The invention provides techniques for issuing electronic seals of certification to online entities, such as online merchants, in a manner that prevents non-certified parties from easily stealing or otherwise misusing the seals. A central seal issuer verifies the credentials, policies or business practices of each merchant and issues a corresponding seal of certification. For each merchant, the seal issuer generates a unique media object representing an electronic seal. The media object includes an encrypted watermark containing an expiration date. The media objects are stored on a seal server such that the merchants can dynamically retrieve and display the electronic seals when accessed by a user. The seal issuer can easily revoke or update the centrally stored seals.