4
votes
3answers
175 views

Are there any differences between PIR, oblivious transfer and differential privacy?

I am trying to make a taxonomy of different purposes of some cryptographic protocols. Generally speaking the purpose of PIR, oblivious transfer and differential privacy it sounds like being invented ...
22
votes
3answers
1k 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 ...
1
vote
1answer
482 views

OpenPGP Public-Key Encrypted Session Key Packet Key ID generation

Im probably just not reading something again, but: RFC 4880 says that a OpenPGP Public-Key Encrypted Session Key Packet (tag 1) is made up of ...
7
votes
2answers
432 views

Toy cipher — does it have a name?

When I was perhaps nine years, I borrowed a book from the library on various maths and CS topics. It outlined various simple ciphers, including one that I used a lot, just for fun. I can't remember ...
7
votes
1answer
6k views

HMAC vs MAC functions

I've read definitions of MAC and HMAC, but can't say I've completely grasped the differences. What are principle differences? When to use one and when the other?(Typical Use Cases)
2
votes
1answer
242 views

Regarding matsui's Paper on Linear Cryptanalysis of DES

I's going throught the the Paper by Matsui on Linear Cryptanalysis of DES . In that he says $NS_{5}(16,15)=12$ And then in the next paragraph he says considering the expansion and permutation phases ...
1
vote
3answers
611 views

Is it possible to use the RSA algorithm, or a variant, for software licensing?

I'm almost a newbie about RSA encryption, so my question could it be banal. I'm beginning to create a licensing system for my software. I need to distribute a crypted license file to many customers. I ...
5
votes
0answers
421 views

How can I find two strings $m_1$ and $m_2$, knowing that I know $m_1 \oplus m_2$? [duplicate]

Possible Duplicate: How does one attack a two-time pad (i.e. one time pad with key reuse)? I recently started to follow the cryptography class of Dan Boneh on coursera.org and the first ...
6
votes
2answers
1k views

Why is ciphertext from low entropy plaintext not compressible?

This comes following a discussion with a colleague. My plaintext file plain consists of a about 100,000 lines of "all work and no play...". It's size is: 2.2 MB. ...
4
votes
2answers
172 views

How can the Schmidt–Samoa cryptosystem uniquely decrypt large messages?

Suppose I choose $p=7$ and $q=11$. This gives a public key of $p^2·q = 539$. However, decryption occurs using a modulus of $p·q=77$. If a person chooses to encrypt $500$ using my public key, how ...
5
votes
2answers
320 views

Tunnels used in md5

I'm reading the paper "Message Freedom in MD4 and MD5 Collisions: Application to APOP" about finding collisions for the MD5 hash algorithm involving the concept of tunnels as described in "Tunnels in ...
10
votes
1answer
756 views

GPG/PGP Verification of Revocation

How does one verify a key revocation? After revoking a key and sending the revocation to MIT's keyserver, I noticed that the key is listed as such: ...
3
votes
3answers
447 views

Can I pre-define the points in Shamir's Secret Sharing algorithm

With Shamir's secret sharing is it possible to pre-define the points returned by the algorithm? For simplicity if I have (k, n) where k=2, and n=4, and I have the points X,Y,Z, and Q. Can I create ...
3
votes
2answers
2k views

How do digital certificates work, and why is it not possible to reverse engineer one from a signed file?

Digital certificates are used quite commonly these days, for signing files. They are used by various operating systems to ensure reliability and security. For example, Android requires that each of ...
3
votes
1answer
224 views

Where can i find relative software for differential cryptanalysis?

Is there any publicly known software for differential cryptanalysis over some cryptographic building blocks as blocks ciphers or cryptographic hash functions
6
votes
4answers
7k views

Difference between encrypting something and hashing something

What is the difference between encrypting something and hashing something? in what situations would I want one or the other?
2
votes
2answers
2k views

RSA Proof of Correctness

Can anyone provide an extended (and well explained) proof of correctness of the RSA Algorithm? And why is it needed? I can't say that this or this helped me much, I'd like a more detailed and newbie ...
10
votes
2answers
725 views

Why would anyone use an elliptic curve with a cofactor > 1?

