Alice loves Bob. She has been getting messages from someone named Bob who is requesting money, because he was mugged in a foreign country. What procedure would Alice use to verify that Bob is actually sending the letter?

Have Bob attach a digital signature. Using Bob's public key decrypt the digital signature, then compare the attached hash value of the message to hash value from her own machine.

Have Bob attach a digital signature. Set up a session key, exchange the shared key, use this key to decrypt the hash of the digital signature.

Have Bob attach a digital signature. Using Alice's public key decrypt the digital signature, then compare the attached hash value of the message to hash value from her own machine.

Explanation

After Bob has created and attached his digital signature (which consists of a the hash value of the message encrypted by his private key) Alice would use the corresponding key pair (Bob's public key) to decrypt the hash value. This hash value would then be used to verify the message, and unless Bob's private key has been compromised Bob would have sent that message. EDIT: Reading through the entirety of all the possible answers takes way too long. Edit: deleted 3 useless answers to reduce reading (EDIT: This leaves out the very important step that Bob has to have his public key verified by a CA. Otherwise a fake Bob could send his own (Steve's) public key and call it Bob's.)