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

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Derived Shared Key vs Distinct Keys?

I've seen a lot of 2-party applications that derive a shared key from distinct keys created by each party. Why is this technique employed? Would it not be better to use those two distinct keys for ...
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401 views

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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2answers
873 views

are CFB and OFB really meant for streaming?

CFB, OFB and other modes are meant for streaming and don't require padding. Are there still limitations such as the text needs to be greater than key length?
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1answer
711 views

Can I secure my key by XORing it with a hashed password?

I'd like to build a simple password-protected symmetric key system. The key-creation process in my system operates as follows: The system creates a 256-bit key purely at random. The user chooses a ...
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2answers
352 views

Combining two keys

What is the most secure way to combine two "random" keys X of size k (k ≤ 512 bits) and Y of size 512 bits in one key Z of size k? Result Z will be used for encryption/decryption with various ...
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1answer
236 views

AES: Is it safe to encrypt same cleartext with same key but with million diferent IV?

The encryption mode that I am using is CBC. Algorithm is AES and the key size is 128bit. I will be encrypting a 36 byte string over 1.3 million times with the same key but with a random IV. My ...
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2k views

Would a “Triple AES” (in the sense of how Triple Des works) serve for a dramatic increase in safety?

The system requires to be as paranoid as possible regarding security. One of the few contemplated changes to the current design is to use multiple encryption. First proposal was to use Serpent on top ...
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1answer
99 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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164 views

Hide message length in symmetric key cryptography

Is there any way to hide message length from adversary in symmetric key cryptography? Suppose we want to hide message length from active and efficient adversaries.
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463 views

Is this a good way to encrypt a file?

I need to encrypt a file, distribute it over an insecure channel, and decrypt it later. Using a symmetric-key algorithm. Here's what I intend to do: Get a password P from the user. Generate a 16-byte ...
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1answer
189 views

Session-specific Symmetric Key Derivation Using SHA256

I was wondering if there were any security implications of deriving session keys from a symmetric key using SHA256 in the following manner: $Ks = SHA256(Nonce||K)$, where $Nonce$ is the session ...
4
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1answer
174 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 "decyphered" message could have been any message ...
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1answer
112 views

Are IVs and salts the same and usable for each other uses?

I'm new in the crypto world and I've just discovered PBKDF (I used to use typed passeword as symmetric key). When using some crypto mode, you're required to ...
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2answers
447 views

Encryption scheme with equivalent keys?

I've long been looking for a symmetric encryption scheme (or algorithm) with equivalent keys. Let me define what I want: Symmetric encryption algorithm with encryption function $E_k$ and inverse ...
4
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1answer
176 views

Is a changing public truecrypt container secure?

I have a Truecrypt container which I want to synchronise between computers (i.e. different people, that I want to share the data with). If I used Dropbox for synchronisation and someone downloaded ...
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0answers
66 views

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 ...
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1answer
839 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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4answers
3k views

Initialization vector in symmetric-key encryption

Can we use symmetric-key algorithms without an initialization vector? I am making an app where both the sender and receiver share a key and there is no way to create an initialization vector for each ...
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2answers
207 views

What is the difference between PBE and symmetric key encryption?

What is the difference between PBE (password based encryption) and symmetric-key algorithms? My understanding is that symmetric-key algorithms like AES and Twofish are used in PBE. Hence PBE is just ...
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753 views

Is there a symmetric-key cryptography based on key establishment techniques?

Most of the current key exchange techniques are based on public-key cryptography. Are there any key exchange/establishment techniques based on symmetric-key cryptography too? In my setup primary ...
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2answers
112 views

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?
3
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1answer
222 views

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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1answer
821 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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2answers
562 views

Are there standards for transport layer security using only symmetric keys?

I have an embedded system with AES-128 implemented in hardware. There is very little flash/RAM and the (8-bit) CPU runs relatively slowly. Public key crypto isn't a viable option. This system is ...
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3answers
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What is the best way to send two floating point numbers to a server on the internet from a small device such as an arduino?

I was thinking about sending some sensitive data (temperature) from an arduino (AVR 8-bit processor) to a server on the internet. But I would like to make sure that it would not be possible for Eve ...
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How to distribute session keys in public key cryptography?

In public key cryptography we can also use session keys which are symmetric. How do the sender (say a server) provides this session key information to its clients? If the sender (here server) ...
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329 views

Ciphers in CBC mode reveal place of change in plaintext

Theoretically, when using a symmetric block cipher in CBC mode, the current block is dependent on the previous block. Suppose one plaintext is encrypted using CBC, and then one bit of it is changed, ...
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395 views

When confusion is applied during encryption?

