Any name or other information, such as signing time, that appears in a document is not by itself necessarily any proof of identity etc. Information placed in the signature appearance is under control of the party creating the signed document In the case of Acrobat the signer can configure the appearance to be what he wants, and similarly in MS Office the signer can provide text / graphics as required. Even if such configuration controls were not available in the application, because of the features that electronic document commonly have of being able to represent information in any form that the author requires, it would not be possible to assure that any automatically created appearance cannot be manually forged by the document signatory.

This fact does not denigrate the importance of the signature appearance in providing a means for the signatory to indicate intent when applying the signature. The very fact that the appearance is under control of the author means that it directly represents the intent of the signatory. The signature appearance must be considered as distinct from the digital signature which is linked to the appearance and provides a means of verifying its authenticity.

The Signature Appearance should be visually verified against the Digital Signature

The digital signature linked to the signature appearance should be used to verify that the signature appearance is authentic As the digital signature is created using a certified key, which is known to belong to an identified person, it can be used to verify the authenticity and integrity of the signed document. However, because the digital signature is invisible, and does not directly appear within the document, it cannot be used to indicate intent through its appearance within the document.

Verification of the signature appearance against the digital signature can be done visually when reading a document by displaying the signature verification information. The reader may visually check the verification information against the signature as it appears in the document.

Consideration had been given to the possibilities of automatically checking the signature appearance against the digital signature by the ETS1 team working on PDF Advanced Electronic Signatures standards. However, it was considered that a fraudulent signer could provide information in a way that may mislead the verification as to what is displayed and how this is linked to the digital signature. Hence it was decided that the surest way is to check the appearance and the verification information visually. Rather the human reader should be assisted in carrying out a visual comparison of the verification information derived from the digital signature against the signature appearance by providing information in a way that the two can be easily related.

Human Understanding of Advanced E-Signature Verification

As verification of the digital signature against the signature appearance is to be carried out by a human, who may not be aware of digital signature technology, the verification needs to be presented in a way that can be clearly understood. Generally, the person reading a document and wishing to verify the signature will have little or no understanding of digital signatures certificates.

The reader clearly needs to be shown whether the signature is valid. However, there will be situations where the validity of the signature is unknown or even the signature is considered invalid. In which case, some basic information needs to be provided to help the reader understand the reason for the signature not being known as valid. If possible this should be provided in simple non-technical terms so that a basic assessment may be made by the reader, but this has to be backed up by detailed information that can be used by an expert in the situation where validity of the signature is of paramount importance (e.g. in case of legal dispute).

The reader also needs to be provided with information regarding certified identity that can be checked against the claimed identity in the signature appearance. This may be provided at a basic level that can be easily related to the identity in the signature appearance but should be backed up by detailed information on the full identity in case of dispute.

If the information displayed in the signature verification is consistent with the appearance of the signature in the document then the user can be assured of the authenticity of the signature as it appears in the document.