# Tagged Questions

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

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### Is there a null HMAC?

I came up with this question while unit testing a function that includes an HMAC. What is the expected behavior of an HMAC with a well-defined key and a set of data with no elements in it? Is there ...
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### Deriving a key for HMAC, using PBKDF2

What is the recommended way to derive a key for an HMAC-SHA256, from a passphrase and salt? I'd like to compute an HMAC-SHA256. I have a passphrase, and a message. I note that RFC-7518 (JWA) ...
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### NIST HMAC Test vectors

I am searching test vectors of the HMAC-SHA256 primitive to verify the correctness of an implementation. Without hesitation I go on the NIST website to see what they offer. While reading the ...
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### Is there any code implementation of the HMAC-DRBG PRNG? [closed]

I'm looking for the code of HMAC-DRBG in either C# or C++, for use in a cryptography related project. I can't find it anywhere. Would appreciate the help! Thanks a ton!
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### Side channel security of HMAC in software

Assume we have a processor with a symmetric co-processor that supports AES. This processor does however not provide co-processor support for hash algorithms such as SHA-1 or SHA-256. I've got the ...
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For a personal project, I'm building a password manager, based off the project in this syllabus from Stanford http://crypto.stanford.edu/~dabo/cs255/hw_and_proj/proj1.pdf Since I don't exactly have ...
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### Difference between AES CMAC and AES HMAC?

Can someone elaborate on how 'signing' is done using AES- CMAC and AES-HMAC? HMAC Signing as I understand: Compute the HMAC( Hash the key and the input concatenated in a special way) Verification: ...
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### Could an HMAC be used as a hash?

MD5 is considered broken and SHA-1 is closely following, but HMACs built around either are still considered relatively secure. It makes me wonder if MD5 and SHA-1 HMACs can be used as secure hashes. ...
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### Is the security of an HMAC equally important as the security of the cipher itself?

I'm a little unclear on how hash based MAC works... So I would like to know, is the security of your hash function as important as the security of the overall cipher? Basically, if you can crack the ...
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### Keys and key-stretching when authenticating requests?

I'm making a REST application and still a bit confused about this issue. basically, to authenticate incoming requests, I give out userId, issued timestamp, expiry timestamp (normally 3 months after ...
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### Why does NaCl's crypto_auth have twice the security level of crypto_onetimeauth?

The crypto_auth function produces a 256-bit HMAC, with a 512-bit version also available. But the crypto_onetimeauth function ...
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### Would it be better to use HKDF or SCrypt for deriving a child key?

Okay so I'm trying to derive a child public key from an EC public key. I've come up with a couple ideas and now I'd like to know which is more secure. Mp: The master key pair, this includes a ...
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### What size should the HMAC key be with SHA-256?

I'm trying to generate a secret key to be used for HMAC SHA-256 signature processing. I've seen many sample of keys with variable length from 32 characters to 96 characters. What is the ironclad rule ...
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### PBEWithMD5AndDES Cipher does not check for integrity first?

I am running FindBugs with findsecbugs plugin to scan java code for security issues. I am getting "Cipher With No Integrity" error for a Cipher function implemented with "PBEWithMD5AndDES". The legend ...
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### Compromise between HMAC and Digital Signature, by encrypting and sending secret key?

For achieving stateless authentication tokens (like JWT), is there a compromise between the performance of HMAC, and public key distribution of Digital Signature? Scenario: Sender is a centralized ...
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### Is it okay to use an HMAC of the plaintext and a (possibly distinct) key as the IV for symmetric cryptography?

I was thinking of how to create an IV for a block cipher that doesn't require stored state, and I came up with the idea of using an HMAC of the (padded) plaintext and a (possibly distinct) key as the ...
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### Execution time for HMAC-MD5 on a 160 bit data using 128 bit key on a 3GHz CPU [closed]

I need to know the execution time for HMAC -MD5 for a 160 bit data using 128 bit key on a 3 Ghz CPU, kindly specify other parameters of the machine like RAM, operating system used etc
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### Comparison of Execution time for HMAC MD5 and a multliplication and pairing operations in G, on same CPU [closed]

I am a beginner in cryptography,One of the module in my project is to make a comparison, between the execution time taken by an HMAC- MD5 operation on 160 bit data, using a 128 bit key and sum of ...
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### Using HMAC as a nonce with AES-CTR encrypt-and-MAC

Suppose I use the following encrypt-and-MAC construction: $E(k_1, k_2, n, m) = E_\text{AES256-CTR}(k_1, n, m) \| \text{HMAC-SHA256}(k_2, m)$, where: $k_1$ and $k_2$ are 256-bit keys $n$ ...
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### Should I authenticate memory data which was loaded from authenticated disk file?

Encrypted fields are loaded from disk file to process memory only after file's HMAC validation is successful. Data is still encrypted in memory and will be decrypted later, on demand, when it needs to ...
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### HMAC vs ECDSA for JWT

I will be implementing JSON web tokens into my website and have a question about implementing them. I have a choice of using two algorithms, HMAC-SHA256 and ECDSA-SHA256. I have used HMAC-SHA256 in ...
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### Is it an overkill to add external integrity check to the AES-GCM encoding?

There is a java library for android which implements integrity checking to predefined encrypting mode(which can be found here). Short class description from author page to avoid further questions ...
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### If a PRG is constructed by iterating a PRF, is it backtracking-resistant?

