# All Questions

270 views

### How are Elliptic Curve Cryptography and Pairing Based Cryptography related?

I have been doing a project that uses the PBC library developed by Ben Lynn. But I am still not clear on how PBC is related to ECC. I know that this is a site for complex crypto QA, but I did not know ...
425 views

### Are RSA signatures deterministic?

If I sign the word HELLO with the mechanism "NONEwithRSA" with the same private key, do I always will have the same signature? A Java example always return ...
701 views

### Master keys for decrypting?

I'm not a Cryptography expert, but i've seen this topic sometimes on the Web: Master decrypt keys. Most of the people said that "Master decrypt key" does not exist. This makes sense if you developed ...
277 views

### Is semantic security important in a hybrid cryptosystem?

RSA doesn't provide semantic security when used unmodified, and neither does the commonly used PKCS#1 v.1.5 padding scheme for encryption. Is this a problem for hybrid cryptosystems at all? My ...
680 views

### What are rounds in the context of ciphers?

Another recent question on security margin included in its answer the discussion of how many rounds are broken so far. What is the definition of a round?
156 views

### GGM PRG construction: Why do we need to change keys all the time with underlying PRG

When constructing a PRF that has n bit input using the GGM PRG, why do we always have to recursively run the PRG using its previous output as a seed key n times. Instead, why don't we run the PRG n ...
284 views

### Finding a collision for a hash function

I'm trying to find a collision for the following (modified) Merkle–Damgård hash function. Suppose we already have a hash function $h : \mathbb{Z}_2^{2·n} \to \mathbb{Z}_2^n$ for fixed length bit ...
378 views

### Why is RSA usually limited to messages up to 1 block

I'm wondering why RSA encryption usually is only used for messages that fit into one block. For larger messages hybrid encryption in combination with symmetric ciphers like AES seem to be the solution ...
2k views

### Why does WPA-PSK not use Diffie-Hellman key exchange?

Is there a reason, why in pre-shared key mode, Wi-Fi session key is not secured with something like Diffie-Hellman but instead is derived from PSK key and some information exchanged in clear during ...
363 views

### Simple homomorphic crypto for 32-bit integers

I'm looking for a simple way to perform homomorphic crypto on 32-bit integers. My only requirement is that I can add and subtract from the plaintext value without actually decrypting it. The crypto ...
367 views

### Break double encryption

Let $E_k$ : {0,1}$^l$ be a block cipher encryption function with block-size $l$ and key-length $n$. In class, we saw that a double encryption with two independent keys $E{}'_{k_1k_2}(x)$ = ...
144 views

### High Power Computing for Crypto Research

Obviously, much of cryptography requires the assumption that adversaries have bounded computing power. So, I was wondering, are there any fields in cryptographic research for which high power ...
134 views

### What's the strategy for future directions in cryptography? Bigger numbers/faster searching, or new methods, say, of factoring?

I'm taking a course in cryptography, and I would value any comments. This is not too technical a question, but more about directions or strategy in cryptography. My question is, is public key ...
2k views

### ANSI X9.31 standards for generating random numbers

The NIST-Recommended Random Number Generator Based on ANSI X9.31 Appendix A.2.4 Using the 3-Key Triple DES and AES Algorithms has 3DES being used three times for each 64-bit block of random data ...
144 views

### Public keys and their protocols

I'm having difficulties understanding exactly what this protocol means: $S \to D : \{N_S , S\}K_D$ $D \to S : \{N_S , N_D \}K_S$ $S \to D : \{N_D \}K_D$ "where $S$ represents the supervisor’s ...
352 views

### Why are protocols often proven secure under the random oracle model instead of a hash assumption?

Is this true that whenever you design a protocol using a hash function, you must prove its security under the random oracle? I mean, is it possible to devise a protocol $P$ using a function $H$, and ...
248 views

### What is an oracle adversary?

I'm trying to figure out block ciphers and this is the first time I've encountered the term "oracle adversary". It appears in the context of defining a PRP and a PRF. Can someone please explain what ...
128 views

### Can you use the same key to provide a signature and a MAC?

Apologies if this is a stupid question, but I'd be grateful if someone could provide an answer. I'm assuming that you need to use different keys to provide these functions, is that correct?
326 views

### Increase number of rounds for SPN and Feistel ciphers

Read a post on Schneiers blog (and again 2011) about increasing the number of rounds for AES from to "AES-128 at 16 rounds, AES-192 at 20 rounds, and AES-256 at 28 rounds" to raise the security. ...
654 views

### Avalanche noise RNG for one-time pad use

I came across this little HRNG widget and was really intrigued as I have been looking for a decent but afordable source for truly random bits to use in a one-time pad. The question is, would a HRNG ...
348 views

### Hash collision resistance requirements for Lamport signatures

According to the original paper, Lamport one-time signature scheme uses two one-way functions: $F$ and $G$. The former one, $F$, is used to create a public key by hashing elements of the private key ...
309 views

### Proving correctness of pseudorandom generator construction given existing pseudorandom generator

Say if I have a given pseudorandom generator G which takes a k-bit input and outputs a 3k-bit number. How should I show that a specific construction using this pseudorandom generator is valid? For ...
174 views

