[Intro. to Computer Security Course Note] Ch 22

Ch22. Internet Security Protocols and Standards

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Secure E-mail

MIME	S/MIME
Extension to the old RFC 822 specification of an Internet mail format - RFC 822 defines a simple heading with To, From, Subject - Assu,es ASCII text format	Secure/Multipurpose Internet Mail Extension Based on RSA Provides the ability to sign and/or encrypt e-mail messages

Typical S/MIME Process for Creating an S/MIME Message

S/MIME Functions

• Enveloped data
  • Encrypted content and associated keys
• Signed data
  • Encoded message + signed digest
• Clear-signed data
  • Cleartext message + signed digest
• Signed and enveloped data
  • Nesting of signed and encrypted entities

Enveloped Data Using Public-key Infrastructure

• M -> 3DES(M) -> X + 3DES(M) -> ELGamal(X + 3DES(M))
  • M: message
  • X: a session encryption key
  • User recipient’s ELGamal’s public key to encrypt X + 3DES(M)
  • ELGamal is based no the Diffie-Hellman public-key exchange algorithm
• Radix-64 is used to convert the ciphertext to ACSII format
• Basic tool that permits widespread use of S/MIME is the public-key certificate
• S/MIMNE uses certificates that conform to the international standard X.509.3

Signed and Clear-signed Data

• Signed data -> Base64(content + sig)
• Clear-signed data -> content + Base674(sig)
• DSS(SHA-1(MEssage), DSS-private-key)
• RSA(SHA-1/MD5(Message), RSA-private-key)
• Radix-64 or Base64 mapping is used to map the signature and message into printable ASCII characters

DomainKeys Identified Mail (DKIM)

• A specification of cryptographically signing e-mail messages
• A proposed Internet standard

Internet Mail Architecture

• User world: MUA
• Transfer world: MHS
  • Composed of MTAs
• 

Why DKIM

• An email authentication technique that is transparent to the end user
• Reasons
  • S/MIME depends on both the sending and receiving users employing S/MIME
    • But, the bulk of incoming mail does not use S/MIME
    • S/MIME signs only the message content
      • Header information concerning origin can be compromised
    • Applied to all mail from cooperating domains
    • Preventing forgers from masquerading as good senders

Simple Example of DKIM Deployment

• DNS = domain name system
• MDA = mail delivery agent
• MSA = mail submission agent
• MTA = message transfer agent
• MUA = message user agent

Secure Sockets Layer (SSL) and Transport Layer Security (TLS)

• Secure Socket Layer (SSL)
  • One of the most widely used security services
• Transport Layer Security (TLS)
  • Becoming Internet standard RFC4346
  • General-purpose service: as a set of protocols that rely on TCP
  • Two implementation choices
    • As part of the underlying protocol suite: transparent to apps
    • Be embedded in specific packages: e.g., most browsers come equipped with SSL

SSL/TLS Protocol Stack

• 

TLS Concepts

• TLS Connection
  • A transport (in the OSI layering model definition) that provides a suitable type of service
  • Peer-to-peer relationships
  • Transient (暫時的)
  • Every connection is associated with one session
• TLS Session
  • An association between a client and a server
  • Created by the handshake protocol
  • Define a set of cryptographic security parameters
  • Used to avoid the expensive negotiation of new security parameters for each connection

TLS Record Protocol Operation

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Handshake Protocol

• TLS 最複雜的部份
• Is used before any application data are transmitted
• Authenticate each other -> Negotiate encryption and MAC algorithms -> Negotiate cyrptographic keys to be used
• Exchanges has four phases

Handshake Protocol Action 1 (phase 1 & 2)

Handshake Protocol Action 2 (phase 3 & 4)

ClientHello (RFC)

struct {
   ProtocolVersion client_version;
   Random random;
   SessionID session_id;
   CipherSuite cipher_suites <2..2^16-2>;
   CompressionMethod compression_methods <1..2^8-1>;
} ClientHello;

