White box (software engineering)

"Open box" redirects here. "Open box" is also the name of the visible symbol ␣ that is sometimes used to indicate a space. See whitespace character.

A white box (or glass box, clear box, or open box) is a subsystem whose internals can be viewed, but usually cannot be altered.

Having access to the subsystem internals in general makes the subsystem easier to understand, but also easier to hack; for example, if a programmer can examine source code, weaknesses in an algorithm are much easier to discover. This makes white box testing much more effective than black box testing, but considerably more difficult due to the sophistication needed on the part of the tester to understand the subsystem.