Cryptographers have studied combining hash functions. See https://scholar.google.com/scholar?q=hash+function+combiner https://scholar.google.com/scholar?q=hash+

Cryptographers have studied combining hash functions. See https://scholar.google.com/scholar?q=hash+function+combiner https://scholar.google.com/scholar?q=hash+function+combiner for a bunch of links. Also see http://eprint.iacr.org/2013/210 http://eprint.iacr.org/2013/210 for an interesting read. People are eager to combine different hash algorithms frequently because they desire to hedge against weaknesses in one of the algorithms. Or, in this case, because they want to shore up an existing 'weakness' in one of the algorithms (for bcrypt, its maximum input size). You are correct that doing this ad-hoc should be an 'anti-pattern'. There are subtle details (see the papers linked above...). However, the idea itself is sound, if handled properly.