Here's a very simple example of a strong algorithm built into a weak protocol. Consider an encryption product designed to allow Alice to send confidential messages to Bob in e-mail. Suppose that the product utilizes the "unbreakable" algorithm E. Even against the "unbreakable" algorithm, Mallory has many ways to intercept Alice's plain text, if the rest of the protocol is weak. For example, Mallory might have ways of intercepting the key, making the "unbreakable" encryption irrelevant (the key might not be stored securely, or might be transmitted without itself having adequate security). Or, the plain text might not travel the whole way as cipher text, but rather travel as vulnerable plain text for part of its trip (say from Alice's workstation to her mail server). Or, once decrypted (or before being encrypted in the first place), the message might be stored insecurely. To use a cliche, Mallory need not attack the "unbreakable" algorithm if the other links in the chain are weaker.