7
votes
2answers
525 views

Can you fake messages from recorded message-history?

Lets assume Eve has a lot of PGP conversation with Alice and records all those messages. Bob is also regularly communicating with PGP with Alice. If Bob doesn't care about not having a signature ...
-2
votes
1answer
32 views

How does the signature checking work in DKIM?

As far as I understand DKIM works like this: DKIM is not used to encrypt anything, the only purpose is to mark mails that are not really sent from the claimed domain as spam. the DKIM-enabled ...
1
vote
0answers
57 views

Implementation Attacks on Hashes

So I am familiar with attacking implementations of block ciphers via side channel attacks, cache-timing attacks, etc. What implementation attacks are there against hashes or hashed based functions ...
-1
votes
1answer
48 views

I am new to Cryptography and I wonder if one can Suggest me Beginner Projects [closed]

Sorry for this kind of a question, but I really like to learn and improve in cryptography. At the moment I am following online lectures from https://class.coursera.org/crypto-011 , can anyone suggest ...
2
votes
2answers
147 views

Chosen plaintext attack on textbook RSA decryption

Let $(n,d)$ be an RSA private key and $(n,e)$ the corresponding private key. Generate a secret random $r$ from $(\mathbb{Z}/n\mathbb{Z})^*$ Compute $C’ = C (r^e) \bmod n$ Submit $C’$ as a chosen ...
-3
votes
1answer
63 views

Is encryption mainly using Diffie-Hellman Key Exchange?

I know how Diffie-Hellman Key Exchange works. Is this the main way of encrypting with PGP, ssh, ssl (https), DKIM, ...? I wonder if Diffie-Hellman is the clue to understand how encryption with ...
-1
votes
5answers
45 views

Why can't you decrypt an encrypted message with just the public key? [duplicate]

How does asymmetric crypto work? For example, if you use PGP in emails, you generate a private key that is known only to you and a public key, which is available to everyone. Is there a simple way ...
-3
votes
0answers
96 views

Deciphering a Custom Encoding Scheme [closed]

I've been tasked with figuring out the strength of an encoding scheme developed by my employer. I've been given a thousand plain-text numerical values along with their corresponding numerical ...
1
vote
1answer
50 views

Decrypting an Affine Cipher with known characters

I am trying to cryptanalyse multiple cipher–texts that I know are encrypted by different Affine ciphers. I have already analysed the frequency that each character occurs, and compared it to a ...
-4
votes
1answer
75 views

How does Diffie-Hellman key exchange work? [closed]

I've been learning about cryptography lately and I'm failing to understand how Diffie-Hellman key exchange works. Can someone please demonstrate using mathematical notation (and if possible, ...
3
votes
1answer
88 views

Multi-key decryption

Let's assume I want to send a secret message to $N$ recipients in an asymmetric way. My message could be encrypted with $N$ different public keys $p_i$ one after another and send to each user ...
6
votes
1answer
596 views

How is SHA1 different from MD5?

On the surface, SHA1 and MD5 look pretty similar. Their diagrams include chunks of bits, bit rotation, xor, and special functions. Their implementations are roughly the same length (at least the ones ...
3
votes
1answer
107 views

Luby-Rackoff theorem confusion

The Luby-Rackoff theorem states that if a round function is a secure pseudorandom function (PRF) then 3 rounds are sufficient to make the block cipher a pseudorandom permutation (PRP). PRPs are ...
0
votes
1answer
89 views

Is it possible to find plaintext from ciphertext if (n) and (a) are known for RSA?

I have a couple of questions pertaining to a RSA problem. I need to decipher some ciphertext and find out what the original plaintext was. Modulus: $n = 2537$ and exponent: $a = 11$ Encrypting ...
1
vote
1answer
56 views

Security implication of exporting private key [on hold]

I have a key pair stored in GPG Keyring. If I export it to an ASCII Armored format, am I removing any layers of security as long as the exported key stays on the same server as the key ring itself? My ...
-2
votes
1answer
78 views

Is there an encryption that is only reversible with a key?

