Presentation on theme: "8: Network Security8-1 Network Security. 8: Network Security8-2 What is network security? Confidentiality: only sender, intended receiver should “understand”"— Presentation transcript:
8: Network Security8-2 What is network security? Confidentiality: only sender, intended receiver should “understand” message contents m sender encrypts message m receiver decrypts message Authentication: sender, receiver want to confirm identity of each other Message Integrity: sender, receiver want to ensure message not altered (in transit, or afterwards) without detection Access and Availability: services must be accessible and available to users
8: Network Security8-3 Friends and enemies: Alice, Bob, Trudy r well-known in network security world r Bob, Alice (lovers!) want to communicate “securely” r Trudy (intruder) may intercept, delete, add messages secure sender secure receiver channel data, control messages data Alice Bob Trudy
8: Network Security8-4 Who might Bob, Alice be? r … well, real-life Bobs and Alices! r Web browser/server for electronic transactions (e.g., on-line purchases) r on-line banking client/server r DNS servers r routers exchanging routing table updates r other examples?
8: Network Security8-5 There are bad guys (and girls) out there! Q: What can a “bad guy” do? A: a lot! m eavesdrop: intercept messages m actively insert messages into connection m impersonation: can fake (spoof) source address in packet (or any field in packet) m hijacking: “take over” ongoing connection by removing sender or receiver, inserting himself in place m denial of service: prevent service from being used by others (e.g., by overloading resources)
8: Network Security8-6 The language of cryptography symmetric key crypto: sender, receiver keys identical public-key crypto: encryption key public, decryption key secret (private) plaintext ciphertext K A encryption algorithm decryption algorithm Alice’s encryption key Bob’s decryption key K B
8: Network Security8-7 Symmetric key cryptography substitution cipher: substituting one thing for another m monoalphabetic cipher: substitute one letter for another plaintext: abcdefghijklmnopqrstuvwxyz ciphertext: mnbvcxzasdfghjklpoiuytrewq Plaintext: bob. i love you. alice ciphertext: nkn. s gktc wky. mgsbc E.g.:
8: Network Security8-8 Symmetric key cryptography symmetric key crypto: Bob and Alice share know same (symmetric) key: K r e.g., key is knowing substitution pattern in mono alphabetic substitution cipher r Q: how do Bob and Alice agree on key value? plaintext ciphertext K A-B encryption algorithm decryption algorithm A-B K plaintext message, m K (m) A-B K (m) A-B m = K ( ) A-B
8: Network Security8-9 Public Key Cryptography symmetric key crypto r requires sender, receiver know shared secret key r Q: how to agree on key in first place (particularly if never “met”)? public key cryptography r radically different approach [Diffie- Hellman76, RSA78] r sender, receiver do not share secret key r public encryption key known to all r private decryption key known only to receiver
8: Network Security8-10 Public key cryptography plaintext message, m ciphertext encryption algorithm decryption algorithm Bob’s public key plaintext message K (m) B + K B + Bob’s private key K B - m = K ( K (m) ) B + B -
8: Network Security8-11 Public key encryption algorithms need K ( ) and K ( ) such that B B.. given public key K, it should be impossible to compute private key K B B Requirements: 1 2 RSA: Rivest, Shamir, Adelson algorithm + - K (K (m)) = m B B - + + -
8: Network Security8-12 RSA: another important property The following property will be very useful later: K ( K (m) ) = m B B - + K ( K (m) ) B B + - = use public key first, followed by private key use private key first, followed by public key Result is the same!
