# All Questions

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### Applying Trapdoor Function directly to plaintext

Suppose we have a secure trapdoor function defined on (G, F, F^-1) where G is a randomized algorithm outputs a key pair (pk, sk). Why would this trapdoor function become insecure if we apply F ...
302 views

### Concatenation of two strong hashes may have striking weakness

For any hash functions $H_0$ and $H_1$, it is easily proved that their concatenation $H_0\|H_1$, defined by $(H_0\|H_1)(X)=H_0(X)\|H_1(X)$, is at least as resistant as the strongest of $H_0$ and $H_1$ ...
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### What happens when a RC4 stream gets corrupted?

I want to encrypt a large file using RC4. But what happens if the encrypted file gets corrupted (bytes modified or lost)? Can I still decrypt the rest of the file correctly? If not, what is the best ...
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### Making PRFs out of PRGs

Is it possible that we take a $PRG$ $G(k)$ of stretch $n\cdot2^n$, and read its output as the table of a $PRF$ $F(K)$ with input and output size of $n$? Intuitively it sounds possible however I read ...
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### What practical uses can random hash collisions be put to?

Using magic, I've built a machine that instantly generates hash collisions. To try it out, I set the dial to SHA256 and typed in "e7cf3ef4f17c3999a94f2c6f612e8a888e5b1026878e4e19398b23bd38ec221a". I ...
29 views

### Is RSA-OAEP secure when e=3? [duplicate]

Is RSA-OAEP secure when the exponent is equal to 3?
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### Practical strength of non-2^n RSA key lengths [duplicate]

It has for many years been popular to use RSA keys that have lengths that are powers of two. E.g. 1024-bit, 2048-bit and 4096-bit key lengths are all popular for use with OpenPGP implementations such ...
116 views

### Why not send the identity in the encrypted message?

I've recently started reading the book "Applied Cryptography" written by Bruce Schneier. Quite a few times I stumbled across protocols who send the identity outside of the encrypted message (see step ...
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### Resynchronizing brute-force attack against stream ciphers

There is a stream cipher without any known cryptographic weaknesses. The attacker knows parts of the key so she can reduce the key-space to a size that can be practically brute-forced. However the ...
216 views

### Given only RSA public key parameters, what security issues might I be able to detect?

I'm attempting to automate some checks against a large list of .NET assemblies, and want to include a test to see if the RSA parameters used to sign the assemblies are sensible. However, since I don't ...
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### CCA-attack is possible in RSA, but how the decryption key $d$ is known to anyone

CCA-attack is possible in text-book RSA, but how the decryption key $d$ is known to anyone, as it is kept secret with receiver.
377 views

### What is the most light-weight symmetric cipher thats still usefull?

I need a very light encryption scheme which costs almost no performance during encryption (while decryption may be slow). I found 5 algorithms: RC4, DES, LED, PRESENT, and Piccolo. But how do I ...
191 views

### Where I can find a list of certified software / hardware RNGs compliant to NIST SP 800-22?

I need to use NIST SP 800-22 approved software / hardware to generate RSA key pair. Where I can find a list of certified software / hardware RNGs compliant to NIST SP 800-22? And how is the SP 800-...
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I have an embedded device. It produces larges files (200 MB). I want the device to encrypt the file before writing to disk, so I gave them all a 16-byte random key. How can I transform the large ...
290 views

### Steganography Decryption From Completely Black Image

I am trying to decrypt a completely black gif that I know has been encrypted by some sort of steganography. I've tried subtracting the bytes of the images with each other, and so far I have gotten a ...
122 views

### why do files downloaded with http and https have the same size?

while working with Encryption mostly cipher text is always larger the plain text, why isn't that the case when downloading a file using https ? even that compression isn't much used anymore because if ...
68 views

### Requested to encrypt data with AES256, digital certification, timestamped

at my job we have been tasked by gov to encrypt some data we send to them. The "law" says we have to do the title above, but they only need us to do the AES256 thing because the data is not really ...
227 views

### Differential Fault Analysis of AES

Let's say: an error is induced into an implementation of the AES-128 to produce a faulty ciphertext. The error is a one-byte eror and is always induced on the last state byte, during round9, before ...
81 views

### RSA Decryption from samples

I am working on a project for a client which involves decryption of data which was encrypted using RSA. The system in question retains three independent copies of each data-set, but each copy is ...
1k views

### decrypt a saml 2.0 response having aes 128 and rsa [closed]

I need to decrypt a saml 2.0 response. But, the response object has reference to aes 128 and rsa algorithms, and I am having hard time in finding a way to decrypt. Below is the structure of the ...
198 views

### Rijndael S-boxes: Where do the $\mu$ and $\nu$ polynomial ring elements come from?

I've asked some other questions before about Rijndael's S-boxes, and step by step I'm coming to an understanding; but those steps often guide me to new questions. I did some lines of code to ...
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### Statistical saturation attack on block ciphers [duplicate]

I was wondering if anyone around here could give me some explanation on this type of attacks. Pretty much the only thing that I could find is this What I really need is basic explanation of the ...
220 views

### Why does consecutive permutations or consecutive substitutions not enhance security?

My professor stated that permutations which are done repeatedly does not further enhance security than just one permutation. He also stated that fact this applies to consecutive substitutions as well. ...
56 views

### Encryption with a known plaintext

I'm building an application where users can be a part of groups, and can obtain and distribute URLs from the group page that allow anyone who clicks on them to join said group. Groups should not ...
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### What is the benefit of applying the tweak a second time using XTS?

