Using the same DES key
The client and server compute the same DES encryption key by using public key cryptography.
For any client A and server B, a key called the common key can only be deduced by A and B. This key is . The client derives the common key by computing the following formula:
KAB = PKBSKA
where K is the common Key, PK is the Public Key, and SK is the Secret Key, and each of these keys is a 128-bit number. The server derives the same common key by computing the following formula:
KAB = PKASKB
Only the server and client can calculate this common key since doing so requires knowing one secret key or the other. Because the common key has 128 bits, and DES uses a 56-bit key, the client and server extract 56 bits from the common key to form the DES key.