An X.509 certificate is a digital document that certifies the ownership of a cryptographic public key. The certificate contains information about the key, its owner (subject), the issuer, and the issuer's digital signature, which verifies the certificate's content. The public key can be used to communicate securely with the certificate subject if the certificate signature is valid and the software trusts the issuer. The X.509 standard defines the most common certificate format, which is further detailed in RFC 5280.
The certificate in public key infrastructure (PKI) is issued by a certificate authority (CA), which is typically a company that charges its customers for issuing certificates. Individuals in a web of trust scheme can also issue certificates.
A certificate can be used for TLS over HTTPS, email encryption, code signing, digital signatures, and other things. A certificate can serve one or more functions.
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