hash_hkdf expands high-entropy input key material into a cryptographic key using HKDF with an optional salt and context string.
The examples explain the data format, expected results and the difference between this API and nearby cryptographic operations.
What It Does
The operation is intended for: hash_hkdf expands high-entropy input key material into a cryptographic key using HKDF with an optional salt and context string.
Part of core PHP or a normally bundled standard extension; check your deployment configuration.
Syntax and Return Value
hash_hkdf(string $algo, string $key, int $length = ..., string $info = ..., string $salt = ...): string| Parameter | Purpose |
|---|---|
| $algo (string) | The exact algorithm identifier supported by this API. |
| $key (string) | Raw key bytes in the required algorithm-specific length; protect them from disclosure. |
| $length (int) | Requested output length; consult byte-versus-text units for the chosen API. |
| $info (string) | Use the PHP signature above to determine the exact expected input type; validate untrusted inputs before calling. |
| $salt (string) | A salt with the exact required length; use random_bytes for password-derived keys. |
The declared return type is shown above. Errors can include false results or exceptions depending on this API: HKDF expects cryptographically strong input key material; PBKDF2 is intentionally iterative and needs a salt and tuned iteration count.
Example 1: Basic Operation
Inspect the expected result: do not print secrets or reuse illustrative credentials in production.
<?php
$ikm = random_bytes(32);
$derived = hash_hkdf("sha256", $ikm, 32, "session-v1");
echo strlen($derived);Example 2: A Different Use Case
Compare domain-separated HKDF outputs or verify determinism for identical inputs.
<?php
$material = random_bytes(32);
$a = hash_hkdf('sha256', $material, 32, 'encryption');
$b = hash_hkdf('sha256', $material, 32, 'authentication');
var_dump(!hash_equals($a, $b));Practical Example: Checking the Result
Compare domain-separated HKDF outputs or verify determinism for identical inputs.
<?php
$material = random_bytes(32);
$first = hash_hkdf('sha256', $material, 32, 'application-v1');
$second = hash_hkdf('sha256', $material, 32, 'application-v1');
var_dump(hash_equals($first, $second));Important Behavior and Edge Cases
HKDF expects cryptographically strong input key material; PBKDF2 is intentionally iterative and needs a salt and tuned iteration count.
Distinguish digest encoding from secret-key authentication and keep fixed-length expected authentication values for comparisons.
Comparison with Related Functions
| Approach | Key distinction |
|---|---|
| hash_hkdf | hash_hkdf expands high-entropy input key material into a cryptographic key using HKDF with an optional salt and context string. |
| Related technique | Use password_hash for storing login passwords, not a general-purpose digest or manually encoded derived key. |
Common Beginner Mistakes
| Mistake | Correct approach |
|---|---|
| Confusing this operation with a neighboring cryptographic or security mechanism | Use password_hash for storing login passwords, not a general-purpose digest or manually encoded derived key. |
| Ignoring a security prerequisite or failure case | HKDF expects cryptographically strong input key material; PBKDF2 is intentionally iterative and needs a salt and tuned iteration count. |
Key Takeaways
- hash_hkdf expands high-entropy input key material into a cryptographic key using HKDF with an optional salt and context string.
- HKDF expects cryptographically strong input key material; PBKDF2 is intentionally iterative and needs a salt and tuned iteration count.
- Use password_hash for storing login passwords, not a general-purpose digest or manually encoded derived key.
Apply this behavior to a real application only after confirming the required extension, configuration and failure handling on the deployment runtime.