Cryptographic Hashing: SHA-256

You have used hashing since the hashmap tier: a hash function turns a key into a number so you can index into a bucket array in O(1). That hash function has one job, spread keys out to minimize collisions, and nothing about it needs to be secret or hard to reverse. In fact a hashmap's hash for small integers is often just the integer itself. Level 30 introduces a completely different flavor of hash function, one built for security rather than speed of lookup: SHA-256.

• -One-way (preimage resistance): given a hash output, there is no feasible way to compute an input that produces it, other than trying inputs one by one.
• -Collision resistant: it is computationally infeasible to find two different inputs that produce the same output, unlike a hashmap, where collisions are expected and just handled with chaining or probing.
• -Deterministic and fixed-size: the same input always produces the same output, and every output is 256 bits regardless of input size. Hashing one byte or one gigabyte both produce a digest of the exact same length.

Change a single bit of the input and the output should change unpredictably and completely, roughly half the output bits flip, with no discernible relationship to which input bit changed. That property, called the avalanche effect, is what makes a hash useful as an integrity check: if a file's SHA-256 digest matches the expected value, you know the file is byte-for-byte identical to what produced that digest, because even a one-character edit would have produced a wildly different hash.

• -Password storage: never store a password directly, store a salted hash of it, so a database breach does not hand out plaintext passwords.
• -Integrity verification: software downloads publish a SHA-256 checksum so you can confirm the file was not corrupted or tampered with in transit.
• -Blockchains: each block includes the hash of the previous block, so tampering with any historical block changes its hash and breaks every subsequent link.
• -Content-addressed storage, like Git: objects are named by the hash of their content, so identical content always gets the same, verifiable name.