Plug in an SDR and the first question that should come before "what is that signal" is "am I allowed to listen to it, and if I can hear it, does that mean anything is unlocked?" The honest answer is a set of clean distinctions that a lot of hobby forums blur: hearing is not the same as using; a scanner picking up a signal does not mean the signal is for you; and - the one that trips up almost everyone - a digital transmission is not an encrypted one. This piece lays out those lines for two jurisdictions, as general information rather than legal advice.
//The United States: Hearing vs Using
US federal law (the Electronic Communications Privacy Act, 18 U.S.C. 2511) makes it not unlawful to intercept radio communications that are "readily accessible to the general public," and then lists what that includes: public broadcasts; marine and aeronautical systems; amateur, CB and GMRS; and government, public-safety and land-mobile systems that are readily accessible. In plain terms, receiving the ordinary open spectrum is lawful.
Two boundaries sit around that. First, the definition of "readily accessible" in 2510(16) explicitly excludes communications that are scrambled or encrypted, that use modulation withheld for privacy, that ride a subcarrier, or that run over a common-carrier system - so those fall outside the exception. Second, and separately, it is an offence to divulge or use the contents of a communication you know was unlawfully intercepted (2511(1)(c) and (d)). Hearing the open spectrum is fine; passing on or acting on something you were not entitled to intercept is a different matter.
There is also a hardware rule people cite with the wrong year: scanners manufactured for or imported into the US must be incapable of receiving the old analog cellular bands. That comes from Public Law 102-556 in 1992, with the FCC's implementing rules effective in 1994 - not "the 1993 law." It is largely vestigial now, because modern cellular is digital and encrypted, but the statute stands.
//Australia: What Is Actually Settled
Australia is the place to be most careful, because the commonly repeated "it's illegal to divulge what you hear" does not map cleanly onto a current Commonwealth statute, and this site does not state things it cannot source. What a careful reading of the Radiocommunications Act 1992 supports - as set out in published analysis by an Australian emergency-law academic - is that operating a receive-only scanner is generally not a Commonwealth offence: the offences in the Act turn on transmitting or on operating specified devices, and a passive receiver is generally not caught.
What is not clearly established is a radio-specific Commonwealth "divulge" offence. The Telecommunications (Interception and Access) Act is built around communications passing over a telecommunications system - phone and internet carriage - rather than over-the-air broadcast or land-mobile radio, so it does not obviously supply one. The honest position, then, is narrow: listening with a receive-only scanner is generally not a Commonwealth offence, but recording, acting on, or republishing what you hear can engage other laws - state surveillance-devices legislation, the interception regime where a telecommunications system is involved - and some states have their own restrictions on scanner possession or use. "Legal to hear" is not "legal to do anything with," and the specifics are state-dependent.
//The Misconception That Matters Most: Digital Is Not Encrypted
Tune across public-safety radio and much of it sounds robotic and garbled rather than like analog voice. People hear that and assume it is encrypted and off-limits. It usually is not. P25, DMR and NXDN are digital modulation and voice-coding schemes - a modern replacement for analog FM - and digital is not the same thing as encrypted. A matching digital decoder turns that robotic sound into perfectly clear speech, because nothing is scrambled; it is just encoded.
Encryption is a separate, optional layer an agency can switch on - AES-256 is the current P25 standard, with older DES still around - and it is a deliberate choice, system by system, not an inherent property of going digital. Field studies of public-safety radio have found a great deal of digital traffic transmitted in the clear, even where encryption was available, though adoption of encryption has grown over time. So: robotic audio means digital, which is usually decodable; genuinely encrypted traffic is a distinct, smaller category - and the one that both US and Australian law put firmly out of bounds to intercept.
//And the Other One: "Privacy Codes" Are Squelch Tones
On FRS/GMRS radios and in CTCSS or "Private Line" systems, you will see "privacy codes." They offer no privacy. A CTCSS tone is a sub-audible signal that tells your receiver's squelch to open only for transmissions carrying the same tone - it keeps you from hearing other users on a shared channel, nothing more. Everyone transmitting is in the clear; switch your receiver to monitor mode, ignore the tone, and you hear all of it. The word "privacy" in the manual is a marketing label on a channel-sharing feature, not security of any kind.
//Sourcing and Framing
The US framework is from 18 U.S.C. 2511(2)(g) (the readily-accessible exception), 2510(16) (which excludes encrypted/scrambled and certain private communications), and 2511(1)(c)/(d) (divulging or using unlawfully intercepted content); the cellular-scanner hardware rule is Public Law 102-556 (1992), FCC rules effective 1994. The Australian position - receive-only scanning generally not a Commonwealth offence, with no clearly established radio-specific "divulge" offence and state law doing the real work - follows published Australian emergency-law analysis of the Radiocommunications Act 1992 and the structure of the Telecommunications (Interception and Access) Act; it is deliberately stated narrowly rather than overclaimed. The digital-versus-encrypted distinction and the prevalence of unencrypted public-safety digital traffic are from Matt Blaze and colleagues' published work on P25; the CTCSS "privacy code" correction is standard radio reference material. This is general information only, and nothing here is tied to any particular operational context.