Blockchain technology is now hitting the mainstream, and countless human interactions, legitimate and illegitimate, are being recorded permanently—and visibly— into distributed digital ledgers. Police surveillance of day-to-day transactions will never have been easier. Blockchain’s open, shared digital architecture thus challenges us to reassess two core premises of modern Fourth Amendment doctrine: that a “reasonable expectation of privacy” upholds the Amendment’s promise of a right to be “secure” against “unreasonable searches,” and that “a reasonable expectation of privacy” is tantamount to total secrecy. This article argues that these current doctrines rest on physical-world analogies that do not hold in blockchain’s unique digital space. Instead, blockchain can create security against “unreasonable searches,” even for data that are shared or public, because blockchain’s open distributed architecture does the work in digital space that privacy does in physical space to advance Fourth Amendment values such as security, control of information, free expression, and personal autonomy. The article also evaluates textualist approaches to blockchain, concluding that the twenty-first century’s latest technology shows how the eighteenth-century text’s focus on ownership and control may be a better means to achieve fundamental human ends than privacy-as-secrecy. Finally, the article proposes an analytical framework for Fourth Amendment protections for distributed ledgers—corresponding to the levels to which blockchain users evince control of their data—that is grounded in text and theory and that is administratively practicable for courts.