12
votes
2answers
429 views

Why xor is a linear operation but ordinary adding is not

I'm new in cryptography and try to read some articles in this field. Many of these articles talk about non-linear S-boxes, and nothing more on what they mean by their non-linearity. I have a simple ...
12
votes
4answers
686 views

About Cryptography in a Character Language

Suppose I had a message in Chinese (or another non-phonetic language) and I wanted to encipher it. Some of the simplest encryptions in English are substitution ciphers, but such ciphers don't seem ...
12
votes
2answers
661 views

Why do we append the length of the message in SHA-1 pre-processing?

As we know, SHA-1 is irreversible, so why do we append the length of the message to the preimage?
12
votes
2answers
452 views

Is digest=HASH(HASH(a)+HASH(b)) equivalent to publishing two digests?

Is combining digests (created using a hash function) using arithmetic addition, and then hashing and publishing the result, less secure than publishing the set of digests? Does the answer change if ...
12
votes
3answers
1k views

Can you create a strong blockcipher with small blocksize, given a strong blockcipher of conventional blocksize?

Suppose I want a strong 20-bit blockcipher. In other words, I want a function that takes a key (suppose the key is 128 bits), and implements a permutation from 20 bits to 20 bits. The set of ...
12
votes
3answers
716 views

Can I use a one time pad key twice with random plaintext?

I understand the basics of OTP: $|\text{key space}| = |\text{plaintext space}|$ implies perfect security, key reuse destroys this. Cryptanalysis on the $N$-Time Pad for $N > 1$ involves finding ...
12
votes
3answers
1k views

Can ECDSA signatures be safely made “deterministic”?

Using the terminology of the ECDSA wikipedia page, ECDSA (and DSA) signatures require a random k value for each signature which ensures that the signature is different each time even if the message ...
12
votes
3answers
1k views

How practical are side-channel attacks and how much of a concern are they?

I see a lot of research in very sophisticated side-channel attacks on crypto systems. Most (but definitely not all) seem to follow a trend, namely, the crypto system does something very dumb like ...
12
votes
1answer
738 views

Should I use the first or last bits from a SHA-256 hash?

I have the need for a hexadecimal token that is smaller than the normal length of the hexadecimal representation of a SHA-256 hash. Should I take the first bits or the last bits? Which of them ...
12
votes
1answer
1k views

Is truncating a SHA512 hash to the first 160 bits as secure as using SHA1?

I am from a web development background (I don't know an awful lot about cryptography or how the algorithms themselves work), so I am asking this question in simple terms. Consider a hash of the word ...
12
votes
2answers
344 views

Are safe primes $p=2^k \pm s$ with $s$ small less recommandable than others as a discrete log modulus?

I take the definition of safe prime as: a prime $p$ is safe when $(p-1)/2$ is prime. Safe primes of appropriate size are the standard choice for the modulus of cryptosystems related to the discrete ...
12
votes
1answer
393 views

What exactly is the base for the KECCAK (SHA3) claim that a security strength of 256 bits is “post-quantum sufficient”?

On page 14 of "Keccak and the SHA-3 Standardization" (February 6, 2013) it says: Instantiation of a sponge function the permutation KECCAK-f 7 permutations: b → ...
12
votes
2answers
2k views

What is wrong with using SHA1 in digital signatures? Why is a robust hash function needed?

For the purposes of signing and verifying signatures, what is the value of the hash function? Why would it matter if SHA1 is later determined to be easy to break? Since a Public/Private key process ...
12
votes
1answer
448 views

Why should I use an Initialization Vector (IV) when I have unique keys?

I took a look at this question. My question is not the same. I've unique keys encrypting (in CBC mode, AES-256) each plaintext, i.e. I do not use a key to encrypt more than one plaintext. Is it ...
12
votes
3answers
686 views

What is the ideal cipher model?

What is the ideal cipher model? What assumptions does it make about a block cipher? How does it relate to assuming that my block cipher is a pseudo-random permutation (PRP)? When is the ideal ...
12
votes
1answer
697 views

Why choose an authenticated encryption mode instead of a separate MAC?

What are cryptographic reasons to choose an authenticated-encryption mode of operation (such as GCM) over a traditional encryption mode plus an independent MAC, or vice versa? Assume there is no ...
12
votes
1answer
15k views

What is safer: ZipCrypto or AES-256?

