# All Questions

473 views

### AES block cipher modes of operation

have a project in which I have to implement an en/de-cryption structure using a standard AES block of 128-bits in VHDL and I think I'm a bit confused. So I'd like to ask some questions about AES and ...
1k views

### Why initialize SHA1 with specific buffer?

SHA-1 is initialize with a specific buffer: h0 = 0x67452301 h1 = 0xEFCDAB89 h2 = 0x98BADCFE h3 = 0x10325476 h4 = 0xC3D2E1F0? Why?
449 views

### WIN LM Password Hashes (using John the ripper) [closed]

In a problem set, we've been asked to use John The Ripper to crack a list of Win LANMAN passwords. They are of the following form: 953A55E36AB85C0B9DA92C18555E42E1:0169295E0A6A6B35B2DD9932D10C1978 ...
114 views

### Self authenticating tamper resistant first meetings

I'm not sure what the precise words I am looking for are so I apologize if there is a better way to phrase this. What I am thinking of is ways to transfer an identity in a way that it is tamper ...
367 views

### Please punch holes in this authentication scheme

Folks, I am undertaking a crypto course in Uni. Our professor asked us to try and design some authentication schemes which are resistant to active attacks. This was just a in class exercise to get us ...
207 views

### Relationship between Elliptic Curve Discrete Log, Integer Discrete Log, and Integer Factorization

I am trying to look into a relation between the following three problems which are widely used to build public crypto systems: Integer Discrete log Elliptic Curve Discrete log Integer Factorization ...
405 views

### How do we prove that AES, DES etc. are secure?

How do we prove that AES, DES etc. are secure? I've read about them and know the basics on how they work, but I still don't quite understand how we can be sure they are secure? What are the proofs?
468 views

### Making counter (CTR) mode robust against application state loss

Counter (CTR) mode, which is a block cipher mode of operation, has some desirable qualities (no padding, parallel encryption and decryption), but at the cost of failing badly when non-unique counter ...
226 views

### AES-CTR with ephemeral keys vs IV

Let's say that we have a key exchange of some sort, which leaves A and B sharing the long-term secret $S$. Then, A and B want to use AES-CTR for the communication, which leaves us with several ...
157 views

### SHA-1 colission resistance proof

I'm searching Why the SHA-1 is collision resistance? Anybody could help me with the proof or say me where I can find this proof?
381 views

### Question about modifying the MD5 plain text to cause a collision

I was trying to understand MD5 and got stuck with this question, its from Michael sipser's book on Information security Principles and Practice, 2nd edition The MD5 collision in Problem 25 is said ...
140 views

### Reversing N CRC steps

Starting from a known CRC result(let's call it CRC1), could I undo the CRC operations for last N bits, so I could obtain the CRC (let's call it CRC2) of the message sequence without the last N bits. ...
626 views

### Encrypting 8 times with 8-bit key beneficial?

I was trying to understand cryptography from the book Information Security by Mark Stamp and I am confused by something. Suppose that Alice has a secure block cipher, but the cipher only uses an ...
129 views

### Why are huge prime numbers important in cryptography?

I read an article the other day about the search for prime numbers. According to the article and several online sources the biggest prime number is over 17 million digits! This made me wonder why ...
851 views

### How does one deal with a negative “d” in RSA?

I'm trying to write my own RSA implementation using the textbook approach — which I know is not optimal — of picking primes $p$ and $q$, then computing Euler's totient $phi$, then randomly picking an ...
838 views

### Is it possible to use the RSA algorithm, or a variant, for software licensing?

I'm almost a newbie about RSA encryption, so my question could it be banal. I'm beginning to create a licensing system for my software. I need to distribute a crypted license file to many customers. I ...
2k views

### Practical disadvantages of GCM mode encryption

It seems that GCM mode encryption has a clear advantage over CBC + HMAC in the sense that it only requires a single key. But it seems that there are some experts here that do not trust it enough to ...
195 views

### Curve parameter for hyperelliptic curve cryptography?

RFC5639 defines some curve parameter for Elliptic curve cryptography. Aren't there any curve parameter database for Hyperelliptic curve cryptography? What I can only find was that written in this ...
438 views

### What is adversary's advantage in cryptography and why we use it?

The definition of adversary's advantage seems a bit odd for me and I am wondering why do we use it to measure the power of an adversary rather than just use the probability of a PPT adversary ...
1k views

### Can an RSA private key have several public keys?

Ok, so my crpto lecturer in Uni posed this question at the end of the RSA key generation lecture as a brain teaser. I have been thinking about this and I think I have come up with a way ( I am aware ...
229 views

### Stream cipher malleability

Considering a stream cipher that produces a ciphertext $c$ from a message $m$ and a key $k$ is it possible to apply operations (multiplication and/or addition) directly to $c$ without knowing the key ...
489 views

### How hard/easy will be to break a SHA-1 or SHA-256 hash if the attacker knows part of the original text?

I'm creating an algorithm to hash some text variables (20/30 characters long) to send them across an insecure channel. I'm not dealing with high security data, so no credits card numbers or any other ...
700 views

### Can two rc4 encoded messages be XORed and decrypted if encrypted with the same key?

If I have the following: E1 = RC4(M1, K); E2 = RC4(M2, K); If two different messages M1 and M2 are encrypted with the same key, K, can the key be recovered by ...
1k views

### Cryptographic Challenge: How to Say Something Confidentially to Snowden?

The Snowden situation raises an intereting cryptograpic problem. At present, how can something be sent confidentially to Snowden? Claim: I have no particular political orientation. The above ...
323 views

