protonURL is an information-sharing tool designed around "one-time reading." Its core purpose is to let users share confidential, secret, or non-confidential content via a link, and the content in that link is deleted after it is read for the first time, thereby reducing the risk of long-term exposure of the information.
In terms of main capabilities, the official description includes the following points: first, secure and confidential sharing, used to share private or non-private information in the form of a link, with the content deleted the first time it is read; second, proof of deletion, used to prove that the content of a certain URL has been permanently destroyed—possibly after being read, or after expiration. Since version 2, each proof is cryptographically signed using protonURL's ECDSA private key, and anyone can independently verify it using the published public key, without relying on trust in the server. For proofs generated before the v2 update, they contain a SHA-256 hash of the content metadata, which serves as a server-side audit trail, but requires trusting the protonURL server.
In terms of usage, users can create protonURL links to share content. In addition, there is also a way to provide prefilled links: one can create a link in the form https://protonurl.com?prefilled=TG9yZW0gaXBzdW0KZG9sb3Igc2l0IGFtZXQ= so that the text box is prefilled with content, thereby making protonURL ready to be created.
The content of the "prefilled" field must be encoded using base64. The official PHP example code is: <a href="https://protonurl.com/?prefilled=<?php echo base64_encode("Login: my-login\nPassword: my-password"); ?>" target="_blank">Bouton</a>.
A language can also be included in the link so that visitors jump directly to the corresponding language, for example: <a href="https://protonurl.com/ en ?prefilled=<?php echo base64_encode("Login: my-login\nPassword: my-password"); ?>" target="_blank">Bouton</a>.
The official notice specifically reminds that this feature is intended for experienced users, and these prefilled links should be used with caution, and should only be shown to people who already know the content, or to the intended recipients of that content, and not to the public. The base64-encoded "prefilled" content is not encrypted; the encoding is only used to simplify the creation and sharing of links. The official statement says that it assumes no responsibility if the system is misused.
The main applicable scenarios include: situations where passwords, login credentials, private information, etc. need to be transmitted in the form of a link, and it is desired that the content be destroyed after the other party reads it once; and scenarios where an independently verifiable proof of deletion is needed before or after sharing to confirm that the content has been permanently destroyed.
In terms of limitations, the official description clearly states that base64-encoded prefilled content does not have encryption protection and is only for convenient link creation and sharing; prefilled links should be used with caution, only for people who already know the content or who are intended to receive it, and should not be made public. In addition, proofs generated in the v1 period require trusting the protonURL server, and only signed proofs from v2 onward support independent verification without requiring server trust.