I know confusion is used in cryptography to make it more difficult to identify any relationship between the ciphertext and the symmetric key. I want to know when the confusion is applied during ...
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69 views

Misuse-resistant authenticated encryption based on SHA-3/Blake2b and AES-CTR/ChaCha20

I am wondering if the following is a useful misuse-resistant authenticated encryption scheme: Parameters (shared by many users): Choose a hash function $H$, such as Blake2 or Keccak. SHA2 could ...
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1answer
465 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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1answer
543 views

Why is RSA usually limited to messages up to 1 block

I'm wondering why RSA encryption usually is only used for messages that fit into one block. For larger messages hybrid encryption in combination with symmetric ciphers like AES seem to be the solution ...
3
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1answer
516 views

Is symmetric encryption vulnerable to plain-text-attacks?

Imagine that Bob sends a message to Alice for symmetric encryption to send to Charlie. (Only Alice and Charlie know the key.) Alice sends the encrypted message back to Bob to send to Charlie. Can Bob ...
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77 views

Multi-target attacks on AES-CTR with a random nonce

128-bit block ciphers are vulnerable to multi-target attacks where the attacker seeks to attack a collection of keys instead of a single key. A simple example: Generate keys $k_1, k_2, ...
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129 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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148 views

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 ...
3
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767 views

What are the pros/cons of using symmetric crypto vs. hash in a commitment scheme?

In a commitment scheme, are there any differences on using a symmetric cipher versus using a hash? If at the "opening", I have to reveal $r$ (a random number concatenated with the messaged at the ...
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1answer
307 views

Can we decrypt in this order when the message is encrypted twice?

If we encrypt a message twice with symmetric key $k_1$ first and then $k_2$ like $E_{k2}\{E_{k1}\{m\}\}$ , ideally we should decrypt with $k_2$ first and then $k_1$ but is it possible to decrypt with ...
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1answer
714 views

Symmetric encryption mode where ciphertext size is plain text size

I've had many questions on Stackoverflow on how to minimize the output of a cipher - during encryption of course - to the same size as the input. Obviously this is possible for a single block of ...
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1answer
363 views

Is appending the hash of the plaintext to the end of an encrypted message sufficient to ensure integrity?

Is appending the hash of the plaintext to the end of an encrypted message sufficient to ensure integrity? My reasoning is that the attacker doesn't know all the plaintext, so by having a hash, the ...
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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 ...
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603 views

Serpent cipher : Osvik S-Boxes confusion and test vectors

I'm having hard time with the implementation of the S-Boxes by Osvik found in this paper: Speeding up Serpent. At the end of the paper, all the s-boxes are given and then, I just implement them. ...
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1answer
2k views

What's the difference between the long term key and the session keys?

I'm currently studying secret key cryptography, and I've come across the terms "long-term key" and "session key". What's the difference between these two kinds of keys?
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305 views

efficient authentication with broadcast encryption?

(I am not a crypto expert, so I apologize if the terminology I use is incorrect.) Suppose I have the following simple broadcast encryption scheme for securely sending content to $n$ authorized ...
3
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1answer
147 views

Symmetric encryption using RSA for random generated password

I'm fairly new to encryption, so I wanted to check the logic for a .net solution I'm working on that encrypts data between two parties. the receiving (decrypting) party will have an RSA key pair ...
3
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1answer
124 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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1answer
180 views

Indistinguishable encryptions in the presence of an eavesdropper equivalence

I'm trying to prove that definition 5 and definition 6 in this document are equivalent. This is what I've done at the moment: Asume that the scheme has Indistinguishable encryptions in the presence of ...
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Should I use distinct MACs for each user, or should I use a *direction* flag when communicating?

Alice and Bob want to communicate using a stream cipher. At the beggining, they create a session key $K_s$ and exchange it via some secure channel (using some asymmetric algorithm). After every ...
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814 views

Generating an IV for ESP 3DES-CBC

I have an application written in C, running in Linux. It uses IPsec (ESP) (manually putting the IPSec packet together) to send certain packets. The algorithm used ...
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1answer
309 views

Is this an acceptable authenticated encryption?

After much deliberation, I've decided to use a normal Encrypt-then-MAC scheme instead of an authenticated encryption mode such as GCM as authenticated encryption primitive. This is due to the lack of ...
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473 views

What is the best way to store sensitive information on the client?

I am very inexperienced with cryptography and would like some advice on how to store some sensitive information on client machines. I have some information stored in a central database on my server. ...