Tagged Questions

HMAC is a method for constructing a message authentication code based on a cryptographic hash function.

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1answer
123 views

Why concatenate the key a second time in HMAC? [duplicate]

On Wikipedia it is said that HMAC is basically (without taking into account padding): $H(key\ \Vert\ H(key\ \Vert\ message))$ where $\Vert$ denotes concatenation. I understand the need to hash ...
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1answer
181 views

What is the best way to share a shared secret key over HTTPS to be used for HMAC authentication?

I am writing an application in Java that will be using SHA-1 HMAC message authentication. From what I understand, a connection over HTTPS is considered secure enough to share a secret key in plain ...
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Does “use of SHA-256 in cryptography” imply use of SHA-256 both in HMAC and DH key exchange, assuming these are the only uses of hash in system? [closed]

I am beginner to Cryptography. And recently I came across the requirement like "use of SHA-256 in cryptography". Does "use of SHA-256 in cryptography" imply use of SHA-256 both in HMAC and DH key ...
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0answers
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Why don't we use H(m||k) as a MAC? [duplicate]

HMAC seems a bit complicated. Why can't we use $H(m||k)$ as a MAC? Unlike $H(k||m)$, length extension attacks won't work. Is there some other obvious attack?
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1answer
334 views

GCM vs CTR+HMAC tradeoffs

So these days I see everyone using AES-GCM. What are its advantages over simple CTR+HMAC modes? Is it speed? Or ciphertext length? And what are the security tradeoffs, both in terms of practical ...
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1answer
166 views

Is it necessary to use single hash function for signature generation and HMAC?

I am trying to figure out if we need to use single hash function across all the places in a ssh session. i.e – signature generation/verification and HMAC calculation/validation. Is it possible to use ...
3
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2answers
281 views

Breaking a PRNG Scheme

Assume the following scheme for a PRNG generating decimals in [0,100) Given a message and a key, compute tag := HMAC512(message, key). Take the first 5 hexadecimal characters of the tag. If the ...
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1answer
72 views

Keyed-hash of a lot of small numbers

Let's say I have a list of several million numbers, with only 6-9 digits of secret information per number (this means that an attacker might efficiently guess the other digits). I have to store the ...
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1answer
283 views

Using HMAC-SHA256 with short passwords and sliced outputs for short-lived secure hashing

I don't have much background in cryptography, so forgive me if I'm using the wrong mathematical terms to explain my needs. Following this question, I learned that taking a random secret 16-bytes ...
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2answers
306 views

Can I use HMAC-SHA1 in counter mode to make a stream cipher?

Say I have an embedded device which is only capable of doing HMAC-SHA1 transformations (that device is, in fact, a YubiKey). Would it be secure to feed it like a block cipher in counter mode to get a ...
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1answer
343 views

How does the secret key in an HMAC prevent modification of the HMAC?

Just as a preface: I'm not implementing a HMAC - I just want to understand it, as it is part of of my Computer Science course. When using Hash-based message authentication codes, I understand that ...
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1answer
60 views

Using HMAC to secure a “widget”

Let's say I have a web "widget" that a customer could include on their website. They would include the widget by adding an iframe that points to a URL on my site. I want to ensure that my widget can ...
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2answers
255 views

Game traffic encryption AES CTR + HMAC

I was asked to implement traffic encryption between the game server and clients. Currently, the traffic is not encrypted in any way, which lets players use cheats to listen to the traffic in order to ...
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0answers
73 views

When do I need to renew keys for Salsa20 or HMAC?

I currently have a system which uses two keys sourced from a single key subjected to HKDF. One key is used in the Salsa20 stream ...
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0answers
187 views

Is there any analysis of freebsd's “geli” encrypted geometry provider?

I've done a few obvious searches on google and google scholar and not turned up any security analysis of FreeBSD's geli. Is there some out there, or has it gone unanalyzed since its introduction ...
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1answer
209 views

Why does WPA2-PSK use 4-way handshake instead of 2-way handshake

I don't seem to grasp why is the handshake for PTK in WPA2-PSK 4-way and not 2-way. Isn't it enough to just have these two steps: First party sends Nonce1 to the other party Second party sends to ...
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1answer
199 views

Is this new server API authentication protocol secure?

