SSL/TLS Protocols

 

 

SSL (Secure Socket Layer, developed by Netscape ) &


TLS (Transport Layer Security, is an IETF standard)

 

Both are  almost  the same.
They run as a user-level processes on top of  TCP/IP.

 

The Basic Protocol:

{=============================================
Alice                                                                                             Bob

 

I want to talk, ciphers I support, Ra                              >

<                                      Certificate, cipher I choose, Rb


Choose secret S, compute K= f (S,Ra,Rb):
{S}Bob , {keyed hash of handshake msgs|CLNT}                         >

                                                 Compute K= f(S,Ra,Rb):
<                                     {keyed hash of handshake msgs|SRVR}

 

 <           Data protected with keys derived from K         >

==============================================}


Keys:

·       Alice chooses a random number S, as pre-master secret.

·       It is shuffled with Ra and Rb to produce a master secret K.

·       Ra and Rb are 32 octets long. The first 4 are the UNIX time (number of seconds since Jan 1, 1970).

This is to ensure that Rs are always different.

·       The master secret is shuffled with Rs to produce six (6) keys:

ü Three (3) for each side for  encryption, integrity, and IV.

ü The three keys used for transmission are known as the write keys

ü The three keys used for reception are known as the read kyes

ü Thus Alice's write keys are Bob's read keys and vice versa.

 

·       To ensure that the keyed hash Alice sends is different from the keyed hash Bob sends, 

Alice includes the string "CLNT" and the Bob include "SRVR" in the hash.

(To avoid the Reflection attack!).

 

·       Note that:

ü Alice has authenticated Bob

(Since Bob has the secret key that corresponds to the certificate’s public key).

ü Bob has no idea to whom he's talking to.

ü In SSL it is optional for the server to authenticate the client, if the client has a certificate.

ü Normally the server authenticates the client using:

 <name, password>  

sent securely over the ssl connection.

 

Session Resumption
 

If the server supports session resumption, it sends session_id for the client at the beginning.  

 

{===========================================
Alice                                                                                   Bob

I want to talk, ciphers I support, Ra                              >
<                    session_id, certificate, cipher I choose, Rb


choose secret S, compute K= f(S,Ra,Rb):
{S}Bob, {keyed hash of handshake msgs|CLNT}          >
                                                 compute K= f(S,Ra,Rb):
<                                   {keyed hash of handshake msgs|SRVR}

<          data protected with keys derived from K         >

===========================================}
 

Session resumption, if both sides remember the session_id:

{=========================================
Alice                                                                                 Bob

session_id, ciphers, Ra                                                     >

<                     session_id, cipher, Rb, {keyed hash of  msgs}

{keyed hash of  msgs}                                                       >

<          data protected with keys derived from K         >

=========================================}

Note that they still have to negotiate ciphers,

But the pre-master secret S is the same (since it is expensive to generate).


Encrypted Records
 

SeqNum | Header | Data  - >  HMAC <-  integriy key
                       ||           ||                   ||
                      V          V                  V
                 Header | Data |     HMAC    |    pad   - > ENCRYPT < -  encryption key
                       ||                                                                     ||
                      V                                                                    V
                 Header |                        encrypted-integrity-protected record
 

If block cipher is used, the IV is used to encrypt the first record.

The final block of each record is used as the IV for the next record.


Connection Closure 

ü The sender should send  close_notify   message to signal the other end that it has no more data to send.

ü The purpose is to prevent a trunctation attack  in which the attacker inserts a TCP FIN segment before the sender has finished sending data forcing the receiver to think that all data has been received.

ü If a party receives FIN without first receiving close_notify it must mark the session as not resumable.

 

 

HTTP Over SSL - https




HTTP:                                                    

HTTP (Hyper Text Transfer Protocol) is the Web basic transport protocol.

The basic unit of HTTP interaction is the request/response pair:

ü The client opens a TCP connection to the server and writes the request.

ü The server writes back the response and indicates the end of response either with:

length header or by closing the connection.


HTTPS:

 

The client makes a connection to the server; Negotiates an SSL connection;

and transmits http data over the established secure connection.

 

 

 


 
 OpenSSL

s_server   & s_client

 

v    s_server  :

 

Examples:

 

ü S1

 

openssl   s_server  -no_dhe -accept 10234  -cert server_cert.pem  -key  server_privatekey.pem

 

ü S2

 

openssl   s_server  -no_dhe  -accept 10234 -cert  server_cert.pem  -key server_privatekey.pem   -Verify 2  –CAfile  ca_cert.pem 

 

 

-Verify : the server demand a certificate from the  client, the depth of the chain should not exceed 2.

 

-CAfile :  specify the trusted certificate.

 

 

 

v    s_client  :

Examples:

 

ü C1

 

openssl   s_client   -connect  localhost:10234  -verify 2   -CAfile  ca_cert.pem 

 

ü C2

 

      openssl   s_client   -connect  localhost:10234  -verify 2   -CAfile   ca_cert.pem   -cert  client_cert.pem  -key client_privatekey.pem

 

 

 

Testing:

 

ü Test1

 

%  S1 

% C1  

 

Will be successful: Typing  any line in C1 it will appear in S1

ü Test1

 

%  S2 

 % C2  

 

Will be successful: Typing  any line in C2 it will appear in S2

ü Test1

 

%  S2 

% C1  

 

Will NOT be successful:  S2 demands a certificate from C1.