### How could block encryption in counter mode be secure from viewpoint of entropy?

A good block cipher e.g. AES running in counter mode is commonly considered to be secure. Assuming a perfectly random key, one could in this case from $128$ bits generate up to $2^{128}·128$ bits for ...
3k views

### How to decrypt AES in CBC

i am having problems understanding how CBC works! I know that it is a chain and the IV is XORed with the PT but then what? I mean we encrypt it with key k and the result is the CT which we use for the ...
560 views

### Can I make a cipher (ex: Vigenère) harder to break?

The Vigenère cipher can relatively easy be broken when the key size is small compared to the size of the message. One first finds the length of the key, and then uses frequency analysis to actually ...
267 views

### Conforming Randomness To An Alphabet

Imagine that we're trying to create a function to generate a random string conforming to a user-supplied alphabet. That way, users can generate random strings with given characters. Something like: ...
992 views

### AES GCM implementation in c#

I am implementing an AES cipher in GCM mode in c# and have a few questions. My code is based on the code found here for reference. I'll copy in the relevant portions. ...
159 views

### Format of NONCE in Initialization Vector (IV)

When we talk about a Number used ONCE (NONCE) in Initialization Vector (IV), is it required to use numbers only? Is is possible to use letters or special characters?
626 views

### Replay attack prevention under strict conditions

Say I have a request-response protocol and the task to make it immune to replay attacks. The requirements are as follows: The defense mechanism cannot rely on the clocks of the parties. The defense ...
322 views

### How to properly add ECDSA private keys?

I'm currently working on an application that requires me to add two ECDSA private keys in order to make a new private key. The result has to have the property, that its corresponding public key is the ...
247 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 ...
220 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 ...
819 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 ...
497 views

### RSA finding the inverse of the public exponent

I have a very basic doubt in RSA key generation and its usage. In RSA key generation you choose two large prime numbers of a very large order. Then you multiply them.(eq $p \cdot q = N$) Now, ...
158 views

### Signature scheme with two private keys, neither derivable from the other

Is there any scheme, ideally one widely used or at least widely available, where you can treat both the signing and verifying keys as secret? Basically, the functionality I'm looking for is this: ...
210 views

### Is there a cumulative commitment scheme?

For a certain application I need a commitment scheme where each user could make a commitment, and a single verification operation could verify all the commitments simultaneously, faster than single ...
1k views

### Dictionary attack on pass-phrases on common algorithms

I don't yet perfectly understand the difference between brute-force and dictionary attack since this differentiates one attacking the key and another attacking password: apparently attacking passwords ...
43 views

### Key space vs Cardinality of 1024-bit RSA

I have been trying to figure out the encryption key space, and cardinality of 1024-bit RSA. From my understanding, the key generated from 1024-bit will be primes greater than $2^{1023}$ but less than ...
51 views

### Secure ElGamal with OAEP

Is it possible to make ElGamal IND-CCA2 using OAEP or OAEP+? The reason I ask is that I recently answered this question and it came to my mind that OAEP or OAEP+ might be possible solutions. Note ...
81 views

### How multiplicative property of RSA can be exploited

It is known that RSA has the multiplicative property $E(m_1) \cdot E(m_2) = E(m_1 \cdot m_2)$ because $m_1^e \cdot m_2^e = (m_1 \cdot m_2)^e \pmod n$ But how can this property be used to break ...
100 views

In cases where a large complex data structure contains elements that are a mix of confidential and not-so-confidential data, what would be the pros & cons of: encrypting the entire structure ...
94 views

### New to cryptology, studying Message Authentication Codes

I understand the basic operational theory behind MAC, but I'm getting tripped up on some of the finer details. Also the differences between MAC, HMAC and MIC are getting lost on me, including the ...
89 views

### Can Poly1305-AES be used with AES-256?

I'm reading through Bernstein's The Poly1305-AES message-authentication code. The MAC is predicated on 16-byte block ciphers like AES and produces 16-byte authentication tags. However, Bernstein does ...
51 views

### Is it secure to transmit a short plaintext with its MAC?

I have a short (32-bit) message that needs to be authenticated. Due to the constraints of my platform, I would like to use Speck32/64 (32-bit block, 64-bit key) with a Matyas-Meyer-Oseas one-way ...
118 views

### AES GCM vs other encryption modes in embedded environment for network traffic

I'm doing a lot of research since I'm doing an internship for a company where I need to upgrade their security level. Their own protocol needs a cryptographic upgrade and I'm leaning towards AES since ...
71 views

### Is there any advantage on encrypting the CMAC together with the message?

I'm reading a protocol specification where the procedure is to generate a CMAC, take the first 4 bytes of it, append this authentication tag to the message and then encrypt the message + CMAC together ...
139 views

### Why are some $x$ coordinates unsuitable for an ECDSA generator point?

For Bitcoin's ECDSA curve (secp256k1, where $a=0$, $b=7$), why can't the generator point's first coordinate be $x=0$? That is, the point on the curve would be $(0,y)$ where $y$ satisfies \$y^2 = 0^3 + ...
87 views

### Formal security of recycled random blinding in a Paillier scheme

This question is a follow-up/variant on a previous question. Supposing that we are trying to generate a large number of (indistinguishable) ciphertexts of a given plaintext and want to avoid the ...