(CNN) Mystery has swirled around the two pages of Anne Frank's diary where brown paper was pasted over the writing. But today it was revealed what lay behind the covered up pages.

Thanks to modern image processing technology, the writing from 1942 was uncovered, and four "dirty jokes" appeared.

"I'll use this spoiled page to write down 'dirty' jokes," Anne wrote on September 28, 1942, just two months after she went into hiding.

Reserachers used digital technology to uncover the writing.

She was 13 when she went into hiding from the Nazis with her family. She kept a diary throughout her time in hiding, and after her death, her father, Otto, published it in 1947.

In 2016, the diary was rephotographed by the Anne Frank House and the hidden writing was deciphered.

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