Chapter 11: Cipher Techniques

Chapter 11: Cipher Techniques

Chapter 11: Cipher Techniques Some Problems Types of Ciphers Networks Examples June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-1 Overview Problems What can go wrong if you naively use ciphers Cipher types

Stream or block ciphers? Networks Link vs end-to-end use Examples Privacy-Enhanced Electronic Mail (PEM) Secure Socket Layer (SSL) Security at the Network Layer (IPsec) June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-2 Problems Using cipher requires knowledge of environment, and threats in the environment, in which cipher will be used Is the set of possible messages small? Do the messages exhibit regularities that remain after encipherment?

Can an active wiretapper rearrange or change parts of the message? June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-3 Attack #1: Precomputation Set of possible messages M small Public key cipher f used Idea: precompute set of possible ciphertexts f(M), build table (m, f(m)) When ciphertext f(m) appears, use table to find m Also called forward searches June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-4

Example Cathy knows Alice will send Bob one of two messages: enciphered BUY, or enciphered SELL Using public key eBob, Cathy precomputes m1 = { BUY } eBob, m2 = { SELL } eBob Cathy sees Alice send Bob m2 Cathy knows Alice sent SELL June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-5 May Not Be Obvious Digitized sound Seems like far too many possible plaintexts Initial calculations suggest 232 such plaintexts Analysis of redundancy in human speech reduced this to about 100,000 ( 217)

This is small enough to worry about precomputation attacks June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-6 Misordered Blocks Alice sends Bob message nBob = 77, eBob = 17, dBob = 53 Message is LIVE (11 08 21 04) Enciphered message is 44 57 21 16 Eve intercepts it, rearranges blocks Now enciphered message is 16 21 57 44 Bob gets enciphered message, deciphers it He sees EVIL June 1, 2004

Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-7 Notes Digitally signing each block wont stop this attack Two approaches: Cryptographically hash the entire message and sign it Place sequence numbers in each block of message, so recipient can tell intended order Then you sign each block June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-8 Statistical Regularities If plaintext repeats, ciphertext may too

Example using DES: input (in hex): 3231 3433 3635 3837 3231 3433 3635 3837 corresponding output (in hex): ef7c 4bb2 b4ce 6f3b ef7c 4bb2 b4ce 6f3b Fix: cascade blocks together (chaining) More details later June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-9 What These Mean Use of strong cryptosystems, well-chosen (or random) keys not enough to be secure Other factors:

Protocols directing use of cryptosystems Ancillary information added by protocols Implementation (not discussed here) Maintenance and operation (not discussed here) June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-10 Stream, Block Ciphers E encipherment function Ek(b) encipherment of message b with key k In what follows, m = b1b2 , each bi of fixed length Block cipher Ek(m) = Ek(b1)Ek(b2)

Stream cipher k = k1k2 Ek(m) = Ek1(b1)Ek2(b2) If k1k2 repeats itself, cipher is periodic and the kength of its period is one cycle of k1k2 June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-11 Examples Vigenre cipher bi = 1 character, k = k1k2 where ki = 1 character Each bi enciphered using ki mod length(k) Stream cipher DES bi = 64 bits, k = 56 bits Each bi enciphered separately using k Block cipher June 1, 2004

Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-12 Stream Ciphers Often (try to) implement one-time pad by xoring each bit of key with one bit of message Example: m = 00101 k = 10010 c = 10111 But how to generate a good key? June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-13

Synchronous Stream Ciphers n-stage Linear Feedback Shift Register: consists of n bit register r = r0rn1 n bit tap sequence t = t0tn1 Use: Use rn1 as key bit Compute x = r0t0 rn1tn1 Shift r one bit to right, dropping rn1, x becomes r0 June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-14 Operation r0 rn1

bi ci r0 rn1 ri = ri1, 0

Example 4-stage LFSR; t = 1001 r ki new bit computation new r 0010 0 01001001 = 0 0001 0001 1 01000011 = 1 1000 1000 0 11000001 = 1 1100 1100 0 11100001 = 1

1110 1110 0 11101001 = 1 1111 1111 1 11101011 = 0 0111 0 0 11101011 = 1 1011 Key sequence has period of 15 (010001111010110) June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-16 NLFSR

n-stage Non-Linear Feedback Shift Register: consists of n bit register r = r0rn1 Use: Use rn1 as key bit Compute x = f(r0, , rn1); f is any function Shift r one bit to right, dropping rn1, x becomes r0 Note same operation as LFSR but more general bit replacement function June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-17 Example 4-stage NLFSR; f(r0, r1, r2, r3) = (r0 & r2) | r3 r ki

new bit computation new r 1100 0110 0011 1001 1100 0110 0011 0 0 1 1 0 0 1 (1 (0 (0

