Key agreement method for establishing an encrypted transmission through an optical link between a first optical node and a second optical node wherein the protection key comprises a value of a physical impairment parameter undergone by the optical signal transmitted through the optical link (N1-N2), the value of the physical impairment parameter being dependent on the length of the link and being identical for both sides of transmission of the optical signal within the optical link (N1-N2), the method comprising the following steps: - applying a first random value of the physical impairment parameter at the first optical node, - sending a signal with the applied first random value from the first optical node to the second optical node through the optical link, - determining, at the second optical node, the received value of the physical impairment parameter corresponding to the addition of the first random value and the physical impairment undergone along the optical link, - applying a second random value of the physical impairment parameter at the second optical node, - sending a signal with the applied second random value from the second optical node to the first optical node through the optical link, - determining, at the first optical node, the received value of the physical impairment parameter corresponding to the addition of the second random value and the physical impairment undergone along the optical link - selecting, for both optical nodes, a protection key by adding at each node their respective determined value received over the optical link with their respective applied random value, the protection key corresponding therefore to the first random value, the second random value and the value of the physical impairment undergone along the optical link wherein the physical impairment parameter corresponds to the polarization mode dispersion differential group delay "PMD DGD". - applying a first random value of the physical impairment parameter at the first optical node, - sending a signal with the applied first random value from the first optical node to the second optical node through the optical link, - determining, at the second optical node, the received value of the physical impairment parameter corresponding to the addition of the first random value and the physical impairment undergone along the optical link, - applying a second random value of the physical impairment parameter at the second optical node, - sending a signal with the applied second random value from the second optical node to the first optical node through the optical link, - determining, at the first optical node, the received value of the physical impairment parameter corresponding to the addition of the second random value and the physical impairment undergone along the optical link - selecting, for both optical nodes, a protection key by adding at each node their respective determined value received over the optical link with their respective applied random value, the protection key corresponding therefore to the first random value, the second random value and the value of the physical impairment undergone along the optical link wherein the physical impairment parameter corresponds to the polarization mode dispersion differential group delay "PMD DGD".