Sat, 06 Feb 2010
Cosmetic carbon copy: The opposite of blind carbon copy
I propose a new feature for email software: Cosmetic carbon copy. CCC works very similarly to today's carbon copy feature: If you put an address in the CCC: list, when the recipients open the message, they can see the address. The one difference is that with "cosmetic" carbon copy, the CCC:d address never recieves the message.
An example might illustrate the situation. Let's say someone (Alice) wants to invite Bob to a movie but doesn't want Charlie to come. She might send this email:
From: alice@example.com To: bob@example.com CCC: charlie@example.com Subject: MOOOVIEEE!
Hey all my friends, join me for a movie tonight!
When Alice sends the email, Bob recives a copy. Bob thinks that Charlie received a copy, as he will be in the CC list. But Alice's mail software never sent Charlie a copy. So Alice can relax, knowing that Bob won't forward a copy to Charlie, since Bob thinks he already received it.
Alice and Bob will meet up without Charlie, and Alice will quietly sigh in relief.
How can this work?
Cosmetic carbon copy works on the same principle as blind carbon copy: the contents of the message are fundamentally independent from the recipients.
Saavy email users have used the "blind carbon copy" feature for years. When you place an address in the BCC list, that address will receive a copy of the email even though other recipients won't see the address listed. This allows for a form of privacy between the sender of the email and that hidden recipient.
This works because, like postal mail, email messages are delivered using an "envelope" that contains the actual recipients. When you compose a message in Alpine, Gmail, or Thunderbird, you are writing the contents of the envelope. When you hit send, the mail software looks for email addresses that should receive a copy. It creates one envelope per address, stuffs the letter inside, and sends the whole thing on its way through the Internet using a protocol called SMTP. When the recipient (Bob)'s email system receives it, the letter is pulled out of the envelope and placed in an inbox.
Which means the envelope is entirely invisible to Bob, the recipient. Most mail software puts information about the envelope in email headers. Since each email recipient gets a separate envelope, In the case of cosmetic carbon copy, Alice's letter claims that Charlie was on the CC: list. But Bob can never learn that Charlie's copy was never sent.
So to implement cosmetic carbon copy, Alice's mail program must understand this rule:
- For each address in the CCC list, copy the message to the CC: list in the email message.
- Create no new envelopes when sending the message.
Because of the last bullet point, this "carbon copying" is purely cosmetic rather than functional.
A possible privacy problem
You might wonder, what happens when Bob hits reply-all to say "Yes, I'm coming"? Then (oh no!) Charlie might see the invitation.
Not a problem: Most of my friends use Gmail, and Gmail users almost always hit Reply instead of Reply-All. Gmail used to have an experimental option called "Reply All by Default", but thankfully they removed that option.
A future version of this specification might suggest mangling the last "." in the BCC:d email addresses with the ONE DOT LEADER character, which looks like "." but isn't one. That way, even if Bob clicks "Reply all," Charlie's email address is subtly incorrect and will bounce.
Future work
The story I've told here is marginally simplified so it's understandable to a less-technical audience. Those who want a more technical version can request I submit an RFC. Perhaps in two months?
(File under "games to play with email.")
