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.

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1answer
93 views

How does the key size per data bit influence the security?

The likelihood of breaking, for instance, an AES-128 cipher is 100% after $2^{128}$ tries in brute force, meaning that I've got to try $2^{128}$ keys to certainly break it. What if I (hypothetically) ...
1
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0answers
91 views

Random numbers for Master Key and Key_block

Exploration through RFCs concludes that random numbers used in the creation of Master Key and Key Block(Key material for Encryption Key and Mac Secret) are from ServerHello.random and ...
0
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1answer
135 views

Prime factors of non-random keys

I have been working on a challenge i found on the internet. It is as follows: You've stumbled onto a significant vulnerability in a commonly used cryptographic library. It turns out that the ...
2
votes
1answer
109 views

Secrete message sharing using N private keys

I'm looking for an encryption scheme that could possibly meet the following requirements. Let, $X = plaintext$ $M = (pub, priv)$, where $priv == (Ky + Kz)$ Given ...
1
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2answers
163 views

Cryptanalysis of an certain algorithm

Is an attacker of an cryptographic algorithm allowed to change order of sub-keys but not changing their number in attack? Would it be called an attack on broken algorithm of cipher?
3
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1answer
250 views

How to protect against ephemeral key reuse in some signature schemes?

Given that some signature schemes (for instance Elgamal) use an ephemeral key that cannot be reused ever. How do people usually protect against this? I'll elaborate more: if you reuse the ephemeral ...
2
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4answers
998 views

One time pad key exchange

If two parties want to communicate with securely using OTP , how is the key ( that is lengthy as plain text) shared with other party for decryption ?
2
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1answer
108 views

Why is the security of block cipher not a function of key and tweak?

The proofs and security of block ciphers constructed especially of Luby Rackoff Constructions and Feistel Networks is based on the number of queries and round functions. The security measure is always ...
3
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1answer
497 views

How to calculate entropy of a combined key

I know how to calculate the entropy of a key that relates to its selection process. For example if the key space is $1000$, entropy of a randomly chosen key is $1000$. Suppose now you have two keys ...
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2answers
153 views

How to use HKDF to combine two keys

I have heard that to combine two keys HKDF is better than using XOR function. But if you look at the HKDF RFC I could not figure out how to use it for concatenating two keys. What I can see is HKDF ...
2
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1answer
117 views

Proof of shared secret through key derivation

Alice gives a random key $K$ (e.g. 32 bytes long) to Bob through a secure channel. Bob want to prove to Alice through an unsecured channel that he knows the key. Is it secure for Bob to send ...
3
votes
1answer
141 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
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1answer
590 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
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1answer
378 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 ...
5
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2answers
1k 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
129 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 ...
3
votes
2answers
1k 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 = ...
2
votes
2answers
169 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
298 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 ...
8
votes
2answers
3k 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
184 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 ...
1
vote
1answer
88 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 ...
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2answers
171 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 ...
9
votes
2answers
989 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
312 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
2k 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 ...
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0answers
164 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 ...
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2answers
116 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
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2answers
135 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 ...
3
votes
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?
2
votes
2answers
611 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
231 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
475 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 ...
2
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1answer
105 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
680 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
2k 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
454 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
190 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
835 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 ...
3
votes
1answer
656 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 ...
2
votes
2answers
249 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
988 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 ...
6
votes
2answers
2k 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
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2answers
3k views
4
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1answer
242 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, ...
3
votes
1answer
76 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 ...
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0answers
312 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
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0answers
105 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
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1answer
189 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 ...
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5answers
1k 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.