Source : Free On-Line Dictionary of Computing

synchronous key encryption
     
         Data {encryption} using two
        interlocking keys where enything encoded using one key may be
        decoded using the other key.  This means if someone makes one
        of the two keys publicly available (as in {public-key
        encryption}) and keeps the other private, then anyone may send
        them a message or data that only they can decode, giving
        privacy, and furthermore, the sender may also encrypt that
        same message additionally with their own private key, making
        it impossible to read without decoding first with *their*
        _public_ key by the receiver, this gives authenticity.
     
        It is a very powerful system.  One cannot determine one key
        from the other, nor can they crack the encryption by computing
        all combinations, because, depending on the size of the keys
        (sometimes as large as 1024 bytes, though having grown from
        smaller versions in popular implementations of the software
        which does this), the amount of computing power required to
        crack the code is unavailable, even supercomputers would take
        more than a hundred years to crack it.
     
        {PGP} is a publicly availble software implementation written
        by Phil Zimmermann.
     
        (1994-10-10)