Provided are methods, apparatus and computer programs for generating and using conditional electronic signatures enabling the parties to an online communication or transaction to link conditions unambiguously with signed data items, and enabling verification of the authenticity of the data item and conditions and verification of the identity of the signing party. A data item and one or more conditions are separately hashed, the resulting digests (hash values) are concatenated, and the concatenand is further hashed to produce a final digest. The final digest is encrypted using the private key of a public/private key encryption scheme to produce a conditional digital signature. The data item and conditions can each be verified. Verification includes decrypting the conditional signature and comparing the decrypted result with a separately generated final digest. A solution is also disclosed for propagating information to interested parties when a first is countermanded at a second review.