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DES (data encryption std) known by ANSI as the data encryption algorithm (DEA) has been a w-wide std for 20 years. uses a 56 bit key (expressed as a 64 bit number). DES can allegedly be cracked and it gets less secure with time. DES, an encrption block cypher is well known and widely used. Export of DES in hw or sw is strictly regulated by the US gov’t and the NSA (Nat’l Security Agency, No Such Agency, Never Say Anything).
Kerberos uses DES. It’s a trusted 3rd party authentication protocol designed for tcp/ip nws. 3rd party=kerberos sec srvr (which acts as a trusted arbitrator).
IDEA was invented in 1990 and uses a 128 bit key. It hasn’t replaced DES partly cause of its patent and has to be licensed for commercial apps - used by PGP.
The problem with a shared private key is that it can be more easily compromised. Nonrepudiation is more common with private key. Private key are great for encrypting your own private files.