# All Questions

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### HMAC tag to verify the key (verification in the reverse direction)

A quick question. Are there any problems with the following HMAC use-case: We have two entities ($A$ and $B$), both of them have a secret ($A$ has $S_0$ and $B$ has $S_1$) $A$ generates a random ...
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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 ...
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### 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 ...
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### 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 ...
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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 ...
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### Workaround to implementing Forward Secrecy

In the case where I am self-signing my digital certificates, if I just change my certificate (generate a new one) every week or month, will this have (sort of similar to) the same effect as ...
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### CTR HMAC SHA256 implementation

After trying to invent my own AES mode, I decided to just implement something that's known to work, namely CTR with HMAC SHA256. From what I understand, I can use the IV as a counter, and the IV ...
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### Keeping the key and IV safe when AES256 encrypting publicly known data

I am encrypting some publicly known data, and some private data using the same key, using AES256 with CBC. An attacker does not have the ability to choose what the public data is. Is this safe? (It ...
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### Find the prime factorization in the DLP

Suppose we have $g^x \equiv h \pmod N$ Where $N = pq$ and $p$, $q$ are distinct primes. Also $(p-1)/2$ and $(q-1)/2$ are primes. If we know what $x$, $g$ and $h$ are, is it possible for us to know ...
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### How to compare between two cryptographic algorithms in terms of security? [closed]

How to compare between two cryptographic algorithms (e.g. SHA-1 and SipHash) in terms of security? That is, how could one prove that algorithm X is more secure than algorithm Y?
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### Are there cryptographic schemes that mask as unencrypted traffic?

Are there cryptographic schemes that make encrypted traffic look like unencrypted traffic?
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### Would the Fiat Shamir identification scheme be more secure if I design it with an exponents higher than 2?

Let's say both Prover and Verifier would use nth power instead of 2nd when creating public keys and when verifying. I know it would slow it down but would that cause the protocol to be more secure?
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### Drawback of ElGamal encryption

I am trying to find out some disadvantages of ElGamal cryptography but I'm not able to figure out what's wrong with the algorithm. The only one I found is that a known-plain text attack is possible in ...
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### Solve the congruence using Pohlig-Hellman algorithm

Use the Pohlig-Hellman algorithm to compute a solution to: $3^x\equiv 2 \pmod {65537}$ My attempt: $p-1 = 65537-1 = 65536= 2^{16}$ $x= 2^0x_0+2^1x_1+2^2x_2+...+2^{15}x_{15}$ For $x_0$: ...
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### Is the result of HMAC-sha256 distinguishable from random noise?

Is if the result of HMAC-sha256 distinguishable from random noise? If so, given: a collection of 1000 keys, and 1000 messages of random noise, 100 signed by each of only 10 of those keys Can ...
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### Schnorr's identication protocol

Schnorr's identification protocol is said to be a honest verifier zero knowledge protocol. Does it imply it will not work if the verifier is dishonest? I'm wondering if this is the case, it would ...
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### Linear Recurrence, keystreams, and periods

I am trying to understand the following problem: Consider the following linear recurrence over $Z_2$ of degree four: $z_{i+4} = (z_{i+3} + z_{i+2} + z_{i+1} + z_{i}) \bmod 2$ i >= 0. ...
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### Convert numeric message from RSA ciphertext to alphabetic

I have a set of numeric values that are supposed to represent a message encoded via textbook RSA. For example: 60889 37946 13359 61629 35960 13747 I've decrypted ...
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### Why would using a random seed with other variables be bad for ecrypting if you can't guess the key?

I asked this on stack exchange with it being code based, but someone suggested to ask here. Basically, I've done a script in python and wanted an option to encrypt the input, but I couldn't get any ...
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### Implications of reversing CBC encryption

