Researchers have published for the first time writing on two pages of Anne Frank's diary which she covered over with brown paper, discovering dirty jokes and a teenager's interest in sex.
Brown paper had been used to cover the two pages in Anne's first diary, but digital and image processing technology was used to decipher their contents.
The young Jew, then aged 13, and her family had only been in hiding from the Nazis in Amsterdam for two months.
She listed four dirty jokes, and added 33 lines about sex education and prostitutes. These new discoveries "bring us even closer to the girl and the writer Anne Frank," added Leopold.
It is not known why she covered up the two pages with brown paper, but several times in her diaries she voices fears that others might pry into her writings.
On October 3, 1942, she wrote: "Daddy is grumbling again and threatening to take away my diary. Oh, horror of horrors! From now on, I'm going to hide it." The Frank family lived in a modest brick building from 1934 in the Dutch capital, after fleeing rising anti-Semitism in neighbouring Germany, until they went into hiding.
On July 6, 1942, they retreated into the secret annexe of another home as the Nazis were rounding up Jewish families following their invasion of the Netherlands in 1940.
In her diary, Anne chronicled her life in hiding until August 1944, when her family was most likely betrayed and sent to Nazi concentration camps.
Researchers were at pains to point out that it is not the first time that Frank wrote about sex in her diary.
From her writings it was clear "she had gleaned information on the subject of sexuality from her parents, especially her father, from her friend Jacqueline and from books," the foundation said.
Some months after covering over the first entry "she emphasised the importance of good and thorough sex education, and she didn't understand why adults were so secretive about it," it added.
Defending the decision to release the newly-found texts, the foundation said "over the decades Anne has grown to become the worldwide symbol of the Holocaust, and Anne the girl has increasingly faded into the background".
"These - literally - uncovered texts bring the inquisitive and in many respects precocious teenager back into the foreground." Anne died at Bergen-Belsen in Germany in early 1945, aged 15, less than a year after her capture and just before the end of the war.
Her diary, written during her time in hiding, is one of the most moving testimonies of the war, and more than 30 million copies have been sold.