2
votes
2answers
173 views

How can I solve congruence modulo N?

I am trying to solve congruences of the form $$J_A \cdot a^e\equiv 1 \pmod n$$ where $n=pq$ for $p,q$ prime and $\gcd(e,\varphi(n))=\gcd(J_A,n)=1$ Solve for $a\in \mathbb{Z}$, in terms of $n,J_A$ ...
6
votes
1answer
770 views

How does the secret key in an HMAC prevent modification of the HMAC?

Just as a preface: I'm not implementing a HMAC - I just want to understand it, as it is part of of my Computer Science course. When using Hash-based message authentication codes, I understand that ...
6
votes
2answers
2k views

Strength of MD5 in finding duplicate files

Why are there a lot of duplicate file finder applications which are using MD5 Algorithm? What is the strength of MD5 in terms of searching duplicate files in hard disk or flash driver or any other ...
4
votes
1answer
146 views

Why has the sponge construction's generic collision finding attack a complexity of O(min(2^(-n/2) , 2^(-c/2)))?

I'm trying to learn about sponge function for creating hash functions and generic attacks on it. I'm looking for the collision finding attack scenario which leads to $O(\min(2^{-n/2} , 2^{-c/2}))$ ...
6
votes
3answers
490 views

Are there other digital certificate formats than X.509?

Hi I am a little new to security, but in researching digital certificates it seems the only format people describe is X.509. Are there other formats? If so what are they and where can I find ...
5
votes
1answer
461 views

What could “look for the constants that you might find in RC6” mean?

In this presentation at the 30c3 (long but extremely interesting and well-documented), Jacob Appelbaum gave this verbal advice (circa 43'30"): look for the constants that you might find in RC6 ...
3
votes
2answers
387 views

unique one-way hash

For a 10-digits numeric domain (swedish social security numbers), is there a hash function with the following properties? no two numbers result in the same hash it is not possible to deduce the ...
2
votes
1answer
112 views

Cryptographic Arithmetic Toolbox/Software [closed]

This term I have many cryptography courses treating finite fields. I was wondering if there is any good software that could help doing basic operations in galois fields etc. I already googled but ...
1
vote
2answers
379 views

Where is my private key stored when I communicate to a website that uses SSL/TLS?

If I communicate with a website that uses SSL/TLS, my browser will generate a private key. Would the browser store that key on the hard disk or would store just it in the RAM?
1
vote
0answers
328 views

Random Number Generation with a Entropy pool versus Seed

To give a little background I'm currently implementing a crypto library in JavaScript. I have started porting across Linux's random number generator because its both publicly scrutinized, it is ...
6
votes
3answers
2k views

A timestamping authority (digital notary)

Does there currently exist a free online service that accepts a file, hashes it, takes an authoritatively chosen timestamp (from one or more time services), signs these and sends this signed message ...
2
votes
0answers
77 views

How do you distribute secret shares without knowing who to first distribute them to?

Figure 1 on page 1249 of the “Multiparty Computation Secure Against Continual Memory Leakage” paper shows $m$ committees are elected in step 1 and then later in step 3 are each given a secret share. ...
4
votes
2answers
430 views

Can an RSA public key be generated without ever knowing the factors? [duplicate]

Context: http://blog.cryptographyengineering.com/2013/12/a-few-more-notes-on-nsa-random-number.html Assume I wanted to use RSA as the basis for a hash function or DRBG. Also assume that my construct ...
3
votes
2answers
268 views

Should I bother implementing DH + AES, or leave it to a library like Crypto++

I'm making a C++ back end for an application that'll heavily rely on AES-256 and DH. Should I trust third party libraries, or spend (lots of) time to learn about cryptographic implementation and write ...
0
votes
1answer
62 views

How to select parameters based on some condition

I am implementing a protocol for authentication where parameters need to be selected with $gcd(\mu,\phi)=1$ where $\phi=(p−1)(q−1)$. What is the best way to select the parameters while implementing
13
votes
2answers
1k views

Why xor is a linear operation but ordinary adding is not

I'm new in cryptography and try to read some articles in this field. Many of these articles talk about non-linear S-boxes, and nothing more on what they mean by their non-linearity. I have a simple ...
5
votes
1answer
117 views

Has a Two Key TDES encryption ever been successfully attacked?

