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Verifiability is an essential quality for any signed document, whether in digital or physical form. It means that a document's authenticity can be confirmed by others, such as a trusted third party.
In the case of digital signatures, verifiability is achieved through cryptographic methods.

These methods involve the use of public keys, which are part of a public key infrastructure. When a document is signed digitally, a unique signature is created using the sender's private key. The recipient can then verify this signature by comparing it with the sender's public key, ensuring that the document has not been tampered with.

• **In digital signatures: Uses cryptographic technology**
• **In handwritten signatures: Relies on expert analysis**

Verifiability reassures the recipient that the signer is legitimate and the content is original, thus maintaining trust in communication and transactions.

Non-forgeability means that once a document has been signed, that signature cannot be duplicated by anyone other than the original signer. This is critical in preventing fraud and unauthorized access to sensitive information.
In digital documents, non-forgeability is ensured through the use of cryptographic algorithms.

These algorithms, along with private keys, ensure that even if a third party intercepts the message, they cannot reproduce the signature without access to the private key of the signer.

• **In digital signatures: Prevents unauthorized replication**
• **In handwritten signatures: Requires security measures**

This concept is vital for maintaining integrity across legal documents, contracts, and confidential communications. By assuring the non-replicability of a signature, parties can confidently rely on the legitimacy of signed documents.

Public key authentication is a security measure that uses a pair of keys—a public key and a private key—to verify identity and protect information. This method is widely used in digital communications to establish secure connections.
When a message or document is sent, the sender uses a private key to generate a digital signature.

The recipient uses the corresponding public key to authenticate the signature, ensuring the message is legitimate and was indeed sent by the stated sender.

• **Protects confidentiality in communications**
• **Ensures sender's identity is verified**

Public key authentication not only helps in verifying the sender's identity but also in safeguarding the data from unauthorized access. This process is crucial for online transactions, secure email exchanges, and any digital communication where trust and verification are required.