ServerHello (RFC)

struct {
   ProtocolVersion server_version;
   Random random;
   SessionID session_id;
   CipherSuite cipher_suite;
   CompressionMethod compression_method;
} ServerHello;

Four TLS specific protocols

• Change Cipher Spec Protocol
  • One of four TLS specific protocols that use the TLS Record Protocol
  • Simplest
  • Consists of a single message which consists of a single byte with the value 1
  • Sole purpose of this message is to cause pending state to be copied into the current state
  • Hence updating the cipher suite in use
• Alert Protocol
  • Two bytes
    • 1^st^ byte: warning (1), fatal (2)
      • For fatal, TLS implementation terminates the TLS connection
      • For other TLS connections using the same TSL session, they may continue, but no new TLS connections may be established
    • 2^nd^: specific alert code (RFC5246-appendix)
      • close_notify(0), unexpected_message(10), etc
• Heartbeat Protocol
  • A periodic signal generated to indicate normal operation or to synchronize other parts of a system
  • Typically used to monitor the availability of a protocol entity
  • Runs on top of the TLS Record Protocol
  • Established during phase 1 of the handshake protocol
  • Each peer indicated whether it supports heartbeats
  • Serves two purposes:
  • Assures the sender that the recipient is still alive
  • Generates activity across the connection during idle periods

SSL/TLS Attacks

The Heartbleed Exploit (Source: BAE Systems)

HTTPS (HTTP over SSL/TLS)

• Secure communication between a Web browser and a Web server
• Build into all modern Web browsers
  • Search engines do not support HTTPS
• Connection initiation and closure
  • HTTP client act as TLS client
  • TLS handshake -> HTTP request
  • Three levels: HTTP, TLS session, TCP

IPSec v.s. SSL/TLS

Applications of IPSec

• Secure branch office connectivity over the Internet
• Secure remote access over the Internet
• Establishing extranet and internet connectivity with partners
• Enhancing electronic commerce security

Benefits of IPSec

• Strong security
• Resistant to bypass at a firewall
• Transparent to apps
  • No need to change software
• Transparent to end user
  • No need to train users on security mechanisms
• Routing apps: prevent attackers from disrupting communications or diverting some traffic
  • A router advertisement from an authorized router
  • A neighbor advertisement from an authorized router
  • A redirect message from the router to which the initial packet was sent
  • A routing update is nor forged
• The Scope of IPSec
  • Two main functions
    • Encapsulating Security Payload (ESP): a combined authentication/encryption function
    • A key exchange function
  • VPN: both authentication and encryption are generally desired
  • Authentication Header (AH): authentication-only function (deprecated)

Security Associations

• A key concept of IPSec
  • One-way relationship between a sender and a receiver
  • Two-way secure exchange: two SAs are required
• Uniquely identified by three parameters
  • Security parameter index (SPI)
  • IP destination address
  • Protocol identifier: AH or ESP
• Characterized by the following parameters
  • Sequence number counter: 32-bit
  • Sequence counter overflow: A flag -> whether overflow -> an auditable event
  • Antireplay window: defining a sliding window
  • AH information
    • Algorithm, keys, key lifetimes
  • ESP information
    • Algorithm, keys, init values, key lifetimes
  • Lifetime of this security association
  • IPSec protocol mode: tunnel or transport
  • Path MTU

Two IPSec Operation Modes

• Transport and tunnel modes
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• Transport Mode	Tunnel Mode
  Provides protection to the payload of an IP packet Typically used for end-to-end communication __between two hosts__ ESP protects the __IP payload__ but not the IP header	Provides protection to the __entire IP packet__ Entire original packet travels through a tunnel from one point to another Used when one or both ends of a security association are a security gateway Hosts on networks behind firewalls may engage in secure communications without implementing IPSec

Encapsulating Security Payload

• Providing authentication and confidentiality services
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IPSec: AH + ESP

• IP AH only
  • 
• IP AH + ESP
  • Transport mode
    • 
  • Tunnel mode
    • 

IPv4 and IPv6 Security