8: Network Security8-13 Authentication Goal: Bob wants Alice to “prove” her identity to him Protocol ap1.0: Alice says “I am Alice” Failure scenario?? “I am Alice”
8: Network Security8-14 Authentication Goal: Bob wants Alice to “prove” her identity to him Protocol ap1.0: Alice says “I am Alice” in a network, Bob can not “see” Alice, so Trudy simply declares herself to be Alice “I am Alice”
8: Network Security8-15 Authentication: another try Protocol ap2.0: Alice says “I am Alice” in an IP packet containing her source IP address Failure scenario?? “I am Alice” Alice’s IP address
8: Network Security8-16 Authentication: another try Protocol ap2.0: Alice says “I am Alice” in an IP packet containing her source IP address Trudy can create a packet “spoofing” Alice’s address “I am Alice” Alice’s IP address
8: Network Security8-17 Authentication: another try Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it. Failure scenario?? “I’m Alice” Alice’s IP addr Alice’s password OK Alice’s IP addr
8: Network Security8-18 Authentication: another try Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it. playback attack: Trudy records Alice’s packet and later plays it back to Bob “I’m Alice” Alice’s IP addr Alice’s password OK Alice’s IP addr “I’m Alice” Alice’s IP addr Alice’s password
8: Network Security8-19 Authentication: yet another try Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. Failure scenario?? “I’m Alice” Alice’s IP addr encrypted password OK Alice’s IP addr
8: Network Security8-20 Authentication: another try Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. record and playback still works! “I’m Alice” Alice’s IP addr encrypted password OK Alice’s IP addr “I’m Alice” Alice’s IP addr encrypted password
8: Network Security8-21 Authentication: yet another try Goal: avoid playback attack Failures, drawbacks? Nonce: number (R) used only once –in-a-lifetime ap4.0: to prove Alice “live”, Bob sends Alice nonce, R. Alice must return R, encrypted with shared secret key “I am Alice” R K (R) A-B Alice is live, and only Alice knows key to encrypt nonce, so it must be Alice!
8: Network Security8-22 Authentication: ap5.0 ap4.0 requires shared symmetric key r can we authenticate using public key techniques? ap5.0: use nonce, public key cryptography “I am Alice” R Bob computes K (R) A - “send me your public key” K A + (K (R)) = R A - K A + and knows only Alice could have the private key, that encrypted R such that (K (R)) = R A - K A +
8: Network Security8-23 ap5.0: security hole Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice) I am Alice R T K (R) - Send me your public key T K + A K (R) - Send me your public key A K + T K (m) + T m = K (K (m)) + T - Trudy gets sends m to Alice encrypted with Alice’s public key A K (m) + A m = K (K (m)) + A - R
8: Network Security8-24 ap5.0: security hole Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice) Difficult to detect: Bob receives everything that Alice sends, and vice versa. (e.g., so Bob, Alice can meet one week later and recall conversation) problem is that Trudy receives all messages as well!
8: Network Security8-25 Digital Signatures Cryptographic technique analogous to hand- written signatures. r sender (Bob) digitally signs document, establishing he is document owner/creator. r verifiable, nonforgeable: recipient (Alice) can prove to someone that Bob, and no one else (including Alice), must have signed document
8: Network Security8-26 Digital Signatures Simple digital signature for message m: r Bob signs m by encrypting with his private key K B, creating “signed” message, K B (m) - - Dear Alice Oh, how I have missed you. I think of you all the time! …(blah blah blah) Bob Bob’s message, m Public key encryption algorithm Bob’s private key K B - Bob’s message, m, signed (encrypted) with his private key K B - (m)
8: Network Security8-27 Digital Signatures (more) r Suppose Alice receives msg m, digital signature K B (m) r Alice verifies m signed by Bob by applying Bob’s public key K B to K B (m) then checks K B (K B (m) ) = m. r If K B (K B (m) ) = m, whoever signed m must have used Bob’s private key. + + - - -- + Alice thus verifies that: ü Bob signed m. ü No one else signed m. ü Bob signed m and not m’. Non-repudiation: Alice can take m, and signature K B (m) to court and prove that Bob signed m. -
8: Network Security8-28 Message Digests Computationally expensive to public-key-encrypt long messages Goal: fixed-length, easy- to-compute digital “fingerprint” r apply hash function H to m, get fixed size message digest, H(m). Hash function properties: r produces fixed-size msg digest (fingerprint) r given message digest x, computationally infeasible to find m such that x = H(m) large message m H: Hash Function H(m)
8: Network Security8-29 large message m H: Hash function H(m) digital signature (encrypt) Bob’s private key K B - + Bob sends digitally signed message: Alice verifies signature and integrity of digitally signed message: K B (H(m)) - encrypted msg digest K B (H(m)) - encrypted msg digest large message m H: Hash function H(m) digital signature (decrypt) H(m) Bob’s public key K B + equal ? Digital signature = signed message digest
