Unless you want to hack on the code, you probably want to install from https://chrome.google.com/webstore/detail/starttls-explainer/gclfifmlfchoicneobnmpjbcaieeolgn
A gmail extension that shows an icon explaining the security of the TLS encryption of the email you received. For no encryption, it shows a scary icon. For RC4, it shows a lame yellow icon. Otherwise, it shows a green lock. In the latter two cases, hovering over the icon tells you the actual cipher in a tooltip.
This uses the InboxSDK: https://www.inboxsdk.com
This only looks at the last connection: the one to Google's servers. This has some obvious negatives, but this is kinda how SSL icons work in browsers anyhow. Additionally, there is too much noise if we don't do that: often, emails would be sent in the clear from backend to frontend email server which then sends the actual message to gmail.
Additionally, I am not sure but I might want to add these icons in the "list view" and "thread view" instead of to each individual message.
<img src=https://www.dropbox.com/s/tjl7o5al8z8xcdc/Screenshot%202015-08-12%2014.41.09.png?dl=1 width=250px>
<img src=https://www.dropbox.com/s/rdh3wwy74izminv/Screenshot%202015-08-12%2014.41.01.png?dl=1 width=250px>
<img src=https://www.dropbox.com/s/traw89vkazdrgm1/Screenshot%202015-08-12%2014.47.33.png?dl=1 width=250px>
<img src=https://www.dropbox.com/s/jw8n248ffym05c6/Screenshot%202015-08-12%2014.56.02.png?dl=1 width=250px>
