11
votes
1answer
782 views

Why do new versions of TLS use an explicit IV for CBC suites?

SSL 3.0 and TLS 1.0 used an insecure scheme to generate implicit IVs when encrypting records in CBC mode: They used the last part of the previous record, a value that can be predicted by the attacker. ...
6
votes
3answers
657 views

Lamport signature: How many signatures are need to forge a signature?

Lamport signature: Signing the message Note that now Alice's private key is used and should never be used again. The other 256 random numbers that she did not use for the signature she must never ...
1
vote
1answer
365 views

ECIES protocol - what does the || operation mean?

I am studying elliptic curves problems, which also includes study of related protocols such as ECIES. A there is a problem I don't understand operation $||$. What this operation mean? Some stuff is ...
2
votes
2answers
230 views

Algorithm Design for only Mutual Information Sharing

Bob and Alice each have a bit string they want to keep private. They each want to know what the bitwise AND of their two strings would be without telling the other or anyone else listening to their ...
1
vote
2answers
443 views

Real life collision when only using truncated hash

For MD5 two different inputs are known that produce the same 128 bit hash value. However, these inputs are artificially created for this specific purpose. For normal, real life inputs I believe no ...
1
vote
1answer
187 views

NTRUEncrypt - Choose the initial random polynomial

I'm trying to implement NTRUEncrypt but encounter some problems. I've finished all the basic functions that are needed for the scheme base on this Document, but I can't find an algorithm to check the ...
2
votes
1answer
321 views

DGK Cryptosystem Encryption Speedup

Following @poncho's nice clarification of the RSA speedup here, let's see if I'm able to do the same in the case of the DGK cryptosystem: We have pk = (n, g, h, u), sk = (p, q, $v_p$, $v_q$) which ...
3
votes
2answers
1k views

Difficulty of breaking RSA for a given key size

Is it true that breaking a 1024-bit RSA key is as difficult as breaking a 128 bit symmetric key (e.g. AES)? I know that breaking a RSA key is equivalent to factoring the modulus $N$. To factor it, you ...
1
vote
1answer
99 views

Determine complexity of a SAT problem

Is there a standard way to determine a complexity of the specified SAT problem? I'm researching algebraic cryptanalysis and came to solving multivariate quadratic equation systems using CryptoMiniSat. ...
7
votes
1answer
868 views

Why Addition Mod 32?

I was looking at the algorithm for Twofish, and I noticed that in some places a XOR is used, but in others, they use "addition modulo-32." What makes modulo-32 special? Why not always use XOR? Why not ...
4
votes
4answers
1k views

Assistance Cracking Classical Cipher

Below is the cipher text I am trying to break and as you can see its rather short which is why I am having so much trouble. ...
2
votes
1answer
190 views

Can A PRNG Be Used To Generate Multiple Private Keys for ECDSA?

Suppose Alice seeds a cryptographically strong Pseudo Random Number Generator with a truly random number, and keeps it secret. Alice then uses the output of the PRNG to generate several 256-bit ...
3
votes
1answer
413 views

Is XOR in a CBC-like mode secure?

Assuming that $K_{n}$, $P_{n}$, and $C_{n}$ are individual bytes of the key, plaintext, and ciphertext respectively. The first byte of ciphertext is computed like this: $C_{1} = K_{1} \oplus P_{1}$ ...
1
vote
2answers
3k views

Initialization vector length insufficient in AES

I use AES with AES/CBC/PKCS5Padding with the following encryption and decryption code sections in Java: ...
4
votes
1answer
453 views

How were the AES key and block length subsets of Rijndael selected?

My intuition tells me it's a trade off between speed and security, but how did the standardisation process select these three seemingly arbitrary key lengths (namely, AES-128, AES-192, AES-256).
3
votes
3answers
2k views

Is it fair to assume that SHA1 collisions won't occur on a set of <100k strings

I'm building a system that has to take file paths, and generate a unique name for each one. I'm planning on using SHA1 as the hash function. My question is: do I have to deal with possible collisions ...
4
votes
2answers
2k views

Why DSA cannot be used for encryption?

Here it is mentioned that DSA cannot be used for encrypt. But Both RSA and DSA can be used to generate public and private keys, right? (Or am I wrong?). Then why can't I use the DSA public key to ...
5
votes
1answer
4k views

Chinese Remainder Theorem and RSA

Wikipedia has a nice section regarding the speedup of the RSA decryption using the Chinese Remainder Theorem here. I need to understand the implementation of a similar speedup for the encryption ...
5
votes
3answers
1k views

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 ...
-1
votes
1answer
199 views

How can we find Public key have only 8 or 16bits? How many messages does Eve need to know the Public key in RSA?

Suppose Alice sends messages to Bob by encrypting the messages with Bob's public key. Eve knows that the data is encrypted using RSA, but does not know the public key. Can Eve figure out the public ...
0
votes
1answer
3k views

How does one implement the Inverse of AES' MixColumns

I got an answer in the related question about Mixcolumn for encryption, but how about decryption? what will I do? Because it said that I will used this: During decryption the Mix Column the ...
6
votes
1answer
706 views

How can disk encryption systems (like Truecrypt) resist frequency analysis when they allow random access?

I don't understand how disk encryption (e.g. TrueCrypt) is supposed to resist frequency analysis. If blocks can be randomly accessed (which they can), doesn't that mean that frequency-domain ...
3
votes
0answers
242 views

RSA reencryption scheme

Is there such an algorithm that would allow to upgrade the encryption of a RSA-encrypted message without revealing the plaintext to the untrusted user? Basically, such an algorithm would probably be ...
1
vote
1answer
149 views

Is it possible take a piece of data in secret?

I want something like this, but in a digital sense: You and others walk into a room. Everyone knows who each of you are and everyone is doing their best to figure out what piece of paper each ...
8
votes
1answer
1k views

How many RSA keys before a collision?

I was wondering how many possible private/public keys exist? If a million people – for whatever reason – would try to generate 5 keys each in the same minute (on the same date and time) is there a ...
1
vote
1answer
437 views

Recovering the key in an ElGamal Signature Scheme variant

From page 318 in Stinson's "Cryptography: Theory and Practice", question 7.3: Suppose that Alice is using ElGamal Signature Scheme. In order to save time in generating >the random numbers k that ...
2
votes
2answers
289 views

Order of hashing concatenation

A = hash("blue" + X); B = hash("pink" + X); If A and the literals are known and X is unknown, are there attacks on B aside from the attacks on directly on hash()? ...
2
votes
2answers
292 views

How do I demonstrate that a PRNG not designed for cryptography is not suitable for generating passwords?

This is a replication of this question on Stack Overflow. There's class Random in .NET runtime which is designed for use as a cheap fast source of pseudo-random ...
2
votes
1answer
223 views

Public Key Encryption with forward secrecy

Is there a known non-trivial system with plausibly secure public key encryption where: 1. $\;\;$ the $\:$ time $\:$ function is efficiently computable 2. $\;\;$ [pubkey,privkey] $\:$ pairs are ...
4
votes
1answer
158 views

Do public/private pairs work both ways?

Public/private key algorithms such as RSA encrypt a message with a private key but only decrypted with another (mathematically related) key. Can the public key decrypt the messages encrypted with ...
2
votes
2answers
5k views

OTP from Sony BIOS password recover

From Dogbert's blog: Sony has a line of laptops ("Vaio") which compete mainly in the high value market segments. They implemented a master password bypass which is rather sane in comparison to the ...
5
votes
2answers
237 views

Is my HMAC secure if I have a complete series of HMAC'd prefix strings

Let's say I have a long sentence, like "The quick brown fox jumped over the lazy dog." Let's further say that I need to keep this string encrypted, so I use an HMAC. Let's further further say I want ...
7
votes
1answer
1k views

Generating Random Primes

Although this has been extensively discussed around here, I'm curious whether my approach makes sense, or I should just stick to "the standard version". I'm implementing some homomorphic encryption ...
1
vote
3answers
774 views

Which attacks can be avoided by the use of OFB instead of ECB?

