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.

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.

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 or AES-256), in which case GopherTrunk can identify and follow the call 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

• Cryptanalysis
• Cipher
• Encryption
• Obfuscation