The syntax of the language is similar to Pascal. CRLs are lists of certificates that are not valid because the accompanying private key has been lost, stolen, or compromised. In this way, another user masquerading as the legitimate user will not be validated when he attempts to use her private key to sign a file. The public key is used to encrypt a message or verify a signature, and the private key is used to decrypt the message or to sign a document. Once a file has been encrypted with a user's public key, only the corresponding private key can decrypt the file.
Symmetric cryptography uses one key for both encryption and decryption.
If a person possesses the key, he can both encrypt and decrypt messages. Unlike public keys, this single secret key cannot be shared with anyone except people who should be permitted to decrypt as well and encrypt messages. Public-key cryptography holds a major advantage over traditional symmetric cryptography. Because of the nature of a single secret key, symmetric cryptography cannot be used for communications over an insecure channel unless a secure channel also exists for the key to be distributed to the valid users. It only makes sense that if a secure channel exists, there is no need to encrypt communications and send them via an insecure channel.