# All Questions

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### If I send a signed email, how does the end user get the public key?

I am sending a signed email using Python and smtplib. I have successfully signed the email using this example: http://proj.badc.rl.ac.uk/svn/ndg-security/branches/Dependencies/m2crypto/doc/howto.smime....
89 views

### Combining public key signature schemes and private key encryption

I'm working on my bachelor thesis and I have some questions. I want to combine a CPA-secure private key encryption scheme and some unforgeable public key signature scheme (in my case some sanitizable ...
297 views

### Do public/private pairs work both ways?

Public/private key algorithms such as RSA encrypt a message with a private key but only decrypted with another (mathematically related) key. Can the public key decrypt the messages encrypted with ...
130 views

### Image sharing without data overhead

The idea is to share $n$ images among $n$ persons so that all images can be reconstructed by someone in possession of all shares. However, there must not be any data overhead (which means the shares ...
27 views

### how MitM compute server secret number in logjam

I would like to know how man in the middle can compute the server secret number if the prime is 512 bit in length and if knowing ($g$) parameter as in the following figure help him or not? is the man ...
95 views

### Changing encryption key without revealing the original key

Say that Alice encrypts a $\text{plaintext}$ with $\text{key}_{m}$ and gives the $\text{ciphertext}_m$ to Bob. Alice wants to send several gigabytes of $\text{plaintext}$ to Eve but she is on a mobile ...
40 views

### Which compound algorithms would weaken cipher, not strengthen it?

OK, in school, some of the lecturers always say "if you compound the crypto, it may make the cipher weaker". So, which combinations of algorithms would do this? Why? Can anyone show the mathematical ...
35 views

### A simple keyed message authentication code using XOR

Suppose I have a key that is at least 32 characters in length and provides at least 128-bits of entropy. If I picked from the character set of [a-hjkmnp-z2-9-], ...
410 views

### Why can the last block contain a full block of padding in CBC Encryption?

I'm trying to understand the SSL Poodle Attack and I'm wondering why the last block of a CBC Record can be full of padding? Wouldn't that mean that the useful data was already a multiple of the key ...
30 views

### Recryption of ciphertext with a different key

Let's have $P$ some sensitive piece of data and $K_1$a secret value, both known to Alice, but not to Bob. $K_1(P)$ means $P$ encrypted using $K_1$. Alice sends $K_1(P)$ to Bob. Bob keeps the value of ...
115 views

### Signing MD5 hashes with RSA? [duplicate]

Lets say I need to update a project and allow anyone to distribute the updates, however I need it to be secure, so I hash the data and sign the hash, this project uses MD5. How likely would it be ...
57 views

### Encoding issue - PKCS#11 CKM_RSA_PKCS wrapping unwrapping EME-PKCS1-v1_5

I am trying to figure out why I am not able to get compatible format of EME-PKCS1-v1_5 3DES key encrypted by public RSA key. I need to send to a third party 3DES key encrypted using their public RSA ...
24 views

### What is the benefit of artificially padding messages? [duplicate]

For ECB, CBC, and CFB, padding is required when the plaintext does not fill out a complete multiple of the block size. However, I have heard that is good practice to pad all messages, even if the ...
308 views

### RSA: Recover e given factorization of N and plaintext/ciphertext pair

Is there a way to recover the public exponent e (assume in this case it is in fact not public) used in an encryption if I know the following: Factorization of <...
20 views

### Randomness and reversibility of a sequence generated by taking sha256 hashes of an predictably-changing input?

Let's say we generate a sentence in a way like for example this: Fix an initial seed. Attach '1' at the end of the seed and compute sha256 of such a string. This hash is our first number. Now attach 2 ...
427 views

### What is cryptographic agility?

I keep seeing the term "Cryptographic Agility" referenced. What does it mean? For instance: https://msdn.microsoft.com/en-us/library/windows/desktop/bb204775(v=vs.85).aspx One of the key value ...
56 views

### what is DHE_EXPORT cipher suite

I need to know what is exactly DHE_EXPORT cipher suite and why it is still supported by some servers. Is there any implementation or details about this weak crypto?
46 views

### Why does Apple create a certificate for my push notification key [closed]

Brief rundown of how Apple Push Notifications (APN) work: Generate a keypair, generate a CSR for it Upload the latter to your Apple Developer account and associate it with your mobile app In the ...
37 views

### Sodium crypto_aead_aes256gcm_encrypt ciphertext length > plaintext length

Using the crypto_aead_aes256gcm functions from Sodium produces a ciphertext of "at most mlen (plaintext length) + crypto_aead_aes256gcm_ABYTES bytes (16 bytes) are put into c (ciphertext), and the ...
80 views

### DGHV Bootstrap - How can I recrypt my cipherbit [closed]

I'm implementing this homomorphic crypto-system : http://eprint.iacr.org/2011/440 I'm using SAGE math. I don't get how the bootstrap is done in this scheme, in the appendix A, they say ...
6k views

### Why is OTP not vulnerable to brute-force attacks?

I saw this question on the book Understanding Cryptography. At first glance it seems as though an exhaustive key search is possible against an OTP system. Given is a short message, let’s say 5 ...
77 views

### How to calculate the probability to get two random numbers equal from the same generator

I have deterministic random bit generator (DRBG) to generate a random numbers in which it is 10 byte value and I want to turn this algorithm twice, what is the probability to get the two random equal ...
465 views

### How is the message digest related to Signatures and Encryption?

I was studying about 'Data Security' and I found out this one thing confusing. What is an message digest? I got it is the hash of the message. Is the message here referring to the plaintext or ...
122 views

### Algebraic structures in RSA

Why do we need a field for RSA and what are the two operations in this field? Why can't we have a ring or group for example? Because in a group you also have inverse elements.
69 views

