A cryptographic hash algorithm is a function which takes a variable size input and produces a fixed size output. The algorithm makes it difficult to predict the output for a given input, find two inputs with the same output, or reconstruct the input from the output.

learn more… | top users | synonyms (1)

3
votes
1answer
251 views

Password hashing in /etc/shadow

Passwords hashed under Unix/Linux systems can use several hashing algorithms. ...
3
votes
2answers
1k views

Difference between salted hash and keyed hashing?

A cryptographic salt is additional input other than message itself for a hash function so that it prevents attacker from launching dictionary attacks . Usually the salt is stored along with the hash ...
3
votes
3answers
245 views

Allowing the user to choose the hashing formula at the registration

I was thinking of a potential new way to increase someone's account security. The idea was allowing the user to choose one of the hash formulas available in the software (example: "MD5", "SHA512" and ...
3
votes
3answers
163 views

Hash Family Example

I am reading my lecture and I found this definition: Let be $F$ be a family of hash functions, that is, a parameterized set $$F=\{g_k:\{0,1\}^{*}\rightarrow\{0,1\}^{n}|k\in K\},$$ where $n\in ...
3
votes
2answers
745 views

RSA digital signature vs authenticated cipher

I want to provide both confidentiality and integrity for data at rest (many large files stored on disk). I plan to encrypt the data using AES, which will cover the confidentiality requirement. So ...
3
votes
2answers
474 views

How does using salt reduce rainbow table attack?

I could create a rainbow table for a particular salt value and still create a successful attack similar to one without salt value.
3
votes
2answers
160 views

Is there an advantage to storing keys split between several hashes?

I have a question about the way to store a key or password that was used for encryption, so that the application can check if the user put in the right key for decryption. If I make a mistake, please ...
3
votes
2answers
522 views

Looking for examples for “proof by reduction”

Im looking for examples for a proof by reduction. For example: Let $A=(Gen, H)$ be a hash function. We define a new Hash function $A'=(Gen',H')$ with Gen=Gen' $H'_s(x)=H_s(H_s(x))$ It should be ...
3
votes
1answer
1k views

“Signing” with public key

For this question, the following caveats and assumptions hold: There exists a 2048-bit RSA key pair used exclusively for signing/verification The private key is kept completely private There exists ...
3
votes
1answer
214 views

A set of key pairs and one hash to secure them

I have a simple problem: I have a set of users' ECDSA key pairs, and say I want to encrypt them with a simple algorithm. I have access to one variable that uniquely identifies the user, so I hash it ...
3
votes
1answer
82 views

What is the “compression function” in Merkle-Damgård?

Is the "compression function" in Merkle-Damgård just a collision-resistant, one-way hash function but one that operates only on fixed size inputs? If so, is MD just a way to extend it to work on ...
3
votes
1answer
173 views

Where did the SHAKEs come from in SHA3?

Where did SHAKE128 and SHAKE256 originate from? I am trying to find them in the original Keccak documentation but can't find them. Is it some special mode of Keccak referenced in the documentation? ...
3
votes
2answers
701 views

Reversing DJB2 Hashes

I have some spare time, and a few hundred DJB2-hashed values sitting around. I thought I'd try to do something "useful" and invert DJB2, such that I could calculate the plaintext of the hashes (which ...
3
votes
1answer
190 views

KDF followed by a Hash?

It is my understanding that a KDF adds entropy, whereas a hash loses information. I've read that KDF should be used to store passwords. I don't understand why we don't use a KDF and then a hash, so ...
3
votes
1answer
267 views

Should HMAC or NMAC or plain Keccak be used for a secure MAC?

After some digging around in the HMAC spec I found this (paraphrased): Step 1: If the length of Key equals the block size of the hash function (512 bits/64 Bytes for SHA-256), set the key equal ...
3
votes
3answers
496 views

What are the pros/cons of using symmetric crypto vs. hash in a commitment scheme?

