Cryptography Chapter3
Public-Key and Message Authentication
Henric Johnson Blekinge Institute of Technology, Sweden http://www.its.bth.se/staff/hjo/ [e-mail]
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OUTLINE
Approache to Me s ssageAuthe ntication S cureHash Functions and HMAC e Public-Ke C y ryptography Principle s Public-Ke C y ryptography Algorithm s Digital S ignature s Ke Manage e y m nt
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Authentication
quire e - m beableto ve that: m nts ust rify Re 1. Me ssagecam fromappare source e nt author, 2. C nts havenot be n alte d, onte e re 3. S e e it was se at a ce om tim s, nt rtain se nce que . or
tim or e
ction against activeattack (falsification of data and Prote transactions)
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Approaches to Message Authentication
ntication Using C ntional Encryption onve Authe
nde ce r y Only these r and re ive should sharea ke
ssageAuthe ntication without Me ssageEncryption Me
ntication tag is ge rate and appe d to e ne d nde ach An authe m ssage e
ssageAuthe ntication C ode Me
alculatetheMACas a function of them ssageand theke e y. C MAC= F(K, M)
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One-way HASH function
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One-way HASH function
e t d fore m d S cre valueis adde be thehash and re ove be transm fore ission.
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Secure HASH Functions
Purposeof theHAS function is to producea H "finge rprint. Prope s of a HAS function H : rtie H
1. 2. 3. 4. H can beapplie to a block of data at any size d H produce a fixe le s d ngth output H(x) is e to com asy putefor any give x. n For any give block x, it is com n putationally infe asibleto find x such that H(x) = h 5. For any give block x, it is com n putationally infe asibleto find with H(y) = H(x). yx 6. I t is com putationally infe asibleto find any pair (x, y) such that H(x) = H(y)
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Simple Hash Function
-bit r ach One circular shift on thehash valueafte e block is proce d would im sse prove
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Message Digest Generation Using SHA-1
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SHA-1 Processing of single 512-Bit Block
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Other Secure HASH functions
SHA-1 Digest length Basic unit of processing Number of steps 160 bits 512 bits MD5 128 bits 512 bits RIPEMD160 160 bits 512 bits
Maximum message size
80 (4 rounds 64 (4 160 (5 of 20) rounds of paired 16) rounds of 16) 264-1 bits
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HMAC
rive , Usea MACde d froma cryptographic hash code such as S HA-1. Motivations:
ryptographic hash functions e cute faste in softwarethan xe s r C e ncryptoin algorithm such as DES s ly Library codefor cryptographic hash functions is wide available xport re strictions fromtheUS No e
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HMAC Johnson
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Public-Key Structure
Henric Cryptography Principles
ys que s y Theuseof two ke has conse nce in: ke distribution, confide ntiality and authe ntication. m die Thesche ehas six ingre nts (seeFigure3.7)
Plainte xt Encryption algorithm Public and privateke y C rte iphe xt De cryption algorithm
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Encryption using PublicKey system
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Authentication using PublicKey System
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Applications for Public-Key Cryptosystems
e gorie Thre cate s: Encryption/decryption: The sender encrypts a message with the recipient's public key. Digital signature: The sender "signs" a message with its private key. Key echange: Two sides cooperate two exhange a session key.
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Requirements for PublicKey Cryptography
1. Computationally easy for a party B to generate a pair (public key KUb, private ke KRb) y 2. Easy for se r to ge rateciphe xt: nde ne rte 3. Easy for there C =rEKUb (M ) ciphe ct using ce to de ive crypt rte privateke y:
M = DKRb (C ) = DKRb [ E KUb ( M )]
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Requirements for PublicKey Cryptography
1. Computationally infeasible to determine private key (KRb) knowing public ke (KUb) y 2. C putationally infe om asibleto re r m ssageM, cove e knowing KUb and ciphe xt C rte 3. Eithe of thetwo ke can beuse for e r ys d ncryption, with theothe use for de r d cryption:
M = DKRb [ EKUb ( M )] = DKUb [ EKRb ( M )]
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Public-Key Cryptographic Algorithms
A -He an RS and Diffie llm RSA - Ron Rives, Adi Shamir and Len Adleman at MIT, in 1977.
A r RS is a block ciphe ost ly ...