The Fourth Amendment to the US Constitution prohibits unreasonable searches and seizures and requires warrants based on probable cause. Ratified 1791. Interpreted by two centuries of Supreme Court jurisprudence into the operational doctrine that governs every digital search in the United States: warrant requirements, exceptions (border, exigent circumstances, plain view, third-party doctrine, consent), and the standards by which evidence gets suppressed when those rules are broken.
What it means in practice
The shape of digital privacy in the United States is the shape of the Fourth Amendment exceptions. The third-party doctrine (Smith v. Maryland, 1979) held that information voluntarily shared with a third party (your phone records, your bank statements) loses Fourth Amendment protection because you had no reasonable expectation of privacy in it. That doctrine governed digital surveillance for forty years until Carpenter v. United States (2018) carved out cell-site location data as warrant-required despite being held by carriers. The border exception (US v. Ramsey, 1977; reaffirmed in US v. Cano, 2019, with Ninth Circuit limits on forensic searches) authorizes warrantless device search at any port of entry. Knowing which exception applies to your situation is the work.
Who it affects, and how
Affects every US person on US soil, plus non-citizens at US borders and ports of entry. The protections are weaker for the latter and weaker still for non-citizens abroad communicating with US persons (FISA Section 702 territory). The strongest protection sits in the home: warrants for residential search require probable cause, particularity, and judicial approval. The weakest sits at the border: any device, no warrant, no probable cause, sometimes no documented reason. The Fifth Amendment’s self-incrimination protection extends to passcode disclosure in some courts (alphanumeric), not in others (biometric typically not protected, though the Ninth and Fifth Circuits split on this through 2025).
What you can change today
Three concrete actions. First, before any border crossing, switch from biometric unlock (Face ID, Touch ID) to alphanumeric passcode of 10+ characters and power the device fully off so it lands in BFU state on the next boot. Second, for sensitive at-home work, document chain of custody on devices that might face civil discovery (timestamped photos of storage locations, written log of who accessed what when). Third, if law enforcement contacts you about a digital matter, stop talking and call a lawyer; the Fourth Amendment’s exclusionary rule only protects you if the lawyer can build the suppression motion later.