What would be the security implications if we reverse CBC encyption? Here is a picture of CBC encryption: Here is a picture of CBC decryption: My guess is that the encryption would work if we ran ...
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I built a system that allowed users to generate an unlimited number of email addresses. Instead of storing each one in a db, I created addresses dynamically and made the address an hmac signed ...
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So I have to answer these questions, but I honestly have no idea what I am doing or where to begin. Can I get some advice on how to tackle these? Consider a PRP candidate function $f$ on $K = X = ... 1answer 66 views ### Is it necessary to encrypt nonce in cryptographic communication? Server and client have a pre-shared key K. Assuming that that pre-shared key is safe, so nobody other than server/client can encrypt/decrypt. Client sends a nonce N to server, server then sends back ... 1answer 141 views ### What prime lengths are used for RSA? Let's assume we have an RSA 2048 that was produced by two primes multiplication. What is the digit's difference in those 2 factors? For example one prime is 1024 digits and second one 1024 or, one is ... 1answer 45 views ### Requirement for the exponent generation during DH protocol [duplicate] The setting is classic DH protocol. Alice computes$A = g^a \bmod p$. Alice then sends$A$to Bob. Then Bob will send$B = g^b \bmod p$to Alice. Are there any requirements for the values$a$and ... 1answer 154 views ### Symmetric mutual authentication with client using a derived secret I'm attempting to find a client/server authentication protocol that allows the client and server to authenticate each other when the client doesn't know the server secret but does have a sensitive key ... 1answer 107 views ### Java RSA Padding: no libraries used I'm trying to implement RSA without using any java libraries using the algorithm from wikipedia HERE So, anyone can clearly see that it is a raw RSA, and can directly be attacked. The question is ... 1answer 75 views ### Prime modulus for RSA and sharing a secret? According to this paper entitled "Using Commutative Encryption to Share a Secret" they define their modulus to be a large prime p, which is public. Both exponents are private in this case. According ... 1answer 46 views ### Stream cipher key length to send I was being argued that stream cipher's key that is the length of the message must be sent to the destination for them to be able to decrypt the message. My point is don't you only have to send the ... 1answer 67 views ### Weak Boneh-Boyen Signature in Composite order group In the paper Ring Signatures of Sub-linear Size Without Random Oracles, authors have remarked that the scheme is instantiable in composite order setting, too. I am attaching the following reference ... 1answer 36 views ### Using Permutation polynomial to compute a MAC Is the following MAC secure? For a block$y_i$in a file, we defined a MAC as follows:$Mac_i:PRF(k,i) \cdot g^{y_i \cdot r_i} \bmod p$. Where$p$is a prime number,$g \in \mathbb{G}$,$PRF(k,i)$is ... 1answer 38 views ### How do you derive the lambda and beta values for endomorphism on the secp256k1 curve? You can see a little background about this on this bitcointalk post by the late Hal Finney.$\beta$and$\lambda$are the values on the secp256k1 curve such that:$\begin{align} \lambda^3 &= 1 ... 1answer 97 views ### What is the difference between a hash function and a pseudorandom function? [duplicate] Read the title. I've seen in RFCs that some MAC functions are called "pseudorandom functions". What are those? How are they different than hash functions? Why can't a hash function be used instead? 2answers 117 views ### Multiple encryption of part of block Is there any security risk in encrypting parts of blocks twice? I'd like to get around block sizes and make encrypted messages just as long as the unencrypted ones: Block 1: Byte 0 - 7 Block 2: Byte ... 1answer 69 views ### XOR cipher with three different ciphertexts and repeated key, key length known. How do I find the plaintexts? Let us say we have three different plaintexts (all alphabets, A-Z):x$,$y$and$z$, each of length$21$. Let the key,$a$, be also of length$21$. Now, what we have is$x \oplus a$,$y \oplus a\$ ...
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I'm trying to solve a question about one time pads but I'm not sure if my assumptions are correct. I am to assume that I have a one time pad with perfect secrecy. This one time pad is used to ...
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### Is there a reference that prove that the AES Key Schedule generate random looking round keys?

Starting from uniformly random generated AES master key, is there a reference that prove that an specific roundkey can be considered as uniformly random generated as well ?
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### How do I communicate the value of the initialization vector to the end user? Should it be part of the encrypted message?

I am taking a cryptography class and our first homework is to implement a 32-bit block cipher. I implemented a simple block-cipher that uses CBC. Currently, my implementation reads both the 32-bit IV ...
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### Proving that an encryption scheme is susceptible to certain attacks

I'm currently trying to prove the following: Where p is a prime number of cryptographic size, prove that: e(m) = am + b (mod p) where a and b are private is open to a known plaintext attack e(m) = ...
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### Will current encryption always remain secure? Is there mathematical proof?

Current encryption algorithms, such as AES, RSA, elliptic curve, etc. work based on known mathematical problems. I am specifically interested in the RSA. Will such security always remain secure? One ...
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### Security issues for integrating AAA and OTP services [closed]

What are the main security issues for integrating AAA and OTP services. In our organization with about 1000 clients, we want to integrate the services. What are the main security issues about the ...
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### Why the data length is shifted 3-bit in MD4 Java implementation? [closed]

In RFC 1320, It is stated that "A 64-bit representation of b (the length of the message before the padding bits were added) is appended to the result of the previous step. In the unlikely event that b ...
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### wireless network security protocol

In network security, encryption and integrity done by the same algorithm that is AES-CCM in that MIC generated by CBC-MAC and encryption done by CTR mode. So suppose I want to do encryption with any ...
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### Why should I use X.509 Authentication over PKC based Authentication?

So I have two examples and I don't understand why one would be chosen over the other. Example 1. A sends EPUb[r1] to B (asks B to prove knowledge of ...
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### How to implement HMAC SHA-256

I am trying to figure out how the HMAC SHA-256 hashing algorithm works. I know that we have to use the following: H (K ^ opad)|| H ((K ^ ipad)|| text)). The ...
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### Help converting sha256 to 32-bit stream

I need some help with hash value calculation. I am a bit confused by this statement that I found on w3 website on calculation of a DigestValue for xmldsig ( ...
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### Nonces being used in SSL and MAC to prevent replay attacks

I'm a bit confused in the way nonces are used in these processes to prevent replay attacks. Heres How I think it works during SSL: Nonces are exchanged during stage one of the handshake protocol. ...
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### Updating the secret key used for HMAC

I am designing a distributed system, composed by a set of physically distributed embedded devices (8 bit cores with less than 1 KB of RAM). The devices need to be able to authenticate the originator ...