In a commitment scheme, are there any differences on using a symmetric cipher versus using a hash? If at the "opening", I have to reveal $r$ (a random number concatenated with the messaged at the ...
3
votes
1answer
90 views

Hashing a message with a length that isn't a multiple of 8 bits

NIST FIPS 180-4. “5.1. Padding the Message.” Suppose that the length of the message, $M$, is $l$ bits. Is there any library that can receive a message of length (in bits) which is not a multiple ...
3
votes
2answers
1k views

Why does second pre-image resistance imply pre-image resistance

I am studying hash functions. I can understand why collision resistance implies second preimage resistance, but I don't get why second preimage resistance should imply first preimage resistance. ...
3
votes
1answer
215 views

How to use salt if I am sending hashed password?

If have application that is sending username and hashed password to server for authentication, how would I use salt for this scenario ?
3
votes
1answer
120 views

Hashing a password with the password?

I was wondering if hashing a password with the password would be a good way of encrypting the password. So, the user must know his/her password to get the same result as the one in the database. Also, ...
3
votes
1answer
172 views

DIfferent inner and outer hash functions for NIST Recommended HMAC?

The NIST recommended HMAC uses $$\operatorname{HMAC}_k(text) = H_\mathrm{out}( (k \oplus \mathrm{opad}) \operatorname\| H_\mathrm{in}((k \oplus \mathrm{ipad}) \operatorname\| text) )$$ Is it ...
3
votes
1answer
233 views

Is there a known vulnerability when using identical key and salt with PBKDF2?

For optimum security, a random salt should be chosen for PBKDF2. I came across a scenario where I produce a hash of a random file that serves as the "password". Due to the nature of files, this has ...
3
votes
1answer
346 views

What type of hash functions provides non-malleability of hash digests?

I want to use a hash function for commitments. I don't want an attacker to construct a commitment related to a previously published (but still unopened) commitment. A simple deterministic commitment ...
3
votes
1answer
75 views

Save the last cycle in GCM GHASH calculation

I need to implement GCM GHASH in hardware, using FPGA. The data bus is 640 bits, so I will use 5 adder/multiplier blocks in parallel. The message size is fixed and AAD is 0. A little example: ...
3
votes
2answers
114 views

Memory-expensive hash from an array of hashes?

Why can't people make a hash function that requires dozens of mega bytes of memory instead of cpu to avoid all the cracking and hacking? What I have in mind myself is this: We have an input $X$, we ...
3
votes
1answer
181 views

Is appending the hash of the plaintext to the end of an encrypted message sufficient to ensure integrity?

Is appending the hash of the plaintext to the end of an encrypted message sufficient to ensure integrity? My reasoning is that the attacker doesn't know all the plaintext, so by having a hash, the ...
3
votes
1answer
365 views

Using GMAC for Authentication without encrypting the message

I am pretty new to cryptography and have a sample of GCM mode of operation for AES. PFB the code ...
3
votes
1answer
103 views

Is there a practical upper limit for password rehashing?

When using PBKDF2, is there a practical upper limit to the iteration count above which we lose security? Note: If you answer “No”, that's fine. But if you answer: “There can't be an upper limit“, ...
3
votes
1answer
183 views

Does adding a salt to the message before HMAC processing add any security?

An HMAC function is designed to verify message integrity, i.e., that the message has not been tampered with. It is generally (always?) implemented by using a hashing function ...
3
votes
1answer
260 views

Is SHA-256 safe when used in this way?

I am wondering whether an attacker can gain any useful information from having several hashes of an unknown plaintext that differs in relatively few bits. For example, if provided with: ...
3
votes
1answer
194 views

Increased security weaknesses when truncating SHA1

Given the following preconditions: A plaintext A (~512bit length), that is hashed by using SHA-1 and truncated to (the first) 64 bits. I know both, the plaintext and the truncated hash, and want ...
3
votes
1answer
359 views

File Hash of Encrypted File

Simple question. I have a source file and I have encrypted it with a certain public key. If I take this exact same source file, encrypt it with the exact same key a few hours later would the file hash ...
3
votes
2answers
148 views

Feedback requested on a method of posting a message without revealing the author

So I was thinking about variations on the Dining Cryptographers problem - In some cases, it's useful to be able to post a message without revealing the source, but with the additional constraint of ...
3
votes
3answers
237 views

Stream cipher with Diffie-Hellman and Cryptographic Hash Function? [duplicate]

I know: Never do your own crypto. But this question is just theoretical. Assume you do a Diffie-Hellman Key Exchange with a server to produce a shared secret $x$. Then use a cryptographic hash ...
3
votes
2answers
152 views

How to compare two datasets „anonymously”?