Is there any known instance of a Two Key TDES ever being successfully attacked (in real life), when the key is used to encrypt less than $2^{20}$ words?
-1
votes
1answer
86 views

Hypothetical question about AES output

Consider that you stumble upon 5 different AES output strings. One of these strings can be decrypted with a secret phrase, the other four strings are based on the real string but have had random parts ...
3
votes
2answers
3k views

What characters does AES output?

From what I understand after testing the Crypto-JS file here: http://crypto-js.googlecode.com/svn/tags/3.1.2/build/rollups/aes.js AES creates encrypted strings that contained letters, both ...
5
votes
1answer
2k views

Why does OpenSSL append extra bytes when encrypting with aes-128-ecb?

Plaintext: Attack at dawn!! Hex: 41747461636b206174206461776e2121 Key: ...
2
votes
1answer
81 views

Using HMAC to secure a “widget”

Let's say I have a web "widget" that a customer could include on their website. They would include the widget by adding an iframe that points to a URL on my site. I want to ensure that my widget can ...
3
votes
1answer
224 views

How to attack this authentication protocol from “Cryptography: An introduction”

Consider the following protocol (from the book "Cryptography: An introduction", by Nigel Smart): ...
5
votes
1answer
266 views

What is 'Carry-forward verification' defense against MITM?

Wikipedia article on Man-in-the-middle attack mentions, in the list of defenses against it, some method called "Carry-forward verification", but it does not explain it. I am curious about different ...
2
votes
1answer
107 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
113 views

A confusion about HMQV?

As the full version of HMQV is very long, I will give a graph of it below \begin{array}{@{}l@{}c@{}l@{}} \hat{A} && \hat{B} \\ (a,A=g^a)&&(b,B=g^b) \\ x\in_R[1,q-1],\quad X=g^x ...
4
votes
1answer
318 views

EC ElGamal versus static+ephemeral ECDH

A client application needs to encrypt a UDP datagram for a server with known EC public key $P$. Performing a full ECDH key exchange would defeat the benefit of using UDP as a connectionless protocol. ...
3
votes
1answer
140 views

Security Level Estimate when Cascading/Compositing Ciphers?

Way back when, DES had a theoretical security level 56-bits. 2-key TDEA provides about 80-bits of security, and 3-key TDEA provides about 112-bits of security. Obviously, those security levels are not ...
1
vote
1answer
131 views

Does the key schedule function need to be a one-way function?

For some key schedule $e_n(e_{n-1}(k))$ (where $e_{n-1}(k)$ is the result of the previous round) , does $e$ need to be a one-way function? In the case of DES or Rijndael the key schedule doesn't ...
2
votes
1answer
178 views

Off-the-record communication

Is my protocol correct? For fun, I tried to design a protocol that would allow off-the-record communication. I'm not sure it works, I have a feeling there might be a problem with it that I'm not ...
1
vote
1answer
197 views

How to prove that a ciphertext is encrypting multiplication of two values?

Zero-knowledge proofs with soundness, completness and zero-knowledge enable a prover to convince a verifier that a witness validates successfully a predicate without giving any information about the ...
4
votes
2answers
260 views

CDH problem and Square-DH problem

CDH problem roughly says that choose $U=g^u, V=g^v$ uniformly at random from cyclic group $G$, it's hard to compute $CDH(U,V)=g^{uv}$. Square-DH problem roughly says choose $U=g^u$ uniformly at ...
3
votes
2answers
278 views

Why is time-lapse cryptography not popular?

Time-Lapse Cryptography has been introduced by Michael Rabin and Christopher Thorpe in the paper “Time-Lapse Cryptography”, but it does not seems to be popular. It is a really good paper with an great ...
5
votes
3answers
250 views

Digital identification using steganography, can it be broken?

Digital identification using steganography, can it be broken? I have read a few articles saying that using an I.D. embedded in video, audio, etc can help track down the origin of a copyright ...
4
votes
1answer
1k views

Good entropy source for generating openssl keys

I need to generate a CA (4096-bit RSA) and server keys for openvpn and I want them to be "top quality". Here is my plan: gather entropy from multiple sources (saving individual files): FreeBSD ...
1
vote
2answers
358 views

Why do these Python XTEA implementations require different deltas?

