To elaborate on QM's solution explanation of AReed's solution, the "^" (exclusive-OR) operator, applied to two strings, will treat them as bit vectors, and return a string whose characters each represent 8 bits, in proper order, from the XOR result.

If the two strings have the same character at the same offset, the XOR result will be zero (null byte); the "tr/\0/ /" operation will not only convert the nulls to spaces, but will also return the number of nulls found and converted.

It's okay if the two strings are not the same length, because the shorter one will be padded with nulls as needed (and any value XOR 0 gives back that same value).