Are digital signatures error-proof?

 Digital signatures are a way to make sure that the sender's identity is not mistaken. The sender is confirmed to be who he claims to be. The content signed by the sender too is automatically linked to his identity and he cannot deny that he sent it, because his digital signature appears on it. But if you have never used a digital signature, you may have your own apprehensions.

 As long as the message is hashed and then signed, there is no scope at all for a case of mistaken identity. For example, if you have created a hashed message and then signed it digitally and the document is altered (by anyone, including yourself) in any way, the recipient will fail to match the public key with your private key. This shows that digital signatures by themselves are error-proof not only with respect to identity but also content. Of course, when the verification process fails, you would not be able to know if it was just a transmission error or a forgery attempt. 

Validity of Digital Signatures

How long a digital signature remains valid depends on who has issued it. Usually the validity ranges from 1-2 years. Every issuing company has its own product specification that varies according to its target market. The good news though is that digitally signed documents can be time-stamped digitally, so that even after the key expires, the signature can be validated.

 This comes in handy when you have digitally signed a contract or agreement that will stay in force beyond the validity period of the signature, e.g. long-term leases. All parties to the contract must retain the time-stamp copy.  If the document is time-stamped, then even if the signer’s key is compromised later on, the contract’s validity can be proved by this time-stamp.