I still doubt my approach is the fastest, so I'd really appreciate if you helped me find a ... 4answers 1k views ### Is there a way to generate sound one-time pads out of public information? Is there a way to 'mangle' a public data-source (for example, the current date in YYYYMMDD or the top New York Times headline) to form a one-time pad that will sufficiently hide the pad's source? ... 2answers 877 views ### Two-timing a one-time pad [duplicate] Is a one-time pad still breakable on a depth of 2+ if the plaintext is completely random? I'm assuming this idea is wrong, mostly because it occurred to me, but let me walk you through my logic so ... 1answer 567 views ### Diffie-Hellman: Does the size of the prime modulus depend on the potential size of the secret? I'm writing my own cryptosystem that uses One Time Pad and Diffie-Hellman to generate keys for the OTP. I am using Rails on the server side and Javascript for the client side. Please don't tell me ... 1answer 191 views ### 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 one-... 1answer 500 views ### Can two people with different one time pads securely exchange a message like this? Alice has a message, generates a one time pad, encrypts her message and sends it to Bob. Bob generates his own one time pad, encrypts the message again, and sends it back to Alice. Alice then decrypts ... 2answers 611 views ### Is there a companion algorithm for OTP to ensure integrity and/or authentication? One-Time Padding is (theoretically) perfect encryption algorithm. Let's assume that these problems are solved: Fast generation of independent and identically distributed random variables Perfect key ... 1answer 381 views ### Having 4096 bit keys and short (< 256 byte) messages, can I simply use the key as OTP? Assumptions: Alice and Bob use (cryptographically secure, hardware-rng generated) 4096 bit keys. Each message is encrypted using a new key; meaning: once a key is used, it's destroyed. Alice and Bob ... 2answers 1k views ### Is OTP useful in modern electronic communication? I have seen from time to time questions about various OTP schemes and "OTP" (i.e. stream cipher) schemes. The most common OTP troubles appear to be: How to extend size of previously generated key or ... 3answers 416 views ### One-Time Pad Distribution of Keys Between Two Parties Consider the One-Time Pad (OTP). Suppose two parties, A and B, generate a completely random secret key in a real-life meeting, and they keep this secret. Now A wants to send some message to B using ... 2answers 445 views ### Re-using parts of a one time pad to encrypt a new one? The main problem with OTP's is the fact that they can only be used once. Would it be possible to use a random number that's, say, 512 digits, then use 256 of those for the (non-random) message, the ... 2answers 16k views ### How to decode an OTP message? First off, I'm extremely sorry if this comes off as a "homework dump", but to clarify, it isn't. I don't want the answer, I want to know how to get to the answer. My cryptography professor gave us an ... 4answers 1k views ### With a true random number generator at hand, how to implement one-time pad? I have a Raspberry Pi single-board computer that happened to have an hardware true random number generator, based on some quantum effects on the processor silicon, baked into its BCM2835 chip and I ... 2answers 364 views ### Privately Sum Values without Dealers, MPC Suppose n actors each hold a plaintext p_i. We wish to find \sum p_i, without leaking any information about individual p_i. Any actor (or any link in the network) could be controlled by an ... 2answers 399 views ### Can you really insert the text you want in one-time pad? The Wikipedia article "One-time pad ~ Authentication" says : For example, an attacker who knows that the message contains "meet jane and me tomorrow at three thirty pm" at a particular point can ... 1answer 239 views ### Do any (non-hardware) RNGs exist which could be used to create an OTP for crypto purposes? I've stumbled (jobwise) over a system where small messages (512 Bytes or less) are encrypted and decrypted using a simple XOR using a OTP. That OTP is created using a seed based on the individual user ... 3answers 1k views ### Implementing a one-time pad without binary? I found a program which takes a purely alphabetical message, converts the letters to their ASCII values and combines them with the ASCII values of an alphabetical key (wrapping around when it gets to ... 1answer 205 views ### Can OTP still be broken if there are several messages being sent with only a slight variation in a password? If an attacker were able to get a hold of 2 encrypted messages that were both encrypted in a one time pad with a password that were each similar but had one change in the first character, could it ... 2answers 615 views ### Is my one time pad cipher secure? This is a program that I made for encrypting text files. It uses a one time pad cipher to encrypt the files, but I do not know if there is any holes in my program that could be a vulnerability. Is my ... 11answers 3k views ### Algorithm: How to use x and y mouse movement co-ordinates to generate random data? Background: I'm making a program for fun as a learning exercise. I want to generate some actual random key material (not pseudorandom) from a JavaScript program. For my program is just for encrypting/... 2answers 316 views ### Is this OTP scheme safe? I have a message m_1 and I want to encrypt it by XORing it with two keys k_1 and k_2:$$c_1 = m_1 \oplus k_1 \oplus k_2 So far so good. Now I was wondering if I could create a "fake" result ...
I'm reading the proof of the implication "Def 2.1 $\Rightarrow$ Def 2.4" in these slides about Adversarial Indistinguishability and Perfectly-Secret Encryption. I have a doubt in the slide 10. Here it ...