he wrote the song in ten minutes

In interviews Dylan has never reported holding as high an opinion of the song as its popular acclaim would suggest - he has said he wrote the song in ten minutes. He has called it a work song, perhaps in reference to its derivative, rather than inspired, nature of its composition, the melody being derived from the old slave song "No More Auction Block", and some of its lyrical structure from the 1953 song "I Really Don't Want to Know".
An urban legend still circulates that the song was actually written by a high-school student named Lorre Wyatt and subsequently purchased or stolen by Dylan before he gained fame.The legend was made famous when it was published in a Newsweek article in 1963; while the story left the claims unconfirmed, it prompted plenty of speculation. Several members of Wyatt's school and community reported having heard him singing the song and claiming authorship a full year before it was released by Dylan, or made famous by Peter, Paul and Mary. Wyatt even told his teacher that he'd sold the song for $1,000 and donated the money to charity, when asked why he had suddenly stopped performing it.It turned out that the plagiarism claims were completely false. Wyatt had performed the song around Millburn, New Jersey, months before it was made famous, but not before it had been published and credited to Dylan in the songbooks Broadside and Sing Out!. Wyatt finally explained his part in the situation to New Times magazine in 1974. He credited the initial lie to panic that he wasn't pulling his weight as a songwriter in a local band.( more from wiki )