Say we construct a pseudo-random number generator from a pseudo-random function $f$ (using some constant key $k$ and some initial value $v_0$). We do this by feeding the output block of the PRF back ...
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### Is it possible to have self-authenticated message?

Consider a text with $n$ words which $m$ words of it can be replaced with an equivalent word. Therefore we have $2^m$ different texts with the same meaning. For example "You are {so/very} ...
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### Is a nonce considered 'secret' enough for HMAC?

We're implementing a mobile application which communicates with a web service, and on first install the user sends its deviceid to the web service (over TLS, unauthenticated at this point) - which ...
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### Why does HKDF use HMAC(salt, key) instead of HMAC(key, salt)?

I've been looking over the HKDF specs (RFC 5869), and something I noticed is that in the key and salt are reversed: ...
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### Why do cryptographic hashes need to be fixed length?

Why do cryptographic hashes need to have a fixed length output? I know that the shallow answer is that an output that varies by key size or file size can leak information somehow, leading to ...
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### Is it wise to use HOTP (raw bytes) as a key for HMAC?

Assume I have client and server applications with a shared master secret KEY_SECRET already in place. Also assume that the client and server have an implicit shared ...
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### How to key an HMAC with keys from separate mutually untrusted parties?

The problem is as follows: There are two parties, both of whom would like to HMAC a message with a key. Although the keys are known to one another, both parties do not trust that the other one ...
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### Double randomised HMAC verification to prevent timing attack

A way to prevent timing attacks for hash string comparison is to perform additional HMAC signing in order to randomize the verification process (see Double HMAC Verification). In addition to the ...
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### HMAC-SHA256 on large data files

I currently need to encrypt large files (video, over 6 Gbytes) for our customers. To ensure authenticity and integrity, we chose HMAC-256 as we already use it in our internal messaging system. But ...
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### Encrypt large data with RSA through blocks in java

I'm working on a communication protocol where a client sends encrypted data to a server. What happens is that in some point, the client creates a symmetric key used to create data digests through the ...
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### HMAC key storage for a web app?

If one is going to use an HMAC, I.E: http://php.net/manual/en/function.hash-hmac.php, where exactly are they storing the key? Without some sort of secure storage (i.e not in a local file or code), I ...
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### For the scrypt HMAC storage format, why run the input of the hash and the hash itself into the HMAC

Generally if the body is hashed, why provide the body and the hash as input into the HMAC. Instead wouldn't the same security be achieved by hashing the body and providing only the hash as input into ...
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### Is it secure to use TOTP with same key and different time-counters?

If I use TOTP to generate codes with different "expiration" time, using the same key for HMAC, will I create some kind of weakness?
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### Is it safe to use single-block AES for mutual authentication?

I read the question Symmetric mutual authentication with client using a derived secret and its answer which, if I'm not mistaken, assert that it should be safe to use the proposed protocol for mutual ...
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### Is this scheme to derive multiple keys secure?

I need a cryptographic scheme to generate a deterministic chain of private keys that look random to outside observer but can always be reconstructed from a single root key, also: Neither private ...
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### Digital signatures using a partially public key

I'm thinking to sign a message using a key that is made up of a publicly shared key and a privately shared key. The operation for mixing the two keys could be concatenation, if it suffices. To ...
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### Avoiding replay attack on certified announcements

Suppose two agents A and B share a common secret and A makes a regular unique 16-byte ...
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### Is this a proper PBKDF2 key derivation function algorithm?

I've started implementing PBKDF2 algorithm recently and since I'm new in crypto, I would like to ask if my implementation is good. I looked up some documentations and I tried to follow many of them ...
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### Two rounds of SHA-256 HMAC?

In a project we are working on, we have two applications within our datacenter that exchange messages. The messages are protected by an HMAC, and the secret key is stored on a SafeNet device. Update ...
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### A periodic PRNG based on hash function?

I need a PRNG which provides 128 bytes numbers. Let's say that given a symmetric key $K$ and an internal state $s$ I use the PRF HMAC-SHA512 this way: The output is PRNG($K, s$) = ...
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### Is a 256 bit HMAC-SHA256 key compromised if the tail is used for identification?

Using a truncated hash of a key seems to be a safe way to identify the key (i.e. key fingerprint), but what about truncating a key itself in the case of HMAC-SHA256? Example: ...
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### Is calculating HMAC from hashed input a good idea?

Original HMAC algorithm is as follows: H(K XOR opad || H(K XOR ipad || text)) What if text would replaced with H(text)? ...
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### How does the HOTP dynamic truncation function generalize to longer hashes?

HOTP, the HMAC-based One-Time Password algorithm from RFC 4226, uses a "dynamic truncation" function to turn the 20 byte HMAC-SHA-1 value into a 31 bit string. The dynamic truncation (from Section ...
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### What are the implications of a birthday attack on a HMAC?

After collecting approximately $2^{n/2}$ message-tag pairs a collision can be observed. So two different messages (m1 and m2) will have the same tag. This paper states: Then, for any string x, ...
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### Chance to find HMAC key/salt when having knowledge of the hashed data?

I'm not sure how to word this... I'm working with a HMAC (I think of it being a "salted hash"). I know the entire string being hashed, I do NOT know the salt. I also know the first 8 characters of the ...
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### Keys in HMAC and NMAC

I'm quite new to this topic and have several questions concerning HMAC and NMAC: Why does NMAC need two keys? How can it be attacked if we just used some sort of initialization vector instead of the ...