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I have boldly stated that there is no way to 'write protect' an e-mail, but want to be sure of my ground.

Surely if Joe Bloggs receives an e-mail from me (or my e-commerce site or a web application written by me) saying - "I will sell you X for $10 ", he COULD modify that price to, say, $9, and forward it to someone or save te e-mail, but so what ?

The original e-mail will always be accesible and traceable on mail servers and a copy in mail clients of all recepients.

So therefore, there is no need to do this - and anyway it is not possible.

Is this true ?

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This sounds like you're trying to rationalise not doing a particular bit of work. Care to explain the context behind this question? – Polynomial Jan 14 at 12:32
Thanks. Yes. I'm writing a simple calculator whereby the user completes an HTML form, we do some maths on the $_POST-ed array variables and end up with an 'offer', which we then mail to a prospective cc ourselves using the PHP mail($to, $subject, $message, $headers); function. The question has been asked by someone in the client company, and to be honest I have never been asked this before. So just sense checking. – Arthur Mild Jan 14 at 12:40
Interesting. I can think of a few issues, but I'll write up an answer. – Polynomial Jan 14 at 12:45
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Why don't you just sign the email? – CodesInChaos Jan 14 at 12:46
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@CodesInChaos Because, in general, "John from Accounts" isn't going to have a clue what PGP is, let alone how to verify the signature's authenticity. – Polynomial Jan 14 at 12:55
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1 Answer

up vote 7 down vote accepted

Your issue is one of integrity, whereby a recipient can take an email you sent them, alter the contents, then forward them to a 3rd party. This might be problematic if an agent or middle-man decides to bloat up the prices and take a cut off the top.

There are two ways of solving this:

  1. Digitally sign the message using a technology such as PGP, and ensure that all potential recipients are aware that you always sign your mail. This provides proof of authenticity, and gives you grounds to refute claims by any 3rd party.
  2. Provide an out-of-band copy of some or all of the content. This could be achieved by logging quote details on the server, along with a "quote ID", which is sent with the email. This gives you a record of the quote, along with the details that were used and the price that was shown.

I would go for option 2 in your situation, since it doesn't require clients to be tech savvy.

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Thanks for taking the time to write this up. I agree that option 2 is preferential in this situation. Much appreciated. – Arthur Mild Jan 14 at 13:54
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For particular integrity with your quote ID system, sign a concatenation of the previous quote's signature and the current quote on your quote server. Nobody will ever have to see it unless it comes down to a legal challenge, but having a chain like that goes a long way to proving you didn't modify your auth system. Additional help comes from write-only archival and/or a 3rd party timestamp service – Jeff Ferland♦ Jan 14 at 14:06
I'm not sure how #2 will really help. Sleazy middlemen can simply delete the link to your quote page at the same time as they change the value in the forwarded message. – Dan Neely Jan 14 at 16:08
@DanNeely But in that event you have recourse to demonstrate good faith on your behalf, in case the customer comes crying to you. – Polynomial Jan 14 at 16:20

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