I need a secure protocol to authenticate client side users with my server which has an API. I am devising something more secure particularly to resist active MITM attacks where an attacker may modify ...
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1answer
1k views

What does AES256-CTS-HMAC-SHA1-96 mean in relation to Kerberos?

I'm not completely clueless as to what it means, but I'd like to understand it at a higher level. The highest encryption type used by Active Directory domain controllers for Kerberos authentication ...
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1answer
113 views

Encrypted query parameters via HMAC

I have an unsubscribe link, which needs to be passed a customer id, and a product id, producing a query string something like: ?cid=100&pid=2 I would like to ...
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2answers
882 views

HMAC vs Encrypted Hash

What is best to use (for message integrity) between… AES(Data) + HMAC(Data) AES(Data + HMAC(Data)) AES(Data + Hash(Data)) …and why?
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3answers
1k views

How to use HMAC-SHA1, HMAC-SHA2 in conjunction with AES

I understand how to compute HMAC-SHA1, HMAC-SHA2 etcetera. Please explain why/how is the resulting HMAC value is used in conjunction with AES for encryption/decryption and signing/verifying. In ...
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0answers
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How does use of HMAC affect hash function combiners?

In the comments of this question, Ricky Demer proposes a function $HMAX$ to combine two $HMAC$s based on different hash functions: $HMAX_{H0,H1}(⟨k_0,k_1⟩,m) = HMAC_{H0}(k_0,m) \oplus ...
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1answer
2k views

HMAC SHA256 vs RSA SHA256 - which one to use

Salesforce recently announced they are moving away from HMAC SHA256 to RSA SHA256. Why did they make that move?. Any technical factors? Is HMAC really easy to break? ...
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2answers
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Can you get two unpredictable hashes from the same message with, e.g. SHA1?

I'd like to create a database of questions and answers so that an attacker with access to the database can't obtain either the questions or the answers. If H0 and ...
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1answer
117 views

Is it possible to find a preimage for a reduced size hash?

Given a message $M$ and a 256 bit key $K$, perform HMAC($K$, $M$) with a 256 bit hash function resulting in: $a$ = ...
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1answer
227 views

HMAC construction based on the combination of two hash functions

I would like an $\operatorname{HMAC}$ that's based on two different hash functions ($H_1$ and $H_2$), so that a break of the combined $\operatorname{HMAC}$ would imply a break of ...
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5answers
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Using one-way hash functions as the encryption method

Suppose two parties want to communicate securely with each other (Bob and Alice) using a simple messaging system in English. There are approximately 180,000 currently used words in the English ...
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1answer
626 views

HMAC-SHA1 input size

I know that the HMAC is a message authentication code that uses a cryptographic key in conjunction with a hash function (SHA1 , MD5, etc.). The HMAC output is 160 bits for HMAC-SHA160 and 256 bits for ...
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2answers
382 views

Why are HOTP and TOTP implementations all using 6 digits and not more?

I was checking the RFC's after wondering for some time. In RFC 4226 - HOTP: An HMAC-Based One-Time Password Algorithm, it states on the appendix: A simple enhancement in terms of security would be ...
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1answer
229 views

Is this an acceptable authenticated encryption?

After much deliberation, I've decided to use a normal Encrypt-then-MAC scheme instead of an authenticated encryption mode such as GCM as authenticated encryption primitive. This is due to the lack of ...
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1answer
2k views

Is HMAC-MD5 considered secure?

I've read something to the effect that the HMAC construct is able to lessen the problem of collisions in the underlying hash. Does that mean that something like HMAC-MD5 still might be considered ...
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1answer
137 views

The probability of finding a limited set of collisions for HMACSHA256

I need some input on the use of HMAC-SHA256 in an open source project, any help appreciated. The problem I'm solving is session fixation. We have a username, for that username we'd like to generate a ...
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2answers
854 views

How does SafeNet MobilePASS generate passwords?

