How can testimony from a document's signer help prove that the document is genuine?

In the context of law, establishing the authenticity of a document is necessary when its validity is contested. Under Anglo-American legal systems, direct evidence of a document's authenticity can be established through the testimony of the individuals who signed the original document.1

By providing firsthand confirmation, the signer serves as a witness to the document's creation, acting as a form of authentication that verifies the document is genuine. While this is a primary method for proving authenticity, it is not always possible to secure such testimony. In cases where the signer is unavailable, the law permits the use of circumstantial evidence to establish that a document is authentic.21