Chapter 11
Security Protocols
Network Security Threats
Security and Cryptography
Network Security Protocols
Chapter 11
Security Protocols
Network Security
z The combination of low-cost powerful computing
and high-performance networks is a two-edged sword:
z Many powerful new services and applications are enabled z But computer systems and networks become highly
susceptible to a wide variety of security threats
z Network security involves countermeasures to
protect computer systems from intruders
z Firewalls, security protocols, security practices z We will focus on security protocols
Threats, Security Requirements,
and Countermeasures
z Network Security Threats
z Eavesdropping, man-in-the-middle, client and server
imposters
z Denial of Service attacks
z Viruses, worms, and other malicious code
z Network Security Requirements
z Privacy, Integrity, Authentication, Non-Repudiation,
Availability
z Countermeasures
z Communication channel security z Border security
Security Requirements
Security threats motivate the following
requirements:
z
Privacy: information should be readable only by
intended recipient
z
Integrity: recipient can confirm that a message
has not been altered during transmission
z
Authentication: it is possible to verify that sender
or receiver is who he claims to be
z
Non-repudiation: sender cannot deny having
sent a given message.
Client Server Request Response re pla y
Eavesdropping
z Information transmitted over network can be observed
and recorded by eavesdroppers (using a packet sniffer)
z Information can be replayed in attempts to access
server
z Requirements: privacy, authentication,
Client Imposter
Server
Client Imposter
z
Imposters attempt to gain unauthorized
access to server
z Ex. bank account or database of personal records z For example, in IP spoofing imposter sends
packets with false source IP address
Attacker Server
Denial of Service Attack
z Attacker can flood a server with requests,
overloading the server resources
z Results in denial of service to legitimate clients
z Distributed denial of service attack on a server
involves coordinated attack from multiple (usually hijacked) computers
Client Server Imposter
Server Imposter
z
An imposter impersonates a legitimate server
to gain sensitive information from a client
z E.g. bank account number and associated user password
z
Requirements: privacy, authentication,
Client Man in the Server middle
Man-in-the-Middle Attack
z An imposter manages to place itself as man in the
middle
z convincing the server that it is legitimate client
z convincing legitimate client that it is legitimate server z gathering sensitive information and possibly hijacking
session
Client Server Imposter
Malicious Code
z A client becomes infected with malicious code
z Opening attachments in email messages
z Executing code from bulletin boards or other sources
z Virus: code that, when executed, inserts itself in
other programs
z Worms: code that installs copies of itself in other
machines attached to a network
z Many variations of malicious code
Countermeasures
Secure communication channels
z Encryption
z Cryptographic
checksums and hashes
z Authentication
Countermeasures
Secure borders z Firewalls z Virus checking z Intrusion detection z Authentication z Access ControlChapter 11
Security Protocols
Cryptography
z Encryption: transformation
of plaintext message into encrypted (and unreadable) message called ciphertext
z Decryption: recovery of
plaintext from ciphertext
z Cipher: algorithm for
encryption & decryption
z A secret key is required to
perform encryption & decryption
Substitution Ciphers
Substitution Cipher: Map each letter or
numeral into another letter of numeral:
a b c d e f g h i j k l m n o p q r s t u v w x y z
z y x w v u t s r q p o n m l k j i h g f e d c b a
z
Example:
z hvxfirgbÆ security
z
Substitution ciphers are easy to break
z Take histogram of frequency of occurrence of letters in a ciphertext message
Transposition Cipher
Transposition Cipher: Rearrange order of
letters/numerals in a message using a
particular rearrangement:
z
interchange character k with character k+1
z
Example:
z securityÆ esuciryt
z
Transposition Ciphers are easy to break
z Suppose plaintext and ciphertext are known
z Matching of letters in plaintext and ciphertext will reveal transposition mapping
EK(.)
Key K Key K
Plaintext P Ciphertext C=EK(P) P
Encryption Decryption
DK(.)
