1 SSH / SSL Supplementary material

2 Secure Shell (SSH) One of the primary goals of the ARPANET was remote access Several different connections allowed –rlogin –rcp –rsh All data was unencrypted –This was a different world than exists today.

3 SSH SSH is a UNIX-based command interface and protocol for securely accessing a remote computer Suite of four utilities—slogin, ssh, sftp, and scp Can protect against: –IP spoofing –DNS spoofing –Intercepting information

4 SSH Objectives Protect data sent over the network –Negotiate an encryption algorithm between sender and receiver –Use that algorithm and a session key to encrypt / decrypt data sent Provide site authentication –Use public key / fingerprint to ensure identity of remote host. –Relies on locally generated keys, so no certifying authority is generally available.

5 SSH Communications Using password SSH Client SSH Server SSH2? SSH2 Diffie-Helman, etc? Diffie-Helman Serv_Pub_key(S_key) OK S_key(Uname,pwd) OK S_key(data) Send Serv_Pub_Key

6 SSH Wire Shark Trace

SSH Communications Using Public Key Problems with Password Authentication –Passwords can be guessed. –Default allows multiple attempts against account –Only 1 account / password needs to be guessed –Alternate approach is to use public / private keys to authenticate user Public Key Authentication –Create public / private keypair –Ensure that private –Upload public key to server user account: ~.ssh/authorized_keys –ssh –o PreferredAuthentications=publickey server.example.org

SSH Communications Using Public Key SSH Client SSH Server SSH2? SSH2 Diffie-Helman, etc? Diffie-Helman Serv_Pub_key(S_key) OK S_key(Uname) OK S_key(data) Send Serv_Pub_Key Client_Pub_key(Random) Client_Pri_key(msg) Hash(Random)

9 SSH Tunneling Use SSH to create an encrypted channel between remote host and server Use that encrypted channel to carry other traffic. SSH Tunnel www access Web Server Local port Internet LAN

SSH Tunneling ssh –L 12345: :80 –l root homenet.net

11 SSH Tunneling

12 Secure Copy (scp) Allows encrypted transfer of files between machines Download files from server: –scp myfile1.txt password: xxxxx Upload files to server –Scp myfile.txt password: xxxxx

13 SSH Passwordless Login On remote client: –Create key pair. Store in.ssh subdirectory On ssh server: –Modify sshd_config to allow shosts based authentication –Create.shosts file in user’s subdirectory –Copy public key from remote client to.ssh subdirectory/authorized_keys

14 SSH Passwordless Login SSH Client SSH Server SSH2? SSH2 Diffie-Helman, etc? Diffie-Helman Serv_Pub_key(S_key) OK S_key(Uname) OK S_key(data) Send Serv_Pub_Key Client_Pub_key(Random) Client_Pri_key(msg) Hash(Random)

15 SecureSockets Layer (SSL) Transport Layer Security (TLS) Originally developed by Netscape to support encrypted access to web servers. SSL v3 released Served as the basis for IETF standard TLS (1999) Used by major financial institutions for secure commerce over the Internet Early problem with weak keys resolved with longer (128-bit) keys

16 SSL / TLS Application (www) SSL / TLS TCP IP

17 SSL/TLS Handshake SSL Client SSL Server Client hello Ciphers I have Server Hello Cipher I choose Server certificate (S_Pub) OK S_Pub(Session_key) Session_key(data)

18 SSL/TLS Security Depends on integrity of public key certificate Public Key Infrastructure (PKI) –Components necessary to securely distribute public keys –Certificate Authorities: Organizations that certify the relationship between a public key and its owner. –Verisign,Thawte

19 SSL/TLS Implementations SSL v2 – Still in use SSL v3 – Most widely deployed TLS v1 – Starting Deployment OpenSSL – Linux/UNIX toolkit that supports all 3 protocols listed above. Private Communication Technology (PCT) –Developed by Microsoft –Compatible with SSL v2 Versions are not completely compatible

SSL/TLS Vulnerability SSL/TLS supports the concept of session renegotiation due to errors, requests, etc. This feature assumes that the renegotiation is with the original party, and any requests or messages transmitted before the renegotiation are combined (pre-pended) with the requests after renegotiation This behavior can be abused to allow man-in-the-middle attacks Demonstrated with https, but the vulnerability exists with any application that uses SSL/TLS

SSL/TLS Vulnerability Client MITMServer TLS handshake session #1 TLS handshake session #2 GET /ebanking/paymemoney.cgi? Acc=LU ?amount=1000 Ignore-what-comes-now; Trigger renegotiation TLS handshake session #1 continued within the encrypted session #2 Client has authenticated session At app layer (with cookie) GET /ebanking/ Cookie: AS X Server receives: GET /ebanking/paymemoney.cgi? Acc=LU ?amount=1000 Ignore-what-comes-now; GET /ebanking/ Cookie: AS

22 References SSH –SSH Tutorial ( Tutorial – –UNIX Secure Shell – Carasik – McGraw-Hill, 1999 –SSH Agent Forwarding (unixwiz.net/techtips/ssh-agent-forwarding.html)unixwiz.net/techtips/ssh-agent-forwarding.html SSL – –RFCs – 2246, 3546 –SSL Authentication Gap (SSL Gap) ( ) –TLS/SSL renegotiation vulnerability explained ( )

SSH RFCs 4250 The Secure Shell (SSH) Protocol Assigned Numbers. –S. Lehtinen, C. Lonvick, Ed.. January (Format: TXT=44010 bytes) –(Status: PROPOSED STANDARD) 4251 The Secure Shell (SSH) Protocol Architecture. –T. Ylonen, C. Lonvick, Ed.. January (Format: TXT=71750 bytes) –(Status: PROPOSED STANDARD) 4252 The Secure Shell (SSH) Authentication Protocol. – T. Ylonen, C. Lonvick, Ed.. January (Format: TXT=34268 bytes) –(Status: PROPOSED STANDARD) 4253 The Secure Shell (SSH) Transport Layer Protocol. – T. Ylonen, C. Lonvick, Ed.. January (Format: TXT=68263 bytes) –(Status: PROPOSED STANDARD) 4254 The Secure Shell (SSH) Connection Protocol. – T. Ylonen, C. Lonvick, Ed.. January (Format: TXT=50338 bytes) – (Status: PROPOSED STANDARD) 4255 Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints. – J. Schlyter, W. Griffin. January (Format: TXT=18399 bytes) –(Status: PROPOSED STANDARD) 4256 Generic Message Exchange Authentication for the Secure Shell Protocol (SSH). –F. Cusack, M. Forssen. January (Format: TXT=24728 bytes) –(Status: PROPOSED STANDARD) 4344 The Secure Shell (SSH) Transport Layer Encryption Modes. –M. Bellare, T. Kohno, C. Namprempre. January (Format: TXT=27521 bytes) –(Status: PROPOSED STANDARD) 4419 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol. – M. Friedl, N. Provos, W. Simpson. March (Format: TXT=18356 bytes) –(Status: PROPOSED STANDARD) 4716 The Secure Shell (SSH) Public Key File Format –. J. Galbraith, R. Thayer. November (Format: TXT=18395 bytes) – (Status: INFORMATIONAL) 4819 Secure Shell Public Key Subsystem. –J. Galbraith, J. Van Dyke, J. Bright. March (Format: TXT=32794 bytes) –(Status: PROPOSED STANDARD)

24 Summary SSH –Supports secure remote access to hosts –SSH – secure shell –SCP – secure copy –SFTP – secure file transfer SSL –Provides a framework for incorporating secure communications into applications –Uses strong cryptography –Can rely on PKI for reliable sharing of public keys