# All Questions

795 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 ...
438 views

### Known plaintext attack on ElGamal encryption

Assume that Alice uses Bob’s ElGamal public key ( = 2, Yb = 8) to send two messages $M = 17$ and $M' = 37$ using the same random integer $k = 9$. Eve intercepts the ciphertext and somehow she finds ...
90 views

### public key delivery without third party

I am currently working on an anonymous P2P file sharing application and I am elaborating some already existing solutions. Some of them allegedly provide point-to-point encryption using RSA ...
302 views

### Rely on NSA Suite B Cryptography?

NSA's Suite B Cryptography suggests some cryptographic algorithms for encryption, digital signatures, message digests and key agreements. The selected algorithms and their key size are suggested by ...
360 views

### How do the following new (2013) ECC curves compare in security or efficiency? [closed]

I read about the following "safe" ECC curves and notably, secp256 and all the NIST curves are marked as "unsafe" when compared to more modern curves. I need a curve for signing or encryption, (or ...
2k 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 ...
258 views

### Block-cipher based vs Hash based MAC

When it comes to MAC algorithms one can have many choices. CBC-MAC, CMAC, PMAC etc are some MAC algorithms that rely on block ciphers (ex AES) to generate a MAC. On the other hand HMAC is very popular ...
386 views

### Interleaving bytes to make an effectively larger block size

Will applying a 16 byte bit block cipher such as AES over a 256 byte block to in the following way result in a much stronger cipher:- Apply block cipher with first key digest to each 16 byte group. ...
721 views

### Subverting the key generation step in RSA public key cryptography

I am interested in the ways the implementation of an RSA key generation implementation can or cannot be subverted so that the subverter has an advantage, but the owner of the key, and the people who ...
129 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 ...
94 views

### Cryptanalysis of Nonlinear Table Lookup

I am trying to derive a symmetric key based on a master key, combined with a simple string. Based on my limited knowledge, it seems that something like PBKDF2 would do that for me in a well-defined ...
599 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 ...
280 views

### Using multiple secret keys

I have two secret keys. One is a secret key generated by OpenSSL (primary secret key). Second key is generated by performing one way hash operation to GPS co ordinates and time parameters (geo secret ...
64 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 ...
76 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 ...
216 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 ...
90 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 ...
247 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, ...
99 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 ...
149 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 ...
311 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 ...
422 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 ...
171 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 ...
775 views

### Why is AES considered to be secure?

The security of RSA is based on the integer factorization problem, which is a very well defined and understood mathematical problem. This problem must be solved in order to fundamentally break RSA. ...
93 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 ...
144 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 ...
80 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 $e$
151 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) ...
111 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 ...
206 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 ...
121 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. ...
116 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 ...
172 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?
76 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 ...
115 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 ...
85 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 ...
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?
5k views

### How does one implement the Inverse of AES' MixColumns

I got an answer in the related question about Mixcolumn for encryption, but how about decryption? what will I do? Because it said that I will used this: During decryption the Mix Column the ...
309 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 ...
200 views

### How many degrees of freedom up my sleeve?

Now that we know how the MD2 S-box was constructed, I find myself wondering how good a nothing up my sleeve number it really is. It is derived from $\pi$, but not in a very obvious way, even in ...
130 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 ...
7k 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: ...
88 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 ...
266 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 ...
1k views

### Is there any strong enough pen-and-paper or mind cipher?

Some ciphers are talked about at “Is there a secure cryptosystem that can be performed mentally?”, but (at the time of writing) I don't see an answer. Are they strong enough, or are non-computer ...
404 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 ...
196 views

### Building a stream-cipher out of a hash function?

I've already read this: Is it feasible to build a stream cipher from a cryptographic hash function? However, my proposed construction differs… Suppose the hash generates N bits. These bits are split ...
1k views

### How does Truecrypt change password without the need for a complete re-encryption of volume

From what I understand: TrueCrypt takes the password as message Add salt calculates a digest use digest for encryption From what I understand. A good hash function has Second pre-image ...
I understand that: $$x_{montgomery} = \frac{1 + y_{edwards}}{1 - y_{edwards}}$$ Using the libsodium ed25519 implementation, I have tried to write the following: ...
If the public key $(e,n)$ and the private key $(d,n)$ are known, what is the easiest way to find the primes $p$ and $q$? When $n$ and $\phi(n)$ are given this is easy to solve. But I can't manage it ...