Symmetric cryptosystems assume two communicating entities share a pre-established secret key.

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Encryption Oracle Avoidance

I have a case where it may be possible for an adversary to deduce the plaintext inputs that went into creating a AES-128-GCM encrypted message (there is good IV diversity across the encrypted messages,...
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Preserving Confidentiality and Integrity in a Public channel

Let's say I've got two people that are trying to talk in a public channel $A$ and $B$ and they want to preserve their confidentiality and integrity. Which cryptographic algorithm would they want to ...
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82 views

Perfect Secrecy in Symmetric Key Systems

For symmetric key systems does perfect secrecy imply that all ciphers in the cipher space are equiprobable? I would think not, but I am finding it hard to come up with a counter example.
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Symmetric encryption vulnerable when encrypting and decrypting are the same?

To my understanding, symmetric encryption is where encrypting and decrypting data can occur with the same key. It's possible for symmetric encryption that the encrypting and decrypting algorithms are ...
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100 views

Contrasting Key Rotation Requirements : Asymmetric vs. Symmetric Ciphers

I'm trying to contrast key rotation requirements for asymmetric and symmetric ciphers. In the case of symmetric ciphers, we have the results such as the so called "CBC Theorem" (stated on pg. 24 of ...
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506 views

Should AES.decrypt(ct, wrong_key) return blank or wrong result?

I am trying to use AES as cipher for an application I am building. I am using a JavaScript library (https://code.google.com/p/crypto-js/) to perform encryption and decryption with AES. When I ...
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150 views

Session key Exchange, some doubt

I have to find a solution to this problem: I have a network composed by 1 server and some client. The server has a couple of keys (public and private) and it shares a secret key with each client. My ...
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3answers
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Defining Symmetric vs Asymmetric cryptosystems

I'm writing a project on the advantages and disadvantages of symmetric vs asymmetric cryptosystems. At the moment I am trying to define both systems using mathematical notation. Under the heading "...
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Containers for secret key

What 'container' is used normally to store/transport secret keys (for symmetric algorithms)? I have come across PKCS12 format where a public key certificate as well as a private key can be bundled ...
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Found a way to crack AES-128, what now?

Let us assume the following scenario: I have just found a way to crack AES-128 in a reasonable time (1-2 days). How do I publish and prove this? I remember reading about lots of people who cracked ...
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118 views

Generating a key to use with AES

I'm going paperless, and I want to encrypt the documents that I scan. I'd like to use CommonCrypto to perform AES-XTS encryption on each file. I'm a software engineer, and I like the idea of being ...
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861 views

Why is Rijndael key length restricted?

Why is Rijndael restricted to key length in $\lbrace 128, 160, 192, 224, 256\rbrace$ bits (and not larger)? The algorithm looks to me like it would support an arbitrary-sized key (multiple of $32$). ...
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Can I decrypt the message having it encrypted with different keys?

I have some encrypted binary data I want to decrypt. I know nothing about it, but I can get it encrypted with as many different (random, unknown) symmetric keys as I want. The keys are long enough to ...
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1answer
162 views

end-to-end encrypted chat app with symmetric + public key encryption, TLS and cert pinning

A big drawback of end-to-end encrypted messaging services is that, as soon as you lose your device, you lose all your messages. Your private key is gone after all. I was wondering though, wouldn't it ...
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2answers
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Is the security of an HMAC equally important as the security of the cipher itself?

I'm a little unclear on how hash based MAC works... So I would like to know, is the security of your hash function as important as the security of the overall cipher? Basically, if you can crack the ...
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1answer
260 views

Which gives better deterministic encryption SIV or Plain ECB mode?

Lets say , if we encrypt a plain text message $msg$ with key $key$ in below two ways. Which is the below would give better deterministic encryption and why ? AES-ECB($key$ , $msg$) SIV($key$, NIL , $...
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61 views

Symmetric encryption algorithms with large IVs and authentication?

Most modern symmetric ciphers require a nonce/IV. If the nonce is reused, security is lost. If the nonce is large enough, it can be generated securely by using a CSPRNG. This avoids needing to keep ...
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1answer
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Why do we use encrypt-decrypt-encrypt (EDE) in 3DES, rather than encrypting three times?

I'm wondering why we use encrypt-decrypt-encrypt (EDE) sequence in 3DES (also known as DES-EDE, TDES or TDEA) with three keys instead of three times encryption (EEE) with three different keys?
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Is it okay to use an HMAC of the plaintext and a (possibly distinct) key as the IV for symmetric cryptography?

I was thinking of how to create an IV for a block cipher that doesn't require stored state, and I came up with the idea of using an HMAC of the (padded) plaintext and a (possibly distinct) key as the ...
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Symmetrical Block Encryption with Synchronised Remote Key Change

I'm considering a theoretical design of a walkie talkie, with strong encryption. My current idea is to use symmetrical block encryption, but with the key changing at regular time intervals without a ...
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1answer
132 views

Cryptography of numbers

Is there any algorithm capable of encrypt securely (symmetric) N numeric digits in a numeric message of M digits (N < M)? I am especially interested in a specific case in which N = 10 and M = N + 3....
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1answer
116 views

Muchomor cipher

I've been given a task to research and possibly implement so called 'Muchomor' cipher. Has anybody ever heard of this cipher? I've found only one article about it and it was in Russian. I'm quite ...
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1answer
131 views

(Lightweight) Multicast one-way Authentication

Problem: I'm thinking about a lightweight solution to provide source authentication (only one source) to multiple receivers (multicast message). Context: Taking the problem to ground, we can think ...
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How to use a stream cipher to sign a message?

Are there known constructions which allow to sign a message M (let's say it will be sent as clear text) using a stream cipher C and a symmetric secret key K? So, we have M, C, K and we want to send M|...
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1answer
174 views

Probability of guessing random 128-bit AES key [closed]

Consider the following five events: Correctly guessing a random 128-bit AES key on the first try Winning a lottery with 1 million contestants Winning a lottery with 1 million contestants 5 times in ...
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Does CPA attack assumption requires the oracle to have the key?

In CPA model, we assume Eve has access to an oracle who has the correct key and Eve can send query to the oracle to encrypt in CPA the data. Assume this encryption algorithm is a plain ECB mode AES ...
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120 views

A “one time pad” can be thought of a Vigenere cipher with

A "One Time Pad" can be thought of a Vigenere cipher with... An infinitely long key A secure symmetric key Multiple ciphertext alphabets A columnar transposition I had this question come up ...
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What communications security protocols exist that aren't based on PKC?

Lets say that you have two devices that need to communicate in a secure manner. These devices can share secrets before communication takes place (This sharing of secrets occurs over another secure ...
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438 views

What is 'key agility' in relation to symmetric-key encryption?

I sometimes see, in discussions of symmetric ciphers, reference to the 'key agility' of a particular algorithm. It seems to be related to the difficulty of switching encryption keys, but I don't ...
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1answer
50 views

Symmetric-like cyphers with several steps

Are there any good symmetric-ish cyphers that involve three or more steps with independent key material for each step? Formally, there should be a function $K(s)$ that returns a tuple $(k_1,..,k_n)$ ...
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Why does SHA-1 have 80 rounds?

Why does SHA-1 algorithm have exactly 80 rounds? Is it to reduce collisions? If yes, then why do SHA-2 and SHA-3 have a lower number of rounds?
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2answers
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Is it safe to initialize secret keys by just reading /dev/random on Linux?

For a software, I don't want the user to choose weak keys, so I plan to just read the needed number of bytes from /dev/random to create the needed secret keys. Is it safe to do so?
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443 views

Is there a correct way to generate a symmetric key?

