In cryptography, X.509 is a standard defining the format of public key certificates. X.509 certificates are used in many Internet protocols, including TLS/SSL, which is the basis for HTTPS, the secure protocol for browsing the web. They are also used in offline applications, like electronic signatures.
https://en.wikipedia.org/wiki/X.509
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assinatura digital - MD5, SHA-1
criptografia simétrica - DES, AES, RC4
criptografia assimétrica - RSA