i'm test building a bunch of packages for ghc 7.5 head,
and crypto api builds fine if the following changes are made
relax the bytestring constraint to < 0.11
modify the CInt import to CInt(..)
(the new ffi stuff complains if the constructor for a new type isn't in scope)
and also the largewords package which need num constraints in its instance heads, and i've emailed the author about that.
Its not an issue, its currently a warning at compile time for 7.4.* ghc, but for 7.6.* it'll be a compile time error / failure if the constructor isn't in scope.