Types of signatures Problems with direct signatures Validity of scheme depends on the security of the sender's private key sender may later deny sending a certain message Y Private key may actually be stolen from X at time T, so timestamp may not help7 Types of Signatures • Problems with direct signatures: ✓Validity of scheme depends on the security of the sender’s private key  sender may later deny sending a certain message. ✓Private key may actually be stolen from X at time T, so timestamp may not help