Online Certificate Status Protocol (OCSP)
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def
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a protocol that lets a host (e.g., a browser) directly query a certificate authority about the current status of a specific certificate
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how it's used
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messages are usually sent to an OCSP responder via HTTP — easier to support over internet links and more efficient than downloading a full Certificate Revocation List (CRL)
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not all browsers/apps support OCSP, and some that support it don't actually bother checking
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