Like in title: which one of these encryption methods (ZipCrypto, AES-256) is more secure and why? I am asking about it because I'd like to know which should be preferred when compressing files with ...
12
votes
2answers
954 views

Design properties of the Rijndael finite field

So we've already had a question on replacing the Rijndael S-Box. My question is - can we use a different finite field other than the one given by $x^8 + x^4 + x^3 + x + 1$ in $GF(2^8)$. In other ...
12
votes
3answers
435 views

What exactly is the impact of the hidden subgroup problem on cryptography?

I understand my group theory (allegedly), so I can make partial sense of The Hidden Subgroup problem: Given a group $G$, a subgroup $H \leq G$, and a set $X$, we say a function $f : G \Rightarrow ...
11
votes
6answers
5k views

Is Diffie-Hellman mathematically the same as RSA?

Is the Diffie-Hellman key exchange the same as RSA? Diffie Hellman allows key exchange on a observed wire – but so can RSA. Alice and Bob want to exchange a key – Big brother is watching everything. ...
11
votes
3answers
585 views

Are NIST's changes to Keccak/SHA-3 problematic?

NIST is working on standardizing SHA-3. They have selected Keccak as the basis for SHA-3, and they plan to make some small changes to it; the result (with NIST's changes) will be standardized as ...
11
votes
3answers
1k views

How long will my encryption remain private?

This is a basic question in cryptography but I have not found a good, comprehensive answer. It is explained that our keys should expire and we should get new, stronger ones with time, reflecting more ...
11
votes
3answers
478 views

Bitcoin Research [closed]

I have recently been assigned to advise a student on a senior thesis in math. She has taken linear algebra, introductory real analysis, basic cryptography, and abstract algebra. Her interest is in ...
11
votes
4answers
2k views

How can rainbow tables be used for a dictionary attack?

I'm putting together a password policy for my company. I very much want to avoid requiring complex passwords, and would much rather require length. The maximum length I can enforce is 14 characters. ...
11
votes
3answers
1k views

Why does the padding in MD5 contain the message length?

I understand the need for padding in MD5. But why do we append the message length to the padding? I heard it strengthens the hash but how? Please provide an example if possible and how it applies to ...
11
votes
3answers
1k views

Why does SHA-1 have 80 rounds?

Why does SHA-1 algorithm have exactly 80 rounds? Is it to reduce collisions? If yes, then why do SHA-2 and SHA-3 have a lower number of rounds?
11
votes
1answer
4k views

What are the differences between a digital signature, a MAC and a hash?

A message may be accompanied with a digital signature, a MAC or a message hash, as a proof of some kind. Which assurances does each primitive provide to the recipient? What kind of keys are needed?
11
votes
2answers
782 views

Does the elliptic curve (EC) cryptosystem outperform RSA and DL cryptosystems?

Throughout the literature, it is stated that EC cryptosystems outperform RSA and Discrete logarithm cryptosystems, but I cannot understand how ECC would be more efficient than RSA and DL in terms of ...
11
votes
2answers
1k views

What is the most practical fully homomorphic cryptosystem?

Craig Gentry recently gave the first fully homomorphic cryptosystem. Quite a bit of work has been done since extending his work. It seems, however, that no system is practical for real world use. ...
11
votes
2answers
494 views

Do I have to have a different salt for each password?

Should I use a different salt for each password? In my system, there are no user names, only passwords. When a user logins in, he types in one or more passwords and the server compares the results ...
11
votes
2answers
656 views

Are AES-256's related-key weaknesses exploitable if it is used to build a hash?

Assume it is made a hash based on AES-256 encryption (perhaps because this is hardware-accelerated, but no standard hash is); and it is used the Merkle–Damgård structure, that is padding of the ...
11
votes
7answers
2k views

Is calculating a hash code for a large file in parallel less secure than doing it sequentially?

I would like to improve the performance of hashing large files, say for example in the tens of gigabytes in size. Normally, you sequentially hash the bytes of the files using a hash function (say, ...
11
votes
2answers
837 views

Practical consequences of using functional encryption for software obfuscation

I came across this article, which describes a method, developed by UCLA CS professor Amit Sahai et al, for using functional encryption in order to achieve software obfuscation. The paper that the ...
11
votes
1answer
508 views

Is Encrypt+HMAC stronger than AEAD?

