Analysis of individual security aspects of a cipher or algorithm, not the security of a cipher or algorithm in general (which would lean towards “algorithm-design”).

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How does one attack a two-time pad (i.e. one time pad with key reuse)?

My question might appear the same as the question Taking advantage of one-time pad key reuse?, but actually I did read all the answers and none of them helped me with the details I need. I am new to ...
48
votes
7answers
36k views

Taking advantage of one-time pad key reuse?

Suppose Alice wants to send encryptions (under a one-time pad) of $m_1$ and $m_2$ to Bob over a public channel. Alice and Bob have a shared key $k$; however, both messages are the same length as the ...
21
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6answers
2k views

Why should I make my cipher public?

As I understand it, the less people know about the internals of my protocol or cipher, the more secure the protocol is. However Kerckhoffs's principle states that A cryptosystem should be secure ...
19
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2answers
10k views

Definition of Textbook RSA

What is the definition of Textbook RSA? What are some of the properties of textbook RSA? How does it differ from other RSAs?
13
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2answers
1k views

Attacks of the MAC construction $\mathcal{H}(m||k)$ for common hashes $\mathcal{H}$?

Consider a common practically-collision-resistant hash function $\mathcal{H}$ (e.g. SHA-1, SHA-256, SHA-512, RIPEMD-160), perhaps based on the Merkle–Damgård construction as are the first three. We ...
18
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1answer
6k views

Security strength of RSA in relation with the modulus size

NIST SP 800-57 §5.6.1 p.62–64 specifies a correspondence between RSA modulus size $n$ and expected security strength $s$ in bits: ...
15
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3answers
1k 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 ...
11
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2answers
5k 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: ...
10
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3answers
37k views

Possible ways to crack simple substitution ciphers

We had a quiz in class today where we had to break the ciphertext with the key given, but not the algorithm. Suffice to say that I wasn't able to decrypt it within the allotted time of 12 mins and ...
12
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1answer
2k views

Duration for attacking Two-Key Triple-DES Encryption using all RAM ever built?

I am considering attacks on Two-Key Triple-DES Encryption assuming $2^{32}$ known plaintext/ciphertext pairs (that's a mere 32MiB of ciphertext) by the method devised by Paul C. van Oorschot and ...
34
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10answers
8k views

Now that quantum computers have been out for a while, has RSA been cracked?

D-wave systems has released a commercially viable quantum computer. This means in theory, that all asymmetric encryption algorithms — such as RSA — are now useless due to the speed at which quantum ...
12
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4answers
2k views

Is compressing data prior to encryption necessary to reduce plaintext redundancy?

As explained in William Stallings' Book, in PGP encryption is done after compression, since it reduces redundancy. I couldn't relate encryption strength with redundancy. Could anyone explain more on ...
7
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5answers
931 views

How to prove the security of the PRNG?

Are there any realties tests or criterias that prove the security of the PRNG? What kind of tests or criteria?
19
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4answers
16k views

Recommended skills for a job in cryptology [closed]

First let me apologize if this is an ill posed question. Let me also note that I do not in any way seek a comprehensive answer, simply your thoughts on what makes for a valuable asset to a company ...
5
votes
3answers
1k views

A simple block cipher based on the SHA-256 hash function [duplicate]

I've come up with this little routine for doing encryption using the SHA-2 (in this case SHA-256) hash function. As such it is a block cipher with a 256 bit (32 byte) block size and an arbitrary key ...
51
votes
5answers
19k views

Is AES-256 weaker than 192 and 128 bit versions?

From a paper via Schneier on Security's Another AES Attack (emphasis mine): In the case of AES-128, there is no known attack which is faster than the 2128 complexity of exhaustive search. However, ...
7
votes
3answers
3k views

How to attack a classical cipher using known partial plaintext?

I have a ciphertext generated by a classical cipher. I do not know what was cipher used to generate it. I do however have the beginning of the plaintext. What are the cryptanalysis approaches for ...
5
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2answers
1k views

DES — Can I recover the key when I have both ciphertext and the plaintext?

Given a message and DES encrypted form of said message, is it possible to efficiently compute the key used to encrypt the data?
2
votes
1answer
168 views

Mathematical formula for switching the key for OTP?

Instead of generating the random key for the one time pad cipher over and over again, is there a mathematical formula that allows you to switch the key to a new key? The new key must be as random and ...
29
votes
5answers
6k views

How can we reason about the cryptographic capabilities of code-breaking agencies like the NSA or GCHQ?

I have read in Applied Cryptography that the NSA is the largest hardware buyer and the largest mathematician employer in the world. How can we reason about the symmetric ciphers cryptanalysis ...
7
votes
2answers
13k views

How much computing resource is required to brute-force RSA?

It's been over 30 years since Rivest, Shamir and Adleman first publicly described their algorithm for public-key cryptography; and the intelligence community is thought to have known about it for ...
27
votes
2answers
7k views

Why does nobody use (or break) the Camellia Cipher?

If Camellia is of equivalent security and speed to AES, concerns arise. First of all, assuming the above, why is Camellia so rarely used in practice? Why aren't there any breaks in Camellia? Does ...
15
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8answers
5k views

RSA with small exponents?

Just to establish notation with respect to the RSA protocol, let $n = pq$ be the product of two large primes and let $e$ and $d$ be the public and private exponents, respectively ($e$ is the inverse ...
14
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3answers
3k 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 ...
16
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4answers
2k views

Is Wiener's attack on RSA extendable to larger keys with low hamming weight?

