The undelivered message was written in great haste on 4 July 1863 to the Confederate commander in the besieged town of Vicksburg, using a Vigenère cipher, with the usual key phrase "Manchester Bluff" (the failure to change key phrases regularly was a serious Confederate mistake). The message is deciphered as follows (I have corrected a few of the encryption errors, and annotated C=cipher, K=key, and M=message):
C: SEAN WIEUIIUZH DTG CNP LBHXGK OZ BJQB FEQT XZBW JJOYSubtraction of letters (M=C-K) is done as if A=1, B=2, etc. In practice, encryption would have been done using a disk like this:
K: MANC HESTERBLU FFM ANC HESTER BL UFFM ANCH ESTE RBLU
M: GENL PEMBERTON: YOU CAN EXPECT NO HELP FROM THIS SIDE
C: TK FHR TPZWK PVU RYSQ VOUPZXGG OEPH CK UASFKIPW PLVO
K: FF MAN CHEST ERB LUFF MANCHEST ERBL UF FMANCHES TERB
M: OF THE RIVER. LET GENL JOHNSTON KNOW IF POSSIBLE WHEN
C: JIZ HMN NVAEUD XYF DURJ BOVPA SF MLV FYYRDE LVPL
K: LUF FMA NCHEST ERB LUFF MANCH ES TER BLUFFM ANCH
M: YOU CAN ATTACK THE SAME POINT ON THE ENEMYS LINE.
C: MFYSIN XY FQEO NPK M OBPC FYXJFHOHT AS ETOV B OCAJDSVQU
K: ESTERB LU FFMA NCH E STER BLUFFMANC HE STER B LUFFMANCH
M: INFORM ME ALSO AND I WILL ENDEAVOUR TO MAKE A DIVERSION.
C: M ZTZV TPHY DAU FQTI UTTJ J DOGOAIA FLWHTXTI QLTR
K: E STER BLUF FMA NCHE STER B LUFFMAN CHESTERB LUFF
M: I HAVE SENT YOU SOME CAPS. I SUBJOIN DESPATCH FROM
C: SEA LVLFLXFO
K: MAN CHESTERB
M: GEN JOHNSTON
Hat tip: Bayou Renaissance Man.