Securing Email
Bruce Maggs
Separate Suites of Protocols
• Protocols for retrieving email:
POP, IMAP, MAPI (Microsoft Exchange)
• Protocols for sending email:
SMTP
2
POP (Post Office Protocol)
•
•
•
•
•
Current version is POP3, RFC 1939.
Client connects to POP server on port 110
Originally, everything was sent in the clear.
Yes, everything, including user name,
password, and your mail.
Now it’s common to connect instead to port
995 using SSL/TLS, same PKI as Web.
3
POP3 Dialog
S:
C:
S:
C:
S:
C:
S:
C:
S:
C:
S:
S:
S:
S:
C:
S:
S:
C:
S:
C:
S:
<wait for connection on TCP port 110>
<open connection>
+OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
USER mrose
+OK User accepted
PASS tanstaaf
+OK Pass accepted
STAT
+OK 2 320
LIST
+OK 2 messages (320 octets)
1 120
2 200
.
RETR 1
+OK 120 octets
<the POP3 server sends message 1>
QUIT
+OK dewey POP3 server signing off (maildrop empty)
<close connection>
<wait for next connection>
4
Experimenting with a POP3 Server
telnet mail.cs.duke.edu 110
openssl s_client -connect mail.cs.duke.edu:995 -quiet
openssl s_client creates a TLS session between client and
server and supports text-based interaction – useful for debugging
5
Autenticated Post Office Protocol
(APOP, RFC 1460)
S:
C:
S:
C:
S:
<wait for connection on TCP port 110>
<open connection>
+OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
APOP mrose c4c9334bac560ecc979e58001b3e22fb
+OK mrose's maildrop has 2 messages (320 octets)
•
•
•
•
in this example, session is not encrypted
1896.697170952 is a time stamp
shared secret is the password "tanstaaf“
Client applies MD5 algorithm to the string
<1896.697170952@dbc.mtview.ca.us>tanstaaf
• which produces a digest value of
c4c9334bac560ecc979e58001b3e22fb
• Password not sent in the clear, digest can’t be reused
Other authentication methods include SASL and Kerberos.
6
IMAP (Internet Message Access Protocol)
• Current version is IMAP4, RFC 3501.
• Client connects to IMAP server on port 143.
• Originally, user name, password, mail sent in
•
the clear.
Now connect to port 993 using SSL/TLS.
7
IMAP Dialog
S: * OK IMAP4rev1 Service Ready
C: a001 login mrc secret
S: a001 OK LOGIN completed
C: a002 select inbox
S: * 18 EXISTS
S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
S: * 2 RECENT
S: * OK [UNSEEN 17] Message 17 is the first unseen message
S: * OK [UIDVALIDITY 3857529045] UIDs valid
S: a002 OK [READ-WRITE] SELECT completed
C: a003 fetch 12 full
8
More IMAP Authentication Mechanisms (RFC 1731)
Kerberos, S/Key (like APOP example), GSSAPI
(GSSAPI is a standardized API, typically wrapped around Kerberos)
Kerberos example:
S:
C:
S:
C:
* OK IMAP4 Server
A001 AUTHENTICATE KERBEROS_V4
+ AmFYig== random 32-bit number
BAcAQU5EUkVXLkNNVS5FRFUAOCAsho84kLN3/IJmrMG+25a4DT
+nZImJjnTNHJUtxAA+o0KPKfHEcAFs9a3CL5Oebe/ydHJUwYFd
WwuQ1MWiy6IesKvjL5rL9WjXUb9MwT9bpObYLGOKi1Qh
service ticket for the principal imap.hostname@realm and authenticator for client
(encrypted checksum in authenticator contains 32-bit number)
S: + or//EoAADZI=
C: DiAF5A4gA+oOIALuBkAAmw==
S: A001 OK Kerberos V4 authentication successful
9
SMTP (Simple Mail Transfer Protocol)
•
•
•
•
Currently described in RFC 5321
Client connects to SMTP server on port 25
Originally everything was sent in the clear
Originally client could send mail without
authenticating! *** SPAM *** ***FORGERY***
• Port 465 used for SSL/TLS
• Sender’s SMTP server relays mail to receiver’s
SMTP server
• DNS Mail Exchanger (MX) records used to resolve
mail domain name to IP address of SMTP server of
recipient, e.g., bmm@cs.duke.edu
10
DNS MX Record
11
SMTP Dialog
S: 220 smtp.example.com ESMTP Postfix
C: HELO relay.example.org
S: 250 Hello relay.example.org, I am glad to meet you
C: MAIL FROM:<bob@example.org>
S: 250 Ok
C: RCPT TO:<alice@example.com>
S: 250 Ok
C: RCPT TO:<theboss@example.com>
S: 250 Ok
C: DATA
S: 354 End data with <CR><LF>.<CR><LF>
C: From: "Bob Example" <bob@example.org>
C: To: "Alice Example" <alice@example.com>
C: Cc: theboss@example.com
C: Date: Tue, 15 January 2008 16:02:43 -0500
C: Subject: Test message
C:
C: Hello Alice.
