# All Questions

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### Why are the initial states of hashes functions (like SHA-1) often non-zero?

There is already a question asking "Why initialize SHA1 with specific buffer?" and my question follows on from this: Why are the initial states of hash functions often non-zero? For most, I have ...
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### Why have hashes when you have MACs?

It would seem to a naive eye that if you have a MAC, you have a hash function: use a key that all the parties know (such as all-bits-zero). A potential application would be a resource-constrained ...
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### Will varying plaintext compensate for a fixed initialisation vector?

This is a follow-up question to Relative merits of AES ECB and CBC modes for securing data at rest. I need to store encrypted Personal Account Numbers (PANs) in a database. The only encryption option ...
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### Relative merits of AES ECB and CBC modes for securing data at rest

I need to store several million Payment Card Numbers (PCNs) securely in a mainframe database (that is, 'at rest'). I assume that any attacker will have access to all of the stored data. I assume the ...
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### Why does SRP-6a use k = H(N, g) instead of the k = 3 in SRP-6?

I've been reading up on the Secure Remote Pasword protocol (SRP). There are a couple different versions of the protocol (the original published version being designated SRP-3, with two subsequent ...
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### Does AES have any fixed-points?

Is there any pair of 128-bit strings M and K such that AES$_K$(M) = M? If yes, how do I go about (efficiently) finding such a pair?
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### Understanding CRC

There are zillions of articles describing CRC. What can I read to (more deeply) understand what's really going on? Both from an algebraic perspective and a bit-manipulation perspective, I'd like to ...
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### Security of RSA for paranoids with padding?

RSA for Paranoids (RSAP) (in cryptobytes v3n1), also known as Unbalanced RSA, is a variant of RSA proposed in 1995 by Adi Shamir, as a mean to increase the RSA public modulus size while keeping ...
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### What is a tweakable block cipher?

Pretty simple question - but I can't seem to find much information about it. What exactly is a tweakable block cipher? How do they differ from traditional block ciphers? What is the 'tweak'? Is it ...
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### How many bits of hash are realistically needed for key verification?

Say I'm connecting to a web server secured with TLS but with a self-signed certificate. Accordingly, I call the owners of the server and ask them what SHA1 fingerprint they have. They start reading ...
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### Constructing a block-cipher from a hash function

It is possible to use a hash function to construct a block cipher with a structure similar to DES? Because a hash function is one way and a block cipher must be reversible (to decrypt), how is it ...
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### Why are elliptic curve variants of RSA “chiefly of academic interest”?

Yesterday I was thinking about elliptic curve variants of popular protocols/algorithms (ECDH, ECES[1], etc) and the thought occured that I had never seen an elliptic curve variant of RSA. My ...
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### Encryption with “constant” initialization vector considered harmful

I try to get the full reasoning behind the above statement. First, after reading articles here and at wikipedia i understand that using an IV only once is good practice. For stream ciphers not doing ...
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### A fair peer-based coin-flipping protocol?

I found this question on the game programming site and was intrigued. I came up with an answer off the top of my head but I'm no cryptanalyst so it is probably not water-tight. This is how my idea ...
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### Why not use CTR with a randomized IV?

I'm currently reading the chapter of Cryptographic Engineering (Ferguson, Schneier, Kohno 2010) about block cipher modes of operation. They have recommended CBC with random IV instead of CTR due to ...
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### What is the key strength reduction encrypting only 160 bits of data using RSA1024 for signatures?

I am attempting to determine the strength of an incorrectly implemented 1024 bit RSA signature scheme. The weakness in the implementation is that the padding data lacks random numbers. As a result, ...
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### Is it possible to weaken a bitcoin private key by “using” it elsewhere?

What are the increased possibilities (if any) of being able to crack a private key given the following: The associated bitcoin (ECDSA Secp256k1-based) public key is known. The private key has been ...
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### Is multiplicative secret sharing secure?

I suggested mulitiplicative secret sharing in an answer to another question, but noted that I wasn't sure if it was even secure and was hoping someone would comment on the security. Since no one did, ...
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Assume RSA with a public modulus $N$ of $n$ bits, a small odd public exponent $e$, plaintext $M$ a random non-negative integer less than $2^m$ for some integer parameter $m$, with $M\mapsto C=M^e\bmod ... 0answers 421 views ### How can I find two strings$m_1$and$m_2$, knowing that I know$m_1 \oplus m_2$? [duplicate] Possible Duplicate: How does one attack a two-time pad (i.e. one time pad with key reuse)? I recently started to follow the cryptography class of Dan Boneh on coursera.org and the first ... 1answer 2k views ### How large should a Diffie-Hellman p be? In a Diffie-Hellman exchange, the parties need to agree on a prime p and a base g in order to continue. Assuming some ... 3answers 308 views ### Designing a key expander out of ciphers Is it possible to compose or combine$n$ciphers ($cipher_0, cipher_1, ... cipher_n$) in such a way that they generate$n$ciphertexts from the same key,$k$, and that recovering the key,$k$, from ... 1answer 717 views ### AES timing attacks I'm just interested in cryptography, so please don't expect me to be an expert. ;) I recently read about AES cache timing attacks and found it very interesting. I read the article Cache-timing attacks ... 3answers 336 views ### Is there a public key semantically secure cryptosystem for which one can prove in zero knowledge the equivalence of two plaintexts? If Alice encrypts two messages$a$and$b$, such that$x=E(a)$,$y=E(b)$. Can Alice prove (without revealing$a$,$b$or the private key) that$a = b$? Obviously the proof must not be too long and it ... 1answer 176 views ### Why are there$ signs in my passwd file?

