A method for increasing security in a passive optical network including an optical termination device and an optical line termination, wherein said method comprises: a) at said optical line termination: a1) detecting access of said optical termination device to said passive optical network; a2) establishing a connection with said optical termination device; a3) generating a first authentication message comprising a first random number associated to said optical termination device, said first authentication message being configured as a physical layer operations and maintenance message; and a4) transmitting said first authentication message through said established connection; b) at said optical termination device: b1) receiving said first authentication message through said established connection; b2) calculating a first authentication code by using said first random number and a secret code stored at said optical termination device; and b3) generating and transmitting through said established connection a second authentication message comprising said first authentication code, said second authentication message being configured as a physical layer operations and maintenance message; c) at said optical line termination: c1) receiving said second authentication message; c2) calculating a second authentication code by using said first random number and a secret code stored at said optical line termination; and c3) authenticating (215, 216) said optical termination device if said first authentication code matches said second authentication code. a) at said optical line termination: a1) detecting access of said optical termination device to said passive optical network; a2) establishing a connection with said optical termination device; a3) generating a first authentication message comprising a first random number associated to said optical termination device, said first authentication message being configured as a physical layer operations and maintenance message; and a4) transmitting said first authentication message through said established connection; a1) detecting access of said optical termination device to said passive optical network; a2) establishing a connection with said optical termination device; a3) generating a first authentication message comprising a first random number associated to said optical termination device, said first authentication message being configured as a physical layer operations and maintenance message; and a4) transmitting said first authentication message through said established connection; b) at said optical termination device: b1) receiving said first authentication message through said established connection; b2) calculating a first authentication code by using said first random number and a secret code stored at said optical termination device; and b3) generating and transmitting through said established connection a second authentication message comprising said first authentication code, said second authentication message being configured as a physical layer operations and maintenance message; b1) receiving said first authentication message through said established connection; b2) calculating a first authentication code by using said first random number and a secret code stored at said optical termination device; and b3) generating and transmitting through said established connection a second authentication message comprising said first authentication code, said second authentication message being configured as a physical layer operations and maintenance message; c) at said optical line termination: c1) receiving said second authentication message; c2) calculating a second authentication code by using said first random number and a secret code stored at said optical line termination; and c3) authenticating (215, 216) said optical termination device if said first authentication code matches said second authentication code. c1) receiving said second authentication message; c2) calculating a second authentication code by using said first random number and a secret code stored at said optical line termination; and c3) authenticating (215, 216) said optical termination device if said first authentication code matches said second authentication code.