8: Network Security8-30 Hash Function Algorithms r MD5 hash function widely used (RFC 1321) m computes 128-bit message digest in 4-step process. m arbitrary 128-bit string x, appears difficult to construct msg m whose MD5 hash is equal to x. r SHA-1 is also used. m US standard [ NIST, FIPS PUB 180-1] m 160-bit message digest
8: Network Security8-31 Trusted Intermediaries Symmetric key problem: r How do two entities establish shared secret key over network? Solution: r trusted key distribution center (KDC) acting as intermediary between entities Public key problem: r When Alice obtains Bob’s public key (from web site, e-mail, diskette), how does she know it is Bob’s public key, not Trudy’s? Solution: r trusted certification authority (CA)
8: Network Security8-32 Key Distribution Center (KDC) r Alice, Bob need shared symmetric key. r KDC: server shares different secret key with each registered user (many users) r Alice, Bob know own symmetric keys, K A-KDC K B-KDC, for communicating with KDC. K B-KDC K X-KDC K Y-KDC K Z-KDC K P-KDC K B-KDC K A-KDC K P-KDC KDC
8: Network Security8-33 Certification Authorities r Certification authority (CA): binds public key to particular entity, E. r E (person, router) registers its public key with CA. m E provides “proof of identity” to CA. m CA creates certificate binding E to its public key. m certificate containing E’s public key digitally signed by CA – CA says “this is E’s public key” Bob’s public key K B + Bob’s identifying information digital signature (encrypt) CA private key K CA - K B + certificate for Bob’s public key, signed by CA
8: Network Security8-34 Certification Authorities r When Alice wants Bob’s public key: m gets Bob’s certificate (Bob or elsewhere). m apply CA’s public key to Bob’s certificate, get Bob’s public key Bob’s public key K B + digital signature (decrypt) CA public key K CA + K B +
8: Network Security8-35 A certificate contains: r Serial number (unique to issuer) r info about certificate owner, including algorithm and key value itself (not shown) r info about certificate issuer r valid dates r digital signature by issuer
8: Network Security8-36 Secure sockets layer (SSL) r transport layer security to any TCP- based app using SSL services. r used between Web browsers, servers for e-commerce (shttp). r security services: m server authentication m data encryption m client authentication (optional) r server authentication: m SSL-enabled browser includes public keys for trusted CAs. m Browser requests server certificate, issued by trusted CA. m Browser uses CA’s public key to extract server’s public key from certificate. r check your browser’s security menu to see its trusted CAs.
8: Network Security8-37 SSL (continued) Client Server Open secure socket Certificate (CA signed Pub Key) Verify CA trusted Extract Server Pub Key Generate symmetric Session Key Encrypt Session Key with Server Pub Key Encrypted Session Key Extract Session Key (using Private Key) In Java all of this happens behind the scenes! SSLSocket s = (SSLSocket)sslFact.createSocket(host, port);
8: Network Security8-38 SSL Observations r Previous example does not m Show how public/private key pairs are generated Manually m Enable the Server to authenticate the client Client can use trusted certificate, or another scheme such as passwords m Show how the Server receives a signed certificate CA!
8: Network Security8-39 Project 3 r Project 3 Overview m Secure your time/date client and server m Mutual authentication between client and server m Implement a trusted CA that can generate certificates for the client and server (by signing their public keys) r Offline, manual tasks 1. CA, Client, and Server generate public/private keys 2. CA generates self-signed certificate 3. Client and Server import CA certificate
8: Network Security8-40 Project 3 m CA behavior 1. Accepts connections from clients – does not require SSL-based client authentication mNote: The T/D Server can act as a client when connecting to the CA 2. Authenticates clients using passwords – can be sent in plaintext 3. Receives Public Key from client 4. Generates certificate by signing client Public Key with CA Private Key 5. Returns certificate to client
8: Network Security8-41 Project 3 m Server behavior 1. Connect to trusted CA 2. Send password for CA authentication of Server 3. Send Public Key to CA 4. Receive certificate from CA 5. Wait for secure client connection 6. Require client authentication – client must have CA- signed certificate 7. Allow client to request time or date 8. Send response
8: Network Security8-42 Project 3 m Client behavior 1. Connect to trusted CA 2. Send password for CA authentication of Server 3. Send Public Key to CA 4. Receive certificate from CA 5. Securely connect to server 6. Request time or date 7. Receive/display response
8: Network Security8-43 Java and SSL r keytool – command line tool used to generate public/private keys, generate self-signed certificates, and import other trusted certificates r javax.net.ssl java.security java.security.[spec,cert] org.bouncycastle.[x509,jce,asn1] r KeyManager – object that stores my keys/certificates m used during handshake with server r TrustManager – object that stores trusted certificates m can be initialized from same file as KeyStore