I'm thinking about man-in-the-middle attacks during a website login, and started wondering about reversible encryption. What I'm imagining is an encryption that takes input ...
-2
votes
1answer
62 views

Is XORing a SHA256 better than truncating it? [duplicate]

Let say I want to truncate SHA256 to 128 bits. What would be the best way to minimize a probability of a collision and improve collision resistance? Taking the last 16 bytes Taking the first 16 ...
1
vote
2answers
55 views

linear computations over bilinear pairings

Does this hold in asymetric bilinear pairings? $e(x_1,x_2)e(x_3,x_4) = e(x_1x_3,x_2x_4)$, where $x_1,x_3 \in \mathbb{G}_1$ and $x_2,x_4 \in \mathbb{G}_2$ for a bilinear pairing $e$
3
votes
2answers
66 views

Hash or compression function in Merkle tree nodes?

Wikipedia says about a Merkle tree: Nodes further up in the tree are the hashes of their respective children. Usually, a cryptographic hash function such as SHA-2 or SHA-3 is used for the hashing. ...
3
votes
2answers
176 views

Security issues of a MAC-then-Encrypt-then-MAC approach?

Encrypt-then-MAC does provide ciphertext integrity, but no plaintext integrity. With MAC-then-Encrypt it’s the other way around: Plaintext integrity but no ciphertext integrity. What comes to mind is ...
2
votes
1answer
58 views

CTR mode encryption using PRG

In CTR mode encryption, the encryption block is not required to be invertible. Thus, a PRF can be used instead of a PRP. However most of the implementations of CTR mode are based on AES which is a ...
8
votes
3answers
679 views

Entropy of two concatenated random values

In this Intel blog posting, the author claims: The amount of work required to brute-force predict a random value that has n bits of entropy is $O(2^n)$. If you concatenate two values together, the ...
2
votes
1answer
68 views

Are there any cryptographic flaws in my webhook signing process?

Out of band my services have exchanged secret keys. These secret keys are then used to prevent two things of the webhooks (HTTP calls triggered by Service A to Service B when an event occurs in ...
-1
votes
0answers
101 views

Multiparty Computation with ORAM

Hope is not a silly question. I'm really struggling with it and I cannot find an answer anywhere! ORAM works with shared "secure" memory on server that is accessed in an oblivious manner and a client ...
2
votes
2answers
101 views

Computational Complexity - When is it really exponential time?

I'm currently working on the discrete logarithm problem and the relevant attacks. I'm fine on the mathematical side of things, but when it comes to estimation of running times I run into problems. ...
4
votes
3answers
119 views

Do I need to use AE/AEAD in my particular scenario? Or is it redundant?

This is my scenario: A small structured text message (1-2kB) is to be sent over an insecure channel. Everything except for the header portion (which contains a title, timestamp, category and tags) ...
1
vote
1answer
60 views

Verification of signed message with encryption

I have a question about the message that is being encrypted and signed. Say, two users called Tom and Mary, both have a pair of private key and public key. Tom sends a message through a system that ...
6
votes
1answer
97 views

Is HMAC-SHA-1 secure?

Although SHA-1 theoretically has collisions, HMAC-SHA-1 which is based on SHA-1 is still widely used (in TLS for example) and is considered to be secure. How is that possible?
3
votes
2answers
86 views

Combining two keys

What is the most secure way to combine two "random" keys X of size k (k ≤ 512 bits) and Y of size 512 bits in one key Z of size k? Result Z will be used for encryption/decryption with various ...
3
votes
1answer
51 views

Session-specific Symmetric Key Derivation Using SHA256

I was wondering if there were any security implications of deriving session keys from a symmetric key using SHA256 in the following manner: $Ks = SHA256(Nonce||K)$, where $Nonce$ is the session ...
3
votes
0answers
50 views

Streaming mode of operation that is more resistant against nonce reuse

Is it possible to create a streaming mode of operation where the reuse of a nonce does not destroy confidentiality? In this question it would be allowed for the streaming mode to leak if ciphertexts ...
2
votes
1answer
144 views

SHACAL in SHA-256

I was reading about hash functions, namely SHA, and I read that it is made of the Merkle-Damgard construction. And then the text said that the compression function used is based on the Davis-Meyer ...
4
votes
1answer
68 views

$(k, n)$ threshold secret sharing to $(k+k', n + n')$ without reissuing all shares

Given a $(k,n)$ threshold secret sharing used to back up a secret (meaning that the dealer has access to the secret at all times), is it possible to update it to a $(k+k', n+n')$ threshold secret ...
-1
votes
1answer
83 views

Help deciphering caesar cipher [closed]