(1 (1 (0 (0 0110 0011 1001 1100 0110 0011 1001 & & & & & & & 0) 1)

1) 0) 0) 1) 1) | | | | | | | 0 0 1 1 0 0 1 =

= = = = = = 0 0 1 1 0 0 1 Key sequence has period of 4 (0011) June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-18

Eliminating Linearity NLFSRs not common No body of theory about how to design them to have long period Alternate approach: output feedback mode For E encipherment function, k key, r register: Compute r= Ek(r); key bit is rightmost bit of r Set r to r and iterate, repeatedly enciphering register and extracting key bits, until message enciphered Variant: use a counter that is incremented for each encipherment rather than a register Take rightmost bit of Ek(i), where i is number of encipherment June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-19 Self-Synchronous Stream Cipher Take key from message itself (autokey)

Example: Vigenre, key drawn from plaintext key XTHEBOYHASTHEBA plaintextTHEBOYHASTHEBAG ciphertext QALFPNFHSLALFCT Problem: Statistical regularities in plaintext show in key Once you get any part of the message, you can decipher more June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-20 Another Example Take key from ciphertext (autokey) Example: Vigenre, key drawn from ciphertext key plaintext

ciphertext XQXBCQOVVNGNRTT THEBOYHASTHEBAG QXBCQOVVNGNRTTM Problem: Attacker gets key along with ciphertext, so deciphering is trivial June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-21 Variant Cipher feedback mode: 1 bit of ciphertext fed into n bit register Self-healing property: if ciphertext bit received incorrectly, it and next n bits decipher incorrectly; but after that, the ciphertext bits decipher correctly Need to know k, E to decipher ciphertext

r k E Ek(r) mi ci June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-22 Block Ciphers Encipher, decipher multiple bits at once

Each block enciphered independently Problem: identical plaintext blocks produce identical ciphertext blocks Example: two database records MEMBER: HOLLY INCOME $100,000 MEMBER: HEIDI INCOME $100,000 Encipherment: ABCQZRME GHQMRSIB CTXUVYSS RMGRPFQN ABCQZRME ORMPABRZ CTXUVYSS RMGRPFQN June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-23 Solutions Insert information about blocks position into the plaintext block, then encipher Cipher block chaining: Exclusive-or current plaintext block with previous ciphertext block:

c0 = Ek(m0 I) ci = Ek(mi ci1) for i > 0 where I is the initialization vector June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-24 Multiple Encryption Double encipherment: c = Ek(Ek(m)) Effective key length is 2n, if k, k are length n Problem: breaking it requires 2n+1 encryptions, not 22n encryptions Triple encipherment: EDE mode: c = Ek(Dk(Ek(m)) Problem: chosen plaintext attack takes O(2 n) time using 2n ciphertexts Triple encryption mode: c = Ek(Ek(Ek(m))

Best attack requires O(22n) time, O(2n) memory June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-25 Networks and Cryptography ISO/OSI model Conceptually, each host has peer at each layer Peers communicate with peers at same layer June 1, 2004 Application layer Application layer Presentation layer Presentation layer

Session layer Session layer Transport layer Transport layer Netw ork layer Network layer Network layer Data link layer Data link layer Data link layer Physical layer Physical layer

Physical layer Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-26 Link and End-to-End Protocols Link Protocol End-to-End (or E2E) Protocol June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-27 Encryption Link encryption Each host enciphers message so host at next

hop can read it Message can be read at intermediate hosts End-to-end encryption Host enciphers message so host at other end of communication can read it Message cannot be read at intermediate hosts June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-28 Examples TELNET protocol Messages between client, server enciphered, and encipherment, decipherment occur only at these hosts End-to-end protocol PPP Encryption Control Protocol Host gets message, deciphers it Figures out where to forward it

Enciphers it in appropriate key and forwards it Link protocol June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-29 Cryptographic Considerations Link encryption Each host shares key with neighbor Can be set on per-host or per-host-pair basis Windsor, stripe, seaview each have own keys One key for (windsor, stripe); one for (stripe, seaview); one for (windsor, seaview) End-to-end Each host shares key with destination Can be set on per-host or per-host-pair basis Message cannot be read at intermediate nodes June 1, 2004

Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-30 Traffic Analysis Link encryption Can protect headers of packets Possible to hide source and destination Note: may be able to deduce this from traffic flows End-to-end encryption Cannot hide packet headers Intermediate nodes need to route packet Attacker can read source, destination June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-31

Example Protocols Privacy-Enhanced Electronic Mail (PEM) Applications layer protocol Secure Socket Layer (SSL) Transport layer protocol IP Security (IPSec) Network layer protocol June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-32 Goals of PEM 1. Confidentiality Only sender and recipient(s) can read message

2. Origin authentication Identify the sender precisely 3. Data integrity Any changes in message are easy to detect 4. Non-repudiation of origin June 1, 2004 Whenever possible Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-33 Message Handling System UA

MTA June 1, 2004 UA MTA Computer Security: Art and Science 2002-2004 Matt Bishop UA MTA User Agents Message Transfer Agents

Slide #11-34 Design Principles Do not change related existing protocols Cannot alter SMTP Do not change existing software Need compatibility with existing software Make use of PEM optional Available if desired, but email still works without them Some recipients may use it, others not Enable communication without prearrangement Out-of-bands authentication, key exchange problematic June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-35 Basic Design: Keys

Two keys Interchange keys tied to sender, recipients and is static (for some set of messages) Like a public/private key pair Must be available before messages sent Data exchange keys generated for each message Like a session key, session being the message June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-36 Basic Design: Sending Confidentiality m message ks data exchange key kB Bobs interchange key Alice

June 1, 2004 { m } ks || { ks } kB Computer Security: Art and Science 2002-2004 Matt Bishop Bob Slide #11-37 Basic Design: Integrity Integrity and authentication: m message h(m) hash of message m Message Integrity Check (MIC) kA Alices interchange key Alice m { h(m) } kA Bob

Non-repudiation: if kA is Alices private key, this establishes that Alices private key was used to sign the message June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-38 Basic Design: Everything Confidentiality, integrity, authentication: Notations as in previous slides If kA is private key, get non-repudiation too { m } ks || { h(m) } kA || { ks } kB Alice June 1, 2004 Bob Computer Security: Art and Science 2002-2004 Matt Bishop

Slide #11-39 Practical Considerations Limits of SMTP Only ASCII characters, limited length lines Use encoding procedure 1. Map local char representation into canonical format Format meets SMTP requirements 2. Compute and encipher MIC over the canonical format; encipher message if needed 3. Map each 6 bits of result into a character; insert newline after every 64th character 4. Add delimiters around this ASCII message June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-40 Problem

Recipient without PEM-compliant software cannot read it If only integrity and authentication used, should be able to read it Mode MIC-CLEAR allows this Skip step 3 in encoding procedure Problem: some MTAs add blank lines, delete trailing white space, or change end of line character Result: PEM-compliant software reports integrity failure June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-41 PEM vs. PGP Use different ciphers PGP uses IDEA cipher PEM uses DES in CBC mode

Use different certificate models PGP uses general web of trust PEM uses hierarchical certification structure Handle end of line differently PGP remaps end of line if message tagged text, but leaves them alone if message tagged binary PEM always remaps end of line June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-42 SSL Transport layer security Provides confidentiality, integrity, authentication of endpoints Developed by Netscape for WWW browsers and servers Internet protocol version: TLS

Compatible with SSL Not yet formally adopted June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-43 SSL Session Association between two peers May have many associated connections Information for each association: Unique session identifier Peers X.509v3 certificate, if needed Compression method Cipher spec for cipher and MAC

Master secret shared with peer 48 bits June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-44 SSL Connection Describes how data exchanged with peer Information for each connection Random data Write keys (used to encipher data) Write MAC key (used to compute MAC) Initialization vectors for ciphers, if needed

Sequence numbers June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-45 Structure of SSL SSL Alert Protocol SSL Handshake Protocol SSL Application Data Protocol SSL Change Cipher Spec Protocol SSL Record Protocol June 1, 2004

Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-46 Supporting Crypto All parts of SSL use them Initial phase: public key system exchanges keys Messages enciphered using classical ciphers, checksummed using cryptographic checksums Only certain combinations allowed Depends on algorithm for interchange cipher Interchange algorithms: RSA, Diffie-Hellman, Fortezza June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-47

RSA: Cipher, MAC Algorithms Interchange cipher RSA, key 512 bits RSA June 1, 2004 Classical cipher MAC Algorithm none MD5, SHA RC4, 40-bit key MD5 RC2, 40-bit key, CBC mode

