web based shared secret symmetric encryption application

[Cryptoparty Marseille]

Hi!

I'm looking for a decent free and open source application that would
work similarly to "Hushmail Express message"
(https://help.hushmail.com/entries/351388-I-have-received-an-encrypted-email-from-Hushmail-how-do-I-read-it-)
that would allow me to deposit an encrypted message on an apache web
server that would be decrypted by the recipient using a shared secret
(that's already established, so I don't need a key exchange mechanism).

I read Tony's posts and his blog article
(http://tonyarcieri.com/whats-wrong-with-webcrypto) and I'm well aware
that browser based encryption is not optimal but I'm just looking for
something slightly more secure than plain text email that could be used
by an end user not willing to install GPG, not an NSA proof secure system.

The (apache2/debian7) web server is physically under my control, I'm the
only root (I hope! :-) and SSL/TLS is enabled.

Thanks for your advice.

cryptomars

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