HMAC obfuscation method for implementation in untrusted systems

We propose an obfus-cation method for using hash-based message authentication codes (MAC) in untrusted systems. Our method is implemented for MAC in a form H(k,x) = h(k,p₁, h(k,p₂, x)). The main idea is to use inner states of a hash function h. We calculate both h intermediate values in a such way that all key related blocks are already reduced by the hash compression function. That values are h(epad(k,p₁)) and h(epad(k,p₂)), where epad is a key padding algorithm. Then we use them in functions h₁ and h₂, which calculate h with the initial block equalled h(epad(k,p₁)) and h(epad(k,p₂)) respectively. So, accordingly to the following equation, these new functions implement original MAC algorithm: H (k,x) = h(k,p₁ ,h(k,p₂ ,x)) = h₁(h₂(x)).

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