MD5 DES, 40-bit key, CBC mode SHA None MD5, SHA RC4, 128-bit key MD5, SHA IDEA, CBC mode SHA DES, CBC mode SHA DES, EDE mode, CBC mode

SHA Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-48 Diffie-Hellman: Types Diffie-Hellman: certificate contains D-H parameters, signed by a CA DSS or RSA algorithms used to sign Ephemeral Diffie-Hellman: DSS or RSA certificate used to sign D-H parameters Parameters not reused, so not in certificate Anonymous Diffie-Hellman: D-H with neither party authenticated Use is strongly discouraged as it is vulnerable to attacks June 1, 2004

Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-49 D-H: Cipher, MAC Algorithms Interchange cipher Diffie-Hellman, DSS Certificate Classical cipher MAC Algorithm DES, 40-bit key, CBC mode SHA DES, CBC mode SHA DES, EDE mode, CBC mode

SHA Diffie-Hellman, key 512 bits RSA Certificate DES, 40-bit key, CBC mode SHA DES, CBC mode SHA DES, EDE mode, CBC mode SHA June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop

Slide #11-50 Ephemeral D-H: Cipher, MAC Algorithms Interchange cipher Classical cipher MAC Algorithm Ephemeral DiffieHellman, DSS Certificate DES, 40-bit key, CBC mode SHA DES, CBC mode SHA Ephemeral DiffieHellman,

key 512 bits, RSA Certificate DES, 40-bit key, CBC mode SHA DES, CBC mode SHA June 1, 2004 DES, EDE mode, CBC mode SHA DES, EDE mode, CBC mode SHA Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-51 Anonymous D-H: Cipher, MAC

Algorithms Interchange cipher Anonymous D-H, DSS Certificate Classical cipher MAC Algorithm RC4, 40-bit key MD5 RC4, 128-bit key MD5 DES, 40-bit key, CBC mode SHA DES, CBC mode

SHA DES, EDE mode, CBC mode SHA June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-52 Fortezza: Cipher, MAC Algorithms Interchange cipher Fortezza key exchange June 1, 2004 Classical cipher MAC Algorithm

none SHA RC4, 128-bit key MD5 Fortezza, CBC mode SHA Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-53 Digital Signatures RSA Concatenate MD5 and SHA hashes Sign with public key Diffie-Hellman, Fortezza

Compute SHA hash Sign appropriately June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-54 SSL Record Layer Message Compressed blocks Compressed blocks, enciphered, with MAC June 1, 2004 MAC

Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-55 Record Protocol Overview Lowest layer, taking messages from higher Max block size 16,384 bytes Bigger messages split into multiple blocks Construction Block b compressed; call it bc MAC computed for bc If MAC key not selected, no MAC computed bc, MAC enciphered If enciphering key not selected, no enciphering done SSL record header prepended June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop

Slide #11-56 SSL MAC Computation Symbols h hash function (MD5 or SHA) kw write MAC key of entity ipad = 0x36, opad = 0x5C Repeated to block length (from HMAC) seq sequence number SSL_comp message type SSL_len block length MAC h(kw||opad||h(kw||ipad||seq||SSL_comp||SSL_len||block)) June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-57

SSL Handshake Protocol Used to initiate connection Sets up parameters for record protocol 4 rounds Upper layer protocol Invokes Record Protocol Note: what follows assumes client, server using RSA as interchange cryptosystem June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-58 Overview of Rounds 1. Create SSL connection between client, server 2. Server authenticates itself 3. Client validates server, begins key exchange

4. Acknowledgments all around June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-59 Handshake Round 1 Client Client vC v r1, r2 s1 s2 ciphers comps cipher comp June 1, 2004

{ vC || r1 || sid || ciphers || comps } {v || r2 || sid || cipher || comp } Server Server Clients version of SSL Highest version of SSL that Client, Server both understand nonces (timestamp and 28 random bytes) Current session id (0 if new session) Current session id (if s1 = 0, new session id) Ciphers that client understands Compression algorithms that client understand Cipher to be used Compression algorithm to be used Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-60 Handshake Round 2 Client

Client Client Client {certificate } {mod || exp || { h(r1 || r2 || mod || exp) } kS } {ctype || gca } {er2 } Server Server Server Server Note: if Server not to authenticate itself, only last message sent; third step omitted if Server does not need Client certificate kS Servers private key ctype Certificate type requested (by cryptosystem) gca Acceptable certification authorities er2