### Elliptic Curve Cryptography Encryption Results

I'm playing around with a package that does Elliptic Curve Cryptography from http://jecc.sourceforge.net/ Every time I encrypt a value it produced a different result (same private key). However I'm ...
2k views

### Hybrid encryption with RSA and AES versus spliting into multiple RSA messages?

I've done research about this subject, but I can't find the answer I'm looking for. The problem is that the string I want to send doesn't fit into the RSA key that the client have, and during ...
90 views

### Hashing a message with a length that isn't a multiple of 8 bits

NIST FIPS 180-4. “5.1. Padding the Message.” Suppose that the length of the message, $M$, is $l$ bits. Is there any library that can receive a message of length (in bits) which is not a multiple ...
969 views

### My encryption message system & question about security

I made this small tool for make & read easily an encrypted message without nothing to install, just a simple private key like a word or a couple of words to share. (Without say it directly of ...
539 views

### Decrypting a message without the private key using CRT

I am given 5 different encryption modulus, $N$, each ranging from 78 to 88 numbers long. Then for the encryption exponent, each has the same which is 5. Then I am given 5 different encrypted messages, ...
1k views

### Difference between salted hash and keyed hashing?

A cryptographic salt is additional input other than message itself for a hash function so that it prevents attacker from launching dictionary attacks . Usually the salt is stored along with the hash ...
254 views

### Elliptic curve cryptography attack vector

I would expect a complicated answer for what seems like a simple question about Elliptic curve cryptography. I've read several entries here such as "Elliptic curve cryptography related key attacks" ...
205 views

### How should I interpret this note on diffusion of the internal state of a PRNG?

This question is about Tyche, a non-cryptographic PRNG. While the PRNG is not cryptographic the analysis and this question surely is heavily involved with cryptography, so I think it's on-topic here. ...
413 views

### RSA encryption exponent

In RSA public-key material (e, N), why must the encryption exponent e be relatively prime to $\phi(N)$? Why must any public key encryption algorithm resist CPA?
457 views

### Generate key pair deterministically from a particular seed [duplicate]

I've been reading "SQRL Secure QR Login" at Gibson's website, and I'm wondering if there is a way to generate an RSA private/public key-pair based on some cryptographically secure input, so that if ...
258 views

### Re-using parts of a one time pad to encrypt a new one?

The main problem with OTP's is the fact that they can only be used once. Would it be possible to use a random number that's, say, 512 digits, then use 256 of those for the (non-random) message, the ...
98 views

### Paillier Crypto System : Pros and Cons? [closed]

Can you please list the pros and cons of the Paillier crypto system you have encountered or found?
1k views

### Why does ROT13 provide no cryptographic security?

I can understand that ROT13 is not secure for obvious reasons, but I'm looking for the theoretical answer. Wikipedia says "The algorithm provides no cryptographic security.." What does it mean to ...
507 views

### Would RSA make sense if we used no computers?

I was recently wondering - would RSA be useful if we brought it to, say, medieval times? Could you choose the keys so that you could encrypt/decrypt messages quite easily, but factoring the private ...
1k views

### Attribute-based cryptography [closed]

I am trying to create a social network, in which I want to use attribute-based cryptography. One professor told me it is possible to do it only in J2EE, but he can barely code! Some friends of mine ...
175 views

### Shamir's Secret Share Over the Reals

What about using Shamir's secret share over the real numbers leaks information? I know there is a problem with random number generation, and someone suggested it leaks the parity of the polynomial, ...
101 views

### Who's talking to who encryption - does this concept work?

With all the NSA stuff in the news, I've been thinking about how to ensure the "who's talking to who" is just as private as the contents of the messages. I had an idea on the subject, about which I've ...
2k views

### Why is MixColumns omitted from the last round of AES?

All rounds of AES (and Rijndael) have a MixColumns step, save the last round which omits it. DES has a similar feature where the last round differs slightly. The rationale, if I recall correctly, ...
219 views

### how to iteratively calculate a^emod n with modulus n sized 4096 bits

In most sites the exponent of the RSA public key is 24 bits. But the modulus can get to 4096 bits size. I have an accelerator that can get max. 2112 bit size modulus. It calculates ...
265 views

### Verifying Self-signed SSL Client Certificates

I have a service and I want to authenticate my clients using SSL client certificates. Now I want the key exchange process to be as simple as possible, kind of like SSH: You generate a keypair and ...
411 views

### How to calculate y value from ((y*y) mod prime) efficiently

i am working ECC-224 bit. can any one tell me, how to calculate y value from ((y*y) mod prime) efficiently for large bit numbers.
447 views

### Why is proof-by-reduction needed (for Elgamal proof of security, for example)?

The textbook proof for Elgamal encryption basically reduces to the Decisional Diffie-Hellman assumption (DDH). Elgamal: $Gen(.): x \xleftarrow{R} \mathbb{Z}_p$; \$Enc(m,g^x): r \xleftarrow{R} ...
332 views

### Rijndael: explanation of Rcon on Wikipedia?

I stumbled onto the explanation of Rijndael Rcon on Wikipedia, and I can't follow it. The example for Rcon indicates that Rcon(9) is 0x1b. That could make sense… ...
369 views

### How to make a message into a polynomial on NTRU encryption?

How to make a message into a polynomial on NTRU encryption? for example how we can conver 'hello world' to polynomial... i have read the converting operation on IEEE Std 1363.1- 2008, but i can't ...