Montrer patte blanche translates as showing one’s credentials or proving one's identity.
Word for word, montrer patte blanche means 'to show the white paw'.
The expression comes from the French fable The Wolf, The Goat and The Kid, written by Jean de La Fontaine in 1668.
In the story a mother goat leaves her baby goat at home alone.
She tells him not to answer the door unless he can see through the window that the caller has a white paw, or hoof, like her own.
When the wolf comes calling, the baby goat asks it to montrer patte blanche.
When the wolf cannot, the baby goat does not let the wolf in and stays safe.
Another version of the story written by the Brothers Grimm has a less positive outcome.
In their telling the clever wolf dips his paw in flour to colour it white, and tricks its way inside to eat the baby goat.