Ok, I hope this question makes some sense because I am not so sure how to word it any differently… Imagine the following situation: There are 10 defined colors (blue, orange, yellow etc.) There are ...
3
votes
1answer
865 views

Why are the Davies-Meyer and Miyaguchi-Preneel constructions secure?

The Davies-Meyer compression function $h(H, m) = E_m(H) \oplus H$ is said to be secure. So too is the Miyaguchi-Preneel compression function $h(H, m) = E_m(H) \oplus m \oplus H$. Why are these ...
3
votes
1answer
768 views

What happens to the entropy of a password when you hash it?

For example, if the entropy of a password is 30 bits, what is the entropy of the password when you hash it with MD5?
3
votes
1answer
168 views

Is there a format preserving cryptographically secure hash?

I want a way to 'scramble' strings (like words, names) in a predictable way but I don't ever want to (myself or anyone else) be able to 'unscramble' the strings. I found that perhaps what I am ...
3
votes
2answers
85 views

Security concern about reducing hash value using modulo operation

As stated in the title, what I am looking for is information about a "technique" that I would like to use in some of my algorithms. Sometimes I need to map a hash function's result into a range of ...
3
votes
1answer
112 views

Is this an acceptable All-or-Nothing Transform?

I was thinking about AONTs, and designed the one below, I call it CHANT for Chained-Hash All-or-Nothing Transform; it's my very first shot at something of the sort, and was hoping I could get your ...
3
votes
1answer
495 views

Cryptographic hash function for 32-bit length input keys

Since I'm unfamiliar with cryptography area I haven't been able to find cryptographic hash functions for 32 bit integer keys. It seems all such functions (as I saw here) are designed to digest a ...
3
votes
1answer
115 views

Key-dependent encryption in TAHOE-LAFS

TAHOE-LAFS uses a form of "key-dependent encryption", in that a private key SK is encrypted with the truncated hash of the private key: ...
3
votes
1answer
111 views

Combining two hashing functions

I have been working on a Bloom Filter implementation recently and after a discussion with a co-worker about how many hashing functions to use I told him I was limited with using hashing functions that ...
3
votes
1answer
233 views

Is there a cryptographic hash function that can be performed with pencil and paper?

Imagine I'm signing up for the 99th new web site this month. I somehow take my secret key (which I have written down on a card in my wallet) and the domain name of the site and feed them both into ...
3
votes
2answers
225 views

Parallel-resistant proof-of-work scheme?

I am looking for a proof-of-work scheme which cannot be effectively parallelized. For example, in hashcash (and by extension bitcoin) you have some collision-resistant hash function $f()$, a target ...
3
votes
4answers
317 views

Is it possible to anonymize web traffic so that the IP Address cannot be determined while still being able to determine distinct IPs?

I'd like to find a way to anonymize IP addresses in web logs to ensure user privacy given the following requirements: If given an IP address, there is no way to look up which requests came from the ...
3
votes
2answers
480 views

What does “message schedule” mean in SHA-256?

I am trying to understand the sha-256 algorithm from FIPS 180-2. I understood the padding and parsing of the message string. However after that it states (page 15): For $i = 1$ to $N$: { ...
3
votes
2answers
355 views

Order of hashing concatenation

A = hash("blue" + X); B = hash("pink" + X); If A and the literals are known and X is unknown, are there attacks on B aside from the attacks on directly on hash()? ...
3
votes
2answers
153 views

Can a secret message be securely transmitted within a MAC tag?

A sender wants to transmit an ultra secret code $M$ which could be either 'go', 'stop' or 'wait'. This could be any selection of code words really and adopted for any use such as transmitting short ...
3
votes
1answer
109 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 ...