It is far better to use a single function that is as strong as the additive bit strengths of the sub functions. There is not even a question about this. For e

It is far better to use a single function that is as strong as the additive bit strengths of the sub functions. There is not even a question about this. For example, sha1(md5) has several flaws that jump out to crypto people. For example, a flaw in the inner function will not be protected by the outer sha1 function. In fact, the outer function will be rendered useless by an error or weakness in the inner md5 function. If I can create an md5 collision (which I just may be able to do...) then the outer sha1 is worthless. On the other hand, if there is an error in the outer sha1, the inner md5's value as a construct is reduced - since its usefulness is wholly subject to the output of the sha1. In addition, your construct f(g(data)), for many f.g implementations, clearly has a meet in the middle attack. What you are creating with your stack of ten layers is a stack that _you_ cannot break, not a secure crypto stack.