From his FAQ:
"About My Areas of Non-Expertise (e.g., Applied Cryptography)
I believe that sound practice of Cryptography has to be based on firm theoretical foundations. Still, this necessary condition is not sufficient, and expertise in applied (or practical) aspects of cryptography requires more than understanding of the theoretical foundations of Cryptography. For example, although I consider myself an expert on the theoretical foundations of Cryptography, I have very little knowledge of the applied (or practical) aspects of cryptography. In particular, please do not ask me to evaluate the security of any specific construct.
About Consulting
My above statement of limited knowledge of the applied (or practical) aspects of cryptography means that I'm not the right person to ask for consulting regarding the latter aspect. On top of this, I am not interested at all in consulting to or being involved in any commercial enterprise. My main reason is my unwillingness to make any commitment to spend time on anything other than my research and personal interests. Unfortunately, for lack of time, I properly pursue only a tiny part of my research and personal interests, and I have no intentions of making this part even smaller."
I believe that sound practice of Cryptography has to be based on firm theoretical foundations. Still, this necessary condition is not sufficient, and expertise in applied (or practical) aspects of cryptography requires more than understanding of the theoretical foundations of Cryptography. For example, although I consider myself an expert on the theoretical foundations of Cryptography, I have very little knowledge of the applied (or practical) aspects of cryptography. In particular, please do not ask me to evaluate the security of any specific construct.
About Consulting
My above statement of limited knowledge of the applied (or practical) aspects of cryptography means that I'm not the right person to ask for consulting regarding the latter aspect. On top of this, I am not interested at all in consulting to or being involved in any commercial enterprise. My main reason is my unwillingness to make any commitment to spend time on anything other than my research and personal interests. Unfortunately, for lack of time, I properly pursue only a tiny part of my research and personal interests, and I have no intentions of making this part even smaller."