Secret Key Cryptography
z Sender encrypts P by applying mapping EK which
depends on secret key K: C = EK(P)
z Receiver decrypts C by applying inverse mapping
What makes a good cipher?
z Algorithm should be easy to implement and deploy
on large scale
z Algorithm should be difficult to break: 1. Number of keys should be very large
z Attacker cannot try all possible keys
2. The secret key should be very hard to derive from
intercepted messages
z Even if a large number of plaintext & corresponding
cyphertexts are known to the attacker
z Examples of secret key methods discussed later:
z Data Encryption Standard (DES) and Triple DES z Advanced Encryption Standard (AES)
Security using Secret Key
Cryptography
z
Privacy: secret key renders messages
confidential
z
Integrity: alteration of the cyphertext will be
detected, because the decrypted message
will be gibberish
z When privacy is not required, encryption of the entire message is overkill because much
processing involved
z We will see that cryptographic checksums provide integrity and require less processing
Sender (John) Receiver (Jane) Ek(r) r Ek(r´) r´
John to Jane, “let’s talk”
Authentication using Secret Key
Cryptography
z Send message identifying self z Send response with
encrypted r
z Can now authenticate receiver
by issuing a challenge
z Reply with challenge that
contains random number r,
nonce = number once
z Apply secret key to decrypt
message. If decrypted number is r then the transmitter is
Cryptographic Checksums and
Hashes
z
Transmitter calculates a fixed number of bits
(crypto checksum/hash) that depends on
secret key
K: H
K(P)
z
Receiver recalculates hash from received
message & compares to received hash
Message Crypto Checksum Calculator CrytoChk Message K P P HK(P)
What makes a Good Hash?
z To be secure, it must be very difficult to find a
message that generates a given hash
z If not difficult, an attacker could produce a message and
corresponding hash that would be accepted as valid
z Suppose message is M bits long and hash is m bits
long, and m<<M
z For each given hash value there are 2M/m messages that
give that hash
z How long does it take to find a match?
z Probability that a random message generates given hash
is 2-msince there are 2m hashes
Example
z M = 1000, m = 128
z Number of possible messages: 21000 z Number of possible hashes: 2128
z For each hash value there are 21000/2128 = 2872
messages that generate the hash
z A randomly selected message produces a desired
hash value with probability 2-128
z If each attempt requires 1 microsecond, time to find
matching message to a hash is:
Some Hashing Algorithms
z Message Digest 5 (MD5)
z Pad message to be multiple of 512 bits z Initialize 128 buffer to given value
z Modify buffer content according to next 512 bits z Repeat until all blocks done
z Buffer holds 128 bit hash
z Keyed MD5
z Pad message to be multiple of 512 bits
z Attach and append secret key to padded message prior to
performing hash function
z Could also append/attach other information such as sender ID
z Secure Hash Algorithm 1 (SHA-1)
z Produce a 160-bit hash; more secure than MD5 z Keyed version available
Hashed Message Authentication
Code Method
z
HMAC improves strength of a hash code
z Pad secret key with zeros to length of 512 bits and X-OR with 64 repetitions of 00110110
z Pad message to multiple of 512 bits
z Calculate hash of padded key followed by padded message, 128 bits for MD5, 160 bits for SHA-1
z Pad hash to 512 bits
z Pad secret key with zeros to 512 bits and X-OR with 64 repetitions of 01011010
z Calculate hash of padded key and padded hash z Result is final hash
EK1(.)
Public key K1 Private key K2 Plaintext P Ciphertext C = EK1(P) P
Encryption Decryption
DK2(.)
Public Key Cryptography
z
Public key cryptography provides privacy
using two different keys:
z Public key K1 available to all for encrypting messages to a certain user: C = EK1(P)
z Private key K2 for user to decrypt messages: P = DK2(EK1(P))
What makes a good public key
algorithm?
