Digitally signing an email is a way of assuring the recipient that the content, while not encrypted, has not been modified in transit, it’s effectively a personal cryptographic certification of the content and attributes of the mail. If the “From:” address is re-written, for example a signed mail is sent to a distribution list and then forwarded on to each of the members of the list with a new “From:” address (usually the address of the list) then the contents will have been modified and the signature will no longer match. The same is true of any content within the mail, if it is intercepted and modified in transit, then the end-user should receive a warning that the signature no longer matches. In a post-PRISM world though, more people are beginning to pay attention to how they can secure their email communication completely from prying eyes. Simply signing will not achieve this, as mails not encrypted – merely “certified” – are still sent in the clear. Full-blown mail encryption is the answer, as Edward Snowden asserted in his recent Q&A, “Encryption works. Properly implemented strong crypto systems are one of the few things that you can rely on“.
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