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1

As @yyyyyy already commented: if you are able to successfully apply a length extension attack, you would be able to compute $$\operatorname{SHA256}(\mathit{secret}\Vert\mathit{padding}\Vert\mathit{data})$$ But computing $\operatorname{SHA256}(\mathit{secret}\Vert\mathit{data})$ for freely chosen $\mathit{data}$ without the $\mathit{padding}$ in between is ...


4

I think the attack by Preneel and van Oorschot (MDx-MAC and Building Fast MACs from Hash Functions, CRYPTO'95) in Proposition 4 applies. It was cited by PulpSpy in reply to my question about H(pass||length(data)||data). With fixed-length data, that amounts to a known suffix for the key. Proposition 4 states in a nutshell that for a generic construction ...



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