10
votes
1answer
468 views

How to choose constants in a cryptographic function?

A number of cryptographic functions have constants built in. For example, the constants used in RFC 2104 for HMAC, or the constants used in s-boxes (e.g., DES and AES), or MD5. In general, how are ...
10
votes
2answers
976 views

Encryption scheme for social-network-like data sharing data via untrusted server?

I am thinking quite a lot lately abut the problem of secure, privacy-preserving social networking. Distributing the network among trusted, preferably self-hosted servers (like Diaspora, GNU Social ...
10
votes
1answer
1k views

Windows 8/Server 2012: Passes FIPS-140-2 despite failing AES-GCM for IV != 96 bits long?

Background Microsoft certifies Windows 7/8 as well as Server 2008 R2 and 2012 to be FIPS-140-2 compliant. Actually they certify just a small crypto core, bcrypt.dll (the library, which is unrelated ...
10
votes
1answer
1k views

Why do the elliptic curves recommended by NIST use 521 bits rather than 512?

Wikipedia says in reference to the elliptic curves officially recommended by NIST in FIPS 186-3: Five prime fields for certain primes p of sizes 192, 224, 256, 384, and 521 bits. For each of the ...
10
votes
2answers
1k views

Measuring entropy for a ciphertext only attack

When bruteforcing a password (e.g. the common attacks on DES), where you have ciphertext only, you need a way to assess whether a decrypted plaintext is the right one. I believe the EFF DES machine ...
10
votes
1answer
2k views

SHA-256: (Probabilistic?) partial preimage possible?

Currently busying myself with the Bitcoin "mining" algorithm, I am wondering if the process really cannot be simplified. For reference, the algorithm is basically SHA-256d: $success := SHA256( ...
10
votes
2answers
4k views

How does order-preserving encryption work?

Order-preserving encryption (OPE) is, apparently, a method of encrypting data so that it's possible to make efficient inequality comparisons on the encrypted items without decrypting them. I've been ...
10
votes
1answer
217 views

Shadowed identity in cryptography

I was trying to implement zero knowledge protocol for authentication based on the paper "A Practical Zero-Knowledge Protocol Fitted to Security Microprocessor Minimizing Both Transmission and Memory". ...
10
votes
2answers
853 views

Which blind signature schemes exist, and how do they compare?

I'm looking into blind signature schemes for use as digital cash. I have come across blinded RSA, and Lucre(DH based). Are there other schemes available, and how do they compare? I suspect there ...
10
votes
1answer
708 views

How would one crack a weak but unknown encryption protocol?

I asked a question on security.stackexchange, but was told it would be a better fit here: http://security.stackexchange.com/questions/32779/how-would-one-crack-a-weak-but-unknown-encryption-protocol ...
10
votes
3answers
212 views

Collision or second preimage for the ChaCha core?

Daniel J. Bernstein's ChaCha core is an evolution of the Salsa20 core. Both are functions over the set of 512-bit bitstrings, partitioned as sixteen 32-bit words. Can we exhibit collisions, or ...
10
votes
1answer
5k views

How does a birthday attack on a hashing algorithm work?

A "normal", brute-force attack on a cryptographic hashing algorithm $H$ should have a complexity of about $2^{n}$ for a hash algorithm with an output length of $n$ bits. That means it takes about ...
10
votes
1answer
433 views

Making ECDSA public keys one bit shorter

In compressed form, ECDSA public keys are one bit larger than the curve size. There are some situations where this extra bit is extremely inelegant. So I had the following idea as a way to remove it. ...
10
votes
1answer
1k views

How can I make my cipher show the avalanche effect?

I am a beginner in cryptography. I designed an password based encryption-decryption algorithm, which uses a random salt and a password to encrypt a message. I'm using SHA-512 for hashing, matrix ...
10
votes
1answer
458 views

Hash function from narrower block cipher operated in CBC-encryption mode?

I am trying to build a public hash function (thus collision-resistant and preimage-resistant, and more generally behaving like a random oracle), with input a message $M$ of fixed size $|M|=m\cdot b$ ...
10
votes
2answers
418 views

How should one implement a delegated shared trust protocol?

