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Questions tagged [hkdf]

HKDF is a key derivation function specified in RFC5869.

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Can someone clarify two things about the HKDF by Krawczyk?

I got a question about the HKDF Scheme by Hugo Krawczyk. On the following link you can find a small explanation of the HKDF-Scheme and some short informations. The HKDF specification itself has been ...
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1answer
2k views

PBKDF vs HKDF for pretty long key

I'm developing a messenger application with encrypted chats. In the first version of the app I've used PBKDF2 (10000 iterations, SHA1, random salt) to extend a short user password and generate keys ...
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4answers
1k views

Which risks are associated with deriving multiple keys from the same DH secret Z?

NIST recommends Krawczyk's HMAC-based key derivation function (HKDF) in SP-800-56C (PDF). HKDF shall e.g. be used to create keys from shared secrets after Diffie Hellman key establishment. NIST ...
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1answer
994 views

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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1answer
429 views

repeated use of HKDF-extract on the same PRK

Regarding HKDF (RFC 5869) - HKDF has two steps, extract and expand. Some answers on this site suggest that you can extract once, and then generate multiple keys from the resulting PRK by multiple ...
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2k views

How is HKDF-Expand better than a simple hash?

Why do we need HKDF-Expand if we can simply hash the pseudorandom key to make it longer? The docs (in the link) say: The second stage "expands" the pseudorandom key to the desired length... Is ...
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1answer
376 views

multiple keys via HKDF - whats better, one or two applications of HKDF-extract

Assume for the sake of the question that I have two variable-length bit strings, each with 128 bit cryptographic randomness, and I want to extract two 128 bit keys via HKDF-SHA256. Which alternative ...
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1answer
504 views

Key collision in scrypt and hkdf

I am developing a mostly-offline authorization system that authorizes a user using an deterministically generated AuthKey derived from a MasterKey derived from a high-entropy chunk of data (128 bits) ...
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1answer
1k views

What information to include is the 'info' input for HKDF?

The RFC states the following: 3.2. The 'info' Input to HKDF While the 'info' value is optional in the definition of HKDF, it is often of great importance in applications. Its main objective ...
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1answer
890 views

Difference between RFC-5869 (HKDF) and SP800-108 (Nist's HMAC-based KDF spec)?

I'm trying to find the difference between the two aforementioned algorithms. It seems that one is either a subset of the other. Could someone explain the similarities/differences?
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1answer
872 views

Computing IV for CBC from PBKDF2 + HKDF

Note that this question is somewhat similar to Can I use my random IV (for AES) as a salt for PBKDF2? My current encryption format computes two random PBKDF2 salts (encryption and HMAC, 8 bytes each) ...
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1answer
872 views

Clarification needed in TLS 1.2 key derivation process

Reference to TLS 1.2 standard documentation section 6.3 regarding the key generation here: To generate the key material, compute ...
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1answer
416 views

Faster alternative to HKDF

HKDF is used in one of my projects to derive separate keys from a common master key. But profiling has shown it to be the performance bottleneck under certain circumstances. Is there any faster ...
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1answer
447 views

Some Questions about the Veracrypt hash functions

Veracrypt has always been my favourite encryption tool since it came out . But still there are some things about the tool I don't understand . Why is there the possibility to use sha256 or sha512 . ...
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1answer
237 views

Negating the risk of a random 64-bit nonce

Given a stream cipher such as Chacha20, which uses a 64-bit nonce, it would be unsafe to use a random nonce because of the risk of accidental repetition (which if used twice with the same key would be ...
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1answer
316 views

KDF vs Stream Cipher

I was wondering why someone would go and use HKDF (or any KDF) when they can use a stream cipher instead. It seems like a stream cipher will always be one-way, whereas a KDF might not be (see AES or ...
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1answer
562 views

What is the difference between KDFs for key derivation vs password stretching?