z EK1 and DK2 should be readily implementable z Inverse relationship should hold:
z P = DK2(EK1(P)) and sometimes P = EK1(DK2(P))
z K1 is a relatively small number of bits and K2 is
usually a large number of bits
z It is extremely difficult to decrypt EK1(P) without K2 z It should not be possible to deduce K2 from K1
z Example: RSA public key cryptography (discussed
Integrity using Public Key
Cryptography
z
Integrity:
z Any one can send messages using public key, so integrity not assured directly
z For integrity, transmitter:
1. encodes P with its private key K2΄ to obtain P΄ = DK2΄ P) 2. encodes P΄ using receiver’s public key: C = EK1(P΄)
z Receiver:
1. decrypts C, DK2(EK1(P΄)) = P΄
2. decrypts P΄ using transmitters public key, EK1΄(DK2΄(P))
= P
Sender Receiver
EK1(r) r
John to Jane, “let’s talk”
Authentication using Public Key
Cryptography
z Transmitter identifies itself
z Receiver sends a nonce encoded using the sender’s
public key in a challenge message
z Transmitter uses its private key to recover the
nonce, and it returns the unencrypted nonce
Digital Signatures using Public
Key Cryptography
z Digital signatures provide nonrepudiation
z User “signs” a message that cannot be repudiated
z Digital signature obtained as follows:
z Transmitter obtains a hash of the message
z Transmitter encrypts the hash using its private key; result
is the digital signature
z Transmitter sends message and signature
z To check the signature:
z Receiver obtains hash of message
z Receiver decrypts signature using sender’s public key z Receiver compares hash computed from message and
hash obtained from signature
Secret Key vs. Public Key
z Public key systems have more capabilities
z Secret key: privacy, integrity, authentication z Public key: all of above + digital signature
z Public key algorithms are more complex
z Require more processing and hence much slower than
secret key
z Practice:
z Use public key method during session setup to establish a
session key
z Use secret key cryptography during session using the
Example: Pretty Good Privacy
(PGP)
z PGP developed by Phillip Zimmerman to provide
secure email
z http://www.philzimmermann.com/index.shtml z http://www.pgpi.org
z Notorious for becoming publicly available for
download over Internet in violation of US export restrictions
z Uses public key cryptography to provide
z Privacy, integrity, authentication, digital signature
z De facto standard for email security
Key Distribution in Secret Key
Systems
z
Every pair of users requires a separate
shared secret key
z N(N – 1) keys for N users; Grows quickly with N z Similar to full-mesh connections for N users
z
Solution: Introduce Key Distribution Centers
z Each users has shared key with the KDC z User A has shared key KKA with KDC
z User B has shared key KKB with KDC
z KDC provides shared key when A & B need to communicate
KDC
A B
C D
Key Distribution Center
z User A contacts the KDC to
request a key for use with user B. z KDC:
z Authenticates user A
z Selects a key KAB and encrypts it to produce EKA(KAB) and EKB(KAB).
z KDC sends both versions of the encrypted key to A.
z User A contacts user B and provides a ticket in the form of EKB(KAB)
z Users A & B both have KAB
request
EKA(KAB), EKB(KAB) challenge
response
Example: Kerberos
z Kerberos: authentication service for users
to access servers over network
z KDC has secret key with every user
z At login, user supplies ID and password
z KDC authenticates user & generates
session key
z Session key & ticket-granting ticket (TGT)
is sent to user encrypted using shared secret key
z To access a particular server, user sends
request to KDC with server name and TGT
z KDC decrypts TGT to recover session key
& then returns ticket to client for desired server
Key Distribution in Public Key
Systems
z In public key only one pair of keys per user
z Key distribution problem: How to determine whether
an advertised public key is not from an imposter?
