Editing Hash Functions and Digital Signatures (section)

== <span style="color: #FFFFFF;">Applying</span> ==
'''Modeling 'The Integrity Check' (How Hashing works):'''
<syntaxhighlight lang="python">
import hashlib

def get_file_fingerprint(data_string):
    """
    Creates a SHA-256 hash of a string.
    """
    return hashlib.sha256(data_string.encode()).hexdigest()

original = "Pay Alice $100"
tampered = "Pay Alice $900"

print(f"Original Hash: {get_file_fingerprint(original)}")
print(f"Tampered Hash: {get_file_fingerprint(tampered)}")
# Note how the two hashes are completely different, 
# even though only one digit changed!
</syntaxhighlight>

; Signature Landmarks
: '''Blockchain (Bitcoin)''' → Every block contains the hash of the previous block; if you change one transaction in history, every hash in the future "breaks," making it impossible to cheat.
: '''Software Updates''' → When your phone downloads an update, it checks the digital signature to ensure it really came from Apple or Google and not a virus.
: '''Git (Version Control)''' → Every commit in Git is named after its hash; this ensures that your source code can never be silently corrupted.
: '''Password Storage''' → Websites never store your actual password; they only store the hash. When you log in, they hash your input and see if it matches the stored fingerprint.
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