• message authentication is concerned with:
  • protecting the integrity of a message
  • validating identity of originator
  • non-repudiation of origin (dispute resolution)
• electronic equivalent of a signature on a message
• an authenticator, signature, or message authentication code (MAC) is sent along with the message
• the MAC is generated via some algorithm which depends on both the message and some (public or private) key known only to the sender and receiver
• the message may be of any length
• the MAC may be of any length, but more often is some fixed size, requiring the use of some hash function to condense the message to the required size if this is not acheived by the authentication scheme
• need to consider replay problems with message and MAC
  • require a message sequence number, timestamp or negotiated random values

Authentication using Private-key Ciphers

• if a message is being encrypted using a session key known only to the sender and receiver, then the message may also be authenticated
  • since only sender or receiver could have created it
  • any interference will corrupt the message (provided it includes sufficient redundancy to detect change)
  • but this does not provide non-repudiation since it is impossible to prove who created the message
• message authentication may also be done using the standard modes of use of a block cipher
  • sometimes do not want to send encrypted messages
  • can use either CBC or CFB modes and send final block, since this will depend on all previous bits of the message
  • no hash function is required, since this method accepts arbitrary length input and produces a fixed output
  • usually use a fixed known IV
  • this is the approached used in Australian EFT standards AS8205
  • major disadvantage is small size of resulting MAC since 64-bits is probably too small

• hashing functions are used to condense an arbitrary length message to a fixed size, usually for subsequent signature by a digital signature algorithm
• good cryptographic hash function h should have the following properties:
  • h should destroy all homomorphic structures in the underlying public key cryptosystem (be unable to compute hash value of 2 messages combined given their individual hash values)
  • h should be computed on the entire message
  • h should be a one-way function so that messages are not disclosed by their signatures
  • it should be computationally infeasible given a message and its hash value to compute another message with the same hash value
  • should resist birthday attacks (finding any 2 messages with the same hash value, perhaps by iterating through minor permutations of 2 messages [1])
• it is usually assumed that the hash function is public and not keyed
• traditional CRCs do not satisfy the above requirements
• length should be large enough to resist birthday attacks (64-bits is now regarded as too small, 128-512 proposed)

Snefru

• a one-way hash function designed by Ralph Merkle
• creates 128 or 256 bit long hash values (let m be length)
• uses an algorithm H which hashes 512-bits to m-bits, taking the first m output bits of H as the hash value
  • H is based on a reversible block cipher E operating on 512-bit blocks
  • H is the last m-bits of the output of E XOR'd with the first m-bits of the input of E
  • E is composed of several passes, each pass has 64 rounds of an S-box lookup and XOR
  • E can use 2 to 8 passes
• overview of algorithm
  • break message into 512-m bit chunks
  • each chunk has the previous hash value appended (assuming an IV of 0)
  • H is computed on this value, giving a new hash value
  • after the last block (0 padded to size as needed) the hash value is appended to a message length value and H computed on this, the resulting value being the MAC
• Snefru has been broken by a birthday attack by Biham and Shamir for 128-bit hashes, and possibly for 256-bit when 2 to 4 passes are used in E
• Merkle recommends 8 passes, but this is slow

MD2, MD4 and MD5

• family of one-way hash functions by Ronald Rivest
• MD2 is the oldest, produces a 128-bit hash value, and is regarded as slower and less secure than MD4 and MD5
• MD4 produces a 128-bit hash of the message, using bit operations on 32-bit operands for fast implementation

• MD4 overview
  • pad message so its length is 448 mod 512
  • append a 64-bit message length value to message
  • initialise the 4-word (128-bit) buffer (A,B,C,D)
  • process the message in 16-word (512-bit) chunks, using 3 rounds of 16 bit operations each on the chunk & buffer
  • output hash value is the final buffer value
• some progress at cryptanalysing MD4 has been made, with a small number of collisions having been found
• MD5 was designed as a strengthened version, using four rounds, a little more complex than in MD4 [2]
• a little progress at cryptanalysing MD5 has been made with a small number of collisions having been found
• both MD4 and MD5 are still in use and considered secure in most practical applications
• both are specified as Internet standards (MD4 in RFC1320, MD5 in RFC1321)