z Certification Authority (CA)
z Issues digitally signed certificate that provides
z User’s name & public key
z Certificate serial #, expiration date
z Certificates can be stored in publicly accessible directories z To communicate with B, a user contacts the CA to obtain
the certificate for B
z Users are configured to have the CA’s public key, which
Transmitter A Receiver B T = gx R = gy K = Rxmod p = gxymod p K = Tymod p = gxymod p
Key Generation: Diffie-Hellman
Exchange
z Generate keys instead of distributing keys
z Diffie-Hellman exchange to create a shared key z A & B pick p a large prime #, and generator g < p
z A picks x and sends T = gx to B; B picks y and sends R = gy z Secret key is K = (gx)y= (gy)x which are calculated by A & B
z Eavesdropper that obtains p, g, T, R cannot obtain x and y
Transmitter A Man in the middle C Receiver B T R' T' R K1 = R´x = gxy´ K1 = Ty´ = gxy´ K2 = R x´ K2 = T´ y = gx´y = gx´y
Man-in-the-Middle Attack
z An intruder C can interpose itself between A & B
z C establishes a shared key K1 with A and a shared key K2
with B
z C can then intercept, decipher, and re-encrypt all
communications
z Need mutual authentication between A & B
z Alternative: Community agrees on g & p; users publish their
Diffie-Hellman Complexity
z
Diffie-Hellman exchange involves
computation of powers of large numbers
z Large number of multiplications implies heavy computational burden
Chapter 11
Security Protocols
Direct Connections to Internet
z Computers A & B communicate across the Internet z Exposure to eavesdropping, imposters, DoS
z Can encrypt some transmitted information
z But IP headers need to be visible to routers & hence others
z Eavesdropper can gather variety of usage information & deduce
nature of interaction
z Choice of which layer to apply security: IP, transport, or
application layer
Internet
Gateway-to-Gateway
z Computers A and B have gateways interposed between their
internal network and Internet
z Gateway can be a firewall
z Controls external access to internal network
z Packet filtering according to various header fields
z IP addresses, port numbers, ICMP types, fields within payload
z Secure tunnels can be established between gateways
z All internal information including headers can be encrypted
Internet
Remote user to Gateway
z Mobile host needs access to internal network z Gateway must provide user with access while
barring intruders from accessing internal network
z May also need to protect identity of mobile user z IP-address of mobile user changes
Firewall Options
z Firewalls can operate at different layers
z IP-layer filtering cannot operate on payload contents
z Circuit-Level Gateways
z Direct client-to-server TCP connections not allowed z Relays TCP segments between actual client & actual
server
z Application-Level Gateways or Proxies
z Interposed between actual client and actual server
z Performs authentication and determines what features are
available to client
Protocol Layer Options
z Security Services can be provided at different layers of
the protocol stack
z Data Link Layer security
z Point-to-point security between directly-connected devices,
e.g. wireless LAN security
z IP-Layer security
z Security service between IP-layer & Transport layer z End-to-end security across an internet, e.g. IPsec
z Transport Layer security
z Security service between Transport & Application Layers z E.g. Secure Sockets Layer & Transport Layer Security
Network Security Services
z
Integrity Service: information received from
network has not been altered during
transmission
z
Authentication Service: the receiver can
authenticate that information came from
purported sender
z
Privacy Service: information is readable only by
intended recipient
z
In applications that require network security,
integrity & authentication essential; privacy not
always justified
IP Security (IPsec)
.
z IPsec defined in RFCs 2401, 2402, 2406
z Provides authentication, integrity, confidentiality, and
access control at the IP layer
z Provides a key management protocol to provide
automatic key distribution techniques.
z Security service can be provided between a pair of
communication nodes, where the node can be a host or a gateway (router or firewall).
z Two protocols & two modes to provide traffic security:
z Authentication Header and Encapsulating Security Payload z Transport mode or tunnel mode
Security Association
z A Security Association (SA) is a logical simplex
connection between two network-layer entities
z Two SA’s required for bidirectional secure
communication
z SA is specified by
z A unique identifier
z Security services to be used
