3
votes
1answer
74 views

Deriving a key for HMAC, using PBKDF2

What is the recommended way to derive a key for an HMAC-SHA256, from a passphrase and salt? I'd like to compute an HMAC-SHA256. I have a passphrase, and a message. I note that RFC-7518 (JWA) ...
15
votes
1answer
1k views

How does a non-prime modulus for Diffie-Hellman allow for a backdoor?

Recently someone found that a Diffie-Hellman modulus used in a unix tool (socat) was not prime. This led some people to shout "backdoor". What I don't understand ...
1
vote
2answers
59 views

NIST HMAC Test vectors

I am searching test vectors of the HMAC-SHA256 primitive to verify the correctness of an implementation. Without hesitation I go on the NIST website to see what they offer. While reading the ...
1
vote
1answer
32 views

Is this the correct procedure for cycle walking using an FPE?

To encrypt a value from a set (say a range of years from 1001 to 2001) Rank the set {0 = 1001, 1 = 1002, 2 = 1003...999 = 2001} Take the value you want to encrypt from the set (year = 1995) and ...
3
votes
1answer
136 views

How to determine if padding has been used while encrypting a message with RSA?

I am trying to decrypt a RSA encrypted text file. I have a 2048 bit public key. Analyzing the public key with openssl reveals the exponent used is 3. ...
3
votes
2answers
111 views

Classification of a stream cipher

How would one classify a stream cipher that is neither synchronous, nor self synchronizing? I would have thought asynchronous, but from all the sources I can find, asynchronous is synonymous with ...
1
vote
1answer
55 views

Find the minimum Adversary's Advantage

I am wondering how to solve the problem below: Suppose we have a $MAC$ scheme $I=(S,V)$. There is another algorithm that can find a pair of texts $(m0,m1)$, with $m0≠m1$ such that $S(k,m0)=S(k,m1)$ ...
3
votes
1answer
98 views

How does RIPEMD160 pad the message?

For RIPEMD hashing algorithm on hardware I am not exactly getting how padding works So as per my understanding padding will work like explained below: For ripemd 160 message should be multiple of ...
10
votes
2answers
5k views

Can you explain “weak keys” for DES?

A weak key for DES is a key $K$ such that $DES_{k_1}(DES_{k_2}(x))=x$ for all $x$. I don't get why are the 4 keys $k_1||k_2$: $1^{112}$, $0^{112}$, $0^{56}||1^{56}$, $1^{56}||0^{56}$ considered as ...
0
votes
3answers
122 views

Algorithms for Cyrillic encryption

I am working on a cryptography application which is to be used in my country. I tried looking up to see if there are any algorithms that support the Cyrillic alphabet, but I found none. Do you know of ...
3
votes
2answers
77 views

Are there any practical methods of steganography?

There isn't a whole lot of information on steganography in the modern age, from what I can tell. A google search for steganography usually brings up a list of very impractical methods (e.g. tattooing ...
1
vote
1answer
399 views

MiTM on RSA public key encryption

a) Alice just generated a new pair of public and private RSA keys (pkA , skA ) = ((N, e), d) and would like to start a private conversation with Bob. How can Alice interact with Bob and exchange a ...
3
votes
1answer
268 views

Key exchange and man in the middle issue. Secure solution

I am trying to design MiTM protected key exchange algorithm. For instance I am using RSA asymmetric keys. Alice and Bob respectively have each their own pair of keys. And they should be able to ...
8
votes
1answer
120 views

Side channel security of HMAC in software

Assume we have a processor with a symmetric co-processor that supports AES. This processor does however not provide co-processor support for hash algorithms such as SHA-1 or SHA-256. I've got the ...
3
votes
1answer
89 views

What is the correct name for bytes which are to be signed?

I have some bytes, and I sign them with my private key using ECDSA. The resultant (71) bytes are the signature. But what is the name of the bytes I have signed? (If this were encryption, they would ...
0
votes
1answer
69 views

Secure blinding factor switching at malicious server-side (Switching in One Time pad)

Suppose all the values and operations are defined over a finite field $\mathbb{F}_p$ where $p$ is a large prime number. Assume all values are non-zero. Here $(z)^{-1}$ denotes multiplicative inverse ...
6
votes
2answers
170 views

Advantages of making streamcipher stream depend on plaintext?

