hashlib — Secure Hashes and Message Digests

This module implements a common interface to many different secure hash and message digest algorithms.

import hashlib

Basic Hashing

Data sent to hashlib must be passed as bytes, not standard strings.

import hashlib

# 1. Create a hash object
m = hashlib.sha256()

# 2. Add data (must be encoded to bytes)
m.update(b"Nobody inspects")
m.update(b" the spammish repetition")

# 3. Get the digest
print(m.digest())      # Returns bytes
print(m.hexdigest())   # Returns a hexadecimal string (most common)
# '031edd7d41651593c5fe5c006fa5752b37fddff7bc4e843aa6af0c950f4b9406'

One-liner Hashing

import hashlib

# MD5 (Warning: considered cryptographically broken, use only for checksums)
hash_str = hashlib.md5(b"Hello World").hexdigest()

# SHA-256 (Recommended for general secure hashing)
hash_str = hashlib.sha256(b"Hello World").hexdigest()

# SHA-512
hash_str = hashlib.sha512(b"Hello World").hexdigest()

Checking Available Algorithms

import hashlib

# Algorithms guaranteed to be available on all platforms
print(hashlib.algorithms_guaranteed)
# {'sha224', 'sha256', 'sha384', 'sha512', 'md5', 'sha1', 'blake2b', 'blake2s'}

# Algorithms available on your specific system via OpenSSL
print(hashlib.algorithms_available)

Hashing Files Efficiently

To hash a large file without loading the entire file into memory, read it in chunks:

import hashlib

def sha256_file(filename):
    sha256_hash = hashlib.sha256()
    
    with open(filename, "rb") as f:
        # Read and update hash in 4K chunks
        for byte_block in iter(lambda: f.read(4096), b""):
            sha256_hash.update(byte_block)
            
    return sha256_hash.hexdigest()

print(sha256_file("large_video.mp4"))

Password Hashing (pbkdf2)

For hashing passwords securely, you should use key derivation functions like pbkdf2_hmac. (Note: The secrets module or external libraries like bcrypt are often better alternatives).

import hashlib
import os

password = b"super_secret_password"
salt = os.urandom(32) # A random salt is required for security

# Use SHA-256 with 100,000 iterations
key = hashlib.pbkdf2_hmac('sha256', password, salt, 100000)

print(key.hex())

Official Documentation

hashlib — Secure hashes and message digests

API Reference

Hash Algorithms Construction

Hash Object Methods

Key Derivation