Hashing and digital signatures

Hashing protects integrity

• We hashed passwords for secrecy. Hashing also protects integrity — proving data was not changed.
• Change even one character of the input, and the hash changes completely.

import hashlib
print(hashlib.sha256(b"hello").hexdigest()[:16])
print(hashlib.sha256(b"hellp").hexdigest()[:16])   # totally different

Checking a download

• Websites publish the hash of a file. After downloading, you hash your copy and compare.
• If the two hashes match, the file arrived intact. If not, it was corrupted or tampered with.

Digital signatures

• A digital signature proves who sent something and that it was not changed.
• The sender hashes the message and encrypts that hash with their private key.
• Anyone can check it with the sender's public key — only the real sender could have made it.

Your turn

• Compare the hashes of an original and a received message. True means the message is intact.

Covers: A-Level 17.1 (digital certification), 6.2 (integrity).

Common mistakes

• A hash is one-way — you cannot get the original back from it.
• A digital signature proves who sent a message and that it was not changed.