In cryptography, an elliptic curve is a group based on a finite field $GF(p^k)$; this group has $n$ elements on it, and we work on a prime-sized subgroup of size $q$. We denote the value $h = n/q$ as ...
2
votes
1answer
137 views

Separate Read and Write Keys in TLS Key Material

Why does the TLS protocol use different symmetric keys for receiving and sending data? Isn't it enough to have a single key used for both reading and writing?
1
vote
1answer
79 views

Incorporating known ciphertext into new message

Which ciphers $c_k$ enable one to define a joining function $f_{c,k}$ such that: $$ f_{c,k}(c_k(p_1), p_2) = c_k(p_1 + p_2) $$ That is, the result of joining some known ciphertext $c_k(p_1)$ with a ...
1
vote
3answers
211 views

Looking for cipher that uses one ciphertext

I'm new to Cryptography so please free to edit my question for clarity and add the appropriate tag. I'm trying to research if the following is possible. Suppose John has a message that he ...
9
votes
2answers
9k views

What's the fundamental difference between Diffie-Hellman and RSA?

What is the difference in the purpose of DH and RSA? Aren't they both public-key encryption?
1
vote
1answer
412 views

How to represent a 32-byte SHA2 hash in the shortest possible string?

I'm calculating a SHA2 hash of a certain sensitive key value. I need to store files on disk using this hash a directory path prefix. So lets say I hash the key value 150023, I get a 32-byte value ...
4
votes
3answers
1k views

Why does CBC decryption with a wrong IV still give readable results?

While developing some code that uses the .NET AesManaged algorithm, I made some mistakes but was surprised at the results. My encryption was correct. I was generating a random IV block and writing ...
4
votes
3answers
595 views

What algorithm would give the shortest ciphertext for very short plaintexts?

I have to encrypt some very short messages (approx. 5 characters) and I would like the ciphertext to be as short as possible. I am currently using AES but the initialization vector (IV) is making my ...
3
votes
1answer
241 views

Does ShaCrypt have the properties which make BCrypt slow on GPUs?

Sha256Crypt / Sha512Crypt are designed based on ideas of BCrypt but use SHA instead of Blowfish because of political requirements. The Sha*Crypt algorithms have the advantage that they are used in ...
3
votes
2answers
170 views

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 ...
2
votes
3answers
487 views

Encryption vs. decryption of same plaintext

Does the same encryption algorithm given the same plaintext always produces the same ciphertext? Does this cause any security flaws?
4
votes
3answers
1k views

Hash function in PBKDF2

From this excellent answer I learned (correct me if I am wrong) that when writing a block cipher with say key size 128 bit, one has to pad the password given (variable size) so that it becomes exactly ...
10
votes
4answers
8k views

What is the difference between known-plaintext attack and chosen-plaintext attack?

I am very confused between the concept of known-plaintext attack and chosen-plaintext attack. It seems to me that these two are the same thing, but it definitely is not. Can anyone explain to me how ...
2
votes
1answer
197 views

Transparent cipher

I'm trying to implement this protocol: Alice has her permanent key $K_1$. She computes $E(K_1, P)$. She wants to share $P$ with Bob, Carol and Dave. For each of them she: 2.1. Generates $K_2$. ...
3
votes
1answer
709 views

Okay to use OpenSSL to encrypt then sign a message?

I am planning on encrypting a message then signing the message so the recipient can authenticate the source. Reading this article has me concerned that I'm doing something wrong. My current plan is ...
4
votes
2answers
554 views

How did LinkedIn “salt” all their passwords?

First, just to make sure I understand "salting" correctly: You randomly generate a string to append to the password before hashing it, so as to increase its length and make precomputed tables much ...
2
votes
1answer
330 views

Padding for the TEA

Sorry if this is a noob questions, but I finally figured out how to implement the Tiny Encryption Algorithm in C++. My question is what to do about padding the key and the plaintext? I know that ...
5
votes
2answers
238 views

Does a hash function in chaining mode or in counter mode make a better pseudo-random number generator?

