4
votes
2answers
200 views

Authenticated encryption without padding

I want to encrypt data and protect its integrity and confidentiality. However, I cannot increase the length of the data. Are there any cipher modes of operation which provide confidentiality and ...
4
votes
2answers
304 views

Perfect Forward Secrecy with Pre-shared Key

I've recently been looking at how to do perfect forward secrecy on a unidirectional connection (server can only push messages to client, client cannot respond). What I've come up with is the idea of ...
4
votes
0answers
161 views

Computing IV for CBC from PBKDF2 + HKDF

Note that this question is somewhat similar to Can I use my random IV (for AES) as a salt for PBKDF2? My current encryption format computes two random PBKDF2 salts (encryption and HMAC, 8 bytes each) ...
4
votes
2answers
384 views

Can an RSA public key be generated without ever knowing the factors? [duplicate]

Context: http://blog.cryptographyengineering.com/2013/12/a-few-more-notes-on-nsa-random-number.html Assume I wanted to use RSA as the basis for a hash function or DRBG. Also assume that my construct ...
4
votes
1answer
169 views

Unforgeability and type of adversary

When trying to prove security of asymmetric signature, for instance for existential (or strong) unforgeability against chosen messages attack, do we need to consider the signer as a possible ...
4
votes
2answers
200 views

Do test vectors ensure a cipher is free of backdoors?

If a cipher implementation passes unit tests using test vectors from some trusted source (these for AES, for example), then can we say that it is fully conforming to the specification, and must have ...
4
votes
1answer
179 views

Keyed digest function with odds of collision below the birthday bound?

I wonder if it is possible to devise a function $F(K,S,R_S)\mapsto D$ where: $K$ is some key (I have freedom on $K$, it could even be generated by a trusted party); $S$ is in $\{0,1\}^s$, say ...
4
votes
3answers
171 views

Why use a timestamp and how can someone know it's the correct one?

Let's say A wants to send a message, so everyone who gets the message, can be assured that it's from A. A then sends a message ...
4
votes
1answer
194 views

Can two people with different one time pads securely exchange a message like this?

Alice has a message, generates a one time pad, encrypts her message and sends it to Bob. Bob generates his own one time pad, encrypts the message again, and sends it back to Alice. Alice then decrypts ...
4
votes
2answers
182 views

How to Distribute the Shares using Secret Sharing for arbitrary Monotone Access Structures

Consider 4 people, $A$,$B$,$C$,$D$, & a secret $s\in\{0,1\}^k$. Construct a scheme which enables the following subsets of people to retrieve the secret $\{A,B\}$, $\{A,C\}$, $\{B,C,D\}$. I know ...
4
votes
1answer
169 views

Proof that MACing a hash of the message is also a secure MAC

I found a theorem that says: Let $MAC = (S,V)$ be a MAC for short messages over $(K,M,T)$. Let $H: M^{big} → M$. Define $MAC^{big} = (S^{big},V^{big})$ over $(K,M^{big},T)$ as: $S^{big}(k,m) = ...
4
votes
3answers
244 views

What is the difference between a 'cipher' and a 'mode of operation'?

What is the difference between the term cipher (a name like RIJNDAEL) and mode of operation (like ECB)? Aren't these both terms for the encryption/decryption technique?
4
votes
1answer
124 views

Non-interactive proof that an element is in a subgroup

I am just reading the DAA paper (http://eprint.iacr.org/2004/205.pdf, Appendix A). A party $\mathcal{I}$ generates two group elements $g' \in \mathrm{QR}_n$ and $h = g'^r \bmod n$ with $r \in_R \left| ...
4
votes
1answer
423 views

How much entropy is lost via hashing when you add known or low entropy data?

(I updated the title, as I think there was some confusion as to the question) Here's the question: For example, if I have a bit stream that is 64K bytes long and there is about 16 *8 bits worth of ...
4
votes
2answers
1k views

How can I remove my personal data from my PGP public key?

According to this Q&A-discussion it is possible to remove all personal data (name and mail address) that is attached to a public key. What steps do I have to follow in order to remove all ...
4
votes
1answer
147 views

Is my identity exposed when publishing my public key or encrypting with PGP?

