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.2 This is why key length matters: a longer key exponentially enlarges the search an attacker must perform. The bound is only an upper limit, though — it holds only if the effective key space really is that large. A cipher with a short key (DES’s 56 bits), predictable key generation, or a mathematical weakness that lets an attacker rule out most keys at once has a much smaller effective space than its nominal length suggests.
Variants
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.
Systems also distinguish a traffic key (the short-lived key that actually encrypts a call) from a key-encryption key (used only to wrap traffic keys for delivery), and they schedule periodic rekeying so that compromise of one key exposes only a limited window.
In practice
Because all the secrecy is concentrated in the key, key management — generating, distributing, storing, and retiring keys — is the hard operational core of any deployed cryptosystem. In land-mobile radio, keys are typically loaded into radios with a physical key loader / KFD and can be refreshed remotely by over-the-air rekeying; each key carries a key ID and algorithm ID so the receiver knows which key and cipher a given transmission used. By Kerckhoffs’s principle, a system should remain secure even if everything except the key is public — so protecting the key is the whole security argument.
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. ↩
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Key size — Wikipedia, for how key length bounds brute-force effort and the notion of effective vs nominal key strength. ↩