Techniques for conducting secure online transactions using captcha images as watermarks are provided. Some techniques combine a trusted, secure device that utilizes a key to generate identifiers with a watermarked transaction verification request page to provide secure online transactions. The trusted, secure devices are provided to users for use in transacting with a transaction provider. In processing a transaction requested by a user, the transaction provider determines an identifier that should be being currently generated by the user's trusted, secure device, creates a captcha image of the identifier, and watermarks a transaction verification request page using the captcha image. The transaction provider then requests that the user verify the transaction described in the transaction verification request page by providing the next identifier that is generated by the user's trusted, secure device to the transaction provider.