Just to add the ones I know of:

https://www.mailvelope.com/

https://github.com/google/end-to-end

Beware:

• They use their own key store. => You can not reuse the one from other clients like pnupg.
• You can not use them on public computers, e.g. from an USB stick.

mailvelop now can import/export keys and lookup public keys on keyservers.

HI, I had this same question and I looked into it a bit. While mailvelope has an API (roundcube uses this for example) for better integration, just displaying the gpg encrypted message should work. Mailvelope will detect it and decrypt it with the keys available. I've tested message sending with mailvelope in SOGo, this already works fine.

Complete PGP support seems very achievable, what do you think?

An example of this can be seen in rainloop. Here are rainloop and SOGo side by side with the mailvelope extension disabled. Rainloop works great with mailvelope.

chrome_2020-08-10_17-42-12.png (188,236 bytes)   

chrome_2020-08-10_17-42-12.png (188,236 bytes)   

chrome_2020-08-10_17-47-19.png (17,178 bytes)   

chrome_2020-08-10_17-47-19.png (17,178 bytes)