Digital signature to e-mail.

[Roberto Alcantara <roberto () fortalnet com br>]

Guys, this is just one idea, and I would like know your comments about 
this.
I will write one sendmail milter to test, just to fun :)

Regards,
Roberto.



http://www.eletronica.org/softlab/userkeys/userkeys_english.htm

** BETA TEXT VERSION **
Give me feedback about text :)

Roberto Alcântara
roberto () eletronica org
2005, May 15.


Digital Signature to E-mail in Server Side

Objective: Offer one way to provide server side authentication
with digital signature, to return-path address ('MAIL FROM'
argument, RFC2821). SMTP protocol was not developer with
security objectives. Your idea it's great to delivery messages,
but very bad to security purpose. The main STMP deficiency is
not able to cryptography message and not have security origin
identification. Our propose is solve origin identification
problem, using asymmetric cryptography, MTA filters and DNS
to store public key.

Motivation: Some options was developed to solve identification
and cryptographic problem, like PGP and GNUPG. Althought
excellent options they had not become popular, therefore they
need new software ('plugin') in client side and did not have
safe and distribuided place for public keys publication. Other
solution like Domain Keys[1] providers a mechanism for verifying
both the domain of email sender and the integrity of the sent
message, but not the sender.


How it works:

Setup: Each protected e-mail (user@domain) have one public and
private key are stored in server side. Public key is stored in
user.userkeys.domain in TXT DNS record (RFC1035). User names
with dot will have some extra characters to fix url. Private
key is stored in secure local database (User Key Database,
UKD), with username/mail from/private key. Each client have
one password to access your SMTP account (SMTP Authentication,
RFC2554).

Sending: e-mail client connect to SMTP server using
authentication and send your message. Server will look in your
UKD if this user have permission to send messages from informed
'MAIL FROM'. After this, server will sign message and add this
signature in e-mail header.

Receiving: When MTU delivery message to MX server,
this will find signature header. One DNS query will
be sent to user.userkeys.domain looking public key to
user@domain address. With public key, server can check sender
identification, adding result in message header or body. One
additional flag in DNS entry say what e-mail owner recommend
MX do when signature can't be found or signature check fail.


Cryptographic problem can be solve with asymmetric keys
but not with this solution, because comunication between Client
and SMTP server it is not safe to envelope of a message.


[1] http://antispam.yahoo.com/domainkeys