z Cryptographic algorithms to be used z How shared keys will be established z Other attributes such as lifetime
Integrity & Authentication Service
z Integrity can be ascertained by sending a
cryptographic checksum or hash of message
z Authentication also provided if hash covers:
z Shared secret key, sender’s identity & message
z Fields that are changed while packet traverses Internet are
set to zero in calculation of hash
z To protect against replay attacks, message should
carry a sequence number that is covered by the hash
z Receiver accepts a packet only once
z Receiver maintains a window of packets it accepts
z Receiver recalculates hash and compares to hash in
Authentication Header
z
Inserted between regular header & payload
z
Packet header contains field indicating
presence of authentication header
z
Authentication header includes:
z Security association ID z Sequence number z Cryptographic hash Packet header Authentication header Packet payload
Tunneling
z A tunnel can be created by encapsulating a packet
within another packet
z Inner packet header carries original source address z Entire contents of inner packet covered by hash
z Outer packet header carries gateway’s address
New header Authentication header Packet payload
Authenticated except for changeable fields in new header
Original header In tunnel mode Internet Tunnel
Privacy Service
z Privacy requires encryption of message
z Encryption header identifies security association &
sequence number
z Encryption can cover payload + padding:
Packet + pad payload Packet header Encryption header Encrypted Encrypted Packet + pad payload New header Authentication header Encryption header
z Authentication header can be used to detect alteration of
In tunnel mode New header Encryption header Original header Encrypted Packet payload
Privacy Service in Tunnel Mode
z In tunnel mode, entire original packet is encrypted
and unreadable to eavesdroppers
z All original packet header fields are unreadable z Only gateway packet header is visible
z It is also possible to use tunnel mode between
trusted routers while traversing untrusted segments of the Internet
Setting up a Security Association
z To setup security association, computers must:
z Agree on security services that will be provided z Agree on cryptographic algorithms
z Authenticate each other
z Establish a shared secret key
z Last two steps are difficult; possible approaches:
z Manual set up of shared key between pair of users z Use Key Distribution Center
z Contact a Certificate Authority
z Internet Key Exchange (RFC 2409) for IPsec
z Assumes parties have a name/identity for other party as
well as a pre-established shared secret (secret key or private key)
Internet Key Exchange (IKE)
Initiator Host Responder Host
HDR, SA Cookie Request HDR, SA Cookie Response Contains Ci Proposes Security Association options Contains Ci & Cr Selects SA options Select random # Ci: initiator’s cookie
Check to see if Ci already in use; If not, generate Cr, responder’s cookie;
Associate Cr with initiator’s address
Check Ci & address against list; Associate (Ci, Cr) with SA;
record SA as “unauthenticated”
Internet Key Exchange
Initiator Host Responder Host
HDR, KE, Ni Key Request HDR, KE, Nr Key Response T=gx mod p Nonce Ni Initiate Diffie-Hellman exchange
Check responder cookie, discard if not valid; If valid identify SA with (Ci, Cr) & record as “unauthenticated” R=gy mod p Nonce Nr
Calculate K=(gy)x mod p Calculate K=(gx)y mod p
Calculate secret string of bits SKEYID known only to initiator & responder
Calculate secret string of bits SKEYID known only to initiator & responder
Internet Key Exchange
Initiator Host Responder Host
HDR, {IDi, Sigi} Signature Request HDR, {IDr, Sigr} Signature Request Prepare signature based on SKEYID, T, R, Ci, Cr, the SA field, initiator ID SKEYID, T, R, Ci,
Cr, SA, IDi Hash of info in HDR
encrypted
Authenticates initiator
comparing decrypted hash to recalculated hash.
If agree, SA declared authenticated.
Prepares signature based on SKEYID, T, R, Ci, Cr, the SA field, responder IDr SKEYID, T, R, Ci, Cr, SA, IDr Hash of info in HDR Authenticate initiator. If successful, SA declared authenticated.
SKEYID & Cookies
z SKEYID for authentication, based on:
z Shared key that results from Diffie-Hellman z Pre-shared key
z Pre-configured secret key
z Private part of a public key pair
z Nonces and/or cookies
z Cookies
z To counteract denial-of-service attacks
z A user that wants to make a connection requests must first
request a cookie
z Connections requests are only accepted from users that
have a valid cookie, and hence that must receive packets at the IP address from which they sent the request
IPv4 Header AH Upper Layer (e.g., TCP or UDP)