We use MobilePASS at work but the latest version of the android client seems to be buggy so I wanted to have a go at implementing the algorithm myself. You can download the client to play with here: ...
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1answer
155 views

Securely generating passwords based on a secret and a public identifier

I'm trying to create a good scheme for generating passwords in my systems management scripts. What I'd like to do is have a secret key (let's say hello123), which I ...
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2answers
248 views

Recommended authenticated stream cipher for minimum overhead?

I'm running my TCP/IP protocol secured by encrypting each packet with AES128/CBC, bundling a SHA256 HMAC over that packet. This causes quite a bit of space overhead for small packets, so at first I ...
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2answers
308 views

When using HMAC, does key encoding matter?

Given: H is a good hash function with block size L. K is a key of length >= L (recommended by RFC 2104). Khex and Kbase64 are ASCII encodings of K. In the HMAC algorithm, is there a good reason to ...
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2answers
225 views

HMAC and assumptions on the cryptographic hash

According to Wikipedia, a cryptographic hash function has the following properties: Pre-image resistance: Given $h$, it's difficult to find any message $m$ such that $h = H(m)$. Second pre-image ...
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2answers
215 views

Proving HMAC collision probability bounds?

Could someone point to results or proofs about the probability of $$HMAC(k, m_1) = HMAC (k, m_2)$$ assuming the underlying hash function is SHA-256? Would those probabilities be higher/lower if $m_1$ ...
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1answer
401 views

With HMAC, can an attacker recover the key, given many known plaintext/tag pairs?

Given many pairs of $(m, t)$, can the attacker compute the key $k$ satisfying $\text{HMAC}(k,m) = t$? (Assume that $k$ was chosen at random.)
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1answer
147 views

Is this algorithm secure?

I recently found this site, proposing a hashing algorithm for passwords. They describe the following: pad the password on both sides with SHA1(email) to ...
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1answer
295 views

Alternatives to HMAC + CBC?

I'm looking at using HMAC + CBC. The combination looks like this: ciphertext = AES256(text, k1) data = HMAC-SHA256(iv | ciphertext, k2) | iv | ciphertext Where: ...
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2answers
132 views

Does knowledge of original file size provide a cryptographic weakness?

I'm encrypting files using an AES-256 CBC cipher and an SHA-256 HMAC every 64KB in the file. Since AES CBC needs an amount of input bytes to be divisible by 16, I use PKCS#7 padding to bring the ...
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2answers
320 views

Is it worth applying a MAC on data in a HSM?

I'm building a basic HSM out of an Arduino, and am using the following scheme to store data: Master symmetric key $k_m$ stored in firmware (secure bit set to prevent trivial extraction). Secondary ...
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1answer
1k views

SHA256 HMAC brute force with chosen plaintext attacks

This is a follow up to Is It Possible To Reconstruct a Cryptographic Hash's Key I am using a SHA-256 HMAC function on a single-word input: sha256hmac(privatekey,word) = output. The private key length ...
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2answers
2k 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 ...
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2answers
203 views

How secure is passing a MAC to Python's random.seed before using random.choice to generate a MAC?

I'd like to use Python's random.choice seeded with a HMAC-SHA1 tag to generate a MAC encoded in a variable set of chars. ...
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2answers
3k views

AES+CTR+HMAC Encryption and Authentication on an Arduino

In my project we would like to encrypt and authenticate the the communication channel between our server and our Arduino nodes, which relies on an underlying TCP channel. We have chosen AES in CTR ...
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2answers
298 views

Case insensitive verification of HMAC / Base64 signature

I am signing a short message using the base64 of an HMAC, like this (python): ...
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2answers
551 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 ...
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1answer
293 views

Single-purpose symmetric encryption scheme for single files

I'm writing a simple password manager program that will encrypt/decrypt a single file (it's size will most likely stay under a few K). This is my initial file format design: ...