I remember I read that a MAC can be considered as a PRF.

Can a variable-length input keyed-PRF be considered as a MAC?

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    $\begingroup$ I'd note that the first statement is incorrect: a secure MAC is not necessarily a secure PRF. Consider any secure MAC and add a single zero bit in the end. The result is clearly not random, but equally secure as a MAC. $\endgroup$ – Dmitry Khovratovich Jul 3 '14 at 12:49

Usually. $\:$ A variable-length input keyed-PRF is a [PRF's output length]-bit MAC.
In particular, a variable-length input keyed-PRF with long output is automatically a MAC.

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    $\begingroup$ I would add that we are talking about secure PRFs and secure MACs. $\endgroup$ – Dmitry Khovratovich Jul 3 '14 at 12:47

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