End round 2 message June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-61 Handshake Round 3 Client { pre } Server Both Client, Server compute master secret master: master = MD5(pre || SHA(A || pre || r1 || r2) || MD5(pre || SHA(BB || pre || r1 || r2) || MD5(pre || SHA(CCC || pre || r1 || r2) Client { h(master || opad || h(msgs || master | ipad)) }

Server msgs Concatenation of previous messages sent/received this handshake opad, ipad As above June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-62 Handshake Round 4 Client sends change cipher spec message using that protocol Client Server { h(master || opad || h(msgs || 0x434C4E54 || master || ipad )) }

Client Server Server sends change cipher spec message using that protocol Server Client Client { h(master || opad || h(msgs || master | ipad)) } Server msgs Concatenation of messages sent/received this handshake in previous rounds (does notinclude these messages) opad, ipad, master As above June 1, 2004 Computer Security: Art and Science

2002-2004 Matt Bishop Slide #11-63 SSL Change Cipher Spec Protocol Send single byte In handshake, new parameters considered pending until this byte received Old parameters in use, so cannot just switch to new ones June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-64 SSL Alert Protocol Closure alert Sender will send no more messages Pending data delivered; new messages ignored

Error alerts Warning: connection remains open Fatal error: connection torn down as soon as sent or received June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-65 SSL Alert Protocol Errors Always fatal errors: unexpected_message, bad_record_mac, decompression_failure, handshake_failure, illegal_parameter May be warnings or fatal errors: no_certificate, bad_certificate, unsupported_certificate, certificate_revoked, certificate_expired, certificate_unknown June 1, 2004

Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-66 SSL Application Data Protocol Passes data from application to SSL Record Protocol layer June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-67 IPsec Network layer security Provides confidentiality, integrity, authentication of endpoints, replay detection Protects all messages sent along a path

dest IP IP+IPsec gw2 IP gw1 src security gateway June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-68 IPsec Transport Mode IP

header encapsulated data body Encapsulate IP packet data area Use IP to send IPsec-wrapped data packet Note: IP header not protected June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-69 IPsec Tunnel Mode IP header encapsulated data body

Encapsulate IP packet (IP header and IP data) Use IP to send IPsec-wrapped packet Note: IP header protected June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-70 IPsec Protocols Authentication Header (AH) Message integrity Origin authentication Anti-replay Encapsulating Security Payload (ESP) Confidentiality Others provided by AH June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop

Slide #11-71 IPsec Architecture Security Policy Database (SPD) Says how to handle messages (discard them, add security services, forward message unchanged) SPD associated with network interface SPD determines appropriate entry from packet attributes Including source, destination, transport protocol June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-72 Example Goals Discard SMTP packets from host 192.168.2.9 Forward packets from 192.168.19.7 without change

SPD entries src 192.168.2.9, dest 10.1.2.3 to 10.1.2.103, port 25, discard src 192.168.19.7, dest 10.1.2.3 to 10.1.2.103, port 25, bypass dest 10.1.2.3 to 10.1.2.103, port 25, apply IPsec Note: entries scanned in order If no match for packet, it is discarded June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-73 IPsec Architecture Security Association (SA) Association between peers for security services Identified uniquely by dest address, security protocol (AH or ESP), unique 32-bit number (security parameter index, or SPI) Unidirectional

Can apply different services in either direction SA uses either ESP or AH; if both required, 2 SAs needed June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-74 SA Database (SAD) Entry describes SA; some fields for all packets: AH algorithm identifier, keys When SA uses AH ESP encipherment algorithm identifier, keys When SA uses confidentiality from ESP ESP authentication algorithm identifier, keys When SA uses authentication, integrity from ESP SA lifetime (time for deletion or max byte count)

IPsec mode (tunnel, transport, either) June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-75 SAD Fields Antireplay (inbound only) When SA uses antireplay feature Sequence number counter (outbound only) Generates AH or ESP sequence number Sequence counter overflow field Stops traffic over this SA if sequence counter overflows Aging variables Used to detect time-outs June 1, 2004 Computer Security: Art and Science

2002-2004 Matt Bishop Slide #11-76 IPsec Architecture Packet arrives Look in SPD Find appropriate entry Get dest address, security protocol, SPI Find associated SA in SAD Use dest address, security protocol, SPI Apply security services in SA (if any) June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-77 SA Bundles and Nesting Sequence of SAs that IPsec applies to packets

