The private key, regardless of the key system utilized, is typically specific to a party or computer system.  It is mathematically impossible for the holder of any public key to use it to figure out what the private key is.

Regardless of the key system utilized, physical controls must exist to protect the confidentiality and access to the key(s). In addition, the key itself must be strong enough for the intended application. The appropriate encryption key may vary depending on how sensitive the transmitted or stored data is, with stronger keys utilized for highly confidential or sensitive data. Stronger encryption may also be necessary to protect data that is in an open environment, such as on a Web server, for long time periods. Because the strength of the key is determined by its length, the longer the key, the harder it is for high-speed computers to break the code.