Secure communication method

An error correction protocol utilising as a security measure the bit error rate of a high bandwidth random data stream (111) broadcast by an optical transmission system (106). A first user (104a) and a second user (104b) receive the random data stream (111) and extract independently a keychain at an extraction rate chosen to memory bound an enemy. The protocol allocates a portion of the extracted data for the purpose of masking the result of an error correction function communicated between the users. The first user applies an error correction function to a key part of its keychain and codes the result using another part of its keychain. The coded result is sent to the second user, who decodes it and uses the decoded result to recover the corresponding key part of its keychain. The respective key parts can be used as a shared secret.