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Why Luke Skywalker call Obi-Wan “Ben”?

In Star Wars Episode IV: A New Hope, Luke Skywalker known Obi-Wan Knobi under the name of "Ben" :

Well, I stumbled across a recording while I was cleaning him. He says that he belongs to someone named Obi-Wan Kenobi. I thought he might have meant old Ben. Do you know what he's talking about?

From where this "Ben" nickname/alias/false identity come from?