Consider the following (probably naive) scenario. Alice, who is very limited in her knowledge of security in general (clueless about securing a private key for example), wishes to delegate certain ...
10
votes
1answer
417 views

Shared secret: Generating Random Permutation

-- or: How to Play Poker Without a Dealer I know this question is long but it's a really interesting theoretical problem about shared secrets and multi-party computation. General Problem: "Shared ...
10
votes
1answer
1k views

Bleichenbacher 1998 “Million message attack” on RSA

I have been reading Bleichenbacher's 1998 paper on a forged message attack on RSA. The paper assumes access to an Oracle that takes a ciphertext $c$ and will check the decrypted text for valid PKCS #1 ...
10
votes
0answers
499 views

Who first published the interest of more than two prime factors in RSA?

Multi-prime RSA is now a well known technique: it uses $k>2$ distinct secret prime factors in the public RSA modulus, with the advantage that, using the CRT, we can gain a speed boost in ...
9
votes
3answers
3k views

Why shouldn't I use ECB encryption?

I'm using Java to generate encrypted strings, and I get this warning at build time: ECB encryption mode should not be used So I'm wondering why I shouldn't use ECB and what I can use instead?
9
votes
4answers
2k views

Is the one-time-pad a secure system according to modern definitions?

Occasionally I hear people say that one-time pads are "useless" or even "broken". "modern cryptography knows more security definitions, under some of which the one-time pad is completely ...
9
votes
4answers
2k views

Is it feasible to build a stream cipher from a cryptographic hash function?

A few years ago I devised a symmetric-key system that worked like so: ...
9
votes
1answer
3k views

Why do all SSH-RSA Keys begin with “AAAAB3NzaC1yc”?

My Friend and I have been generating a few ssh2-rsa keys and noticed that all the public keys began with "AAAAB3NzaC1yc". The similarity extended to "AAAAB3NzaC1yc2EAAAABIwAAAQEA" between two keys I ...
9
votes
2answers
2k views

Is there a way to make RC4 (ARCFOUR) secure, or is it completely broken?

I need a method to authenticate a process with another in order to establish interprocess communication between them, to prevent malicious processes from trying to hook onto the system. Currently I ...
9
votes
2answers
4k views

Using the same secret key for encryption and authentication in a Encrypt-then-MAC scheme

Is it a weakness to use a single shared secret for protecting messages using a Encrypt-then-MAC scheme? Assuming a system is using AES-256-CBC and a SHA1-HMAC and the same secret key for both ...
9
votes
2answers
692 views

Encrypt-then-MAC: Do I need to authenticate the IV?

In the setting of Encrypt-then-MAC, do I need to include the IV in what I'm HMACing, or is authenticating just the AES-encrypted data sufficient?
9
votes
4answers
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 ...
9
votes
3answers
599 views

Is key size the only barrier to the adoption of the McEliece cryptosystem, or is it considered broken/potentially vulnerable?

A recent paper showed that the McEliece cryptosystem is not, unlike RSA and other cryptosystems, weakened as drastically by quantum computing because strong Fourier sampling cannot solve the hidden ...
9
votes
4answers
3k views

Encrypting small values with RSA private key

I'm looking for best practices when it comes to encrypting small (< 128 bytes) amounts of data with the RSA private key. Signing it would make the resulting payload too large.
9
votes
4answers
2k views

Why is the IV passed in the clear when it can be easily encrypted?

The initialization vector (IV) is exclusive or'd against the plain text before encryption for the first block sent in order to prevent an attacker from learning that duplicate message blocks are being ...
9
votes
3answers
5k views

Why hash the message before signing it with RSA?

The diagram below illustrates the process of digitally signing a message with RSA: As diagram shows, the message is first hashed, and the signature is then computed on the hash, rather than on the ...
9
votes
3answers
496 views

Should I delete cryptographic data from memory?

When dealing with cryptographic data, is there a chance that attacker recovers portion of data I used in my program? Should I try to delete every bit of keys and encrypted data that I used, overwrite ...
9
votes
7answers
3k views

Where can I begin to study the math behind modern cryptography?

Why do I start the word buffers in MD5 with those values specifically? What in the world motivated the decision to use the numbers 1, 1, 2, 3 for the mix columns stagein AES? And what kind of ...
9
votes
4answers
10k views

Why is padding used for RSA encryption given that it is not a block cipher?