Let's say I create my PGP keys with my appropriate name and email address. Will these personal information be exposed if I publish my public key? When I encrypt a message for sending it to my ...
4
votes
2answers
242 views

CTR mode nonce with aggressive key rotation policy

I'm writing a small encryption library which will be required to use the shortest possible amount of overhead for describing ciphertexts. I want to use CTR mode with a 6-byte random nonce. Normally ...
4
votes
1answer
140 views

Does EAX use the same keys for encryption and authentication?

I am actually checking use of EAX AEAD mode, and following EAX spec review (scheme definition for my part), my question is: what about the derivation of the authentication and encryption keys? In the ...
4
votes
2answers
1k views

Finding a keylength in a repeating key XOR cipher

In an old cryptography FAQ, I found the following step described for determining a the length of the key a cipher was repeatedly XORed against: Discover the length of the key by counting ...
4
votes
3answers
265 views

Why are elliptic curves better than cyclic groups?

The set of points of an elliptic curve over a finite field is isomorphic to the direct product of two cyclic groups (i.e. $E(F_{p^n}) \cong Z_{s} \times Z_{t})$. What is the advantage of representing ...
4
votes
1answer
188 views

How hard is it to generate a partial RSA fingerprint collision?

When I use SSH to log into a new system, I get asked to verify that the fingerprint (a 32 hex digits string) of the hosts's RSA key is correct. How much if it must I actually compare (by hand/eye) to ...
4
votes
1answer
125 views

low-exponent RSA

I have questions from http://link.springer.com/chapter/10.1007%2F3-540-68339-9_1 Suppose we have 2 messages $m_1$ and $m_2$ related by a known relation $m_2=m_1+1$. Suppose further the messages are ...
4
votes
1answer
471 views

Hill-cipher, disordered alphabet

I am going to apply a simple substitution cipher to my input, then encrypt the result with a Hill cipher. How can this be broken, in a chosen-plaintext threat model? In other words, instead of the ...
4
votes
2answers
652 views

Why are RSA key sizes almost always a power of two?

I know that other bit sizes are possible, e.g. this HTTPS server seems to have a 9000 bit key https://www.ssllabs.com/ssltest/analyze.html?d=qqq.gg, but it's very rare that one sees a key not of size ...
4
votes
2answers
481 views

Diffie-Hellman key agreement with both Server Authentication and Perfect Forward Secrecy

I am trying to find the protocol with the least overhead, which still meets the following requirements: Server Authentication of server identity to client. The client has an authentic copy of the ...
4
votes
2answers
4k views

What is the difference between MAC and HMAC?

In reference to this question, what are the "stronger security properties" that HMAC provides over MAC. I got that MAC requires an IV whereas HMAC doesn't. I also understood that MAC may reveal ...
4
votes
3answers
231 views

What is a block cipher?

I have read about block ciphers, but I do not understand the topic completely. Is a block cipher a way to send data or a way to encrypt data? How is the data divided into multiply blocks before it ...
4
votes
1answer
315 views

HMAC for short messages

I am using HMAC (hmac-sha256 to be precise) message authentication without encryption. My question is, is having a short message make it easy to determine what my secret key is? My C# code is as ...
4
votes
1answer
252 views

A situation where security by obscurity might be the best solution - or am I wrong?

Imagine the following situation. There is an application for a microcontroller-driven system. The profits are made from manufacturing and selling the device itself. However, it is expected, that the ...
4
votes
1answer
454 views

Can I use my random IV (for AES) as a salt for PBKDF2?

Also, where do I store my salt (can I just store it at the beginning of the encrypted file)?
4
votes
1answer
145 views

What authentication techniques can be used by QKD Systems?

As it is well known that QKD systems are prone to simple MITM attacks… what authentication techniques are being used in QKD to prevent those attacks in practice?
4
votes
2answers
917 views

Homomorphic cryptosystems in RSA

Hopefully Crypto can help me understand homomorphic cryptosystems. I'm designing a high score server for a game I made, and because of facets in the language i'm using, the player would be able to ...
4
votes
4answers
569 views

Which block cipher modes of operation allow a predictable IV?

