He hammered home one point which I agree with in principal:  don’t implement your own crypto.  However, from a practical standpoint, that’s misleading.  The message should be:  don’t implement your own protocols. 

What do I mean by that?  Suppose I’m designing a new authentication scheme and I naively choose DES as my encryption algorithm.  As a result, an attacker could conceivably sniff my traffic and brute-force the key.  But suppose further that, in designing my authentication protocol, I forgot to guard against replay attacks (which have nothing to do with my choice of crypto algorithm).  Now, instead of brute-forcing the key, the attacker can simply resend the packets to achieve a successful authentication.  The latter attack is way easier.

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