This is a SA bundle Nest tunnel mode SAs This is iterated tunneling June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-78 Example: Nested Tunnels Group in A.org needs to communicate with group in B.org Gateways of A, B use IPsec mechanisms But the information must be secret to everyone except the two groups, even secret from other people in A.org and B.org Inner tunnel: a SA between the hosts of the two groups Outer tunnel: the SA between the two gateways

June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-79 Example: Systems gwA.A.org hostA.A.org SA in tunnel mode (outer tunnel) SA in tunnel mode (inner tunnel) June 1, 2004 hostB.B.org gwB.B.org Computer Security: Art and Science

2002-2004 Matt Bishop Slide #11-80 Example: Packets IP header from gwA AH header from gwA ESP header from gwA IP header from

hostA AH header from hostA ESP header from hostA IP header from hostA Transport layer headers, data Packet generated on hostA

Encapsulated by hostAs IPsec mechanisms Again encapsulated by gwAs IPsec mechanisms Above diagram shows headers, but as you go left, everything to the right would be enciphered and authenticated, etc. June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-81 AH Protocol Parameters in AH header Length of header SPI of SA applying protocol Sequence number (anti-replay) Integrity value check

Two steps Check that replay is not occurring Check authentication data June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-82 Sender Check sequence number will not cycle Increment sequence number Compute IVC of packet Includes IP header, AH header, packet data IP header: include all fields that will not change in transit; assume all others are 0 AH header: authentication data field set to 0 for this Packet data includes encapsulated data, higher level protocol data June 1, 2004 Computer Security: Art and Science

2002-2004 Matt Bishop Slide #11-83 Recipient Assume AH header found Get SPI, destination address Find associated SA in SAD If no associated SA, discard packet If antireplay not used Verify IVC is correct If not, discard June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-84 Recipient, Using Antireplay Check packet beyond low end of sliding window Check IVC of packet

Check packets slot not occupied If any of these is false, discard packet current window June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-85 AH Miscellany All implementations must support: HMAC_MD5 HMAC_SHA-1 May support other algorithms June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop

Slide #11-86 ESP Protocol Parameters in ESP header SPI of SA applying protocol Sequence number (anti-replay) Generic payload data field Padding and length of padding Contents depends on ESP services enabled; may be an initialization vector for a chaining cipher, for example Used also to pad packet to length required by cipher Optional authentication data field June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop

Slide #11-87 Sender Add ESP header Includes whatever padding needed Encipher result Do not encipher SPI, sequence numbers If authentication desired, compute as for AH protocol except over ESP header, payload and not encapsulating IP header June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-88 Recipient Assume ESP header found Get SPI, destination address

Find associated SA in SAD If no associated SA, discard packet If authentication used Do IVC, antireplay verification as for AH Only ESP, payload are considered; not IP header Note authentication data inserted after encipherment, so no deciphering need be done June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-89 Recipient If confidentiality used Decipher enciphered portion of ESP heaser

Process padding Decipher payload If SA is transport mode, IP header and payload treated as original IP packet If SA is tunnel mode, payload is an encapsulated IP packet and so is treated as original IP packet June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-90 ESP Miscellany Must use at least one of confidentiality, authentication services Synchronization material must be in payload Packets may not arrive in order, so if not, packets following a missing packet may not be decipherable Implementations of ESP assume classical cryptosystem

Implementations of public key systems usually far slower than implementations of classical systems Not required June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-91 More ESP Miscellany All implementations must support (encipherment algorithms): DES in CBC mode NULL algorithm (identity; no encipherment) All implementations must support (integrity algorithms): HMAC_MD5 HMAC_SHA-1 NULL algorithm (no MAC computed) Both cannot be NULL at the same time

June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-92 Which to Use: PEM, SSL, IPsec What do the security services apply to? If applicable to one application and application layer mechanisms available, use that PEM for electronic mail If more generic services needed, look to lower layers SSL for transport layer, end-to-end mechanism IPsec for network layer, either end-to-end or link mechanisms, for connectionless channels as well as connections If endpoint is host, SSL and IPsec sufficient; if endpoint is user, application layer mechanism such as PEM needed June 1, 2004

Computer Security: Art and Science 2002-2004 Matt Bishop Slide #11-93 Key Points Key management critical to effective use of cryptosystems Different levels of keys (session vs. interchange) Keys need infrastructure to identify holders, allow revoking Key escrowing complicates infrastructure Digital signatures provide integrity of origin and content Much easier with public key cryptosystems than with classical cryptosystems June 1, 2004 Computer Security: Art and Science 2002-2004 Matt Bishop

Slide #11-94

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