Octet i of the transformed data ("transformed-octet-i") is the XOR of octet i of the original data ("original-octet-i") with octet at index i modulo 4 of the masking key ("masking-key-octet-j"):
Octet i of the transformed data ("transformed-octet-i") is the XOR of octet i of the original data ("original-octet-i") with octet at index i modulo 4 of the masking key ("masking-key-octet-j"):
j = i MOD 4 transformed-octet-i = original-octet-i XOR masking-key-octet-j
http://tools.ietf.org/html/rfc6455