computer security lecture 6 ch.11 cryptographic hash functions prepared by dr. lamiaa elshenawy
TRANSCRIPT
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Computer Security
Lecture 6Ch.11
Cryptographic Hash FunctionsPrepared by Dr. Lamiaa Elshenawy
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Cryptographic Hash Functions
• Applications of Cryptographic Hash Functions Message Authentication Digital Signatures Other Applications• Two Simple Hash Functions• Requirements and Security Security Requirements for Cryptographic Hash Functions Brute-Force Attacks Cryptanalysis• Hash Functions Based on Cipher Block Chaining• Secure Hash Algorithm (SHA) SHA-512 Logic SHA-512 Round Function• 11.6 SHA-3
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Cryptographic Hash FunctionsKey Points
• Hash function Variable-length message Fixed-length message digest
• Cryptographic hash functions iterative use of a compression function
• Compression function
Specific function (SHA)
Symmetric block cipher (Whirlpool)
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Cryptographic Hash Functions
• Hash function Computationally infeasible
One-way property Collision-free property
H“Hash function”M H(M)
Data Integrity
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Cryptographic Hash FunctionsBlock diagram
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Cryptographic Hash FunctionsApplications
1. Message Authentication2. Digital Signatures3. Other Applications (Internet protocols)
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Cryptographic Hash FunctionsMessage Authentication
• Message authentication Mechanism or service used to verify the integrity of a message.
No modificationNo insertionNo deletionNo replay
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Cryptographic Hash FunctionsMessage Authentication
Confidentiality
No Confidentiality
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Cryptographic Hash FunctionsMessage Authentication
Confidentiality
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Cryptographic Hash FunctionsMessage Authentication
• No confidentiality: method (b) has an advantage over methods (a) and (d)
less computation
• Avoid encryption method (c)
Encryption software relatively slow Encryption hardware high cost Time high Encryption algorithms patents cost
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Cryptographic Hash FunctionsMessage Authentication
M: variable-length message K: secret key KOutput: fixed-size
MAC: message authentication code (keyed hash function)
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Cryptographic Hash FunctionsDigital Signatures
Public key
H“Hash
function”M H(M)
Encryption algorithm
Private key
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Cryptographic Hash FunctionsDigital Signatures
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Cryptographic Hash FunctionsDigital Signatures
• Digital signature private-key-encrypted hash code authentication (only the sender could have produced the encrypted hash code, essence of the digital signature technique)
• Confidentiality + Digital signature message + private-key-encrypted hash codeencrypted using a symmetric secret key
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Cryptographic Hash FunctionsDigital Signatures
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Cryptographic Hash FunctionsOther Applications
1. One-way password file2. Intrusion detection3. Virus detection4. Pseudorandom function(PRF) or a
Pseudorandom number generator (PRNG).
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Cryptographic Hash FunctionsPassword
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Cryptographic Hash FunctionsTwo Simple Hash Functions
1. Bit-by-Bit exclusive-OR (XOR) of every block
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Cryptographic Hash FunctionsTwo Simple Hash Functions
2. Rotated exclusive-OR (XOR) of every blocki. Initially set the -bit hash value to zero.ii. Process each successive -bit block of data as
follows: a. Rotate the current hash value to the left by one bit. b. XOR the block into the hash value.
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Cryptographic Hash FunctionsRequirements and Security
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Cryptographic Hash FunctionsSecond Preimage Resistant
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Cryptographic Hash FunctionsSecond Preimage Resistant
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Cryptographic Hash FunctionsRequirements and Security
1. Brute-force attacks bit length (hash value)
2. Cryptanalysis weaknesses in cryptographic algorithm
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Cryptographic Hash FunctionsBrute-Force Attacks
• Preimage and second preimage attacks
y H(y)=h
• Collision resistant attacks
y H(y)
Birthday paradox
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Cryptographic Hash FunctionsBrute-Force Attacks
m: number of hash value bits
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Cryptographic Hash FunctionsCryptanalysis
Merkle–Damgård hash function
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Cryptographic Hash FunctionsCryptanalysis
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Cryptographic Hash FunctionsSecure Hash Algorithm(SHA)
• National Institute of Standards and Technology (NIST)- Federal information processing standard (FIPS 180)-1993
SHA-0 (1993)SHA-1 (1995)SHA-256SHA-384 (2002)SHA-512
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Cryptographic Hash FunctionsSecure Hash Algorithm(SHA)
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Cryptographic Hash FunctionsSHA-512 Logic
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Cryptographic Hash FunctionsSHA-512 Logic
1. Append padding bits (length896 mod 1024)2. Append length (128 bits=16 bytes)3. Initialize hash buffer
4. Process message in 1024-bit blocks5. Output 80
rounds
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Cryptographic Hash FunctionsSHA-512 Logic
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Cryptographic Hash FunctionsSHA-512 Logic
• Let 32 bit quantity 90AB12CD16
• 4 bytes are: 90, AB, 12, CD • Two ways to store in memory
Address Value
1000 90 1001 AB 1002 12 1003 CD
Address Value
1000 CD 1001 12 1002 AB 1003 90
Big -endian
Little -endian
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Cryptographic Hash FunctionsSHA-512 Round Function
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Cryptographic Hash FunctionsSHA-512 Round Function
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Cryptographic Hash FunctionsSHA-512 Round Function
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Cryptographic Hash FunctionsSHA-512 Round Function
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Cryptographic Hash FunctionsSHA-3
• SHA-1& SHA-2 Not broken • NIST competition (2007) SHA-3Replace SHA-2 with SHA-3 in any application by
a simple drop-in substitution.Preserve online nature (SHA-2) small
blocks (no buffer-entire message)
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Example
M=“abc”
ASCII code
Padding bits=896-24=871
Length=24 bits=18 hexadecimal
1024-bit massage
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Cryptographic Hash FunctionsExample
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Cryptographic Hash FunctionsExample
After two rounds
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Example
After 80 rounds
Hash value
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Example
512-bit message digest Finally
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Thank you for your attention