But note: though we're fairly strong on security issues, we are not crypto experts. We've done our best to assemble (digest?) the best available information into this Guide, but we welcome being pointed to the errors of our ways.
* What is a cryptographic hash?
A "hash" (also called a "digest", and informally a "checksum") is a kind of "signature" for a stream of data that represents the contents. The closest real-life analog we can think is "a temper-evident seal on a software package": if you open the box (change the file), it's detected.
Let's first see some examples of hashes at work.
Many Unix and Linux systems provide the md5sum program, which reads a stream of data and produces a fixed, 128-bit number that summarizes that stream using the popular "MD5" method. Here, the "streams of data" are "files" (two of which we see directly, plus one that's too large to display).
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