SHA (Secure Hash Algorithm)

• SHA was designed by NIST & NSA and is the US federal standard for use with the DSA signature scheme (nb the algorithm is SHA, the standard is SHS)
• it produces 160-bit hash values
• SHA overview[3]
  • pad message so its length is a multiple of 512 bits
  • initialise the 5-word (160-bit) buffer (A,B,C,D,E) to
  • (67452301,efcdab89,98badcfe,10325476,c3d2e1f0)
  • process the message in 16-word (512-bit) chunks, using 4 rounds of 20 bit operations each on the chunk & buffer
  • output hash value is the final buffer value
• SHA is a close relative of MD5, sharing much common design, but each having differences
• SHA has very recently been subject to modification following NIST identification of some concerns, the exact nature of which is not public
• current version is regarded as secure

Other Hash Functions

HAVAL

• a variable length one-way hash function designed by Uni of Wollongong and recently published at Auscrypt'92
• it processes messages in 1024-bit blocks, using an 8-word buffer and 3 to 5 rounds of 16 steps each, creating hash values of 128, 160, 192, 224, or 256 bits in length
• uses highly non-linear 7-variable functions in each step
• is faster than MD5
• it not subject to MD5 type analysis, no attack is known

Using Private Key Ciphers

• a large number of "Modes of Use" have been proposed which use a block cipher to create a hash value
• original proposal was by Davies and Meyer
• many other proposals
• most have been broken using a birthday attack
• the design of fast, secure hash functions of this form is still being studied, with many questions unresolved

• public key signature schemes
• the private-key signs (creates) signatures, and the public-key verifies signatures
• only the owner (of the private-key) can create the digital signature, hence it can be used to verify who created a message
• anyone knowing the public key can verify the signature (provided they are confident of the identity of the owner of the public key - the key distribution problem)
• usually don't sign the whole message (doubling the size of information exchanged), but just a hash of the message
• digital signatures can provide non-repudiation of message origin, since an asymmetric algorithm is used in their creation, provided suitable timestamps and redundancies are incorporated in the signature

RSA

• RSA encryption and decryption are commutative, hence it may be used directly as a digital signature scheme
  • given an RSA scheme {(e,R), (d,p,q)}
• to sign a message, compute:
  • S = M^d(mod R)
• to verify a signature, compute:
  • M = S^e(mod R) = M^e.d(mod R) = M(mod R)
• thus know the message was signed by the owner of the public-key
• would seem obvious that a message may be encrypted, then signed using RSA without increasing it size
  • but have blocking problem, since it is encrypted using the receivers modulus, but signed using the senders modulus (which may be smaller)
  • several approaches possible to overcome this
• more commonly use a hash function to create a separate MDC which is then signed

El Gamal Signature Scheme

• whilst the ElGamal encryption algorithm is not commutative, a closely related signature scheme exists
• El Gamal Signature scheme
• given prime p, public random number g, private (key) random number x, compute
  • y = g^x(mod p)
• public key is (y,g,p)
  • nb (g,p) may be shared by many users
  • p must be large enough so discrete log is hard
• private key is (x)
• to sign a message M
  • choose a random number k, GCD(k,p-1)=1
  • compute
  • a = g^k(mod p)
  • use extended Euclidean (inverse) algorithm to solve
  • M = x.a + k.b (mod p-1)
  • the signature is (a,b), k must be kept secret
  • (like ElGamal encryption is double the message size)
• to verify a signature (a,b) confirm:
  • y^a.a^b(mod p) = g^M(mod p)

