GnuPG: An Open Solution to Data Protection
The public-key cipher system utilizes two keys: a private one that is used for decrypting data, and a public one for encrypting data. This eliminates the problem of secure key communication that is inherent in the symmetric cipher system, since the public key may be freely transmitted. It is, however, imperative that the private key be kept just that - private. As such, the hybrid cryptosystem is one in which the core is the public-key cipher and a symmetric cipher is used to protect the private key.
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