Most descriptions that explain how streamciphers work (like the one on Wikipedia), tend to describe a model that boils down to a simple “$ciphertext = plaintext \oplus stream$”, where the stream is ...
3
votes
1answer
77 views

Multi-target attacks on AES-CTR with a random nonce

128-bit block ciphers are vulnerable to multi-target attacks where the attacker seeks to attack a collection of keys instead of a single key. A simple example: Generate keys $k_1, k_2, ...,k_{2^{40}...
4
votes
1answer
35 views

Can the backdoor in Dual_EC_DRBG be used to create a public key stream cipher?

Dual_EC_DRBG has the property that if $Q = e\cdot P$, someone who knows $e$ can break the PRNG. This seems to lead to a public-key stream cipher: Alice chooses a random $P, e$, where $P$ is a ...
5
votes
2answers
95 views

How many polynomials are used in Secure Multiparty Computation?

Secure Multiparty Computation based on Samir Secret sharing methods rely on Polynomials. Imagine corpus of data should be outsourced to bunch of untrusted servers for any computations. Now the data ...
8
votes
1answer
871 views

Advanced Access Content System (AACS) and Subset Difference techniques for Broadcast Encryption

I'm trying to get a grasp on AACS and Subset Difference (archived version with images) for a project I'm working on and am having a hard time coming up with a technically valid layman explanation, let ...
1
vote
2answers
43 views

Is part of an output of secure PRNG also cryptographically secure?

Lets say I generate 192 bits using some secure PRNG. If I now take first 128 bits is it equivalent of generating 128 bits in the first place? P.S. More practical application for me is that I need a ...
1
vote
1answer
170 views

Attacking hashed password by timing attack and other means

This question is inspired by this recent one, which raises a point that need to be made quantitative to be meaningfully discussed. I chose an unusual approach: a new question worded as homework that ...
1
vote
0answers
42 views

Algebraic relationship between Original Key and Sub Key of DES

Does there exist any sorts of algebraic relationship between Original and Sub Key of DES? I understand that by solving K-Maps of 64-bit Input/48-bit Output, I can get 16 algebraic relationships ...
3
votes
2answers
53 views

Determining Involutary Keys in Substitution Ciphers

How would I be able to show that a key $K=(a,b)$ is involutary for an Affine Cipher over the integers modulo n? Similarly, is there a way to generalize the conditions for a key to be involutary for a ...
0
votes
0answers
14 views

Extension of Diffie Hellman for many participants [duplicate]

How to do the Extension of Diffie Hellman Algorithm for Multiple Participants
3
votes
1answer
174 views

How to clock a linear feedback shift register?

I have some trouble understanding what happens when a LFSR is clocked. I have tried many examples but I do not get the correct result. Can anyone point out my errors? I have the following polynomial: ...
1
vote
1answer
471 views

Public key encryption and big files with NaCL

I am trying to encrypt big files using NaCL (actually PyNaCL) - see http://nacl.cr.yp.to/box.html After reading some docs, I came up with this prototype : Bob wants to send a big file to Sarah. They ...
1
vote
1answer
39 views

Key Exchange MAC encryption order of events? Problem

I want to write the sequence of events of how a MAC encryption works for documentation at work. For this purpose I have a document (given by my boss to me) in German and I am trying to translate it ...
1
vote
1answer
76 views

What is the complexity to break second preimage resistance?

In the context an of hash functions we have three required properties. Preimage resistance Second preimage resistance Collision resistance. The bounds for Pre image collision is $O(2^{n-1})$, the ...
1
vote
0answers
67 views

When we use one time pad TWICE in two different ways

Assumption and Notations: All the values defined over the field $\mathbb{F}_p$, where $p$ is a large prime number. We denote multiplicative inverse of value $v$ by $v^{-1}$. All values are non-zero ...
1
vote
0answers
44 views

Need help with PKCS#12 3 key triple des key expansion

I am trying to decrypt PKCS8-Shrouded Key Bag inside a PKCS#12 pfx file. The oid for the encrypted data is PbeWithSHAAnd3-KeyTripleDES CBC. Iteration count and salt has been provided within the pfx ...
3
votes
1answer
84 views

Help with AES decryption with AES-NI Instructions

I am in process of implementing a AES 128 encryption in AVX2 assembly (using AES NI), and am having some problems with decryption logic. I have implemented the encryption sequence without any trouble. ...
0
votes
1answer
93 views