C: This is a test message with 5 header fields and 4 lines in the message body.
C: Your friend,
C: Bob
C: .
S: 250 Ok: queued as 12345
C: QUIT
S: 221 Bye {The server closes the connection}
12
STARTTLS
Instead of using separate ports for SSL/TLS
connections, POP, IMAP, SMTP connections can be
converted from plain text to encryped communications
through the use of the STARTTLS command.
S: <waits for connection on TCP port 25>
C: <opens connection>
S: 220 mail.example.org ESMTP service ready
C: EHLO client.example.org
S: 250-mail.example.org offers a warm hug of welcome
S: 250 STARTTLS
C: STARTTLS
S: 220 Go ahead
C: <starts TLS negotiation>
C & S: <negotiate a TLS session>
C & S: <check result of negotiation>
C: EHLO client.example.org
13
SMTP AUTH Extension
S: 220 smtp.example.com ESMTP Server
C: EHLO client.example.com
S: 250-smtp.example.com Hello client.example.com
S: 250-AUTH GSSAPI DIGEST-MD5
S: 250-ENHANCEDSTATUSCODES
S: 250 STARTTLS
C: STARTTLS
S: 220 Ready to start TLS
... TLS negotiation proceeds. Further commands protected by TLS layer ...
C: EHLO client.example.com
S: 250-smtp.example.com Hello client.example.com
S: 250 AUTH GSSAPI DIGEST-MD5 PLAIN
C: AUTH PLAIN dGVzdAB0ZXN0ADEyMzQ=
S: 235 2.7.0 Authentication successful
Client must log in (authenticate) to send mail.
(In this example user name and password are encoded but not
encrypted in PLAIN string.)
14
Warning!
Mail relayed between two SMTP servers
might not be encrypted.
(Even if sender and recipient connect
securely to their SMTP and IMAP
servers.)
15
SORBS (SPAM and Open-Relay Blocking System)
• Database of bad SMTP (blacklisted) relays accessed
•
through DNS
Mail won’t be accepted from these relays
(Also SPAMHAUS database.)
16
Ways to Prove your SMTP Relay is Legit
• Register reverse DNS name (weak)
• DKIM (DomainKeys Identified Mail) – relay
•
signs headers with its private key, receiver
can verify that sender is in the same domain
(e.g., cs.duke.edu) as relay (no CAs, public
keys distributed through DNS)
SPF (Sender Policy Framework) – whitelist of
which servers are permitted to originate email
for which domains, accessed via DNS
17
Signing Your Email
• PGP (Pretty Good Privacy)
• Certificate signed by certificate
authority, e.g., Comodo.
18
PGP
•
•
•
•
•
•
Developed by Phil Zimmerman
OpenPGP standard described in RFC 4880
Public key bound to user name or email address
No centralized certificate authority in original design
Public keys can be signed by other users attesting to
their authenticty, often at key-signing parties
Supported in Thunderbird, Pine, with plug-in in
Outlook.
19
Comodo
• Verifies that you can receive email at the address
that you want a certificate for.
20
Retrieve Certificate
• Chrome downloads and stores certificate.
21
Install in Thunderbird
• (easier if downloaded to Firefox than Chrome)
22
My First Signed Message
23