usually secret information used as input to various kinds of cryptographic algorithms, like encryption, signature, MAC, to select the concrete transformation done by the algorithm.

learn more… | top users | synonyms

3
votes
1answer
96 views

Calculate entropy of key derived from PBKF2 function

I am using PBKF2 to derive a encryption key from a password. suppose entropy of my password is y and my PBKF2 function has x number of iterations. So how to calculate the entropy of finale key derived ...
-1
votes
1answer
130 views

Guessing encryption algorithm based on key and ciphertext

Is it possible to tell which encryption algorithm was used, assuming I have a key and a encrypted text? To not complicate the situation let assume we are speaking about symmetric algorithms only. ...
1
vote
1answer
173 views

What exactly is inside a private key?

May sound stupid to many, but I would like to have some pointers on what exactly is contained inside a private key. I have decent understanding of public/private keys/certificates (have created them ...
3
votes
2answers
269 views

DES — Can I recover the key when I have both ciphertext and the plaintext?

Given a message and DES encrypted form of said message, is it possible to efficiently compute the key used to encrypt the data?
3
votes
1answer
102 views

Combining two hashing functions

I have been working on a Bloom Filter implementation recently and after a discussion with a co-worker about how many hashing functions to use I told him I was limited with using hashing functions that ...
1
vote
0answers
62 views

When do I need to renew keys for Salsa20 or HMAC?

I currently have a system which uses two keys sourced from a single key subjected to HKDF. One key is used in the Salsa20 stream ...
3
votes
2answers
538 views

Determine the Key given ciphertexts and plaintexts?

Given two sets of plaintexts and their corresponding ciphertexts, how does one determine the key $K$? The ciphertexts are: $$C_1= (P_1 \oplus K_0) + K_1$$ and $$C_2= (P_2 \oplus K_0) + K_1$$ $K = ...
1
vote
3answers
321 views

Does it make sense to block RSA keys under 1024 bits?

A paper I found (http://eprint.iacr.org/2012/064.pdf, which was submitted to a cryptography conference about two years ago), is based on the analysis of some 7.1 million 1024-bit RSA keys published ...
2
votes
2answers
91 views

Could someone reuse client certificates?

To my understanding, if a server “cert+key” (RSA) is compromised, than the SSL/TLS network traffic can be decrypted. What about a client “cert+key” (RSA)? Could someone reuse client certificates? ...
1
vote
1answer
105 views

What is “serial concatenation”? [closed]

I have a third-party point-of-sale API (Speedflow Pay-N-Get) that I am trying to communicate with over HTTP, but I have a problem with it. In a nutshell, a request is encrypted using RSA, and the ...
6
votes
2answers
543 views

Using the same RSA keypair to sign and encrypt

The RSA signature operation is basically the same as encrypting with the private key. In particular, both operations use the same kind of keys. Is it safe to use the same RSA keypair both for ...
3
votes
2answers
101 views

What is the function of the secret key “r” in Poly1305?

Poly1305-AES uses two per-connection keys $k$ (for AES) and $r$ (for Poly1305's compression function) and a per-message nonce n to execute. I've read the original paper on the implementation, and I ...
3
votes
1answer
122 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 ...
1
vote
1answer
73 views

Decimal point in keyspace notations? (eg 2^54.15)?

I was reading up on CryptoCat's (pretty ridiculous) programming issues and how is dramatically reduced the key search space (link to bug synopsis) One thing that caught my eye was So 2^54.15 ...
1
vote
2answers
101 views

Encoding multiple fields with same AES key and vector?

I'd like some advice on cryptography best practices. I need to store on my web app the users' credentials (login & password) for 3rd-party sites. I'm using AES-256 with random key and ...
7
votes
2answers
446 views

When using Curve25519, why does the private key always have a fixed bit at 2^254?

When using Curve25519, the private key always seems to have a fixed bit set at position 2^254. Why is that? Is there any good reason to use a fixed positioned most-significant-bit in the private key? ...
1
vote
2answers
173 views

Why can we always assume that $Gen$ algorithm choose key uniformly?

Talking about a (symmetric) encryption scheme $\Pi=(Gen,Enc,Dec)$ where $Gen$ take the security parameter $n$ as input and generates a key $k$ of length $n$. Generally speaking we say that $Gen$ ...
6
votes
1answer
516 views

Is a private RSA key vulnerable to a chosen plaintext encrypted with that private key?

I grasp that having known plaintext encrypted with the public key doesn't provide a means of discovering the private key (that being the whole point...). But if one were in a situation where they ...
1
vote
0answers
63 views

Perfect Forward Secrecy in MTI/A0

Would the property of perfect forward secrecy still be achieved in MTI/A0 for a session key that was established between C and D when one LL-key aD is revealed so that both LL-keys aC and aD are ...
0
votes
2answers
500 views

If I have the unencrypted text and the encrypted text, can I calculate the key?

If I have the unencrypted text and the encrypted text, can I calculate the key? It seems logical based on my limited knowledge of encryption that I should be able to... but it also seems like too ...
1
vote
2answers
97 views

Password as it relates to various encryption schemes

Suppose you have 2 drives you want to encrypt using different encryption algorithms, as an example: AES256 and Twofish. I am aware that using the same password for both could potentially weaken ...
0
votes
2answers
114 views

The weak link is the password?

Consider a secure modern block cipher like AES/Serpent/Twofish. I hear everyone say that the complexity is $2^{128}$ for a 128 block cipher. But isn't the weak link the actual hashed and salted ...
1
vote
1answer
243 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?
3
votes
4answers
402 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 ...
2
votes
2answers
500 views

Is there an effective way for public key generation in homomorphic encryption schemes?

Is there any way to effectively generate valid distributed public key in schemes like ElGamal or Benaloh for a number of participants? There's no need in private keys since there's no intent to ...
2
votes
1answer
145 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 ...
3
votes
1answer
237 views

Which PRNG security requirements do I need for key generation

Do I need to worry about "predictability" an an allegedly secure CSPRNG if I am just using it to generate key material from time to time? Suppose that for some odd reason, I am less trustful today ...
5
votes
3answers
445 views

Order of cascaded ciphers

Does the order of a block cipher cascade (e.g. in TrueCrypt) make a difference to the security provided, assuming independant keys? For example: ...
2
votes
1answer
91 views

What is the security risk if an encryption key is not random?

Ok, I just jumped into crypto and now struggling with some points… I read lots of resources, most of them mention how important the strength of the encryption key is. However, I could not find much ...
2
votes
1answer
188 views

Since it gives negative power, how is rcon(0) calculated in AES?

I'm trying to calculate Rijndael key schedule "Rcon" values manually. I know how to calculate each value with the equation ...
2
votes
1answer
494 views

Two different approaches for Key Expansion using the AES-256 algorithm?

I am implementing AES 256-bit Algorithm from the Theory given in the book, "Cryptography and Network Security" by Behrouz A. Forouzan. The Algorithm described in the Book in relation to ...
0
votes
1answer
168 views

Key space: Dense and sparse

I'm taking a cryptography class and I come across these terms dense and sparse key space allt he time. What do they mean? As far as I can I understand, dense key space means that there are more ...
1
vote
1answer
148 views

Intuition for random variables in cryptography

I have a simple question about the use of random variables for encryption schemes, to make sure my intuition is correct. We say $P(K = k)$ to represent the probability that a particular key $k$ is ...
1
vote
1answer
227 views

Key-size of encryption method

Is there a definied keysize to be used with the "AES_256-CBC" method in PHP, and what is the size? Some background: I want to store encrypted text into a PostgreSQL database. The user who posts the ...
1
vote
5answers
175 views

Using multiple secret keys

I have two secret keys. One is a secret key generated by openssl(Primary secret key). Second key is generated by performing one way hash operation to GPS co ordinates and time parameters(Geo secret ...
3
votes
1answer
219 views

What's the most secure way to derive a key from a password repeatably?

I'm trying to write a cloud storage application where everything the user uploads is unreadable by the server, encrypted client-side before being sent. Currently, the flow looks like this: User ...
1
vote
2answers
211 views

Is there a theoretical maximum useful keysize given the block-size?

Consider a block-cipher $F$ with a $N$-bit block-size and a $M$-bit key-size. That is, if $k$ is a $M$-bit key, $p$ is a $N$-bit block of plain-text and $c$ is a $N$-bit block of cipher-text, then: ...
2
votes
1answer
583 views

Self-expiring symmetric keys, or: cryptography in absence of secure deletion

I can encrypt some data D using a random symmetric key K, obtaining a ciphertext C, and then encrypt K with my public key Pub and obtain H. So far so good: I can only decrypt C if I have H and my ...
0
votes
2answers
301 views

One Time Pad Question

I have a ciphertext encoded with One Time Pad. I do not know the key. How can I find the possible keys which when decoded, will result in a meaningful plain text?
6
votes
2answers
1k views

AES key expansion: 256bit key

In AES algorithm, in the key schedule, Why does the expansion of a 256 bit key need an extra application of the S-box, unlike the expansion of 128 bit and 192 bit keys ? (The obvious answer would be ...
6
votes
2answers
1k views
4
votes
1answer
178 views

Swapping Key and IV in AES? Safe?

I have an application where I want to be able to send an encrypted file, and then mete out "keys" that allow the receipient to decrypt the file from a certain point to the end of the file. Actually, ...
5
votes
3answers
4k views

AES 256 Encryption - Is it really easy to decrypt if you have the key?

So this might sound like a crazy question but bear with me for a minute. I can't find any info on the internet and so am here, although this might have been a good place to start. I've recently ...
3
votes
1answer
70 views

Modes for encrypting small binary blobs?

Suppose I am building an encrypted document storage system. Suppose each user has a "user key": a symmetric key. Suppose each document is encrypted with a "document key". So, in order to give a ...
1
vote
0answers
151 views

IV and Headers inside cipher text

I am a bit confused about the use of IV, and key hash. Is the key hash and IV stored inside the headers of the cipher text? Isn't it a security risk to store this header information? By key hash I ...
1
vote
0answers
86 views

can permutation matrix be found in parallel processing technique?

Let us say there exists a permutation matrix $n \times n$ for some large $n$. Then, there are $n!$ possible permutations. If one to use brute-force to find the secret permutation among $n!$ ...
4
votes
1answer
135 views

Does EAX use the same keys for encryption and authentication?

I am actually checking use of EAX AEAD mode, and following EAX spec review (scheme definition for my part), my question is: what about the derivation of the authentication and encryption keys? In the ...
8
votes
5answers
890 views

Encrypt a file once with 50 characters password or twice with 25 characters?

What better Encrypt the file once with 50 characters password or encrypt it twice each time with 25 characters password.
1
vote
0answers
93 views

How is key rotation defined?

I'm aware that it's advisable to rotate cryptographic keys periodically. HKDF accepts high-entropy input (IKM - Input Key Material), and expands it to an arbitrary length. It also accepts a salt, ...
2
votes
2answers
184 views

Encrypting and MACing different data with same key

It's a well-known best practice to not use one key to both encrypt and MAC data. In my application, there are instances where I MAC a piece of data without having encrypted it first. Do I still need a ...