All other apps and extensions store your private content somewhere, usually encrypted. SeeOnce does not do this; instead, it achieves forward secrecy by changing encryption keys with every message. This way, you don't have to trust anybody. Not even SeeOnce.
When you get a SeeOnce-locked message, just copy it into SeeOnce and it will open automatically. Then you can reply directly from SeeOnce.
Your friends don't need to have the SeeOnce app. Messages open in a webpage just by clicking them. They can do this on a smartphone if necessary, through mobile apps like this one.
Some people live in places where using encryption is itself compromising. This is why SeeOnce can disguise its output so it looks like normal text. Just supply a sufficiently long cover text after the message is locked. Disguised messages also unlock as soon as you paste them into SeeOnce.
SeeOnce has comprehensive help just one click away.
SeeOnce uses 255-bit standard elliptic curves, which have been vetted against weaknesses by experts. On top of that, SeeOnce uses 256-bit XSalsa20 encryption and 512-bit hash functions to complete the locking process. XSalsa20 is a high-performance, open source cipher, which has been scrutinized by experts for nearly a decade without any practical weaknesses being found.
This is the Android version of SeeOnce, essentially identical to the web version. The following is to check the authenticity of the web version of SeeOnce:
Current version of SeeOnce is: 1.1.1
Made on 7/18/16
Main source: https://seeonce.net
SHA256 string for web source (single html file):
Watch developer F. Ruiz reading this string: