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Cryptography and Network Security Chapter 11

Cryptography and Network Security Chapter 11. Fourth Edition by William Stallings Lecture slides by Lawrie Brown /Mod. & S. Kondakci. Message Authentication. message authentication is concerned with: protecting the integrity of a message validating identity of originator

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Cryptography and Network Security Chapter 11

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  1. Cryptography and Network SecurityChapter 11 Fourth Edition by William Stallings Lecture slides by Lawrie Brown/Mod. & S. Kondakci

  2. Message Authentication • message authentication is concerned with: • protecting the integrity of a message • validating identity of originator • non-repudiation of origin (dispute resolution) • considersa set of security requirements • Three alternative functions used: • message encryption • message authentication code (MAC) • hash function

  3. A Set of Security Requirements • disclosure • traffic analysis • masquerade • content modification • sequence modification • timing modification • source repudiation • destination repudiation

  4. Symmetric-key Message Encryption • message encryption by itself also provides a measure of authentication • if symmetric encryption is used then: • receiver knows sender must have created it • since only sender and receiver know key used they know that content cannot have been altered by others than themselves

  5. public-key Message Encryption • If public-key encryption is used: • encryption provides no confidence of sendersince anyone potentially knows public-key • however if • sender signs message using their private-keythen encrypts with recipients public keythus, have both secrecy and authentication

  6. Message Authentication Code (MAC) • MAC provides assurance that message is unaltered and comes from certain sender. • generated by an algorithm that creates a small fixed-sized block • depending on both message and some key • like encryption though need not be reversible • A MAC is appended to message as a signature • receiver performs same computation on the received message and checks wheathr it matches the MAC appended

  7. Message Authentication Codes • MAC provides authentication but can also use encryption for secrecy • generally use separate keys for each • can compute MAC either before or after encryption: generally regarded as better done before • why use a MAC? • sometimes only authentication is needed • sometimes we need authentication to persist longer than the encryption (eg. archival use)

  8. MAC Properties • A MAC is a cryptographic checksum MAC = CK(M) condenses a variable-length message Musinga secret key Kto a fixed-sized authenticator • A MAC is a many-to-one function • potentially many messages have same MAC • but finding these can be very difficult

  9. Requirements for MACs • Taking into account the types of attacks, MAC should satisfy the following aspects: • knowing a message and MAC, it should be infeasible to find another message with the same MAC • MACs should be uniformly distributed • MAC should depend equally on all bits of the message

  10. Symmetric Ciphers for MACs • Can use any block cipher with chaining mode and use final block as a MAC • Data Authentication Algorithm (DAA) is a widely used MAC based on DES-CBC • using IV=0 and zero-pad of final block • encrypts message using DES in CBC mode • and sends just the final block as the MAC • or the leftmost M bits (16≤M≤64) of final block • but final MAC is now too small for security!

  11. Cipher Block Modes of Operation Cipher Block Chaining Mode (CBC) The input to the encryption algorithm is the XOR of the current plaintext block and the preceding ciphertext block. Repeating pattern of 64-bits are not exposed 11

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  13. CBC Encryption & Decryption

  14. Data Authentication Algorithm (FIPS PUB 113)

  15. Data Authentication Algorithm FIPS PUB 113

  16. Hash Functions • condenses arbitrary message to fixed size h = H(M) • usually assumed that the hash function is public and not keyed • Hashes are used to detect changes to message • can be used in various ways with messages • mostlyused to create digital signatures

  17. Hash Functions & Digital Signatures

  18. Requirements for Hash Functions • can be applied to any sized message M • produces fixed-length output h • is easy to compute h=H(M) for any message M • one-way property: given hit should be infeasible to find xs.t.H(x)=h • weak collision resistance: given xit should be infeasible to find ys.t. H(y)=H(x) • strong collision resistance:it should be infeasible to find any x,ys.t. H(y)=H(x)

  19. Simple Hash Functions • There are several proposals for simple functions • based on XOR of message blocks • not secure since can manipulate any message and either not change hash or change hash also • need a stronger cryptographic function

  20. Secure Hash Algorithm • SHA originally designed by NIST & NSA in 1993 • was revised in 1995 as SHA-1 • US standard for use with DSA signature scheme • standard is FIPS 180-1 1995, also Internet RFC3174 • based on design of MD4 with key differences • produces 160-bit hash values • recent 2005 results on security of SHA-1 have raised concerns on its use in future applications NIST = National Institute of Standards and Technology NSA = National Security Agency

  21. Secure Hash Function

  22. Hash Algorithm Structure

  23. Revised Secure Hash Standard • NIST issued revision FIPS 180-2 in 2002 • adds 3 additional versions of SHA • SHA-256, SHA-384, SHA-512 • designed for compatibility with increased security provided by the AES cipher • structure & detail is similar to SHA-1 • hence analysis should be similar • but security levels are rather higher

  24. SHA-512 Overview

  25. Keyed Hash Functions as MACs • Need a MAC based on a hash function • because hash functions are generally faster • code for cryptographic hash functions widely available • hash includes a key along with message • original proposal: KeyedHash = Hash(Key|Message) • some weaknesses were found with this • eventually led to development of HMAC

  26. HMAC • specified as Internet standard RFC2104 • uses hash function on the message: HMACK = Hash[(K+ XOR opad) || Hash[(K+ XOR ipad)||M)]] • where K+ is the key padded out to size • and opad (5C Hex), ipad (36 Hex)are specified padding constants • overhead is just 3 more hash calculations than the message needs alone • any hash function can be used • eg. MD5, SHA-1, RIPEMD-160, Whirlpool

  27. HMAC Overview ipad = 36 hex opad = 5C hex

  28. HMAC Security • proved security of HMAC relates to that of the underlying hash algorithm • attacking HMAC requires either: • brute force attack on key used • birthday attack (but since keyed would need to observe a very large number of messages) • choose hash function used based on speed verses security constraints

  29. Typical Digital Signature Approach Henric Johnson

  30. Obtaining a User’s Certificate • Characteristics of certificates generated by CA: • Any user with access to the public key of the CA can recover the user public key that was certified. • No part other than the CA can modify the certificate without this being detected. Henric Johnson

  31. X.509 CA Hierarchy Henric Johnson

  32. Revocation of Certificates • Reasons for revocation: • The users secret key is assumed to be compromised. • The user is no longer certified by this CA. • The CA’s certificate is assumed to be compromised. Henric Johnson

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