• given p=11, g=2
• choose private key x=8
• compute
  • y = g^x(mod p) = 2⁸(mod 11) = 3
• public key is y=3,g=2,p=11)
• to sign a message M=5
  • choose random k=9
  • confirm gcd(10,9)=1
  • compute
    • a = g^k(mod p) = 2⁹(mod 11) = 6
  • solve
    • M = x.a+k.b(mod p-1)
    • 5 = 8.6+9.b(mod 10)
    • giving b = 3
  • signature is (a=6,b=3)
• to verify the signature, confirm the following are correct:
  • y^a.a^b(mod p) = g^M(mod p)
  • 3^6.6³(mod 11) = 2⁵(mod 11)

DSA (Digital Signature Algorithm)

• DSA was designed by NIST & NSA and is the US federal standard signature scheme (used with SHA hash alg)
  • DSA is the algorithm, DSS is the standard
  • there was considerable reaction to its announcement!
    • debate over whether RSA should have been used
    • debate over the provision of a signature only alg
• DSA is a variant on the ElGamal and Schnorr algorithms
• description of DSA
  • p = 2^L a prime number, where L= 512 to 1024 bits and is a multiple of 64
  • q a 160 bit prime factor of p-1
  • g = h^(p-1)/q where h is any number less than p-1 with h^(p-1)/q(mod p)> 1
  • x a number less than q
  • y = g^x(mod p)
• to sign a message M
  • generate random k, k<q
  • compute
    • r = (g^k(mod p))(mod q)
    • s = k^-1.SHA(M)+ x.r (mod q)
  • the signature is (r,s)
• to verify a signature:
  • w = s^-1(mod q)
  • u1= (SHA(M).w)(mod q)
  • u2= r.w(mod q)
  • v = (gu1.yu2(mod p))(mod q)
  • if v=r then the signature is verified
• comments on DSA
  • was originally a suggestion to use a common modulus, this would make a tempting target, discouraged
  • it is possible to do both ElGamal and RSA encryption using DSA routines, this was probably not intended :-)
  • DSA is patented with royalty free use, but this patent has been contested, situation unclear
  • Gus Simmons has found a subliminal channel in DSA, could be used to leak the private key from a library - make sure you trust your library implementer

Other Signature Schemes

Fiat-Shamir

• originally a signature scheme, subsequently modified into a zero knowledge proof of identity scheme
• based on difficulty of finding square roots mod pq
• this algorithm is patented

Schnorr

• combines ideas from ElGamal and Fiat-Shamir schemes
• uses exponentiation mod p and mod q
• much of the computation can be completed in a pre-computation phase before signing
• for the same level of security, signatures are significantly smaller than with RSA
• this algorithm is patented

• all cryptographic systems have the problem of how to securely and reliably distribute the keys used
• in many cases, failures in a secure system are due not to breaking the algorithm, but to breaking the key distribution scheme
• ideally the distribution protocol should be formally verified, recent advances make this more achievable
• possible key distribution techniques include:
  • physical delivery by secure courier
    • eg code-books used submarines
    • one-time pads used by diplomatic missions
    • registration name and password for computers
  • authentication key server (private key, eg Kerberos)
    • have an on-line server trusted by all clients
    • server has a unique secret key shared with each client
    • server negotiates keys on behalf of clients
  • public notary (public key, eg SPX)
    • have an off-line server trusted by all clients
    • server has a well known public key
    • server signs public key certificates for each client

• if using a key server, must use some protocol between user and server [4]
• this protocol should be validated, formal techniques exist to acheive this (Ban logic provers [5])

Challenge-Response

• basic technique used to ensure a password is never sent in the clear
• given a client and a server share a key
  • server sends a random challenge vector
  • client encrypts it with private key and returns this
  • server verifies response with copy of private key
• can repeat protocol in other direction to authenticate server to client (2-way authentication)
• in simplest form, keys are physically distributed before secure comminications is required
• in more complex forms, keys are stored in a central trusted key server

Needham-Schroeder

• original third-party key distribution protocol

• given Alice want to communicate with Bob, and have a Key Server S, protocol is:

Message 2 S -> A E_Kas{N_a , B, K_ab, E_Kbs{K_ab, A} }

Message 3 A -> B E_Kbs{K_ab, A}

Message 4 B -> A E_Kab{N_b}

Message 5 A-> B E_Kab{N_b-1}

nb: N_a is a random value chosen by Alice, N_b random chosen by Bob

• after this protocol runs, Alice and Bob share a secret session key K_{ab for secure communication}
• _{unfortunately this protocol includes a fatal flaw - Message3 can be subject to a replay attack with an old compromised session key by an active attacker}
• _{this has been corrected either by:}
  • including a timestamp in messages 1 to 3, which requires synchronised clocks (by Denning & Sacco 81)
  • having A ask B for a random value Jb to be sent to S for return in E_Kbs{Kab, A, Jb} (by Needham & Schroeder 87)
• many other protocols exist but care is needed

Kerberos - An Example of a Key Server

• trusted key server system developed by MIT
• provides centralised third-party authentication in a distributed network
• access control may be provided for
  • each computing resource
  • in either a local or remote network (realm)
• has a Key Distribution Centre (KDC), containing a database of:
  • principles (customers and services)
  • encryption keys
• basic third-party authentication scheme
• KDC provides non-corruptible authentication credentials (tickets or tokens)

Kerberos - Initial User Authentication

• user requests an initial ticket from KDC
• used as basis for all remote access requests

Kerberos - Request for a Remote Service

• user requests access to a remote service
  • obtains a ticket from KDC protected with remote key
  • sends ticket with request to remote server

Kerberos - in practise

• currently have two Kerberos versions
  • 4 : restricted to a single realm
  • 5 : allows inter-realm authentication, in beta test
• Kerberos v5 is an Internet standard
  • specified in RFC1510, and used by many utilities
• to use Kerberos
  • need to have a KDC on your network
  • need to have Kerberised applications running on all participating systems
• major problem - US export restrictions
  • Kerberos cannot be directly distributed outside the US in source format (& binary versions must obscure crypto routine entry points and have no encryption)
  • else crypto libraries must be reimplemented locally

• part of CCITT X.500 directory services
• defines framework for authentication services
• directory may store public-key certificates
• uses public-key cryptography and digital signatures
• algorithms not standardised but RSA is recommended

X.509 Certificate

• issued by a Certification Authority (CA)
• each certificate contains:
  • version
  • serial number (unique within CA)
  • algorithm identifier (used to sign certificate)
  • issuer (CA)
  • period of validity (from - to dates)
  • subject (name of owner)
  • public-key (algorithm, parameters, key)
  • signature (of hash of all fields in certificate)
• any user with access to CA can get any certificate from it
• only the CA can modify a certificate

CA Hierarchy

• CA form a hierarchy
• each CA has certificates for clients and parent
• each client trusts parents certificates
• enable verification of any certificate from one CA by users of all other CAs in hierarchy
• X<<A>> means certificate for A signed by authority X

• X<<W>>W<<V>>V<<Y>>Y<<Z>>Z<<B>>
• B acquires A certificate following chain:
• Z<<Y>>Y<<V>>V<<W>>W<<X>>X<<A>>

Authentication Procedures

• X.509 includes three alternative authentication procedures

One-Way Authentication

• 1 message ( A->B) to establish
  • identity of A and that messages is from A
  • message intended for B
  • integrity & originality of message

Two-Way Authentication

• 2 messages (A->B, B->A) which also establishes
  • identity of B and that replay is from B
  • reply intended for A
  • integrity & originality of reply

Three-Way Authentication

• 3 messages (A->B, B->A, A->B) which enables
  • above authentication without syncronised clocks

• email is one of the most widely used and regarded network services
• currently message contents are not secure
  • may be inspected either in transit
  • or by suitably priviledged users on destination system
• Email Privacy Enhancement Services
  • confidentiality (protection from disclosure)
  • authentication (of originator)
  • message integrity (protection from modification)
  • non-repudiation of origin
  • (protection from denial by sender)
• can't assume real-time access to a trusted key server
• often implement using Email Encapsulation