In AES we use some padded bytes at end of message to fit 128/256 byte blocks. But as RSA is not a block cipher why is padding used? Can the message size be any byte length (is the encrypting agent ...
9
votes
2answers
1k views

Why are the constants so simple in Keccak?

Keccak, the construction selected for SHA-3 is very interesting. It seems unlike other primitives and has chosen very simple constants. (Keccak talk PDF) The initial values of the state in Keccak is ...
9
votes
2answers
4k views

Why is asymmetric cryptography bad for huge data?

I've been told that asymmetric cryptography requires that the message to be encrypted be smaller than its key length. Why is this? I know about hybrid encryption, which uses symmetric encryption to ...
9
votes
3answers
1k views

Is it possible to distinguish a securely-encrypted ciphertext from random noise?

Say I have a bunch of data encrypted with a secure block cipher (such as AES). An attacker has unlimited access to this encrypted data. The attacker doesn't know whether the data is encrypted or if ...
9
votes
2answers
10k views

How can Cipher Block Chaining (CBC) in SSL be attacked?

I am trying to understand how CBC-mode in SSL/TLS can be attacked. I have been looking around online but all examples and explanations are very hard to understand and follow. Can you give a simple ...
9
votes
2answers
659 views

Why Elliptic Curves?

What is the benefit of using elliptic curves over the standard finite field, when the cyclic subgroup we consider of the EC's solution group is just isomorphic to some integer residue class of prime ...
9
votes
3answers
573 views

Can we ensure the security of a crypto-algorithm and -implementaton against acoustic cryptanalysis?

Like people always say: “Attacks only get worse…” — which is why I'm asking early. I have been reading the paper “RSA Key Extraction via Low-Bandwidth Acoustic Cryptanalysis” published December 18, ...
9
votes
3answers
257 views

How hard is to invert the function that computes the middle-bits of (x^2)?

I'm designing a function f that should be moderately hard to invert and very fast to evaluate in a modern CPU. The function will be used in a proof-of-work function. I've read that the middle-bits of ...
9
votes
2answers
3k views

Practical disadvantages of GCM mode encryption

It seems that GCM mode encryption has a clear advantage over CBC + HMAC in the sense that it only requires a single key. But it seems that there are some experts here that do not trust it enough to ...
9
votes
1answer
5k views

Why exactly is Blowfish faster than AES?

I've not been able to understand exactly the reason behind Blowfish being faster than AES. Is it dependent on the block size? Or is it processor dependent? (if Yes, then lets assume that AES ...
9
votes
2answers
4k views

Predicting values from a Linear Congruential Generator

I have learnt that Linear Congruential Random Number Generators are not cryptographically secure - my understanding is that given an LCG of the form: ...
9
votes
3answers
517 views

Is it reasonable to assure that p-1 and q-1 aren't smooth?

I came across the requirement that, in RSA, $p-1$ and $q-1$ shouldn't be smooth, shouldn't consist of lots of small factors. Therefore my question: How complicated is it to check whether $p-1$ is ...
9
votes
1answer
6k views

How strong is the ECDSA algorithm?

Some cryptographic algorithms are as strong as the size of their key is, while other have some weaknesses that limit their strength (such as SHA-1). How strong is the ECDSA algorithm, and does that ...
9
votes
4answers
1k views

How do you find a cheater in Shamir Secret Sharing?

If there are 4 people involved, and every two of them should be able to know the secret (the polynomial is just a line) and you are given f(x) and x for each of those people, and you know one of them ...
9
votes
1answer
1k views

How exactly was the finalist chosen in the NIST AES competition?

I was just reading the Stick Figure Guide to AES and came across an interesting table explaining how the winner was chosen: Unfortunately the NIST site is down so I can't gain further information ...
9
votes
2answers
976 views

Is a small size block cipher usable?

Consider a block cipher with a key of size 128 bits but a small block size, say 32 bits. Is this kind of block cipher secure? I would like to know to what extent I can use a small block cipher, like ...
9
votes
1answer
2k views

What are the advantages of a static ECDH key?

What are the advantages of using "static-ephemeral ECDH" over "ephemeral-ephemeral ECDH"?

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