This was mentioned in the comments to another question: To clarify: This is about key-stretching KDFs. Not the kind for diversifying keys (eg. HKDF) and There are two kinds of KDF, the slow, ...
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1answer
492 views

SHA512 vs HKDF key expansion

I'm working on a secure file storage server. This server allows users to upload and download files based on a "seed". This "seed" is a 128-bit value. From this 128-bit seed, we run SHA512 to derive a ...
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2answers
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How long to wait to feed hashing using SHA 256?

I have a source which generates secrets at $8-10 bits $ per second. I need to use this secret to feed another generator every second. I am wondering how long should I wait while I am concatenating 8 ...
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2answers
167 views

Using HKDF to derive symmetric keys from a hybrid public-key encryption scheme

RFC 5869 describes HMAC-based Extract-and-Expand Key Derivation Function (HKDF). In section 4, entitled "Applications of HKDF", it states that one of the intended uses is: derivation of symmetric ...
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1answer
109 views

Security of HKDF when part of output is exposed

There is a system in which Alice and Bob need to establish a secure communication channel. Their conversation happens via a "public" server whose only purpose is to rely messages between users. This ...
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1answer
947 views

How to calculate entropy of a combined key

I know how to calculate the entropy of a key that relates to its selection process. For example if the key space is $1000$, entropy of a randomly chosen key is $1000$. Suppose now you have two keys $X$...
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1answer
580 views

Using KDF output for password validation

I am currently revising an AES data format. I would like to determine whether a provided password/key is incorrect. Previously this was done by validating the HMAC, but this makes it impossible to ...
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1answer
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KDF based on HMAC-SHA-256

Is KDF based on HMAC-SHA-256(Hashed Message Authentication Code, Secure Hash Algorithm) algorithm a suitable option to generate symmetric key from the secret key? What is the basic funda, behind ...
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1answer
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Generating keys with HKDF from Diffie Hellman agreement

In order to generate multiple session keys from a Diffie Hellman key agreement protocol, I'm trying to use the HKDF to generate this session keys. The HKDF algorithm uses an Extraction-Expand ...
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1answer
157 views

Where can I find official test vectors for NIST SP 800-56C r1 Single Step KDF

NIST 800-56Cr1 describes a single and two (extract-expand) step KDF designed for use in key agreement routines. NIST declares that HKDF (RFC5869) is a version of the two-step variation so test vectors ...
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2answers
652 views

Wrong Test Vector for HKDF with HMAC-SHA256 [closed]

I have implemented HKDF by myself and it seems to work good. I managed to replicate all the test vectors listed here but... except this one: ...
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1answer
40 views

HKDF on output key material of previous HKDF

I understand the typical way to use HKDF is to make one call to HKDF to obtain multiple keys. Is it safe to use HKDF the following way to generate different keys ? If not, why? key0 = HKDF-extract-...
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1answer
147 views

HKDF: ikm, salt and info values

As indicated at Key generation for AES-GCM-256 file encryption I'm currently working on a file encryption software. In the above thread it was suggested that, for performance reasons, I use a ...
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2answers
277 views

Doing RSA-KEM with RSA-OAEP

I am fiddling around with the JS Web Crypto API and creating a hybrid encryption system that uses a symmetric key to encrypt form data with AES256-CBC and and a public/private key-pair to wrap/...
2
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1answer
540 views

Can I use HKDF to improve security of HMAC used with weak hash function?

I am writing a framework which encrypts/decrypts files (or streams, in general) with symmetric key using block cipher (for example, AES). To authenticate encrypted file, I use HMAC, which is ...
2
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1answer
157 views

HKDF - Generating a new, random salt per user?