I have to to find the shift value and the message for the following caesar cipher. OC_VAWDGPM_VHWTVX_VXJOCVZ_GDX_MWO_VWIZVYGWIZ_NODI_ I have already tried it and with the shift value of 21 i get the ...
-3
votes
0answers
37 views

RSA encoding problem help [duplicate]

Write a program to attack the Textbook RSA Encryption Scheme in order to decrypt the encrypted file key_enc.txt which contains the 128 bits AES secret key together with the initialisation vector ...
2
votes
1answer
77 views

RSA Blind Signature Attack

In Information Security Class I learned a non-adaptive chosen ciphertext attack using "blinding" but I just can't follow the proof and therefore can't reconstruct it. From what I've found out, it ...
5
votes
2answers
189 views

What does well-typed mean?

I am currently studying Cryptography and I don't really understand what does 'well-typed' mean when talking about secure cryptographic protocols. I can't find any reasonable explanation on the ...
6
votes
2answers
217 views

Recover from compromised shares with Shamir Secret Sharing

He is a quote from Wikipedia page for Secret Sharing: If the players store their shares on insecure computer servers, an attacker could crack in and steal the shares. If it is not practical to ...
4
votes
1answer
61 views

Diffie-Hellman when $p$ and $g$ are pre-agreed and hidden from Eve

During a regular Diffie–Hellman public key exchange, Alice and Bob agree to use a prime number $p$ and a base $g$… which are both made public and are therefore also assumed to be known by Eve. Alice ...
5
votes
1answer
105 views

Discovering private exponent from public key

I'm going to assume this isn't possible, but I have to ask because I'm trying to fundamentally understand what I've thus far been trying to implement by following an RFC. SRP-6a starts off with ...
0
votes
3answers
80 views

AES256-CBC vs AES256-CTR in SSH [duplicate]

I used AES256-CBC to SSH to a remote server. Recently, it stopped working with the following message: ...
-3
votes
1answer
96 views

Currently i am doing decryption for RSA encoding and i face some problem with it [closed]

I am very new to cryptography so I don’t know much about it. I have been given a very large $N$ value and $E$ value to decrypt a ciphertext which was created using a AES 128 key and a IV by using RSA ...
2
votes
1answer
143 views

CMAC vs HMAC security strength

From the perspective of a Birthday Paradox attack, isn't it true that CMAC based on AES-128 is weaker than HMAC-SHA-1? The attack on CMAC-AES-128 requires about $2^{64}$ operations whereas the same ...
1
vote
0answers
76 views

Hardware Implementation of Pairing over BN curves

I am in the middle of FPGA based Hardware architecture design for the computation of Pairing (particularly R-ate Pairing) over BN curves. Where, the point addition, and point doubling should be ...
5
votes
0answers
60 views

Implementation and Testing of SRP-6a

I have been wracking my brain trying to develop a functioning implementation of SRP-6a in Python to use with a 3rd-party API that claims to use SRP-6a with $N=$ 2048-bit prime and generator of $2$. ...
1
vote
1answer
113 views

Is it insecure to encrypt the wrong plaintext?

Most plaintext must be fed as a string or a binary object to be encrypted. However, it's often possible to mis-decode the plaintext before encrypting it. i.e. If it's received as a string in JSON, ...
4
votes
1answer
187 views

What characters are valid in PGP encrypted and signed messages?

What characters may be present in a PGP encrypted message or a PGP signature? For example, do encrypted messages only use the Roman alphabet + some numbers and symbols? Can the character set be ...
1
vote
0answers
58 views

Algebraic Normal Form Piccolo

I'm trying to verify the claim of paper Improved Algebraic Fault Analysis: A Case Study on Piccolo and with Applications to other Lightweight Block Ciphers about the numbers of equations in algebraic ...
0
votes
0answers
28 views

Question about AES and Cryptsetup [migrated]

I've read that AES can work with three key sizes: 128 bits, 192 bits and 256 bits. I'm trying to set up a LUKS partition with CryptSetup, using “aes-xts-plain64” cipher. Why can I create keys of any ...
1
vote
1answer
130 views

A way around digital signatures?

Scenario: I have to protect the integrity of some files that will be exchanged between entities $A$, $B$, $C$. Entity $A$ creates the file (e.g. configuration) and sends it to $B$ and $C$. Entities ...

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