# All Questions

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### Why can't I break ElGamal encryption by brute-forcing the secret exponent?

I am doing a course on cryptography on coursera and one of the topics covered was the ElGamal Encryption system. I am using the terms as defined in Wikipedia. Alice publishes $g$ and $g^x$. ...
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### How to generate a public key from a private ECDSA key?

Having some specific ECDSA curve and a private key, how does one calculate the public key? I am having a hard time finding the algorithm and equations for it.
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### Is there any semantic difference between predicate encryption and functional encryption?

Predicate encryption(PE) arrived as a descendant of attribute based encryption. Attribute based encryption allows the encryptor to embody a policy to the ciphertext (Ciphertext-Policy based ...
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### Why doesn't this replay attack work on ECDSA?

I've just started working with elliptic curves and ECSDA in particular, so my understanding of the underlying math isn't great. The thing I'm currently stuck on is trying to understand why replay ...
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### S-box with differential uniformity = 2

I read that we do not know if there exists an 8x8 sbox with differential uniformity = 2. I suppose we cannot compute every possible sbox because there are $64!$ possible s-boxes. Am I right? Is ...
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### Public key cryptography - public key encrypts and cannot decrypt?

I understand the basics behind public key cryptography, in that each party has two keys - the public one encrypts, and the private one decrypts. What I cannot figure out is, How does the public key ...
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I've been looking at the weakness with plain/textbook RSA, where the same message is encrypted and sent to multiple destinations. In this case, it is possible to recover the message. Given that an ...
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### Two step encryption

Is there any asymmetric cryptography algorithm which will allow recursive encryption. ...
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### Is there an advantage to storing keys split between several hashes?

I have a question about the way to store a key or password that was used for encryption, so that the application can check if the user put in the right key for decryption. If I make a mistake, please ...
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### Looking for examples for “proof by reduction”

Im looking for examples for a proof by reduction. For example: Let $A=(Gen, H)$ be a hash function. We define a new Hash function $A'=(Gen',H')$ with Gen=Gen' $H'_s(x)=H_s(H_s(x))$ It should be ...
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### Cracking the Beaufort cipher

Is there any easy way to crack a Beaufort cipher? We have a Vigenère table, and are trying to guess the keyword. Any easier way?
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### Impact of algorithms for factoring using elliptic curves over $\mathbb{Q}$

Recently a few papers have appeared that describe a new approach to factoring, using elliptic curves over $\mathbb{Q}$. See, e.g., Factoring integers and computing elliptic curve rational points, ...
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### Is frequency analysis a useful tool against encryption by multiplication?

If I transform natural plaintext by: making each letter two decimal digits, considering the whole as a decimal number; multiplying by the key (some integer constant), giving the ciphertext; would ...
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### Is there a symmetric-key cryptography based on key establishment techniques?

Most of the current key exchange techniques are based on public-key cryptography. Are there any key exchange/establishment techniques based on symmetric-key cryptography too? In my setup primary ...
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### Are there secure stream ciphers that cannot be parallelized?

Are there any stream ciphers (or a deterministic random number generators, that should work as well I guess?) that cannot be parallelized? So for example if I seed it with a specific value, and then ...
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### Attack by replaying messages

Assume Alice and Bob communicate over an insecure channel using one-time perfectly-secret encryption together with one-time secure message authentication code. Say Eve has the ability to eavesdrop and ...
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### Correct method to encrypt data so that it can be decrypted only by Alice and Bob

I need to build a system that stores encrypted transactions. Transaction involves always two parties. Payer and payee. Both must be able to download encrypted transaction from server and decrypt it ...
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### Why not encrypt salt?

Assuming I had to distribute salt+ciphertext together over an insecure channel, isn't it better to store the salt encrypted? By encrypted I mean with a block cipher and with key and IV derived from ...
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### AES CCM vs CCMP

Are the terms AES CCM and AES CCMP are equivalent, or is there any technical differences between the two?
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### bcrypt - collision-resistance against chosen salt and work factor?

Would it be difficult to find $\:$ cost,salt,input,password0,password1 $\:$ such that bcrypt(cost,salt,password0,input) = bcrypt(workfactor,salt,password1,input) and $\:$ password0 != password1 $\:$ ...
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### Is symmetric encryption vulnerable to plain-text-attacks?

Imagine that Bob sends a message to Alice for symmetric encryption to send to Charlie. (Only Alice and Charlie know the key.) Alice sends the encrypted message back to Bob to send to Charlie. Can Bob ...
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### MD5 implementation doubt

In MD5, there are four rounds. After every round, why do we need to add the computed Q values to the initial values and then take this value as input to the next round. For example after the first ...
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### Is quantum key distribution safe against MITM attacks too?

i read this recently: http://www.newscientist.com/article/dn12786-quantum-cryptography-to-protect-swiss-election.html and some parts of this: http://en.wikipedia.org/wiki/Quantum_key_distribution ...
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### are CFB and OFB really meant for streaming?

CFB, OFB and other modes are meant for streaming and don't require padding. Are there still limitations such as the text needs to be greater than key length?
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### Can I secure my key by XORing it with a hashed password?

I'd like to build a simple password-protected symmetric key system. The key-creation process in my system operates as follows: The system creates a 256-bit key purely at random. The user chooses a ...
561 views

### Passwords with same SALT. What does this mean?

If the same SALT is used for many passwords on a Linux server, in what way is that a security risk? Does the mean, that a user (which can change his own password) can calculate other users passwords? ...
1k views

### RSA cracking: The same message is sent to two different people problem

Suppose we have two people: Smith and Jones. Smith public key is e=9, n=179 and Jones public key is e=13, n=179. Bob sends to ...
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### How is de-synchronisation of HOTP solved?

From RFC 4226 I understand how HOTP generates one-time passwords by incrementing a counter and uses the 'look-ahead' window to try to resynchronise (from this counter), if the user tries a few wrong ...
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### A set of key pairs and one hash to secure them

I have a simple problem: I have a set of users' ECDSA key pairs, and say I want to encrypt them with a simple algorithm. I have access to one variable that uniquely identifies the user, so I hash it ...
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### Ballot box with multiple parties. All can read it, or none can read it

I am trying to create a decentralized moderation system for the game Diplomacy, a game for seven players. In each round of a face-to-face game, each player writes their moves on paper then puts it in ...
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### Why does AES-GCM need MAC? (TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256)

"TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256" is just one example for a cipher suite, as far as I'm concerned it means this: it uses the TLS protocol, exchanges keys with Elliptic Curve Diffie Hellman ...