Recently I found out that in the modes CBC and PCBC the IV may be passed in cleartext but never must be predictable. However for this part of my app I rather have the IV be predictable and unique ...
4
votes
2answers
1k views

Why should the RSA private exponent have the same size as the modulus?

Consider the generation of an RSA key pair with a given modulus size $n$ and a known, small public exponent $e$ (typically $e = 3$ or $e = 65537$). A common method is to generate two random primes ...
4
votes
2answers
556 views

Is this a correctly formatted PGP session key packet?

RFC 4880 may be full of information, but it can be incredibly vague at times, so im looking for someone who actually knows the answer to this. Given this public key: ...
4
votes
1answer
185 views

How to communicate authentication tag for GCM?

I have written some code to do AES in GCM. I currently manually append the tag property to the ciphertext, is this the proper way to communicate the authentication tag?
4
votes
1answer
354 views

OpenPGP Signature Packet hashed data

RFC 4880 describes the version 4 signature packet, tag 2, as ...
4
votes
1answer
136 views

Does security under ROM imply exactly what?

I'm not sure I understand really the implications of proofs of security in the random oracle model. Does a proof of security in ROM translate to a reduction of security of the crypto-system to the ...
4
votes
1answer
328 views

How can one share information using the 'host-proof' paradigm?

I am attempting to make a web-based secure password management and sharing utility, both as an academic exercise and to fully understand and feel safe about using it. I really like the idea of a ...
4
votes
1answer
2k views

What is the 'Version Incompatibility" Problem in TLS 1.1/1.2 and 1.0?

It's unclear to me if there are multiple problems, or a single one, (and just what that problem is) with servers incorrectly negotiating down from a client offering 1.1 or 1.2 to something they ...
4
votes
1answer
445 views

Differential cryptanalysis - breaking the last round of FEAL4?

I've been trying to learn cryptanalysis. I've come across this resource which proved very helpful: http://theamazingking.com/crypto-feal.php So far I've been almost successful in breaking FEAL4 ...
4
votes
2answers
356 views

Provable Encryption

Is it possible to encrypt data in a way that it can be proven that the data is encrypted, without revealing the key? Alice chooses some plaintext, then she encrypts it with a certain scheme. She ...
4
votes
3answers
575 views

Malleability of ElGamal and Hashed ElGamal

Question: Suppose A encrypts a number $x$ which indicates her bid on a contract, using ElGamal encryption. Say that the encryption of $x$ produces a ciphertext $c$. Explain how E can modify $c$ to ...
4
votes
1answer
788 views

How does a key wrapping like RFC 3394 secure my cryptographic keys?

So I'm messing around in the BouncyCastle library with the RFC 3394 AES Key Wrap engine and I'm trying to understand the benefit of it. The problem I'm running into is how to store keys securely on a ...
3
votes
1answer
78 views

Partial hash code protocol for security tokens providing signatures

I've read the following two sentences within a beta protocol specification, which references ISO/IEC 7816-8 for PSO:HASH which already contains notions of a "partial hash". The off-card entity is ...
3
votes
1answer
162 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 ...
3
votes
3answers
206 views

How feasible is word-level frequency analysis over English (or any language)?

Say I have some black box which, given any English word, deterministically outputs a token for that word. Assume our black box is implemented using strong cryptography, i.e. the hardness of reversing ...
3
votes
2answers
196 views

Using PBKDF2 twice with different argument order

I'm pretty sure this is a really bad approach (in theory), but one of my clients is doing this and I was wondering… How bad it is to perform pbkdf-2 in this way (with 2000 iterations)? ...
3
votes
0answers
119 views

Is there a flaw in this ECC blind signature scheme?

Recently I've found the following work on the internet: An ECC-Based Blind Signature Scheme The paper claims to be an ECDSA blind signature however it seems that their scheme has a flaw in it. The ...
3
votes
1answer
88 views

Does $i^n=j^n$ for $i, j \in GF(2^q)$ and $i \neq j$ for some $n<2^q-1$

Let $i, j \in GF(2^q)$ and $i \neq j$ and $i,j\neq0$. Is that possible that $i^n=j^n$ for some $n$ such that $0 < n < 2^q-1$? I am looking for a proof if the answer is no, or for a method to ...

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