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Public-key cryptography is a cryptographic system in which keys come in pairs. The transformation performed by one of the keys can only be undone with the other. This gives public-key encryption systems an advantage over symmetric encryption systems in that the encryption key can be made public. They are typically much slower than symmetric algorithms, so they do not scale well for long messages. The only used cryptosystems are RSA (for both signing and encryption), DSA (for signing) and Diffie-Hellman.