Using small private exponents with RSA improves performance. However, it has been shown (Wiener, 1990) that if $\log d \leq \frac14 \log N$, the private exponent $d$ can be reconstructed from the ...
3
votes
2answers
1k views

Difficulty of breaking RSA for a given key size

Is it true that breaking a 1024-bit RSA key is as difficult as breaking a 128 bit symmetric key (e.g. AES)? I know that breaking a RSA key is equivalent to factoring the modulus $N$. To factor it, you ...
5
votes
2answers
888 views

Hill cipher, unknown letter value

I've been struggling on this problem for a while now : the Hill cipher is well-known to be vulnerable to known-plaintext attack due to its linearity. Given a key matrix $K$ of size $n\times n$, one ...
15
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3answers
2k 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?
4
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2answers
3k views

How can I do a brute force (ciphertext only) attack on an CBC-encrypted message?

Given a CBC ciphertext and IV, how can I find the encryption key? We are limited with an 8 chars key, each char in the range of [a..h], so I can generate every possible key (these are only $8^8 = ...
3
votes
1answer
378 views

How do you test the security of your cipher?

I got asked this question and I didn't know what to answer. How do you test the security of your cipher? What comes to my mind now would be to test it with famous attacks: padding attacks, ...
0
votes
2answers
443 views

Shamir Secret sharing - Can share generator keep x values secret?

I'm wondering, in Shamir secret sharing, can generator of the shares, keep the x values which are used in evaluating the polynomial to obtain y values (i.e., the shares) secret, and whenever the ...
2
votes
2answers
450 views

How does compression before encryption leak info about the input?

Apparently current best practices recommend that you do not compress before you encrypt. For example in this blog entry (*): http://sockpuppet.org/blog/2013/07/22/applied-practical-cryptography/ It ...
2
votes
2answers
4k views

How to get the keyword from a keyword cipher?

I was given a ciphertext and now I am trying to break it via looking for the keyword. This is a keyword cipher. So: PlainEnglish: ABCDEFGHIJKLMNOPQRSTUVWXYZ If ...
1
vote
1answer
345 views

Knowing pre-encryption data, can we find the private key CryptoWall used?

I'm just repurposing a question already asked about Cryptolocker for CryptoWall: "If we know exact contents of some of the files prior to them being encrypted, would it be feasible to use those ...
-1
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1answer
402 views

Hash Based Encryption (fast & simple), how well would this compare to AES? [duplicate]

First of all, I know it's a very bad idea to invent your own encryption algorithm. It's better to use existing known, trusted, extensively tested and studied algorithms with a proven track record. The ...
25
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3answers
2k views

Is this password migration strategy secure?

I want to upgrade the security of some existing databases of users' authentication tokens strictly for the purpose of making sure that if the database is stolen, attackers will not be able to guess ...
13
votes
1answer
4k views

repeating-key xor and hamming distance

I read that to break repeating-key xor you can do the following: try a keysize $n$ and compute the hamming distance between the first $n$ bits of the encrypted string and the bits $n+1$ to $2n$ of the ...
12
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3answers
807 views

What is the best way to put a backdoor in an encryption system?

How can you put a backdoor into an encryption algorithm? Are there any techniques that can be used to reduce the time it takes to break a key? I am looking for practical examples encryption schemes ...
24
votes
2answers
2k views

How do I apply differential cryptanalysis to a block cipher?

I have read a lot of summaries of block ciphers particularly with regards to the NIST competitions stating that reduced-round block ciphers are – for example – vulnerable to differential ...
10
votes
3answers
640 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, ...
7
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2answers
8k views

How does a chosen plaintext attack on RSA work?

How can one run a chosen plaintext attack on RSA? If I can send some plaintexts and get the ciphertexts, how can I find a relation between them which helps me to crack another ciphertext?
16
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3answers
13k views

How cryptographically secure was the original WW2 Enigma machine, from a modern viewpoint?

If cryptanalysts today were to crack the original Enigma machine, “how fast” or “how easily” could they do it? What methods would they use? The original cracking was significantly helped by operator ...
14
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3answers
2k views

Webcam random number generator

I have a question about random number generators. I have read from a real random number generator, based on a webcam ("randcam"). My problem is, that I do not really understand how the generation of ...
14
votes
4answers
16k views

What is the effect of the different AES key lengths?

How does a changing key length affects the ciphertext, not only in case of AES, but in general? I know that the key spaces become much larger and the number of rounds in case of AES changes, but is ...
10
votes
2answers
5k views

Can you explain “weak keys” for DES?

A weak key for DES is a key $K$ such that $DES_{k_1}(DES_{k_2}(x))=x$ for all $x$. I don't get why are the 4 keys $k_1||k_2$: $1^{112}$, $0^{112}$, $0^{56}||1^{56}$, $1^{56}||0^{56}$ considered as ...
11
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4answers
3k views

Any practical uses of machine learning for cryptography?

I am about to go study for my masters in machine learning, data mining and high performance computing, but have recently become very interested in cryptography after taking Dan Boneh's Cryptography ...
11
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2answers
785 views

Why is the salt used only once in PBKDF2, while the password is used often?

The purpose of PBKDF2 is to create a derived key (DK) from a master password (PW) and a salt, often using a function like HMAC-SHA256. I have read that the salt should be as random as possible. But ...
11
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2answers
3k views

Can two different pairs of RSA key have the same modulus?

Can $n=pq$ be part of two different pairs of RSA keys? If such keys exist, say $(e_1,n)$ and $(e_2,n)$, how are they related? What will be the security concerns for the two users?
15
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3answers
3k views

Is 80 bits of key size considered safe against brute force attacks?

I came across KATAN Family of Ciphers for small domain input blocks . They cipher arbitrary block lengths 32,48,64 but their key size 80 bits only. Is 80 bits of key size considered safe with ...
14
votes
1answer
560 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 ...