# Tagged Questions

Encryption is the process of transforming plaintext using a cipher into ciphertext to make it unreadable to anyone except those possessing the key. Decryption is the process of transforming that ciphertext back into plaintext, using the key.

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### What's wrong with XOR encryption with hash and an iterated salt

Suppose you'd use the following algorithm to encrypt a message Let $k$ be the key to encrypt with Let $m$ be the message to encrypt Split $m$ into groups of 512 bytes Given a hash function with ...
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### How does this solution guarantee authentication of B and privacy?

The exercise is: What could be changed on the second message " Kya(Kxb(Ra,Ks)) " to guarantee B's authenthicity and also privacy? My teacher's solution was "add a destination" , so it would be ...
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### Need help understanding this exercise's solution

We were given this exercise, which asks us to identify the types of keys used on this diagram: The solution was Kxa -> A's private key Kya -> A's public key KxB -> B's private key My question ...
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### Plain text size limits for AES-GCM mode just 64GB?

Based on NIST SP 800-38D section 5.2.1.1, it seems that the maximum length of plaintext is 2^39-256 bits ~ 64 GB. We've got 100+GB files in genomics that need to be GCM encrypted so are concerned ...
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### Is double AES-CBC encryption with the same key and IV unsafe?

If some plaintext encrypted twice with the same AES function (CBC-mode), same IV and same key, is it possible to retrieve either the key or the plaintext?
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### Term for the ratio of ciphertext to plaintext size?

Is there a term that determines the ratio of size of ciphertext $|c|$ and plaintext $|n|$? I vaguely recollect reading it as scale out factor. Is there a better term? Or a popular term for this?
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### What are the most appropriate PKI based cryptographic protocols for peer to peer communicaton?

I've been trying to find out which is the most appropriate PKI based cryptographic protocol for peer to peer communication. I do understand that they vary with respect to the peer to peer architecture ...
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### Insecure third party connection

My organization processes some PII data and shares it with an outbound connection (outflow only) that is a third party. The data transfer transactions occur without human intervention. That outbound ...
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### Numeric base conversion in cryptosystems?

I have been wondering if numeric base conversion has ever found an application in cryptography. By base conversion, an example that is probably familiar is Base64 encoding. This is an example of ...
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### Does AAD make GCM encryption more secure?

Does AAD make AES GCM encryption more secure? What if we drop AAD in AES-GCM 256? If we drop it, how will it make the encryption less secure?
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### Why is it that most messages seem to be expanded?

Whenever I have seen a more secure version of a message, the message seems to be expanded by some factor. One example is RSA encryption, but whenever a message does not appear to be expanded it is ...
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### Combining secure hashes with insecure hashes?

Assuming I'm a bank which seeks for secure hash functions to use in the process of signing on digital contents. I'd like to explain / prove why using each one of the following hash functions might ...
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### Why does OpenPGP encryption produce different ciphertexts from the same plain text?

With the same plain text to be encrypted and the same public key, OpenPGP tends to produce a different ciphertext every time I run the encryption operation. Why is this?
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### Security Strength of Double Substitution Ciphers

I am wondering why people are using RSA keys when some types of double substitution ciphers seem to be just as secure if not better off. By some types of double substitution ciphers I mean one Which ...
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### is Authentication tag always appended at last to ciphertext in AES-GCM 256? [duplicate]

I am using AES GCM 256 to encrypt/decrypt a UUID string,We are using java 7 + bouncy castle library for this,We are getting authentication tag of 128 bit ,i am confused ,whether it's position is ...
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### X509 Cert exploit CA check

Assumptions Let's theoretically assume that a browser skips the check that a certificate it receives belongs to the stated CA (that's an exercise). How could I exploit this in order to impersonate ...
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### El Gamal special construction

The algorithm: I have to determine if the following algorithm is secure: $p,q \in \mathbb{P}$ and $N = p \cdot q$ $e \in [2, N - 1]$ such that $\gcd(e, N) = 1$ $r_1, r_2$ are random numbers ...
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### Is it possible to extract the decryption key from a locked android device

Is it possible to extract the decryption key from a FDE android device while the screen is locked but the phone is powered on. Is the data only safe when the phone is off ?
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### How to brute force sqlcipher encrypted file with MD5 hash of a 7 bit key?

I have a database file that is encrypted using sqlcipher and MD5 hash. Sqlcipher to my understanding provides 256 bit AES encryption. The key is 7 bits long. How do I brute force it?
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### How does the ring settings of enigma change wiring tables?

EDIT: The model I'm trying to make is "Enigma 1". I learned initially about it from a book called "Code Book" and then looked at it in detail from its wikipedia page. The site wont allow me too add ...
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### multi key encryption

Is there an algorithm which allows using multiple keys for decryption? I had this scenario in mind: think about being a boss of a corperation which has an encrypted data base with loads of secret ...
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### Use MDC to provide origin authentication for public key encryption?

Why can't we use an MDC (“Manipulation Detection Code”, as also handled in this Q&A) to provide origin authentication when using public-key encryption?
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### How to detect what crypto-method is used by Filecoder.Q?

I have 2 different images, one original, and one locked by a malware detected by eset as "Win32/Filecoder.Q". How to detect the encryption method that is used and the key, that is not using any ...
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### Full encryption of IDs, partial decryption with customer specific key

I am looking for a "partial" encryption/decryption algorithm. Let's say I am encrypting the following List of IDs (long values) $[1,2,3,4]$ (with a secret key) to an encrypted string $e$, I would ...
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### Can you still decrypt XTS if half of the cipher text is deleted?

