I don't need a historian, I've got John Surratt himself telling us he met personally with Benjamin.
Now, I'll admit that Surratt goes on to deny any CSA government knowledge of the plot, but there's a great deal of research that's come up in the last 30 years or so. A confession that George Atzerodt had given the Baltimore police, but which was subsequently lost until the 1970s says, "Booth said he had met a party in N. York who would get the Prest. certain. They were going to mine the end of the pres. House, near the War Dept. They knew an entrance to accomplish it through." This refers to the failed Harney plot, which was fully approved by Davis. Then there's Booth's multiple visits to Canada, where he'd meet with Confederate agents and return with large sums of money. The best books on the subject are:Rash, perhaps foolish, but honorable I maintain in its means and ends; actuated by such motives as would under similar circumstances be a sufficient inducement to thousands of southern young men to have embarked in a similar enterprise. Shortly after our abandonment of the abduction scheme, some dispatches came to me which I was compelled to see through to Richmond. They were foreign ones, and had no reference whatever to this affair. I accordingly left home for Richmond, and arrived there safely on the Friday evening before the evacuation of that city. On my arrival I went to [the] Spotswood Hotel, where I was told that Mr. Benjamin, the then Secretary of War of the Confederate States, wanted to see me. I accordingly sought his presence. He asked me if I would carry some dispatches to Canada for him. I replied "yes." That evening he gave me the dispatches and $200 in gold with which to pay my way to Canada.link
Come Retribution: The Confederate Secret Service and the Assassination of Lincoln by Tidwell, Hall & Gaddy
April '65: Confederate Covert Action in the American Civil Warby Tidwell
Blood on the Moon: The Assassination of Abraham Lincolnby Edward Steers
Basically the evidence is pretty undeniable that Booth was an active Confederate agent throughout the war, and that the highest levels of the Confederate government had already approved at least one other plot to assassinate LIncoln.
I don't question that Booth had contacts with Confederate agents in Canada, or that there was discussion about the possible kidnapping of Mr. Lincoln (similar to the Union's efforts to kidnap Davis via Ulrich Dalgren). But we're talking about Judah Benjamin's involvement, not J.W. Booth's. Most professional historians (as opposed to Tidwell, Hall, and Gaddy) still believe that Booth and his group acted alone after the kidnapping plot fell apart. The "highest levels of the Confederate government" would presumably include Jefferson Davis. It is almost certain that Davis would never have approved an assassination attempt upon Lincoln, inasmuch as such an event would have elevated his arch-enemy, V.P.Andrew Johnson, to the U.S. presidency.
Acting as a consultant to the Johnson administration (which desperately wanted to implicate high-ranking Confederates in Lincoln's murder) Judge Francis Lieber was given all War Dept. material and captured Confederate documents to determine whether Davis or his administration were involved in the assassination. Lieber concluded they were not. As with the Kennedy assassination, conspiracy theories abound regarding Mr. Lincoln's death, and probably always will.