Hm, is `hkdf()` defined to accept+return sjcl bitArrays? Looks like it from here.. in that case, then yeah the hex form of the output should have twice as many characters as the "42" you passed in.

FWIW, I *think* (but I'd have to look it up) that sjcl's bitArrays also have a .length, but it's something weird like the number of 32-bit words.