IPsec Authentication Header
z Authentication header (AH) placed after headers
that are examined at every hop
z Presence of AH indicated by protocol value = 51 in
IPv4 header
z Authentication performed over all fields including IP
Next Header Length Reserved Security Parameters Index
0 8 16 31
Sequence Number Authentication Data
Authentication Header Format
z Format used in IPv4 and IPv6
z Next Header indicates next payload after AH
z Length of Authentication data in multiples of 32 bits z SPI = unique ID for security association
z Sequence number for anti-replay protection
z Authentication data contains result of authentication
Encapsulating Security Payload
z
ESP provides:
z Integrity & authentication service
z Privacy service by encryption of payload
z
Authentication data at end is optional
z Placement at ends makes implementation simpler
Security Parameters Index
0 16 24 31 Sequence Number
Payload Data Padding
Pad Length Next Header Authentication Data
ESP Header Format
z Authenticated coverage from SPI until next header field
z Encrypted coverage from payload data field until next header z Protocol type = 50
Secure Sockets Layer (SSL)
z
SSL developed by Netscape Communications
z Operates on top of TCP
z Provides secure connections
z HTTP, FTP, telnet, …
z Electronic ordering & payment; e-mail
z SSL 3.0 submitted to IETF for standardization
z
TLS standardized by IETF (RFC 2246)
Transport Layer Security (TLS)
z TLS protocols operate at two layers
z TLS Record Protocol operates on top of TCP z Protocols on top of TLS Record Protocol
z TLS Handshake Protocol
z TLS Change Cipher Specification Protocol z TLS Alert Protocol TCP TLS Record Protocol Handshake Protocol Change cipher spec Protocol Alert Protocol HTTP Protocol IP
TLS Record Protocol
z
TLS Record protocol provides
z Privacy service through secret key encryption
z Encryption algorithm is negotiated at session setup z Secret keys generated per connection using another
protocol such as Handshake protocol
z Reliability service through keyed message authentication code
z Hash algorithm negotiated at session setup
TLS Handshake Protocol
z TLS Handshake protocol used by client & server
z Negotiate protocol version, encryption algorithm, key
generation method
z Can authenticate each other using public key algorithm z Client & server establish a shared secret
z Multiple secure connections can be set up after session
setup
z Session specified by following parameters
z Session Identifier: byte sequence selected by server z Peer Certificate: certificate of peer
z Compression method: used prior to encryption
z Cipher spec: encryption & message authentication code z Master Secret: 48-byte secret shared by client & server z Is resumable?: flag indicating if new connections can be
Client Server ClientHello
TLS Handshake Process
ServerHello Certificate* ServerKeyExchange* ServerHelloDone Request connection Includes:Version #; Time & date; Session ID (if resuming); Ciphersuite (combinations
of key exchange, encryption, MAC, compression)
Send ServerHello if there is acceptable Ciphersuite
combination; else, send failure alert & close
connection.
* Optional messages
Server Certificate
Server part of handshake done Server part of key exchange:
Diffie-Hellman, gx;; RSA, public key
ServerHello includes:
Version #; Random number; Session ID ; Ciphersuite & compression selections
Compute shared key May contain public key New CipherSpec pending
TLS Record protocol initially specifies no compression or encryption
Client Server
ClientKeyExchange
[ChangeCipherSpec]
Finished
Client’s part of key agreement:
Diffie-Hellman gy; RSA, random #s
Change Cipher protocol
message notifies server that subsequent records protected under new CipherSpec & keys
Server changes CipherSpec
Hash using new CipherSpec; allows server to verify change in Cipherspec
Handshake Protocol continued
Compute shared key
Client Server
Application Data
Handshake Protocol completion
[ChangeCipherSpec] Finished
Notify client that subsequent records protected under new CipherSpec & keys
Client changes CipherSpec
Hash using new CipherSpec; Client verifies new
CipherSpec
TLS Record protocol encapsulates application-layer messages
• Privacy through secret key cryptography • Reliability through MAC
• Fragmentation of application messages into blocks for compression/encryption • Decompression/Decryption/Verification/Reassembly
Client Server
ClientHello
TLS Handshake with Client
Authentication
ServerHello Certificate* ServerKeyExchange* CertificateRequest ServerHelloDone Certificate* ClientKeyExchange CertificateVerify* [ChangeCipherSpec] Finished Application Data [ChangeCipherSpec] FinishedServer requests certificate if client needs to be
