When Alice receives it , she uses it to lock a box containing her message , and sends the locked box to Bob .
First , Alice asks Bob to send his open padlock to her through regular mail , keeping his key to himself .
When Alice receives it , Alice uses it to lock a box containing Alice 's message , and sends the locked box Bob Bob .
First , Alice asks Bob to send his open padlock to her through regular mail , keeping his key to himself .