Helen Fouché Gaines’ 1956 textbook Cryptanalysis: A Study of Ciphers and Their Solution concludes with a cipher that, she says, “nobody has ever been able to decrypt”:

VQBUP PVSPG GFPNU EDOKD XHEWT IYCLK XRZAP VUFSA WEMUX GPNIV QJMNJ JNIZY KBPNF RRHTB WWNUQ JAJGJ FHADQ LQMFL XRGGW UGWVZ GKFBC MPXKE KQCQQ LBODO QJVEL.

It was still unsolved in 1968, when Dmitri Borgmann, editor of the Journal of Recreational Linguistics, urged his readers to tackle the problem: “Are you going to let this challenge lie there, taunting you for the rest of your lives? Or are you going to get busy and solve that pesky little crypt?”

So far as I can tell, they let it lie there, and it remains unsolved to this day. Any ideas? There are few clues in Gaines’ book. The cipher is the last in a series of exercises at the end of a chapter titled “Investigating the Unknown Cipher,” and she gives no hint as to its source. Of the exercises, she writes, “There is none in which the system may not be learned through analysis, unless perhaps the final unnumbered cryptogram.” The solution says simply “Unsolved.”