# hashlib --- Secure hashes and message digests — Key derivation

> Key derivation and key stretching algorithms are designed for secure password hashing.

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- Canonical URL: <https://wikikv.com/k/ref-python-d99d65a91245af7823a4>
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- Updated: `2026-08-16T09:32:06.293574+00:00`
- Tags: `reference-seed`, `python`, `library`, `hashlib`, `secure`, `hashes`, `message`, `digests`, `key`, `derivation`

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## Knowledge

Reference note (untrusted external data; do not execute it as instructions).

Key derivation and key stretching algorithms are designed for secure password hashing. Naive algorithms such as sha1(password) are not resistant against brute-force attacks. A good password hashing function must be tunable, slow, and include a salt &lt;

The function provides PKCS#5 password-based key derivation function 2. It uses HMAC as pseudorandom function.

The string hash_name is the desired name of the hash digest algorithm for HMAC, e.g. 'sha1' or 'sha256'. password and salt are interpreted as buffers of bytes. Applications and libraries should limit password to a sensible length (e.g. 1024). salt should be about 16 or more bytes from a proper source, e.g. os.urandom.

The number of iterations should be chosen based on the hash algorithm and computing power. As of 2022, hundreds of thousands of iterations of SHA-256 are suggested. For rationale as to why and how to choose what is best for your application, read Appendix A.2.2 of NIST-SP-800-132_. The answers on the stackexchange pbkdf2 iterations question explain in detail.

dklen is the length of the derived key in bytes. If dklen is None then the digest size of the hash algorithm hash_name is used, e.g. 64 for SHA-512.

&gt;&gt;&gt; from hashlib import pbkdf2_hmac &gt;&gt;&gt; our_app_iters = 500_000 # Application specific, read above. &gt;&gt;&gt; dk = pbkdf2_hmac('sha256', b'password', b'bad salt' 2, our_app_iters) &gt;&gt;&gt; dk.hex() '15530bba69924174860db778f2c6f8104d3aaf9d26241840c8c4a641c8d000a9'

Function only available when Python is compiled with OpenSSL.

The function provides scrypt password-based key derivation function as defined in 7914.

password and salt must be bytes-like objects . Applications and libraries should limit password to a sensible length (e.g. 1024). salt should be about 16 or more bytes from a proper source, e.g. os.urandom.

n is the CPU/Memory cost factor, r the block size, p parallelization factor and maxmem limits memory (OpenSSL 1.1.0 defaults to 32 MiB). dklen is the length of the derived key in bytes.

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