I have two pseudorandom generators: $f_1$ takes a random seed $l_0||r_0$ $\in \{0,1\}^{160}$ as input and outputs $r_1||r_2|| \dots ||r_k$, where $l_i||r_i = \operatorname{SHA-1}(l_{i-1}||r_{i-1})$ ...
5
votes
1answer
351 views

What's the reason for applying the hash twice when hashing with salt?

One of the typical approaches to computing a salted hash is this: hash(salt+hash(secret)) where hash is something like SHA-256 hash function - taking any size ...
2
votes
2answers
175 views

How hard is to find the operators of an addition knowing the sum of them?

I want to learn whether or no there is a cryptographic primitive,scheme assumption that is based on the following hard problem if it is hard . By hard we mean that we have a polynomial adversary: The ...
1
vote
2answers
142 views

Idea for secure password hash-likes, is this feasible/usable/workable?

(Hand moved from stackoverflow.com,due to suggestion) Since breaking password hashes has become a new passtime for scriptkiddies, I thought of the problem and came up with a novel(?) idea. store ...
3
votes
1answer
195 views

Attack vectors introduced by compilers

I have a question about attacks on the implementation of cryptographic code that are enabled by compilation and compiler-optimisations. I am aware of this. Would anyone be able to point me to other ...
4
votes
1answer
358 views

How secure is my OTP program?

I'm writing an One-Time Pad encryption program, because I got really interested in the idea of " encryption which has been proven to be impossible to crack if used correctly". I'm writing the program ...
8
votes
2answers
2k views

Why do block ciphers need a non-linear component (like an S-box)?

Why is there a requirement of "Non-Linear functions" as a component of many popular block ciphers (e.g. the S-box in DES or 3DES)? How does it make the cipher more secure? The only intuition I have ...
2
votes
1answer
131 views

In S/MIME, are the same certificates always used to sign and encrypt messages?

My assumption is that S/MIME almost always utilizes certificates as follows: My certificate can be used to allow people to encrypt messages and send them to me. My certificate (the same ...
4
votes
1answer
154 views

How does DJB's nistp224 manage to fit compressed points into 224 bits?

DJB's nistp224 program purports to be an implementation of elliptic curve Diffie-Hellman relative to the standard NIST P-224 elliptic curve. To the best of my ...
-1
votes
1answer
397 views

File Encryption/Decryption in ECB mode [duplicate]

Possible Duplicate: Removing Padded Value in Decrypted Message I tried to encrypt and decrypt a file in ECB mode using AES Algorithm but I encountered one problem during decryption, I don't ...
2
votes
0answers
50 views

SHA-512 hash collisions [duplicate]

Possible Duplicate: Is SHA-512 bijective when hashing a single 512-bit block? Is there any guarantee that there are no collisions for all 512-bit input values for SHA-512 (every 512-bit ...
1
vote
1answer
2k views

Removing Padded Value in Decrypted Message

How to remove padded value in Decrypted Message? I am using AES Algorithm. Let's take this as the decrypted message: "abcdefghijklmn " There is a 2 space in ...
0
votes
2answers
186 views

Do these 2 KSA shortcuts weaken RC4?

Alice securely gives Bob a secret key so they can exchange 10 different messages encrypted with RC4. (Messages will include a counter for uniqueness and a MAC for authentication.) For each message ...
4
votes
1answer
142 views

Proof of work scheme with one and only one correct answer

I need a problem that has one and only one solution, such that the solution is hard to find but easy to verify. I need to be able to generate the problem deterministically from a random seed. So ...
1
vote
1answer
1k views

How can 3DES encryption use decryption in the middle step?

I have been reading about the 3DES algorithm and I cannot understand one part. In 3DES we do following operation: $$C=E[K_3,D[K_2,E[K_1,P]]].$$ Where $C$ is ciphertext, $P$ is plaintext, $E$ is DES ...
1
vote
2answers
114 views

How can I protect against the failure of a block or symmetric cipher?

Can I protect against the failure of a block or symmetric cipher by chaining different techniques together? If so what implementation details should I be aware of? Are some combination of ciphers ...

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