That means that to use the card for encryption and digital signatures, a person needs the personal identification number (PIN) code necessary to unlock it. Protecting the private key is the idea behind some smart cards such as the United States Department of Defense's Common Access Card (CAC), which never lets the key leave the card. Someone who possesses the private key can create a valid digital signature. However, a digital signature is forensically identical in both legitimate and forged uses. The role of the certificate authority is to authoritatively state to whom the certificate belongs, meaning that this person or entity possesses the corresponding private key. A public key certificate can be used by anyone to verify digital signatures without a shared secret between the signer and the verifier. įor digital information, the most commonly employed TTP is a certificate authority, which issues public key certificates. This double security makes notaries the preferred form of verification. A notary provides the extra benefit of maintaining independent logs of their transactions, complete with the types of credentials checked, and another signature that can be verified by the forensic analyst. A notary is a witness who verifies an individual's identity by checking other credentials and affixing their certification that the person signing is who they claim to be.
A forensic analyst specializing in handwriting can compare some signature to a known valid signature and assess its legitimacy. The two most common TTPs are forensic analysts and notaries. To mitigate the risk of people repudiating their own signatures, the standard approach is to involve a trusted third party. If the key used to digitally sign a message is not properly safeguarded by the original owner, digital forgery can occur. Verifying the digital origin means that the certified/signed data likely came from someone who possesses the private key corresponding to the signing certificate. If confidentiality is also required, then an encryption scheme can be combined with the digital signature, or some form of authenticated encryption could be used. Note that the goal is not to achieve confidentiality: in both cases (MAC or digital signature), one simply appends a tag to the otherwise plaintext, visible message. The difference between MAC and Digital Signatures, one uses symmetric keys and the other asymmetric keys (provided by the CA).
To achieve non-repudiation one must trust a service (a certificate generated by a trusted third party (TTP) called certificate authority (CA)) which prevents an entity from denying previous commitments or actions (e.g. Thus just providing message integrity and authentication, but not non-repudiation. A misconception is that encrypting, per se, provides authentication "If the message decrypts properly then it is authentic" - Wrong! MAC can be subject to several types of attacks, like: message reordering, block substitution, block repetition. Message Authentication Codes (MAC), useful when the communicating parties have arranged to use a shared secret that they both possess, does not give non-repudiation. The common method to provide non-repudiation in the context of digital communications or storage is Digital Signatures, a more powerful tool that provides non-repudiation in a publicly verifiable manner.
Because of this, data integrity is best asserted when the recipient already possesses the necessary verification information, such as after being mutually authenticated. Even with this safeguard, it is possible to tamper with data in transit, either through a man-in-the-middle attack or phishing. A data hash such as SHA2 usually ensures that the data will not be changed undetectably. Proof of data integrity is typically the easiest of these requirements to accomplish.
In general, non-repudiation involves associating actions or changes with a unique individual.