8: Network Security8-44 Java and SSL r SSLContext – context that specifies keys, certificates, and trusted certificates m initialize context with appropriate KeyManagers and TrustManagers r SSLSocketFactory/SSLServerSocketFact ory – will create socket based on given context m default context will not contain appropriate certificates r SSLSocket – performs regular socket-like communication only with encryption
8: Network Security8-45 Java and SSL r CertificateFactory – can create a certificate from an InputStream m used to create certificate sent from CA r Certificate, PublicKey, PrivateKey – what you’d expect
8: Network Security8-46 Java and SSL r X509Certificate – certificate based on X.509 standard m defines structure of certificate, etc r X509Name – object to represent distinguished name of subject/issuer of certificate r X509EncodedKeySpec/KeyFactory – create public key from byte stream r X509V3CertificateGenerator – use to generate certificate m use specified private key to generate certificate for holder of specified public key
8: Network Security8-47 Firewalls isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others. firewall
8: Network Security8-48 Firewalls: Why prevent denial of service attacks: m SYN flooding: attacker establishes many bogus TCP connections, no resources left for “real” connections. prevent illegal modification/access of internal data. m e.g., attacker replaces CIA’s homepage with something else allow only authorized access to inside network (set of authenticated users/hosts) two types of firewalls: m application-level m packet-filtering
8: Network Security8-49 Packet Filtering r internal network connected to Internet via router firewall r router filters packet-by-packet, decision to forward/drop packet based on: m source IP address, destination IP address m TCP/UDP source and destination port numbers m ICMP message type m TCP SYN and ACK bits Should arriving packet be allowed in? Departing packet let out?
8: Network Security8-50 Packet Filtering r Example 1: block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23. m All incoming and outgoing UDP flows and telnet connections are blocked. r Example 2: Block inbound TCP segments with ACK=0. m Prevents external clients from making TCP connections with internal clients, but allows internal clients to connect to outside.
8: Network Security8-51 Application gateways r Filters packets on application data as well as on IP/TCP/UDP fields. r Example: allow select internal users to telnet outside. host-to-gateway telnet session gateway-to-remote host telnet session application gateway router and filter 1. Require all telnet users to telnet through gateway. 2. For authorized users, gateway sets up telnet connection to dest host. Gateway relays data between 2 connections 3. Router filter blocks all telnet connections not originating from gateway.
8: Network Security8-52 Internet security threats Mapping: m before attacking: “case the joint” – find out what services are implemented on network Use ping to determine what hosts have addresses on network m Port-scanning: try to establish TCP connection to each port in sequence (see what happens) m nmap (http://www.insecure.org/nmap/) mapper: “network exploration and security auditing” Countermeasures?
8: Network Security8-53 Internet security threats Mapping: countermeasures m record traffic entering network m look for suspicious activity (IP addresses, pots being scanned sequentially)
8: Network Security8-54 Internet security threats Packet sniffing: m broadcast media m promiscuous NIC reads all packets passing by m can read all unencrypted data (e.g. passwords) m e.g.: C sniffs B’s packets A B C src:B dest:A payload Countermeasures?
8: Network Security8-55 Internet security threats Packet sniffing: countermeasures m all hosts in organization run software that checks periodically if host interface in promiscuous mode. m one host per segment of broadcast media (switched Ethernet at hub) A B C src:B dest:A payload
8: Network Security8-56 Internet security threats IP Spoofing: m can generate “raw” IP packets directly from application, putting any value into IP source address field m receiver can’t tell if source is spoofed m e.g.: C pretends to be B A B C src:B dest:A payload Countermeasures?
8: Network Security8-57 Internet security threats IP Spoofing: ingress filtering m routers should not forward outgoing packets with invalid source addresses (e.g., datagram source address not in router’s network) m great, but ingress filtering can not be mandated for all networks A B C src:B dest:A payload
8: Network Security8-58 Internet security threats Denial of service (DOS): m flood of maliciously generated packets “swamp” receiver m Distributed DOS (DDOS): multiple coordinated sources swamp receiver m e.g., C and remote host SYN-attack A A B C SYN Countermeasures?
8: Network Security8-59 Internet security threats Denial of service (DOS): countermeasures m filter out flooded packets (e.g., SYN) before reaching host: throw out good with bad m traceback to source of floods (most likely an innocent, compromised machine) A B C SYN