For a file encryption program, I was told to use Output Feedback mode (OFB) instead of ECB (Electronic code book) mode. Which attacks can I avoid by this choice?
1
vote
2answers
333 views

Complexity of ECB and OFB

What is the complexity of ECB in terms of Time and Memory? and also in OFB? I can't find it in the internet, so I decided to ask it in here.
6
votes
4answers
905 views

Creating an encryption key from several other keys and using hash functions

I want to combine two or more keys to create a single encryption key that relies on all of them. What is the proper method for doing that? Simple XOR? Using hash functions? Something else? I ...
10
votes
2answers
2k views

Can two different pairs of RSA key have the same modulus?

Can $n=pq$ be part of two different pairs of RSA keys? If such keys exist, say $(e_1,n)$ and $(e_2,n)$, how are they related? What will be the security concerns for the two users?
15
votes
6answers
563 views

Can I encrypt user input in a way I can't decrypt it for a certain period of time?

I run a baseball league and would like to do silent auctions for free agents. This would require teams to enter their highest bid and the highest bidder at the end of the auction period would win. ...
5
votes
1answer
203 views

One-way hash on encrypted data, result hidden from hasher

I'm looking for a one-way hash function that can be performed by A on an encrypted piece of data E(D) provided by B, without the performer A able to figure out D or H(D). This similar to HMAC(Message, ...
5
votes
2answers
492 views

Practical necessity of semantic security under chosen plain text attack (CPA) in CBC mode

I was not able to understand why we practically need a CPA security in Cipher Block Chaining. (which insist on having a random IV), let say if the encryption is not CPA secure i.e , the adversary can ...
3
votes
1answer
242 views

Has there been any cryptanalysis of RC4-52?

Several websites ( such as Is there a secure cryptosystem that can be performed mentally? ) briefly mention RC4-52 as a modification of standard RC4. RC4-52 has only with 52 instead of 256 elements ...
5
votes
1answer
154 views

Digital signature for an expiring password

I would like to create a publicly available tool that is used to generate a password that gives a limited time access to a resource. My idea is to digitally sign the current date with a private key ...
6
votes
1answer
511 views

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 ...
4
votes
3answers
297 views

Comparison: complexity measures vs. security

Assume that you want to compare several cryptographic primitives (say, encryption schemes), and choose one. You need to consider several complexity measures, such as the key length, encryption time, ...
3
votes
1answer
506 views

Advanced Encryption Standard Algorithm Secret Key

just a follow up question with this question.. How does the key schedule of Rijndael looks for keysizes other than 128 bit? A 256-bit key can still encrypt a block of plaintext in 128 bit? but in ...
2
votes
1answer
589 views

How does Output Feedback mode use the initialization vector?

How can I process Initialization Vector inside the Block Cipher Encryption box (using for example AES) together with the Key? Will I encrypt the Initialization Vector like a block of plaintext? So ...
6
votes
3answers
2k views

How does the key schedule of Rijndael looks for keysizes other than 128 bit?

It said in Wikipedia that: [....] Rijndael can be specified with block and key sizes in any multiple of 32 bits, with a minimum of 128 bits. The blocksize has a maximum of 256 bits, but the ...
5
votes
1answer
544 views

Elliptic curves for ECDSA

i'm trying to implement parameters generation for ECDSA according to SEC1 v2.0: Input: The approximate security level in bits = t is {80, 112, 128, 192, 256} ...
4
votes
1answer
357 views

How was the complexity of the Biclique Attack calculated?

The abstract of the Biclique Attack paper claims: The first key recovery attack on the full AES-128 with computational complexity $2^{126.1}$. What does $2^{126.1}$ mean in this context? And ...
7
votes
1answer
4k views

How strong is the ECDSA algorithm?

Some cryptographic algorithms are as strong as the size of their key is, while other have some weaknesses that limit their strength (such as SHA-1). How strong is the ECDSA algorithm, and does that ...

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