I am trying to get access to my eReader and I managed to get the passwd file. ...
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First, just to make sure I understand "salting" correctly: You randomly generate a string to append to the password before hashing it, so as to increase its length and make precomputed tables much ...
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### ECDSA: How to retrieve a non-random k

I have a question regarding the random $k$ number of ECDSA encryption. As far as I know, it is possible to retrieve $k$ (and thus the private key) from two signed messages if both used the same $k$. ...
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### ECDSA vs RSA: Performance on Android platform and surprising results

For our privacy-preserving protocol, an encrypted channel is established. In order to protect our system from man-in-the-middle attacks, signature-based approach is used. After we've implemented it ...
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### ElGamal in private set intersection: how to handle negative numbers?

I am trying to implement the Efficient Robust Private Set Intersection using Additive ElGamal. For this, from the client side, my inputs are $C = \{1, 2\}$. So, the polynomial is $(x-1)(x-2)$ which ...
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### Attacking 2DES efficiently

Meet in the middle on 2DES uses $2^{56}$ memory. Given the fact that the attacker has only $2^{45}$ memory. How can the attacker adjust the attack so even with this memory limit, it will still be more ...
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### Parallel-resistant proof-of-work scheme?

I am looking for a proof-of-work scheme which cannot be effectively parallelized. For example, in hashcash (and by extension bitcoin) you have some collision-resistant hash function $f()$, a target ...
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### Using a Hash as a secure PRNG

I was just looking at some NIST PRNG recommendations, specifically at Hash_DRBG. I read briefly through the algorithm, and even though it is not overly complex, it still seems unnecessary to me. I ...
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### Relation between attack and attack model for signatures

I would like to know: What is the relationship between an attack and an attack model. For example, let $\Pi$ be the Lamport signature scheme. This signature has it's security based on the one-way ...
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### Timing attack on modular exponentiation

It is known that computing $a^x \bmod N$ takes $O(|x| + \mathrm{pop}(x))$ multiplications modulo $N$, where $|x|$ is the number of bits of $x$ and $\mathrm{pop}(x)$ is the number of $1$ bits (Hamming ...
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### Theoretical pi-based stream cipher

Let's pretend that all digits of pi are known and arbitrarily long sequences of digits are trivial to get. Further, some mathematician proves that there are no patterns in pi. We could create a stream ...
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### Non-cryptographic hash function as MAC for stream ciphers

I understand that for a stream cipher to be useful, there must be a way to verify that the message was not tampered with (bits were flipped by an attacker). So, instead of using some cryptographic ...
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### Advantage of AES(Rijndael) over Twofish and Serpent

I'm trying to figure out a suitable encryption technique and after reading a bit, I figured the current AES 128-bit encryption is suitable for what I'm trying to do. However, this is more due to the ...
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### CPA Secure Chosen plaintext scheme

The example for IND-CPA secure schemes given is generally: for a random r, Enc_k(m) =(r|| E_k(r) XOR m) where E is a PRF But does the role of r and k really matter--i.e. isn't this equally ...
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### Are there secure stream ciphers that cannot be parallelized?

Are there any stream ciphers (or a deterministic random number generators, that should work as well I guess?) that cannot be parallelized? So for example if I seed it with a specific value, and then ...
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### What happens to the entropy of a password when you hash it?

For example, if the entropy of a password is 30 bits, what is the entropy of the password when you hash it with MD5?
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### How does one calculate the scalar multiplication on elliptic curves?

I found this example online: In the elliptic curve group defined by $$y^2 = x^3 + 9x + 17 \quad \text{over } \mathbb{F}_{23},$$ what is the discrete logarithm $k$ of $Q = (4,5)$ to the base ...
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### Security considerations for partially shared password databases

Programs like KeyPass and 1Password store password database files encrypted by a single password. If someone knows the protecting password ("Vault Key"), they can read the entire database ("secrets"). ...
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### Can I pre-define the points in Shamir's Secret Sharing algorithm

With Shamir's secret sharing is it possible to pre-define the points returned by the algorithm? For simplicity if I have (k, n) where k=2, and n=4, and I have the points X,Y,Z, and Q. Can I create ...
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### Does the XML Encryption flaw affect SSL/TLS?

A "practical attack against XML's cipher block chaining (CBC) mode" has been demonstrated: XML Encryption Flaw Leaves Web Services Vulnerable. Does this weakness of CBC-mode which is used here also ...
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### How to calculate complexity of ElGamal cryptosystem?

How to calculate time and space complexity of ElGamal encryption and decryption as there are two exponentiation operation during encryption and one during decryption? Here is my code for encryption: ...
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### What curve and key length to use in ECDSA with BouncyCastle

I'm developing a client/server system in Java which is not interacting with third party software, so I don't have to worry about compatibility. At a certain point, I need the client and server to ...
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### How to calculate the maximum output size for data encrypted with a RSA Private Key?

I have an an encryption algorithm I am working with that looks like this: prv_key_enc(sha1_hash(data)) Where, the RSA parameters are: RSA/ECB/PKCS1Padding 1024 ...
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### Weaker Notion of Target Collision Resistance

I'm reading the paper “Collision-Resistant Hashing? Towards Making UOWHFs Practical” which states: While it might be easy to find a collision $M,M'$ in $F_K$ by making both $M,M'$ depend on $K$ ...
I'm reading the paper “Collision-Resistant Hashing? Towards Making UOWHFs Practical” which states: With an ACR hash function $F$ the key $K$ is announced and the adversary wins if she manages to ...