Field Guide · term

Also known as: crypto

Cryptography is the science of securing information against adversaries — protecting the confidentiality, integrity, authentication, and non-repudiation of data using mathematical techniques.1 It is the constructive counterpart to cryptanalysis, the study of breaking such systems, and it underlies everything from HTTPS to the encrypted voice traffic a scanner encounters on public-safety radio.

cryptography confidentiality integrity authentication
Cryptography pursues several goals at once: keeping data secret, detecting tampering, and proving who sent it.

How it works

Cryptography combines several goals, only one of which is secrecy:

  • Confidentiality — keeping data unreadable to anyone without the key, achieved by encryption with a cipher.
  • Integrity — detecting whether data has been altered, typically with hash functions or message authentication codes.
  • Authentication — proving who produced a message.
  • Non-repudiation — preventing a sender from later denying they sent it, usually via digital signatures.

Modern cryptography rests on Kerckhoffs’s principle: the algorithm is assumed public and all the security lives in the secret key. That distinguishes it from obfuscation, which merely hides a method that anyone who learns it can reverse.

Variants

The field divides into three broad branches:

  • Symmetric-key cryptography uses one shared secret for both encryption and decryption. It is fast and is what protects bulk data — the AES and DES block ciphers, and stream ciphers like RC4, all live here. This is the branch land-mobile radio uses.
  • Public-key cryptography uses a public/private key pair, solving the key-distribution problem and enabling digital signatures; it is the basis of TLS handshakes and PKI.
  • Hashing produces a fixed-size fingerprint of data with no key, underpinning integrity checks and message authentication.

Symmetric systems still need a way to distribute keys securely; on radio networks that job falls to key loaders and over-the-air rekeying.

In practice

A working cryptosystem is more than a cipher: it needs sound key management, correct modes of operation, fresh initialization vectors, and integrity protection, because attackers rarely break the math — they exploit reuse, weak keys, or protocol mistakes. On public-safety radio the practical stack is a symmetric cipher plus a key-management scheme (OTAR, key-ID/algorithm-ID signaling, and a key loader).

Relevance to SDR

A trunked-radio receiver constantly meets the products of cryptography. Voice traffic on DMR and P25 systems may be encrypted (DMR Enhanced Privacy using RC4; P25 voice using DES-OFB, DES-XL, or AES-256; TETRA using the TEA algorithms), in which case GopherTrunk can identify and follow the call — reading its key-ID/algorithm-ID fields — but cannot recover the audio without the key. Other on-air transformations are not cryptography in the security sense: data-link CRCs provide integrity but no secrecy, and the Motorola P25 talker-alias scheme is obfuscation rather than encryption — it was analyzed clean-room in issue #773 using only publicly observable data. Telling these apart is the first step in deciding what a scanner can decode.

Sources

  1. Cryptography — Wikipedia, for the goals of cryptography and its distinction from cryptanalysis. ↩

See also