at Security the Transport Layer
Lecture 6
SSL/TLS
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SSL was developed by Phil Karlton & Netscape.
The standards community wanted their own version free from any patents/restrictions
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Thus was born TLS, which is very similar to SSL
IETF changed the name to avoid showing bias and add a few things
E.g: TLS defines additional alert codes TLS MAC covers compression version field in addition to what SSL MAC covers
TLS v1.0 is very similar to SSL v3.1
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SSL
HTTP
NNTP
FTP
Telnet
Etc.
Other Apps
SSL/TLS
TCP/IP
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Digital certificates and the CA
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Public key cryptography avoids the problem of key distributions but it has its weakness: the public key is
public and anyone can forge a public key announcement
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Solution: public-key certificate:
<a public key and user ID of the key owner>, the whole thing, is signed by a trusted third party (TTP) The TTP is a certificate authority (CA) A user can present her public key to the CA in a secure manner and obtain a certificate The user can then publish her public key Anyone needs this user's public key can obtain the certificate and verify that it is valid by the way of the attached trusted signature
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Client/Server Mutual Authentication
Establish trust with intended recipients s Signed Digital Certificates
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Server authenticates Client (optional) Client authenticates Server
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Secure Sockets Layer (SSL)
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SSL is designed to run in user-level process no need to change the OS (not like IPSec that runs in superuser-level process) SSL uses TCP only (not UDP), hence it is simple and it does not have to worry about timing out and retransmitting lost data (TCP does that for SSL) Applications based on TCP needs to change (even minimum) to work with SSL
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Secure Sockets Layer (SSL)
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The primary goal of the SSL Protocol is
To provide privacy and reliability between two communicating applications. Allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery (Many applications at Application Layer are based on client/server model)
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SSL Secure Client/Server Communication SSL Server authentication allows clients to confirm a server's identity s SSL client authentication allows server to confirm a client's identity (optional) s SSL provides confidentiality and integrity
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Sent information is encrypted Integrity check is carried out to detect tampering
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SSL Secure Client/Server Communication
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To allow Secure Client/Server Communication to happen using SSL:
The peer's identity can be authenticated using certificates [basically using asymmetric (private key cryptography like DES), and public key, cryptography (e.g., RSA, DSS, etc.)]. Encryption is used after an initial handshake to define a secret key. Secret key cryptography is used for data encryption (e.g., DES, RC4, etc.) Integrity check is done using a keyed MAC: Secure hash functions (e.g., SHA, MD5, etc.) are used for MAC computations.
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SSL Main Functions
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Negotiates and employs essential functions for secure transactions
Mutual Authentication Data Encryption Data Integrity
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SSL Structure
Application Layer (http imap etc) ChangeCipher Alert Handshake Record Layer TCP/IP
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SSL/TLS
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Cipher Suite
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For public-key, secret key encryption and certificate verification we need
public-key algorithm Secret key encryption algorithm message digest (hash) algorithm (MD5, SHA-1, etc)
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This collection is called a cipher suite SSL supports many different cipher suites Client and server must decide on which one to use The client offers a choice; the server picks one
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SSL Protocols
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SSL consists of two protocol layers (The Record Layer and its above layer which consists of ChangeCipherSpec protocol , Alert protocol and Handshake protocol.) The SSL record protocol:
takes an application message to be transmitted, fragments the data into manageable blocks, optionally compresses the data, applies MAC, encrypts, adds a header and transmits the resulting unit in TCP segment
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SSL Other Protocols
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The Handshake protocol negotiates all options of the session (most important)
The handshake protocol involves using SSL record protocol to exchange a series of messages between an SSL-enabled server and an SSL-enabled client
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The ChangeCipherSpec protocol indicates the channel is ready for secure communications The Alert protocol indicates errors or other caution conditions have occurred in the connection
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The Changecipherspec Structure
The change cipher spec message is sent by both the client and server to notify the receiving party that subsequent records will be protected under the newly negotiated CipherSpec and keys
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struct {
enum { change_cipher_spec(1), (255) } type;
} ChangeCipherSpec;
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Alert Message Structure
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The Alert protocol specifies and conveys the status of transmitted messages
struct { AlertLevel level; AlertDescription } description; Alert;
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Handshake Protocol
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The TLS Handshake Protocol involves the following steps:
Exchange hello messages to agree on algorithms, exchange random values, and check for session resumption. Exchange the necessary cryptographic parameters to allow the client and server to agree on a premaster secret. Exchange certificates and cryptographic information to allow the client and server to authenticate themselves. Generate a master secret from the premaster secret and exchanged random values. Provide security parameters to the record layer. Allow the client and server to verify that their peer has calculated the same security parameters and that the handshake occurred without tampering by an attacker.
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ClientHello Message Structure
struct {
ProtocolVersion client_version; Random random; SessionID session_id; CipherSuite cipher_suites; CompressionMethod_list compression_methods;
} ClientHello;
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ServerHello Message Structure
struct {
ProtocolVersion server_version; Random random; SessionID session_id; CipherSuite cipher_suite; CompressionMethod_list compression_method;
} ServerHello;
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Message Flow of full Handshake
Client
ClientHello --------> ServerHello Certificate* ServerKeyExchange* CertificateRequest* ServerHelloDone
Server
<-------Certificate* ClientKeyExchange CertificateVerify* [ChangeCipherSpec] Finished -------->
[ChangeCipherSpec] <-------Finished
Application Data <-------> Application Data
* Indicates optional or situationdependent messages that are not Borrowed from rft2246 always sent.
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Client/Server
Server
client1
message
new thread s1 to serve client1 main thread message new thread s2 to serve client2
client2
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Client/Server Socket Connection
Client A #1
accept Original socket
Newly returned socket
#2
Newly returned socket
Client B
Server
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Client/Server with TCP and Socket
Client must contact server server process must first be running server must have created socket (door) that welcomes client's contact Client contacts server by: creating client-local TCP socket specifying IP address, port number of server process
When client creates socket: client TCP establishes connection to server TCP When contacted by client, server TCP creates new socket for server process to communicate with client allows server to talk with multiple clients
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TCP socket programming flow
Client
socket() connect() write()
Connection establishment TCP threeway handshake Data (request)
socket()
well-known port
bind() listen() accept()
Server
read()
Block until connection from client
Process requests
read() close()
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Data (reply) Endoffile notification
write() read() close()
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SSL Applications Simple Demo
/*We can use SSL to build secure Internet application The purpose of this simple program is to show you how to connect to a secure website using SSL and Java Secure Socket Extension */
//TestSSL.java class public class TestS...