• Privacy Enhanced Mail
• Internet standard for security enhancements to Internet (RFC822) email
  • developed by a Working group of the IETF
  • specified in RFC1421, RFC1422, RFC1423, RFC1424
• uses message encapsulation to add features
• confidentiality - DES encryption in CBC mode
• integrity - DES encrypted MIC (MD2/MD5)
• authentication - DES/RSA encrypted MIC
• non-repudiation - RSA encrypted MIC

PEM - Key Management

• central key server (private-key)
  • requires access to on-line server
• public-key certificates
  • uses X.509 Directory Service Strong Authentication to protect key certificates
  • signed by a Certification Authority (CA)
  • CAs form a hierarchy to permit cross-validation of certificates
  • CAs must be licenced by RSA Data Inc.
  • currently only licenced in US/Canada

• Pretty Good Privacy
• widely used de facto secure email standard
  • developed by Phil Zimmermann
  • available on Unix, PC, Macintosh and Amiga systems
  • free!!!!
• confidentiality - IDEA encryption
• integrity - RSA encrypted MIC (MD5)
• authentication & non-repudiation - RSA encrypted MIC
• uses grass-roots key distribution
  • trusted introducers used to validate keys
  • no certification authority hierarchy needed

PGP - In Use

• all PGP functions are performed by a single program
• must be integrated into existing email/news
• each user has a keyring of known keys
  • containing their own public and private keys (protected by a password)
  • public keys given to you directly by a person
  • public keys signed by trusted introducers
• used to sign/encrypt your messages
• used to validate messages received

Sample PGP Message

-----BEGIN PGP SIGNED MESSAGE-----

May all your signals trap
May your references be bounded
All memory aligned
Floats to ints be rounded

Lawrie
-----BEGIN PGP SIGNATURE-----
Version: 2.3
iQBzAgUBLdl1RILpoub8ek7fAQF2nwLuJwVPh8iiFrksXSCe6z37ZdV37pXvsYyz0WAnCBCdpu55yId5/kVhmvusTo10zUHPssPwB99TQq9YsduSfkVeILjfJNJEuUWQkJl8dWvaB+IIEEodF0Xpbc23krnuOA==
=hn90
-----END PGP SIGNATURE-----

PGP - Issues

• were questions of legality, but PGP may now be legally used by anyone in the world:
  • noncommercial use in US/Canada with licenced MIT version
  • commercial use in US/Canada with Viacrypt version
  • noncommercial use outside the US is probably legal with (non US sourced) international version
  • commercial use outside the US requires an IDEA licence for the international version
• is on-going legal battle in US over its original export between US govt and Phil Zimmermann

• SNMP is a widely used network management protocol
• comprises
  • management station
  • management agent with
  • its management information base (MIB)
  • linked by network management protocol (GET,SET)
• SNMP v1 lacks any security (GET and SET open if there)
• SNMP v2 includes security extensions for
  • message authentication (keyed MD5)
  • message secrecy (DES)
• based on the SNMPv2 party (sender & receiver roles)
  • used for access control & key management
  • all associated information stored in a party MIB
• assumes syncronised clocks (within a set interval)

• user authentication (identity verification)
  • convince system of your identity
  • before it can act on your behalf
• sometimes also require that the computer verify its identity with the user
• user authentication is based on three methods
  • what you know
  • what you have
  • what you are
• all then involve some validation of information supplied against a table of possible values based on users claimed identity

Passwords or Pass-phrases

• prompt user for a login name and password
• verify identity by checking that password is correct
• on some (older) systems, password was stored in the clear (this is now regarded as insecure, since breakin compromises all users of the system)
• more often use a one-way function, whose output cannot easily be used to find the input value
  • either takes a fixed sized input (eg 8 chars)
  • or based on a hash function to accept a variable sized input to create the value
• important that passwords are selected with care to reduce risk of exhaustive search

Denning Computer (In)security Fig 2 & 3, pp111-12

One-shot Passwords

• one problem with traditional passwords is caused by eavesdropping theit transfer over an insecure network

• these are passwords used once only
• future values cannot be predicted from older values