I tried to understand the inner working of XTS. What I understood is that it is applying the tweak two times: once on the plaintext and once on the output. What is the benefit of applying the tweak ...
147 views

### Statistical saturation attack on block ciphers

I was wondering if anyone around here could give me some explanation on this type of attacks. Pretty much the only thing that I could find is A Statistical Saturation Attack against the Block Cipher ...
613 views

### How do you break encryption on a message? [closed]

This may seem like a simple question, but I have just started studying cryptography and I can't seem to find any information on this at all. I am trying to write a "simple" encrypted messenger using ...
157 views

### What does variable-base point/scalar multiplication mean in ECC?

I am a bit confuse about the term, variable-base point/scalar multiplication, in Elliptic Curve Cryptography. What I have understood so far. It means that the base or point on EC is variable/unknown. ...
322 views

### Can we reverse a hash when we know part of the input?

While I read that hashes are not meant to be reversible, but assuming we have part/half of the message/input of a hash? Is there any method that we can use to recover the remaining message via this ...
153 views

### Key exchange using hash function

Consider two types of attackers on a key exchange (KE) protocol and subsequent communication between the parties which used this KE instance to establish a secure communication key. Attacker Eve ...
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### Why are twofish or other algorithms not NIST approved, are they still safe?

NIST has a total of 3 approved block ciphers on their website: AES, TDES and skipjack. I get why those are on there (though personally I find TDES a bit iffy) but from my understanding Twofish and ...
209 views

### Why is this a fix to Bleichenbacher's attack?

Looking at the excellent explaination of the attack here: Can you explain Bleichenbacher's CCA attack on PKCS#1 v1.5? I have one additional question: It says that: ""To prevent this attack, SSL ...
260 views

### Block cipher mode with diffusion on ciphertext

As I understand, diffusion property in AES-CBC ensures that one-bit change in plaintext will yield changes in all following blocks in ciphertext upon encryption. I'm looking for algorithm that will ...
43 views

### WPA and WPS: connection method [closed]

Do clients use WPS every time when they connect to a WPS-enabled AP? Or maybe they do it just the first time and then they switch to a normal WPA/WPA2 key exchange? If the latter, it's possible for ...
109 views

### If classical ciphers are used with compressed plaintext, how much does it make frequency analysis attack harder?

Classical ciphers, such as Vigenère cipher, are weak and no longer be used. They can be broken by using frequency analysis, that is a well-known fact. But, the frequency analysis often depends on the ...
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### Attack on key exchange with authentication

Suppose the honest parties - Alice and Bob - exchange public key in person/via telephone so that attacker can only eavesdrop on the line. Alice has hashed the public key using a collision resistance ...
151 views

### Of what use is my code for finding prime numbers of a certain size?

I've developed a bit of Mathematica code that can find primes within a range of numbers. For example, if I wanted all the primes between one million and two million, it could do that. Of what use is ...
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Server A produces signed messages on request. You can tell server A what type of RSA padding to use in the signature (either RSASSA-PKCS1v1_5 or RSASSA-PSS). Either way, the message will be signed ...
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### FileVault AES-XTS decryption with cipher text that's missing a part

My mac was recently upgraded to OS X Yosemite and FileVault2 full disk encryption was turned on. Encryption finished, but things went horribly wrong and key parts of the disk are filled with all 0s (...
759 views

### RSA key pair generation and FIPS 140-2, 186-4

I need to securely generate RSA key pair and I need to have access to private exponent in order to process it further. What's the difference in FIPS 140-2 and FIPS 186-4 in connection with RSA key ...
155 views

### Beginner question about secure communication with one user using DES

I am currently learning about encryption and I was asked a question: If a customer only had a DES system how could I secure messages as DES is not secure. I was researching this and I came across a ...
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### Implementing modular reductions (n*n) [closed]

I'm implementing a public key cryptosystem in a proprietary System On Chip that only allows modular reductions by $n$. However, that cryptosystem requires ($n^2$) modular reductions. Perhaps someone ...
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### What is the difference between any secret sharing scheme and arithmetic secret sharing scheme

It looks like that only difference between two of them the variables. But it does not make sense for me and I could not find exact definition of arithmetic secret sharing scheme. I mean what exactly ...
166 views

### Security of integrity protection with HMAC vs AEAD

I'd like to know how the security of the integrity protection compares between: HMAC-SHA-256 with an 32 byte tag or a truncated 16 byte tag AES-GCM with a 16 byte tag AES-CCM with a 16 byte tag or ...
105 views

### Can CKM_RSA_PKCS signature be made secure by lowering its maximum input size?

The CKM_RSA_PKCS Mechanism in PKCS#11 v2.20 section 12.1.6 PKCS #1 v1.5 RSA is a signature scheme commonly used to implement RSASSA-PKCS1-v1_5 of PKCS#1 signature of data hashed externally to a ...
203 views

### Self verifying hash algorithm

For quite some time I've been thinking about the idea to construct a hashing algorithm that contains its own checksum value, and thereby can verify itself. With hashing algorithms like SHA1 and MD5 ...
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### Why are modes of operation used, what attacks do they prevent?

I know you always need to use a mode of operation when using a block cipher, AES for example, and Wikipedia has a good explanation for what modes of operation are Now I know if i do not use a mode of ...
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### Reordering of multiple signed blocks

I would like to refer you to page 430 of the Handbook of Applied Crypto, where the authors said "... reordering of multiple signed blocks presents a security risk..." This statement was made in ...
Assume you have a finite group $\mathbb{G}$ and an integer $n$. Given $g_1,\dots,g_n,t$ chosen uniformly from $\mathbb{G}$, consider the problem of finding a vector $(a_1,\dots,a_n)\in \mathbb{Z}^n$ ...