I once needed to an XTEA snippet to from C++ to Python and it did not work properly. I then found a function on ActiveState.com and noticed that it looks almost exactly like mine, with one exception - ...
2
votes
1answer
162 views

Significance of repetition in XOR cipher text

I'm attempting to recover plain-text ($P_1, P_2$) from two uneven cipher-text strings ($C_1, C_2$) that I suspect used the same key ($K$). When I XOR $C_1$ and $C_2$ together to obtain ($P_1 + ...
3
votes
1answer
63 views

Oblivious Transfer where neither party learns the index of the message

In a traditional Oblivious Transfer setting, the sender has a list $(x_1, x_2, ... , x_n) \in G$ where $G$ is the chosen group. The receiver has $b \in \mathbb{N}$, such that engaging in the protocol ...
-2
votes
1answer
115 views

What parts of number theory does the RSA algorithm use?

It is said that the RSA algorithm uses number theory. What parts of number theory does it use? I know it uses modular arithmetic and Euler's totient theorem and function. Is that all?
4
votes
1answer
592 views

Coefficients in Shamir's Secret Sharing Scheme

Sorry if this is a stupid question, but: in Shamir's scheme, we construct a polynomial and make our secret $S$ the zero-th coefficient $a_0$. What, if anything, necessitates this - in other words, can ...
2
votes
2answers
188 views

Can the passphrase of a PGP key be deduced from an old backup?

Let's assume that Bob, at some point in time, has created a PGP (key length and other parameters are not relevant at this point). For some reason, he has chosen a very poor passphrase, for instance ...
3
votes
2answers
990 views

Keeping IV secret for AES CFB mode

I'm developing a security/encryption software and I'm using AES CFB (block size: 16 and key size: 32 bytes). I want to know, if I also keep IV (32 bytes) secret like the key itself (32 bytes), would ...
1
vote
1answer
595 views

Forgery Attack Against RSA Digital Signature

I have been reading about attacks against RSA Digital Signature in a book The attacker chooses the signature first and then computes the message. As a consequence, he cannot control the ...
2
votes
2answers
301 views

Two-dimensional S-Box

Consider the following simple cipher: $$c_1 = S(m_1 \oplus k_1) \oplus k_2$$ Where $S$ is S-box, $m_1$ - 16-bit plain text, $k_1$ and $k_2$ is two parts of 16-bit key. If S-box is standard then this ...
3
votes
2answers
119 views

What's the alternative should PKI collapse?

Premises - please check it and critique if necessary: If factoring the products of large numbers suddenly becomes simple, PKI goes up in smoke. Is this correct? If it is correct, it seems to me ...
4
votes
1answer
918 views

What are the advantages of CBC over ECB?

From this question I understand that, for a block cipher, using CBC is better than ECB. It seems that if one only has part of the cipher text, then decryption is difficult because the decryption ...
10
votes
1answer
217 views

Shadowed identity in cryptography

I was trying to implement zero knowledge protocol for authentication based on the paper "A Practical Zero-Knowledge Protocol Fitted to Security Microprocessor Minimizing Both Transmission and Memory". ...
2
votes
1answer
126 views

With Encrypt-then-MAC, and flexible protocols, do I need to authenticate the authentication?

Sorry, the question is not easy to state (or wasn't for me, anyhow). So I'm writing a very flexible format for security. It allows for full choice of cipher, key size, and so on, along with key ...
4
votes
1answer
191 views

How does a backdoor in the RNG enable an attacker to break encryption protocols?

I'm wondering how a backdoor in a random-number generator (as rumored the NSA have placed), actually compromises the security of things like VPNs or TSL/SSL. What I understand is that, if such a ...
4
votes
2answers
255 views

Can we say that if $P=NP$ there is no CPA secure public key encryption?

I've learned that public key encryption is based on the problem of Discrete Log (as regard to group theory) which believed to be hard. But, can we say that it doesn't matter on which problem our ...
3
votes
1answer
292 views

Single Party Encryption, Multi Party Decryption

I am looking for an adequate protocol for public key decryption, which would allow $n$ parties to jointly decrypt a cipher text. The usage scenario for such a protocol would be a mixnet that wouldn't ...

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