Regarding HKDF, the specification says a salt may be re-used. However, I have set up my system to generate a new, long and cryptographically random salt that persists with the user. This salt is not ...
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1answer
49 views

Signing different types of messages with the same asymmetric key

I want to be able to sign different types of messages with the same asymmetric key. (For completeness, I am using Ed25519 for my signatures). Why do I want to do this? The same key is used to sign ...
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1answer
267 views

HKDF entropy extraction

I collect entropy from the following sources: system_entropy = System provided crypto entropy stream (CryptGenRandom on Windows) user_entropy = User-provided entropy - in a form of a byte stream of ...
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1answer
562 views

Key derivation for AES-Encryption key and HMAC

Okay so I have two options for how I would derive two keys for AES encryption and HMAC they are as follows: Run BCrypt on a users passphrase and then use the bouncy castle implementation of HKDF in ...
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1answer
69 views

How to select salt an IKM for HKDF

I have an application where I need to combine a Diffie-Hellman shared secret, with a user's password and derive a key. I'm investigating the possibility of using HKDF to as follows Initial Keying ...
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1answer
72 views

Is using derived keys for HKDF + AES-GCM safe

I have an existing system which takes a cryptographically secure BLOB from user and pass it to lower API which calls HKDF-Expand to get 2 keys: one is effectively discarded in this scenario and second ...
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0answers
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Orignal Krawczyk's HKDF vs latest NIST SP800-56C

I've noticed that the original HKDF in rfc5869 defines a feedback loop. So the pseudo-code is something like: ...
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0answers
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Why would one split HKDF output into multiple parts instead of expanding it multiple times

Reading the implementation details of Signal's approach on double ratchet algorithm I noticed the following key derivation procedure for using AES-CBC with HMAC: HKDF is used (...) to generate 80 ...
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0answers
59 views

Combining multiple hash functions for HKDF

I'm trying to apply the hash function combination method described in https://eprint.iacr.org/2013/210.pdf to a HKDF PRNG construction. Thing is it needs 4 uniformly random parameters/keys chosen by ...
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1answer
454 views

Choosing between simple Hash and HKDF to derive the second key used for MAC

I have an encrypt function that takes a single encryption key and encrypts data but also incorporates an HMAC. So I need to derive a second key from the encryption key to use it for HMAC. You ...
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2answers
133 views

KDF-based password manager

I want to make a password manager that from a single master password generates passwords for my services. For that I thought that HKDF suits my needs, as it is able to generate multiple keys from a ...
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2answers
108 views

Key derivation: bit lengths

This is a follow-up question to HKDF: ikm, salt and info values Based on the feedback, I have now decided to implement my key derivation for AES-GCM-256 file encryption roughly as follows: ...
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1answer
439 views

HMAC vs PKDBF2 as the first step towards HKDF

I'm implementing an application that from a password has to derive two keys, one for authentication with the server, one for encryption. I'm using Java, with JCA and Bouncy Castle. So far, to ...
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1answer
737 views

The number of keys generated in TLS 1.2?

In section 6.3. Key Calculation in the standard documentation here, I see the following: The master secret is expanded into a sequence of secure bytes, which is then split to a client write MAC ...
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1answer
35 views

HKDF: Difference between salt and info

I want to do generate individual AES keys for a number of smart cards, based on a random master key (key diversification) and the serial number of the card. According to the answers to this question ...
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1answer
132 views

Maximum output of HKDF

I got a question with the RFC of HKDF https://tools.ietf.org/html/rfc5869. There is a maximum specified for the output keying material. The length of the OKM shall be <= 255*HashLen. What does it ...
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2answers
1k views

Deriving 2 keys using HKDF

I have an encryption scheme that uses a 256-bit master key, from which 2 separate keys (one for AES-256-CTR encryption and one for a HMAC-SHA256) are derived using HKDF. However, I'm not sure exactly ...
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1answer
265 views

Is there a recommendation or standard for using SCRYPT as an unbounded length KDF and not a fixed length authentication hash?

How would I go about this? I don't want to roll my own but I do really want to use SCRYPT as a KDF for several algorithms, some of which require an output length longer than the SHA-256 hash. ...
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2answers
317 views

How to use HKDF to combine two keys

I have heard that to combine two keys HKDF is better than using XOR function. But if you look at the HKDF RFC I could not figure out how to use it for concatenating two keys. What I can see is HKDF ...