I built this because I didn't trust Slack with API keys, and there are many other cases where I don’t want my shared secrets to be hanging around in chat windows.

Here is exactly how the encryption works- 1) You enter your secret 2) Your message is encrypted and a url is generated. Url contains the key for decrypting which is never passed to server. 3) Your encrypted message is send to server. Note that server don’t get the key to decrypt. 4)You have your URL which you can share. 5) On first time message read through URL, message is destroyed.