SHA1 collisions and the impact for ECDSA signatures

What will it mean for ECDSA using SHA1 when we have practical attacks breaking the collision resistance property of SHA1? [UPDATE] Added a bit more details to be clear. If $(r,s)$ is the ECDSA ...
0
votes
1answer
100 views

Multiparty encryption and decryption

Is it possible to send a single encrypted data message to multiple users so that the users receiving the message can only decrypt certain parts of the message, depending on their rights. For example, ...
-4
votes
1answer
40 views

Computing inverses in a binary field

Please suggest how can i solve the below question What is the inverse of {03} in GF (2^8) with the irreducible polynomial x8+x4+x3+x+1?
4
votes
1answer
188 views

When all shares of a secret are given to adversary as a permuted matrix

Suppose we have a secret $\sigma$. The secret comes from a universe in which the elements are not necessarily distributed uniformly. We split $\sigma$ into $n$ shares $[\sigma_1,...,\sigma_n]$ (...
8
votes
2answers
245 views

Would a Rubik's cube be a trapdoor function?

It's easy to scramble a Rubik's cube but it is hard to solve, unless you know the algorithms for solving it. This makes it a trapdoor function, correct?
0
votes
2answers
45 views

Can an AES-256 key be generated for a large number of cribs? [duplicate]

I have a question in regards to Ransomware type viruses. I know that a number of these viruses use the AES-256 algorithm to encrypt user's files using a randomly generated key. I work in IT support ...
2
votes
4answers
3k views

Is 512-bit RSA still safe for signature generation?

The standard CSP on Windows XP only supports RSA up to 512-bit, which means that it's the maximum key size I can use for authenticity verification of updates. The public key is embedded in the updater,...
1
vote
0answers
113 views

Example of FHEI scheme?

I am reading this paper Toward Secure Multikeyword Top-k Retrieval over Encrypted Cloud Data. I am trying to work out an example of the FHEI (Fully homomorphic Encryption over Integers) scheme ...
1
vote
1answer
122 views

Why does image size increase after steganography?

I am working on a steganography technique. Sometimes I need to send my steganographed images online to my friends but in the process the image gets compressed destroying all the data. I found ways to ...
2
votes
1answer
40 views

Can one passively load-balance HTTPS traffic based on TLS SNI?

Is it theoretically possible to passively snoop into the HTTPS traffic, reliably detect whether or not TLS SNI is attempted to be used by the client, and, if it is, redirect the stream of the ...
0
votes
0answers
59 views

total effective key length of the AESX-192

If I have a AESX-192 be a block cipher which is similar to DESX but has the DES being replaced by AES and the AES key size is 192 bits. How should I compute the total effective key length of the ...
1
vote
2answers
99 views

MAC algorithm difficulties

Can anyone explain to me why this is a bad idea? I cannot see anything wrong with it except that key K should be different for MAC and AES. MAC algorithm consists of letting the MAC of message M (...
1
vote
0answers
23 views

Shamir SS: To associate shares to the public values $x_i$, when the shares are permuted [duplicate]

Suppose we use $(t,n)$ Shamir secret sharing as follows: We share the secret $\beta$ as $S=[s_1,..., s_n]$, where $X=[x_1,...,x_n]$ are the public values. For the sake of simplicity, lets assume ...
2
votes
1answer
62 views

Proving property of Modified Rabin Signature

I'm trying to prove one of the properties of the “Modified Rabin Signature Scheme”: If $gcd(x, n) = 1$, then $x^{\frac{(p−1)(q−1)}{2}} \equiv 1 \pmod n$ This is what I’ve got so far… Proof Let ...
0
votes
1answer
41 views

how to define the Truncated Polynomial?

In NTRUEncryption, I seen the trucated polynimials, but I cannot understand the trunacated polynomial calculation. So, could tell me anyone How do we calculate the truncated polynomial?
38
votes
3answers
34k views

How does one attack a two-time pad (i.e. one time pad with key reuse)?

My question might appear the same as the question Taking advantage of one-time pad key reuse?, but actually I did read all the answers and none of them helped me with the details I need. I am new to ...
3
votes
2answers
96 views

Long passwords and Bcrypt

I need to store passwords for a web application, and was looking at Bcrypt for a possible solution. After some research, it appears only the first 72 bytes influence the final hash output. While it is ...

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