Co-signature and multiple signatures

Co-signature designates the collection of at least two signatures on the same document. Two modes are distinguished: sequential signature (signer B receives the invitation only after A has signed — useful for hierarchical contracts or notarial deeds) and parallel signature (all signers receive the invitation simultaneously — faster for symmetric documents). Co-signature raises the question of inter-round integrity: each PAdES added in the PDF must reference the previous revision via an incremental signature, ensuring that no party has modified the document between two appositions. Certyneo manages signer order, targeted reminders, per-signer expiration deadlines, and automatically detects inter-round modification attempts via SHA-256 hash verification at each step of the audit trail.