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Hash Functions

2. Hash Functions. Why needed?Security of hash functionsPractical hash functions:MD5SHA-1Problems with hash functions. 3. Why hash functions?. Input may be a long stream of charactersOutput is always a fixed sizeDigital signatures use hash functionsAlso known as Message Digest functionsMust

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Hash Functions

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    1. 1 Hash Functions

    2. 2 Hash Functions Why needed? Security of hash functions Practical hash functions: MD5 SHA-1 Problems with hash functions

    3. 3 Why hash functions? Input may be a long stream of characters Output is always a fixed size Digital signatures use hash functions Also known as Message Digest functions Must be a one way function Collision resistant Hash function has no key

    4. 4 Security of Hash Functions Typical output sizes are 128 to 512 bits To get preimage of x with n-bit hash function you need 2n checks Use of higher bit hash function with a lower level security is common Hash functions are vulnerable to birthday attacks

    5. 5 MD5 Developed by Ron Rivest 128-bit hash function Uses 512 bit blocks Uses 128-bit keys or states The 128-bit key is split into 4 subkeys of 32 bits each Four rounds are used to produce the hash In each round addition, XOR, AND, OR, rotation are used

    6. 6 SHA-1 Developed by NSA and standardized by NIST Secure Hash Algorithm SHA-1 is an improvement over SHA-0 SHA-1 is a 160-bit hash function It was developed from MD4 and so has many similarities with MD5 SHA-1 uses a 160-bit state The state is divided into five 32-bit substates SHA-1 uses 4 rounds

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