Looking in some cryptographic algorithms, I've realized that: The way the plain text is encrypted/decrypted is always specified, but what about the key? Every paper I've seen describing the algorithm ...
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Key derivation design to avoid key leaks

I have the first function that generates keys from AES decryption (128-bit key). It takes seed as input: ...
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911 views

Authentication using AES

I need to implement an authentication mechanism in an embedded environment which does not support floating point operations but has an AES accelerator module which allows for encyption/decryption ...
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Secure method for encrypting 32 byte private keys

I am using 32 byte EC private keys on the client side of a web application. These keys should be securely stored on a server database. The keys should be encrypted (AES) before being sent to the ...
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1answer
131 views

Encrypt a message with hash function, shared symmetric key, but no cipher

I am very new to Crypto and need some clarification or input. The question is: Suppose that Alice wants to encrypt a message for Bob, where the message consists of three plaintext blocks, $P_0$, $...
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Combining a keystream generator with a block cipher

I understand that: A stream cipher's security is diluted if the same key is used more than once. This is fixed by using an additional initialization vector in the keystream's computation. A block ...
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Symmetric stream cipher, is it known?

I am thinking on a simple symmetric stream cipher. My first idea is the following: The key is a set of array of random bytes ($\underline{k}_1$ ... $\underline{k}_n$), whose length is $l_1$, $l_2$, ....
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What are the practical limitations of Asymmetric encryption vs Symmetric encryption? [duplicate]

I hear it commonly said that the advantages of Symmetric encryption are speed and no limit to the amount of data that can be symmetrically encrypted. I also hear it commonly said that the advantages ...
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1answer
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Encrypt with one cipher and decrypt with a different cipher?

I recently installed an android app called Secret Space Encryptor and think I found a serious flaw in it. Let me explain: This app provides a variety of different ciphers to use and I used Threefish ...
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CBC - a canonical mode, even though there are streaming modes

Why is CBC considered the canonical mode when there are streaming modes available such as CFB and OFB? One thing that I can think of is that in CBC you can easliy do range-based decryption. All you ...
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How to break this PRNG encryption scheme? [closed]

Suppose we have the following symmetric algorithm: key = random bits G = PRNG(seed: key) input = [0,1,1,0,1,0,0,0, 0,1,1,0,1,0,0,1] // "hi" in ascii The key can ...
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KCV and compatibility with block cipher modes of operation

There has been lately a question on KCV (key check value), value provided by many CRYPTOKI (PKCS#11) implementations. I don't particularly like KCV, but I decided to ask about proper use of KCV. ...
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1answer
190 views

Does the key schedule function need to be a one-way function?

For some key schedule $e_n(e_{n-1}(k))$ (where $e_{n-1}(k)$ is the result of the previous round) , does $e$ need to be a one-way function? In the case of DES or Rijndael the key schedule doesn't ...
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1answer
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Ephemeral Diffie-Hellman in a symmetric crypto context

I'd like forward secrecy in a case where the two sides have a shared long-term symmetric key. I intend to use AES in GCM. It looks like ephemeral Diffie-Hellman is the best way to generate the session ...
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Defining correct secured channel from endpoint to server

I'm new to cryptography and have a question regarding encryption usage. The scenario is as follows: I have an endpoint hardware device (CPU - ARM A7 1.2 ghz) which runs Ubuntu 14.04. This hardware ...
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What is the original SKID3 protocol?

While searching for a mutual authentication protocol I often stumbled upon SKID3. However I encountered different variations of it. The basic structure is the following: (1) A --> B: rA (2) B --> A:...
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Why is Diffie-Hellman considered in the context of public key cryptography?

In all textbooks I used the Diffie-Hellman key exchange is under "public key cryptography". As far as I can see it is a method to exchange a key to be used with a symmetric cryptographic algorithm, ...
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Difference between “ECDH with cofactor key” and “ECDH without cofactor key”?

I need to use “ECDH with a cofactor key” for generating symmetric key. I have a fair idea on how ECDH works, but I don’t understand the cofactor part. What is the difference between ”ECDH with a co-...