There are a few posts that I've come across that seem to infer that using regular encryption and a MAC might be better than using the newer AEAD (ie: AES/GCM) modes. ...
11
votes
3answers
638 views

Where do I securely store the key for a system where the source is visible?

I have a customer with an Access database (ugh!) in which credit cards are stored in plaintext (yikes!), so amongst other changes I'm doing in the app, I'm applying some encryption in there. I've ...
11
votes
1answer
575 views

Is Wikipedia's table about SHA-2 collisions correct?

I was looking a Wikipedia article on SHA-2, and the "Comparison of SHA functions" table seems to indicate that SHA-2 is less secure than SHA-1. Is this true, or is the table wrong / misleading? ...
11
votes
2answers
820 views

Using same keypair for Diffie-Hellman and signing

Are there any security risks using a single key-pair for both key-exchange and signing? I'm mainly interested in using Curve25519 for key-exchange and Ed25519 for signing. But similar combinations, ...
11
votes
2answers
703 views

Random oracle model proofs and programmability

Proving the security of a scheme with the random oracle model (ROM) involves two steps: first you prove that the scheme is secure in an idealized world where a random oracle exists, and then you ...
11
votes
2answers
2k views

Why is MixColumns omitted from the last round of AES?

All rounds of AES (and Rijndael) have a MixColumns step, save the last round which omits it. DES has a similar feature where the last round differs slightly. The rationale, if I recall correctly, ...
11
votes
4answers
590 views

Could one construct a cipher that is secure for friendly parties to use but insecure for hostile parties?

Consider the situation of a nation state (Blue) at war with another nation state (Red). Blue wants to deploy a secure cipher that blue currently can not break, but they are considered that Red could ...
11
votes
1answer
2k views

What is a white-box implementation of a cryptographic algorithm?

What is a white-box implementation? Does a white-box implementation have specific properties?
11
votes
2answers
350 views

Why use a 1-2 Oblivious Transfer instead of a 1 out of n Oblivious Transfer?

When initiating an oblivious transfer, why would someone use a 1-2 oblivious transfer rather than going for an 1 out of n oblivious transfer? Perhaps a slight time overhead for the extra message ...
11
votes
1answer
5k views

What is the difference between PKCS#5 padding and PKCS#7 padding

One runtime platform provides an API that supplies PKCS#5 padding for block cipher modes such as ECB and CBC. These modes have been defined for the triple DES, AES and Blowfish block ciphers. The ...
11
votes
4answers
286 views

Do parts of a hash carry the properties of the entire hash?

When I need to generate unique id's based on some information hashing is typical choice. However, sometimes that id needs to be of a particular size. I've seen a lot of schemes (HMAC-MD5-96 in SSH, ...
11
votes
3answers
1k views

How well does scrypt perform on different architectures / OSes?

The scrypt algorithm seems to be a prominent feature in the "CPU friendly" Bitcoin clones for the proof-of-labor part. I've heard claims that it's relatively slow on Windows and/or Intel compared to ...
11
votes
1answer
483 views

Why did NIST remove The Lempel-Ziv Compression test from the Statistical Test Suite?

NIST removed "The Lempel-Ziv Compression" test from the Statistical Test Suite in revision 2008 and above and has not incorporated it since – see revision 2010. Why was it removed? Does it no longer ...
11
votes
1answer
1k views

Technical details of attack on Android bitcoin usage of SecureRandom

Reports are surfacing that Android's Java SecureRandom class has issues and isn't totally secure. A specific example of how this issue translates to applications is bitcoin, where reports are stating ...
11
votes
1answer
385 views

Do recent announcements about solving the DLP in $GF(2^{6120})$ apply to schemes proposed for cryptographic use?

A recent paper by Göloğlu, Granger, McGuire, and Zumbrägel: Solving a 6120-bit DLP on a Desktop Computer seems to "demonstrate a practical DLP break in the finite field of $2^{6120}$ elements, using ...
11
votes
1answer
5k views

How to choose a padding mode with AES

Depending on the framework you are using, there are various padding modes that can be used with AES encryption. For example, with .NET we can choose PKCS7, ISO10126, ANSIX923, Zeros or None. I ...
11
votes
1answer
483 views

Security of N bit HMAC

Lets say that I am using 128 bit HMAC. How many operations are needed to find "non secure" message. Is birthday attack possible?

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