Also known as: encipherment
Encryption transforms plaintext into ciphertext using a key, so that only holders of the matching key can recover the original message.1 It is the practical means by which cryptography delivers confidentiality, performed by a cipher and reversed by decryption.
How it works
Encryption applies a cipher parameterized by a secret key. In symmetric encryption the same key encrypts and decrypts; in asymmetric (public-key) encryption a public key encrypts and a private key decrypts. Either way, the defining property is that confidentiality depends on the key, not on hiding the algorithm — the essence of Kerckhoffs’s principle.
This is what separates encryption from look-alikes:
- Obfuscation rearranges or hides data with no secret key, so anyone who learns the method can reverse it.
- Scrambling/whitening applies a fixed, public sequence to balance a signal, not to keep it secret.
Only encryption gives security that survives full public knowledge of the method.
Variants
Beyond the symmetric/asymmetric split, encryption is characterized by where and how it is applied. Link (over-the-air) encryption protects a single hop and is what most land-mobile radio uses; end-to-end encryption protects content across every relay so even the infrastructure cannot read it. Voice traffic is usually encrypted with a keystream mode (a block cipher run in OFB, or a native stream cipher) so it can be applied continuously to vocoder frames with no expansion. Each protected transmission signals which key and algorithm it used through key-ID / algorithm-ID fields, and the keys themselves are distributed by a key loader or by over-the-air rekeying.
In practice
Encryption is only as strong as its key management. A perfect cipher is undone by reused keys, predictable initialization vectors, or keys shared insecurely, which is why real systems invest heavily in key loaders, rekeying, and key rotation. On public-safety radio this shows up as OTAR infrastructure, crypto-net planning, and the operational headache of keeping thousands of field radios in sync — problems entirely separate from the cipher math.
Relevance to SDR
For a scanner, encryption is the hard wall. When DMR Enhanced Privacy (RC4) or P25 DES-OFB / AES-256 voice encryption is in use, GopherTrunk can detect the encrypted call, read its key-ID/algorithm-ID, identify the talkgroup, and follow it across the trunked system, but it cannot recover audio without the key — and it does not attempt to. The contrast matters because much of what crosses the air only looks protected: a scrambling sequence is reversible, a CRC is for integrity, and the Motorola talker-alias scheme analyzed in issue #773 is keyless obfuscation rather than encryption. Distinguishing real encryption from the rest tells you immediately what is decodable.
Sources
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Encryption — Wikipedia, for the definition of transforming plaintext to ciphertext under a key and its inverse, decryption. ↩