Securing Email Bruce Maggs Separate Suites of Protocols

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 POP 3, 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

POP 3 Dialog S: C: S: C: S: S: C: S: <wait for connection on TCP port 110> <open connection> +OK POP 3 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 POP 3 server sends message 1> QUIT +OK dewey POP 3 server signing off (maildrop empty) <close connection> <wait for next connection> 4

Experimenting with a POP 3 Server telnet pop. example. com 110 openssl s_client -connect pop. example. com: 995 -quiet 5

APOP (RFC 1460) S: C: S: <wait for connection on TCP port 110> <open connection> +OK POP 3 server ready <1896. 697170952@dbc. mtview. ca. us> APOP mrose c 4 c 9334 bac 560 ecc 979 e 58001 b 3 e 22 fb +OK mrose's maildrop has 2 messages (320 octets) • 1896. 697170952 is a time stamp • shared secret is the password "tanstaaf“ • Client applies MD 5 algorithm to the string <1896. 697170952@dbc. mtview. ca. us>tanstaaf • which produces a digest value of c 4 c 9334 bac 560 ecc 979 e 58001 b 3 e 22 fb • 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 IMAP 4, 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 IMAP 4 rev 1 Service Ready C: a 001 login mrc secret S: a 001 OK LOGIN completed C: a 002 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: a 002 OK [READ-WRITE] SELECT completed C: a 003 fetch 12 full 8

More IMAP Authentication Mechanisms (RFC 1731) Kerberos, GSSAPI, S/Key (like APOP) Kerberos example: S: C: * OK IMAP 4 Server A 001 AUTHENTICATE KERBEROS_V 4 + Am. FYig== (random 32 -bit number) BAc. AQU 5 EUk. VXLk. NNVS 5 FRFUAOCAsho 84 k. LN 3/IJmr. MG+25 a 4 DT +n. ZIm. Jjn. TNHJUtx. AA+o 0 KPKf. HEc. AFs 9 a 3 CL 5 Oebe/yd. HJUw. YFd Wwu. Q 1 MWiy 6 Ies. Kvj. L 5 r. L 9 Wj. XUb 9 Mw. T 9 bp. Ob. YLGOKi 1 Qh (ticket and an authenticator for the principal "imap. hostname@realm") S: + or//Eo. AADZI= C: Di. AF 5 A 4 g. A+o. OIALu. Bk. AAmw== S: A 001 OK Kerberos V 4 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> 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-MD 5 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-MD 5 PLAIN C: AUTH PLAIN d. GVzd. AB 0 ZXN 0 ADEy. Mz. Q= S: 235 2. 7. 0 Authentication successful 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

Proving your SMTP Relay is Legit • Register reverse DNS name • DKIM – relay signs headers with its • private key, receiver can verify that relay is in the same domain (e. g. , cs. duke. edu) as sender SPF – list 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 Open. PGP 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