authenticated Client sends suitable
certificate
If server finds certificate unacceptable; server can send fatal failure alert
message & close connection Client prepares digital
signature based on
messages sent using its private key
Server verifies client has private key
TLS 1.0 & SSL 3.0 Compatibility
z TLS 1.0 allows backwards compatibility with SSL 3.0 z When TLS client sends ClientHello to SSL server:
z Client sends TLS message with {3, 1} in version field to
indicate it also supports SSL 3.0
z Server that supports SSL 3.0 will respond with SSL 3.0
ServerHello message
z A TLS server that handles SSL 3.0 clients
z Accepts SSL 3.0 ClientHello messages & responds with
SSL 3.0 Server Hello message if client message contains {3,0} in version field indicating that it only supports SSL 3.0
Chapter 11
Security Protocols
Data Encryption Standard
z DES adopted by U.S. National Bureau of Standards
in 1977
z Most widely-used secret key system z Efficient hardware implementation
z Encryption: Electronic Codebook (ECB) Mode
z Message broken into 64-bit blocks
z Each 64-bit plaintext block encrypted separtely into 64-bit
cyphertext
z Original version of DES uses a 56-bit key
z Decryption: Encryption operations performed in
Initial permutation Iteration 1 Iteration 2 Iteration 16 32-bit swap Inverse permutation 64-bit plaintext 64-bit ciphertext 48-bit Key 1
Generate 16 per-iteration keys 56-bit key 48-bit Key 2 48-bit Key 16
DES Algorithm
z Initial permutation is independent of key z Final permutation is inverse of initial permutationz Penultimate step swaps
32-bits on left with 32-bits on the right
z Intermediate 16 iterations
apply a different key that is derived from the original 56-bit key
DES Iteration
z 64-bit block divided into Li –1
and Ri –1 halves z Left output Li = Ri –1 z Right output Ri = Li –1 XOR f(Ri –1, Ki) z bitwise XOR z f(.,.) as follows: z Ri –1 expanded to 48 bits
using fixed re-ordering & duplication pattern
z XORed with Ki
z Each resulting group of
6-bits is mapped into 4-bit output according to
substitution mapping
Li-1 Ri-1
L1 Ri
Cipher Block Chaining
z
ECB mode encrypts 64-bit blocks
independently
z Attacker can use knowledge about pattern in
message to attack encrypted sequence of blocks
z
Cipher Block Chaining (CBC) introduces
dependency between consecutive blocks
z Current plaintext block is XORed with preceding ciphertext block
z First plaintext block XORed with an initialization vector that is transmitted to receiver in ciphertext
Decrypt P1 C1 IV Decrypt P2 C2 Decrypt P3 C3 (b) Decryption … Encrypt P1 C1 Encrypt P2 C2 Encrypt P3 C3 IV (a) Encryption …
DES Security
z DES vulnerable to brute-force attack
z 56-bit key is too short
z Has been broken in less than one day using a
specially-designed computer
z Triple-DES (3DES)
z Provides better security z Uses two 56-bit keys
C = EK1(DK2(EK1(P)))
P = DK1(EK2(DK1(P)))
z Instead of “triple encryption”, use
encryption-decryption-encryption
Advanced Encryption Standard
z AES selected in 2001 by U.S. NIST (National
Institute of Standards & Technology)
z Developed by Rijmen and Daemen z Secret key system
z Encryption of 128-bit blocks with keys of size 128, 192, or
256 bits
z Software & efficient hardware implementation z 3.4x1038 keys vs. 7.2x1016 keys for 56-bit DES
z AES is gradually replacing DES/3DES
z See:
RSA Public Key Algorithm
z
Named after Rivest, Shamir, and Adleman
z
Modular arithmetic & factorization of large
numbers
z Let n = pq, where p & q are two large numbers
z n typically several hundred bits long, i.e. 512 bits z Plaintext must be shorter than n
z Find e relatively prime to (p – 1)(q – 1)
z i.e. e has no common factors with (p – 1)(q – 1)
z Public key is {e,n}
z Let d be multiplicative inverse of e
z de = 1 modulo (p – 1)(q – 1)
Encryption & Decryption
z Fact: For P<n and n, p, q, d as above:
Pde mod n = P mod n
z Encryption:
C = Pe mod n
z Result is number less than n and is represented by same
number of bits as key
z Decryption:
Cd mod n = Ped mod n = P mod n = P
z Security stems from fact that it is very difficult to factor
RSA Example
z
Let
p = 5, q = 11
z n = pq = 55 and (p – 1)(q – 1) = 40
z
Let
e = 7, which is relatively prime to 40
z 7d mod 40 = 1, gives d = 23
z
Public key is {7, 55}
RSA Example continued
z Encrypt “RSA”: R=18, S=19, A=1
C1 = 187 mod 55 = 184+2+1 mod 55
= (18 mod 55) (182 mod 55) (184 mod 55) mod 55 = (18) (324 mod 55) (184 mod 55) mod 55
= (18) (49) (492 mod 55) mod 55 = (18)(49)(36) mod 55 = 31752 mod 55 = 17 C2 = 197 mod 55 = 24 C3 = 17 mod 55 = 1 z Decrypt 1723 mod 55 = 1716+4+2+1 mod 55 =18 2423 mod 55 = 19 123 mod 55 = 1