Self-decryption when the confidentiality period expires. Seals are used as letterhead printed on documents to indicate identity or signature. Self-decryption at the expiration of confidentiality. The seal style is self-decryption at the expiration of confidentiality. Generally, seals are stained with pigment before printing. Those that are not stained with pigment and will appear concave and convex after being printed on a flat surface are called steel seals, and some are wax prints printed on wax or wax paint or envelopes.