Hash Function
A function mapping arbitrary input to a fixed-size digest, used for identity, integrity, and addressing.
A hash function maps input of any size to a fixed-size value. The properties that matter are determinism (same input, same digest) and collision resistance (finding two inputs with the same digest should be infeasible).
The uses divide cleanly. Content addressing names a thing by its digest, so identity and integrity are the same fact — the model behind Git's object store and behind content-hashed asset filenames, where a changed file necessarily has a new name and cache invalidation becomes unnecessary. Integrity checking verifies that a download matches what was published. Cache keys summarize a set of inputs so that a change to any of them is detectable; see Fingerprint. Password storage uses deliberately slow functions designed for exactly that, and nothing else.
Two practical cautions. A hash covers exactly what was fed into it — including ordering, so a digest over a list changes when the order changes even if the membership does not, which produces "identical content, different hash" surprises across tools that enumerate in different orders.
And a hash used to derive a value from a small space — a port number, a bucket index — collides regularly. Hashing does not eliminate collisions; it only makes them uniform.
See also10
Caller ID Authentication
Cryptographic attestation of an originating telephone number, and why spoofed calls survive it.
Networks & Distribution17 connections
Determinism
The property that identical inputs produce identical outputs, and the foundation of caching and verification.
Systems & Tooling61 connections
Cache Invalidation
Deciding when a cached value has stopped being correct — famously one of the hard problems.
Web Platform30 connections
Version Control
A system recording the history of a project's files, enabling branching, merging, and recovery.
Version Control & Delivery26 connections
Record and Replay Testing
Capturing a real interaction once and replaying it deterministically in later test runs.
Agents & Language Models14 connections
Provenance
The recorded origin of a change or a claim — who made it, when, and on what evidence.
Method44 connections
LLM-as-Judge
Using a language model to score another model's output against a rubric.
Agents & Language Models12 connections
Write-Ahead Logging
Recording the intent of a change durably before applying it, so a crash can be recovered.
Computation & Algorithms20 connections
Hash Table
A key-value structure that uses a digest of the key to index an array of buckets.
Computation & Algorithms23 connections
Multi-Factor Authentication
Requiring a second kind of proof besides a password to sign in, such as a code, a phone prompt, or a hardware key, and why only the methods tied to the real website stop modern phishing.
Web Platform11 connections
Linked from27
- Audit LogData & Content
An append-only record of who did what, to what, and when, kept to answer questions of accountability long after the fact, and built so that even its administrators can't quietly rewrite it.
- BitTorrentNetworks & Distribution
A peer-to-peer file-sharing protocol from 2001 in which everyone downloading a file also uploads pieces of it to others, so popular files get faster as more people want them, and its legitimate and pirate uses.
- Bloom FilterComputation & Algorithms
A probabilistic set membership structure that can say "definitely not present" but only "probably present."
- Build CacheVersion Control & Delivery
Reusing the outputs of build and test steps whose inputs haven't changed, keyed by a hash of those inputs, so large codebases build in minutes instead of hours.
- Cache InvalidationWeb Platform
Deciding when a cached value has stopped being correct — famously one of the hard problems.
- Caller ID AuthenticationNetworks & Distribution
Cryptographic attestation of an originating telephone number, and why spoofed calls survive it.
- ChecksumNetworks & Distribution
A small value computed from data so that accidental corruption can be detected later, from parity bits and credit card check digits to the CRCs in every Ethernet frame, and useless against deliberate tampering.
- Consistent HashingComputation & Algorithms
A hashing scheme where adding or removing a node remaps only a small fraction of keys, not nearly all of them.
- DeterminismSystems & Tooling
The property that identical inputs produce identical outputs, and the foundation of caching and verification.
- Disk ImageSystems & Tooling
A single file holding the complete contents and structure of a disk, used to install operating systems, run virtual machines, preserve old media, and copy drives exactly for forensic investigation.
- Domain Name SystemSystems & Tooling
The distributed directory translating names to addresses, and the layer where propagation delays live.
- EmulationPlay & Games
Software reimplementation of console hardware, and the fault line between preserving a game and possessing it legally.
- eSIMNetworks & Distribution
A SIM soldered into the device and provisioned over the air, with profiles downloaded rather than cards swapped.
- ETagWeb Platform
An opaque fingerprint of a response's content that lets a client ask a server whether it's changed without re-downloading it.
- FingerprintAgents & Language Models
A hash over everything that determined a result, used to detect when the result has gone stale.
- Game PreservationPlay & Games
The effort to keep games playable against delisting, license expiry, server shutdown and hardware rot.
- Git Object ModelVersion Control & Delivery
Git stores every version of a project as a graph of immutable objects named by the hashes of their contents, blobs for files, trees for directories, and commits for snapshots, which makes history tamper-evident and identical content free to store twice.
- Hash TableComputation & Algorithms
A key-value structure that uses a digest of the key to index an array of buckets.
- LetterlockingMeaning & Society
Folding, slitting and sealing a letter so the sheet becomes its own tamper-evident envelope.
- Multi-Factor AuthenticationWeb Platform
Requiring a second kind of proof besides a password to sign in, such as a code, a phone prompt, or a hardware key, and why only the methods tied to the real website stop modern phishing.
- Personal DataData & Content
Any information that relates to an identifiable person, why it is broader than names and emails, and the engineering habits that privacy law now expects.
- Pseudorandom Number GeneratorComputation & Algorithms
An algorithm that produces a sequence of numbers that looks random but is fully determined by a starting seed, and the separate, stricter kind needed for security.
- Universally Unique IdentifierData & Content
A 128-bit identifier that any machine can generate without coordination and still be practically certain no one else will produce the same one.
- Version ControlVersion Control & Delivery
A system recording the history of a project's files, enabling branching, merging, and recovery.
- Virtual Private NetworkNetworks & Distribution
An encrypted tunnel that presents a remote network as local, and the narrow set of things a commercial one actually hides.
- WebhookNetworks & Distribution
A webhook is an HTTP callback a service pushes to you when an event happens, inverting who initiates the request.
- Write-Ahead LoggingComputation & Algorithms
Recording the intent of a change durably before applying it, so a crash can be recovered.