In this illustration, we see Trudy at top left looking malicious. Below her is a letter, labeled "Message," that reads:

Alice,

Deliver a pizza to me.

Bob

Blue arrows extend rightward and downward from this message.

A blue arrow extends rightward from the message to a blue box labeled "Many-to-one hash function," and another arrow points rightward from the many-to-one hash function to the encryption algorithm. To the right of the algorithm is Trudy's private key, K (-/T); a blue arrow points from the key to the algorithm. A blue arrow points downward from the algorithm to a letter labeled "Signed (using Trudy's private key) message digest." This letter reads:

Fgkopdgoo69cmxw

54psdterma[asofmz

A blue arrow points leftward from the signed message digest to Alice's mailbox, where we also see Alice. In addition, a blue arrow points downward from Trudy's original message to Alice's mailbox.

A blue arrow points downward from Alice's mailbox to a blue box labeled "Alice uses Trudy's public key, thinking it is Bob's, and concludes the message is from Bob." To the left of this box is Trudy's public key, K(+/T); a blue arrow points from the key to the blue box. And finally, a blue arrow points rightward from the blue box to a pizza, having been freshly delivered to Bob, who looks confused.