bxor a binary k against a (longer) binary value

binary_xor.erl

-module(foo).

-export([b/2]).

b(K, B) when is_binary(K) andalso
             is_binary(B) andalso
             byte_size(K) =< byte_size(B) ->
    SizeB = byte_size(B),
    N = (SizeB div byte_size(K)) + 1,
    K2 = lists:duplicate(N, K),
    K3 = list_to_binary(K2),
    K4 = <<K3:SizeB/binary>>,
    binary_xor(K4, B).

binary_xor(A, B) when is_bitstring(A) andalso
                      is_bitstring(B) andalso
                      bit_size(A) =:= bit_size(B) ->
    BitSize = bit_size(A),
    <<IA:BitSize>> = A,
    <<IB:BitSize>> = B,
    I = IA bxor IB,
    <<I:BitSize>>.