1. Computer and Network Security
CSEN 1001
Computer and Network Security
Amr El Mougy
Alaa Gohar
Heba Anwar
**Slides are attributed to William Stallings

2. Lecture (10)
Internet Security

3. Internet Security Protocols and Standards
Secure Sockets Layer (SSL) / Transport Layer Security (TLS)
IPv4 and IPv6 Security

4. Secure Sockets Layer (SSL)
Transport layer security service
originally developed by Netscape
version 3 designed with public input
Subsequently became Internet standard RFC2246: Transport Layer Security (TLS)
Use TCP to provide a reliable end-to-end service
May be provided in underlying protocol suite
Or embedded in specific packages

5. Secure Sockets Layer (SSL)
SSL identifies two main concepts:
Session: an association between client and server that defines security parameters. Created by the Handshake Protocol
Connection: A transport within a session supporting particular services. Every connection is associated with one session
Handshake Protocol
Change Cipher Spec Protocol
Alert Protocol
HTTP
SSL Record Protocol
TCP
A client and a server may have more than one connections within a session
They may also have several sessions but this is not common

6. SSL Record Protocol Services
Message integrity
using a MAC with shared secret key
Confidentiality
using symmetric encryption with a shared secret key defined by Handshake Protocol
AES, IDEA, RC2-40, DES-40, DES, 3DES, Fortezza, RC4-40, RC4-128
message is compressed before encryption

7. Record Protocol Operation

8. SSL Handshake Protocol
Allows server & client to:
authenticate each other
to negotiate encryption & MAC algorithms
to negotiate cryptographic keys to be used
Comprises a series of messages in phases
Establish Security Capabilities
Server Authentication and Key Exchange
Client Authentication and Key Exchange
Finish

9. SSL Handshake Protocol

10. Change Cipher Spec Protocol
One of 3 SSL specific protocols which use the SSL Record protocol
A single message
Causes pending state to become current
Hence updating the cipher suite in use

11. SSL Alert Protocol Conveys SSL-related alerts to peer entity Severity
warning or fatal
Specific alert
fatal: unexpected message, bad record mac, decompression failure, handshake failure, illegal parameter
warning: close notify, no certificate, bad certificate, unsupported certificate, certificate revoked, certificate expired, certificate unknown
Compressed & encrypted like all SSL data

12. HTTPS HTTP over SSL or TLS Uses port 443 instead of port 80
When HTTPS is used, the following are encrypted:
URL of requested object
Contents of object
Contents of browser forms (filled in by user)
Cookies
HTTP header
The client initiates the SSL/TLS session and connection
Once the handshake is complete, the HTTP request can be sent
Multiple TCP connections can be established in one session

13. IPSec Scheduled to be integral component of IPv6, optional in IPv4
Supports strong authentication and encryption at layer 3
Bi-directional tunnel
Packet filtering is primary access control method
Requires Public Key Infrastructure (PKI)

14. IPSec Uses

15. IPSec Functionality Functionality
AH (Authentication Header): integrity and authenticity
ESP (Encrypted Security Payload): confidentiality, optional authentication & integrity
Security Association (for each pair of hosts): determined by destination IP address and the SPI (Security Parameters Index)
Specification of the crypto methods to be used by SPI
Keys to be used by the crypto methods for that SPI
The hosts and other entities associated with this traffic
Key Management
Manual Keying (required)
Key Management Protocols (in flux)

16. IPSec Modes Transport Mode: Tunnel mode:
Provides protection to the payload of the packet (everything that comes after the IP header). Thus, headers are not encrypted
Typically used for end-to-end communications between hosts
Tunnel mode:
Provides protection to the whole packet
After AH or ESP headers are added, the entire packet plus the new headers are encrypted
New outer IP header is then added
Routers in the middle do not read the internal headers
Used when one or both ends of the security association is a gateway, not a host

18. Security Policy (SP) and Security Association (SA)
SA is a one-way relationship between sender & receiver that affords security for traffic flow
Defined by 3 parameters:
Security Parameters Index (SPI)
IP Destination Address
Security Protocol Identifier
Has a number of other parameters
seq no, AH & EH info, lifetime etc.
SP determines how traffic in an SA is handled

19. IP Traffic Processing IPSec Architecture Inbound Packets
Outbound Packets
Inbound Packets

20. Authentication Header
Provides data integrity and authentication
Prevents source address spoofing
Based on MAC
Scope covers payload and parts of the IP header

21. Encapsulating Security Payload Header
Provides confidentiality and optional integrity
Based on symmetric encryption techniques (AES)
Scope depends on transport or tunnel modes

22. Combining Security Associations

23. Whatsapp End-to-End Encryption
Link encryption: data is visible to the server
End-to-end encryption: data is encrypted in the server

24. Whatsapp End-to-End Encryption
New encryption system supports regular and group chats, images, videos, voice messages, files, and voice calls
At the center of this system is the new “Signal Protocol” developed by Open Whisper Systems
Even if a user’s key is physically compromised from a device, an attacker cannot decrypt previously encrypted messages

25. The Signal Protocol: Keys
Each user has three types of public keys:
Long-term identity key generated at installation
Medium-term key generated at installation and rotated periodically. The medium-term key is signed by the identity key
One-time key: generated as needed
In addition, there are three types of session keys:
Root key: 32-byte value used to create chain keys
Chain key: 32-byte value used to create message keys
Message key: 80-byte value used to encrypt messages. Out of these 80 bytes, 32 are used for AES 256, another 32 are used for HMAC-SHA256, and 16 bytes IV

26. The Signal Protocol: Operation
At registration time, the client sends the public identity key, public medium-term key (signed by the private identity key) , and a set of one-time keys
The private keys are never sent
To chat with someone you need to establish a session (any open whatsapp chat is a session)
A session does not need to be re-established unless the app is re-installed

27. The Signal Protocol: Operation
To establish a session, the initiator requests the public keys of the recipient from the server (identity key, signed medium term key, and one of the stored one-time keys)
Once the server returns those keys, the initiator generates a new key and uses its own identity key in addition to the recipient’s public keys to calculate a master secret.
The master secret is then used to create a root key and a chain key (using an algorithm similar to Diffie Hellman)

28. The Signal Protocol: Operation
Now, the initiator can start sending messages to the recipient, even if he/she is offline
To establish the session at the receiving end, the initiator inserts all values necessary for the receiver to calculate the root and chain keys in the header of all messages
The receiver uses this information together with its own private keys to calculate the master secret
The master secret is used as input to the key-derivation function to calculate the root and chain keys

29. The Signal Protocol: Operation
Each message is encrypted with a message key that includes AES256 encryption and HMAC-SHA256 for integrity and authentication
Each time a new message needs to be sent, a new message key is derived from the chain key
The message key cannot be derived from the message
The chain key is also rotated every time a message is sent

30. The Signal Protocol: Special Functions
To transmit media, the sender first saves it in an encrypted cloud store
The sender then sends an encrypted pointer to the receiver to download the media
Group messages in whatsapp are disseminated using server-side fan out (message is copied N times)
Pairwise sessions are created between all members of the group
Each of these session has a different chain key used to create message keys
Thus, the sender encrypts the message N times (one for each member) and the server sends them
Calls are established using encrypted RTP