Is Form Signer (Digital Signature) feasible in Oracle R12 Seeded Screens?

We want digital signature to be applied on certain fields of the screen (Oracle Forms or OAF Pages). For example: PO Quantity, PO Price Field at Purchase Order Line Level. The creater of the purchase order will sign the fields using the token (USB device), which contains the certificate. There will be an image (TICK Mark) against the field which are digitally signed. Then during approval, the approver of Purchase Order will validate the digital signature.

This signature confirms that the document is originated from the signer and has not been altered. For this purpose, Digital Certificates issued by Certificate Authorities are used. Data will be signed by Digital certificate present on external devices such as tokens. This USB tokens will be given to each authorized users.