Also known as: key, secret key
A cryptographic key is the secret value that controls how a cipher transforms data; with the correct key the transformation can be reversed, and without it the data should be infeasible to recover.1 In a sound system the key — not the algorithm — is the only secret.
How it works
A cipher is a fixed, often publicly documented algorithm; the key is the variable secret that selects one specific transformation out of an astronomically large family. Change the key and the same algorithm produces a completely different result.
The strength of a key is measured by its key space — the number of possible keys, set by the key length in bits. An n-bit key has 2ⁿ possibilities, so a 128-bit key has 2¹²⁸ of them, far beyond any feasible brute-force search. This is why key length matters: a longer key exponentially enlarges the search an attacker must perform.
Keys take different forms in different systems:
- A single shared secret in symmetric-key cryptography.
- A public/private pair in public-key cryptography.
- A seed that is expanded into a keystream in a stream cipher.
By Kerckhoffs’s principle, a system should remain secure even if everything except the key is public — so all the secrecy is concentrated in the key, and protecting it is the heart of key management.
Relevance to SDR
Whether GopherTrunk can recover encrypted voice comes down to one thing: the key. The decoder can identify an encrypted P25 or DMR call, read its key-identifier and algorithm fields, and follow the traffic, but the audio stays opaque without the secret key. That is the intended behaviour of a correctly designed symmetric cipher. It also clarifies the difference from reversible scrambling, which has no secret key and so can be undone by anyone who knows the public method.
Sources
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Key (cryptography) — Wikipedia, for the key as the secret parameter and the role of key length and key space. ↩