What is the length extension property of the SHA-256 hash function ?
Added by Teresa T.
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It is part of the SHA-2 family and is widely used for data integrity and security. Show more…
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State the value of the length field in SHA-512 if the length of the message is: 1) 1919 bits 2) 1920 bits 3) 194 bits
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Problem 1. A broken proof of work hash function. In class we discussed using a hash function H : X × Y → {0,1,...,2^n − 1} for a proof of work scheme. Once an x ∈ X and a difficulty level D are published, it should take an expected D evaluations of the hash function to find a y ∈ Y such that H(x,y) < 2^n/D. Suppose that X = Y = {0,1}^m for some m (say m = 512), and consider the hash function H : X '× Y → {0,1,...,2^256 − 1} defined as H(x,y) := SHA256(x ⊕ y). Show that this H is insecure as a proof of work hash. In particular, suppose D is fixed ahead of time. Show that a clever attacker can find a solution y ∈ Y with minimal effort once x ∈ X is published. Hint: the attacker will do most of the work before x is published.
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