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Let's Encrypt

A free, automated certificate authority that issues TLS certificates at no cost, making HTTPS easy and ubiquitous. Its automation (via the ACME protocol) lets sites obtain and renew certificates without manual effort.

Let's Encrypt is a widely used nonprofit certificate authority that provides free, short-lived TLS certificates issued and renewed automatically through the ACME protocol. By removing cost and manual hassle, it dramatically expanded HTTPS adoption across the web. For defenders it matters both as the reason HTTPS is now near-universal and because its ease of use means attackers also obtain valid certificates for malicious sites, so a valid certificate alone does not imply trustworthiness.

Introduced in: Defensive Web Fundamentals

Examples

  • A website obtaining a free TLS certificate from Let's Encrypt.
  • Automatically renewing certificates via the ACME protocol.
  • Recognizing that attackers can also get valid Let's Encrypt certificates.