Encryption in SET protocol I have a rough idea of what is symmetric and asymmetric encryption, but I can't apply the idea to the SET protocol (Secure Electronic Transaction
Specification), so how the symmetric and asymmetric encryption are used in the SET protocol? Also I want to know more about the Preq messages.
how the symmetric and asymmetric encryption are used in the SET protocol?

As per section 3.2 (which goes into further detail) of the SET specification here:
Symmetric Encryption

SET will rely on cryptography to ensure message confidentiality. In SET, message data will
be encrypted using a randomly generated symmetric encryption key. This key, in turn, will
be encrypted using the message recipient’s public key. This is referred to as the “digital
envelope” of the message and is sent to the recipient along with the encrypted message itself.
After receiving the digital envelope, the recipient decrypts it using his or her private key to
obtain the randomly generated symmetric key and then uses the symmetric key to unlock the
original message.

Asymmetric Encryption

SET uses a distinct public/private key pair to create the digital signature. Thus, each SET
participant will possess two asymmetric key pairs: a “key exchange” pair, which is used in
the process of encryption and decryption, and a “signature” pair for the creation and
verification of digital signatures. Note that the roles of the public and private keys are
reversed in the digital signature process where the private key is used to encrypt (sign) and
the public key is used to decrypt (verify the signature).


You can find even more detailed information in the Formal Protocol Definition here which also goes into some detail on PReq messages (on PDF page 84).