A few things about the extension. I don't know how secure the blowfish algorithm is. Next, I don't know whether extensions call home or not. That is, this extension uses all client side code. It would seem that there should not be any interchange between Chrome and a server somewhere, but I just don't know if it's the case.
The extension is simple to use. There is a textarea that holds the encrypted or decrypted text. This textarea is followed by a text field that holds the encryption key. This key is not saved anywhere. No cookies, no local storage, etc. The key field is followed by two buttons: one for encryption and one for decryption. It looks like the following:
To use, simply type or paste the desired unencrypted text in the field, enter a key, and then click "Encrypt". To decrypt, pasted the encrypted string into the field, enter the same key, and then push "Decrypt".
On to the building of this simple encryption extension. The extension consists of five files located in one folder:
The manifest.json file contains the following:
The file index.html contains the following:
The background.js files consists of the following:
Finally, there is the file blowfish.js. There is no need to display the text for it. It is reviewable at its site or here.
The file icon.png is just an image of a key.
Once these files have been created, all that remains is to add the extension to Chrome. Just go to the extension management page, click "Load unpacked extension" and point to the directory.