Cryptographic hashing and integrity
A cryptographic hash function maps data of any length to a fixed-size digest. SHA-256, part of the SHA-2 family, always produces 256 bits, shown as 64 hexadecimal characters. A good hash has three properties: preimage resistance (given a digest, you cannot find an input that produces it), second-preimage resistance (given an input, you cannot find a different one with the same digest), and collision resistance (you cannot find any two inputs with the same digest).
Python
import hashlib
print(hashlib.sha256(b"abc").hexdigest())
first = hashlib.sha256(b"transfer 100 to alice").hexdigest()
second = hashlib.sha256(b"transfer 900 to alice").hexdigest()
differing = sum(x != y for x, y in zip(first, second))
print(len(first), first == second)
print(differing > 40)