Field Guide · term

Also known as: asymmetric cryptography, asymmetric encryption

Public-key cryptography (also asymmetric cryptography) uses a linked pair of keys: a public key that may be shared freely and a private key kept secret.1 Data encrypted with the public key can only be decrypted with the matching private key, so no shared secret has to be distributed in advance.

public key private key plaintext encrypt cipher decrypt plaintext
Two linked keys: the public key encrypts; only the private key decrypts.

How it works

The two keys are generated together and bound by a mathematical relationship — typically one based on a problem believed hard to reverse, such as integer factorisation (RSA) or the elliptic-curve discrete logarithm (ECC). The public key can be published openly; deriving the private key from it is computationally infeasible.

This gives two complementary uses:

  • Encryption — anyone encrypts with the recipient’s public key, and only the holder of the matching private key can decrypt.
  • Digital signatures — the holder signs with the private key, and anyone can verify with the public key, proving authenticity and integrity.

Because the heavy mathematics make asymmetric operations far slower than symmetric ones, real systems usually use public-key cryptography only to agree on or transport a short symmetric key, then switch to a fast symmetric cipher for the bulk data. As with all modern cryptography, its safety rests on Kerckhoffs’s principle: the algorithm is public and only the private key is secret.

Relevance to SDR

Public-key cryptography rarely appears in the over-the-air voice path of conventional trunked-radio protocols, which protect speech with fast symmetric ciphers instead. It is more likely to surface in key management and provisioning — distributing or rekeying the symmetric traffic keys, and in device authentication — rather than in the per-call audio GopherTrunk demodulates. For the purposes of decoding received signals it is mostly background context, but it explains how the symmetric keys that do protect voice get delivered.

Sources

  1. Public-key cryptography — Wikipedia, for the public/private key-pair model and the key-distribution problem it solves. ↩

See also