### Why is it so hard to make a good KDF?

Why are we stuck with only 2-3 KDF functions since KDFs don't imply mathematically conceiving a cryptographic primitive like the inner workings of AES or SHA but instead build a system out of existing ...
51 views

### How would multiple uses of a vigenère cipher be decrypted on paper (classic)?

As far as I can tell (from reading the Wikipedia article about it) decrypting a message encrypted with the vigenère cipher is possible on paper. But what would happen if the message is encrypted ...
104 views

### Anonymity problem after voting with blind signatures

In a distributed voting system (all the messages are public, but signed and maybe encrypted/blinded), having: $d$, $e$, $N$ — registrar key params $m$ — msg $r$ — random blinding factor $m'$ —...
251 views

### Finding Private key in RSA with public key, cipher text and plain text

Is there a known 'non-brute force' method of determining a private key in an RSA system when all other parameters are know? I found the values of a ciphertext ($C$), its corresponding plaintext ($P$) ...
46 views

### Decrypting a file encrypted with AESCRYPT using key instead of password

i have encrypted a file using aescrypt and took memory image while the file was being encrypted and hence have the key that was used for encryption in the image but not the password, can i decrypt the ...
57 views

### Padding password before SHA-512 hashing to avoid info leak via timing

I've noticed that SHA-512 password hashing leaks some information about the password length. There's a weak correlation between password length and time to hash. It's roughly linear, but in steps of ...
47 views

### What's the best way to pad a message?

Currently my padding is adding a string of random characters $l(N)/3 - (l(M) + 1)$ Long and just adding it to the messages, is this sufficient? $l(x)$ = length of a variable
54 views

### Finding d given everything else in RSA [duplicate]

I've got everything else (e, p, q, phi(n), n), but cannot see a way to figure out what d should be. P and q are both 300 digits, so phi(n) and n are about 600 digits, but e is only 5 digits. I do ...
51 views

### I am trying to understand how AES works

I am trying to understand how AES encryption operates. I am trying to run through the steps on paper to understand. So far, I am creating a simple example. I have the plain text "MESSAGE TESTTEXT" ...
17 views

### security of Even-Mansour scheme with keyed shuffle

I was researching the Even-Mansour scheme and had a question that wasn't addressed in the paper that I saw, and google was minimally helpful. What if a permutation that uses secret data like the ...
290 views

### Encryption in which ciphertext is bigger than plaintext?

Is there an encryption which would produce a ciphertext longer than the plaintext? (please ignore block-chaining IV stuff, which would result in a data being generated from the plaintext). The point ...
34 views

### Using quadratic residue to learn the sign of a field element

Given $x' \in \{-x, x\} \bmod q$ (where $q$ could be any prime of my choice), $s$ is a random element in the field, $y = x'\cdot s$ and $y' = \pm\sqrt{(x\cdot s)^2}\bmod q$ (i.e., both solutions to ...
50 views

### Proper forward secrecy [closed]

Currently I have a protocol using a simple RSA to AES handshake. I have been reading more and more and would like to implement proper forward secrecy, but at the same time I'd like to improve the ...
55 views

### Selective and existential unforgability of signature schemes

I understand that one can define EUF-CMA of a signature scheme is terms of a game where the adversary is allowed to query signatures on messages of his choosing, and at the end of the game he must ...
77 views

### Is it acceptable to write the nonce to the encrypted file during AES-256 GCM? [duplicate]

I've used AES-128 in CTR mode in the past and using the same key but different IVs for each file, I wrote the IV to the first 16 bytes. Is it safe to do so with the nonces used in AES-256 GCM? I'm ...
51 views

### Is WPA2(PKS) AES vulnerable to 4 way hand shake and brute force attack?

I can easily crack a WPA TKIP using a dictionary/brute force method, provided that I collect the four way handshake. Is the same method possible on WPA2 AES? I cannot find any articles about '4 way ...
37 views

### I am trying to implement TLS v1.1 {3,1} but get alert 70 on hellomessage to goolge

I am trying to implement TLS on top of the winsock api's from windows. I am using port 443 and have created a socket. I have sent this message in decimal. ...
35 views

### homomorpic encryption [closed]

I am doing project k-nearest neighbor classification over semantically secure encrypted relational data I have doubt for in smin algorithm in 1)Li ← Wi ∗ Φir′; ri′ ∈R ZN this line it the out put is ...
1k views

### How does the Blowfish algorithm key initialization work

I'm having some trouble understanding the Blowfish encryption algorithm. From the Wikipedia article... The secret key is then, byte by byte, cycling the key if necessary, XORed with all the P-...
33 views

### Breaking linear blockcipher - expanding recursive formula

I am trying to break a block cipher that can be defined as follows: We have 12 rounds and at each round we perform the following operation. We split 64 bit data into 2 parts each 32-bits, $\ x_r$ ...
29 views

### Publishing only items your subscriber previously subscribed to, without you knowing which those items are

Let's say you were given a dump from a security breach. You want to share the email addresses affected by the breach with a separate company who pays you for this service so that they can protect ...
419 views

### Given enough RSA ciphertext, is it possible to determine which key was used?

Given enough RSA ciphertext, is it possible to determine which public key was used to generate the ciphertext? Presume an unknown public key that was used to generate RSA ciphertext, and we have ...
130 views

### Is an android encrypted SDcard readable/decryptable easily by a desktop machine?

If I encrypt my SDCard on my android phone using the built-in encryption software, and my phone dies for whatever reason, would I be able to easily by just using the pin/password to access that data ...
121 views

### What is Deterministic Authenticated Encryption?

I came across something known as deterministic authenticated encryption in my studies and a lot of people were associating it with Synthetic IV mode. I am having trouble what exactly DAE is because I ...