If a part of my data is damaged, corrupted, or deleted, is it then still possible to decrypt the rest of the data if you still have the key? The situation: a piece of data (something like a document ...
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### Decrypting an encrypted text out of order

Does there exist a cipher that can be encrypted twice with two different keys and then decrypted twice using either key first? For example: ...
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### AES implementation to encrypt files

I'm making a small python program to encrypt an arbitrary text file. Currently I have a file to store passwords (hashed values), so nothing stored in plaintext. The user enters the command line ...
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### Encrypting and obscuring data between site/user without SSL

Im trying to figure out what the best way to encrypt data sent between a webpage and the user(both ways) is, when hosted in an environment that doesn't support SSL. The purpose of encryption would be ...
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### What does 0…0 and 1…1 mean

I am very new to cryptography but can someone explain what exactly does 0...0 and 1...1 mean in general term. Is it that 1st and ...
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### How to create and validate potential grids when attacking a Playfair cipher?

First up: I am not asking you to decypher anything. This question is about attacking a Playfair cipher; more specifically – how to create potential grids when doing so. I'm currently working on a ...
14k views

### How cryptographically secure was the original WW2 Enigma machine, from a modern viewpoint?

If cryptanalysts today were to crack the original Enigma machine, “how fast” or “how easily” could they do it? What methods would they use? The original cracking was significantly helped by operator ...
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### How does blocksize affect security?

I have recently read about how blockciphers and how stream ciphers work, and although I think I finally got everything right, I'm still wondering how increasing blocksize increases security.
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### How many possible Enigma machine settings?

I'm trying to calculate the amount of settings the enigma machine has. I have found several sites regarding this topic, but it seems like there are two answers to my question. The first answer is 158,...
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### Security of the shared secret key for a smart meter (Linky)

Reading the specs of a new electricity smart meter (called Linky in France), I was surprised about the chosen encryption method (128 bits symmetric key AES), based on a single secret key (called CCC ...
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### Difference between stream cipher and block cipher

A typical stream cipher encrypts plaintext one byte at a time, although a stream cipher may be designed to operate on one bit at a time or on units larger than a byte at a time. A block cipher ...
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### How to approach to solve cipher text with fixed length key (which is repeating)?

I took a project to solve for lecture. In my project for a given cipher text(length 2031), I have a key (Picture of example is given.) which I do not know the length of it. So I started to try to find ...
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### AES 128 bits with 12 bytes IV vector composed of a 4 bytes counter : safe?

I have a 128 bits AES cypher (GCM, Galois/Counter Mode) using a 12-byte initialization vector (IV) composed of: 8 constant bytes: 6 bytes (device ID) 2 bytes (zero) 4 bytes (incrementing frame ...
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### Can I serve data for user without the ability to access them? Which functions for the user remain?

My Problem I want to serve sesible data for users as a webservice that is accessed by the user via the browser. Restrictions: I may not be able to technically acces the data of the user Data is ...
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### How to publish a cipher (concept)?

In the last months I was searching for a subject for my bachelor thesis. I came up with an idea for a new cipher concept that works by combining already known techniques in a (hopefully) new way. So ...
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### Is the one-time pad secure?

I have read about one-time pads (OTP) on Wikipedia. Is this secure? Can I actually use modular addition as ecryption like it said in Wikipedia? And the plaintext is as long as the OTP, so when I send ...
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In one of the hot forums of today on how to convince friends cryptography is good one of the arguments stated is the knife paradox, I was unable to find the answer on Google and my question is what is ...
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### Is there a reduction of security if I XOR Pkcs5S2 and SecureRandom?

I'm using this codereview sample to encrypt data. I would like to provide hardware protected encryption and a user password as the key. Is it acceptable to XOR the KDF Pkcs5S2 in Bouncy Castle with a ...
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### Fixed initialization vector in aes cbc [duplicate]

What are the pros and cons of using fixed iv in aes-cbc encryption? I've read somewhere that it said using fixed iv doesn't really matter?
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### Is $E′(k,m)=E(0^n,m)$ semantically secure?

$(E,D)$ is a (one-time) semantically secure cipher where the message and ciphertext space is $\{0,1\}^n$. $E'(k,m)$ is defined as $E′(k,m)=E(0^n,m)$ The question is whether $E'(k,m)$ is (one-time) ...
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### How do I create a secure encryption scheme using addition?

You have a 4 number long PIN code with each number ranging from 0 to 9 which you wish to encrypt. You are then given a random 4 digit number ranging from 0 to 9999, which when added to the original ...
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### How is the AddRoundKey operation performed in AES-256?

Can anyone explain to me (or send a link to somewhere) how round keys are added in AES-256? I understand that the round keys is added to the state matrix for each round and how XOR works. But since ...
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### Has anybody implemented BGN cryptosystem to multiply two plain texts?

I have tried to use this code as follows, http://stackoverflow.com/questions/33581962/bgn-implementation-in-java but the issue is, when it generate the pairing, the output is in GTfiniteelement (for ...
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### Why is PuTTYgen-created RSA public exponent(e) not in {3,17,65537}?

As far as I know, RSA public exponent(e) should be one of {3,17,65537}. However, I found PuTTYgen-created RSA public exponent(e) is 0x25(37) by default,as follows, (PuTTYgen version: 0.66) I am ...