SEAN WIEUIIUZH DTG CNP LBHXGK OZ BJQB FEQT XZBW JJOY TK FHR TPZWK PVU RYSQ VOUPZXGG OEPH CK UASFKIPW PLVO JIZ HMN NVAEUD XYF DURJ BOVPA SF MLV FYYRDE LVPL MFYSIN XY FQEO NPK M OBPC FYXJFHOHT AS ETOV B OCAJDSVQU M ZTZV TPHY DAU FQTI UTTJ J DOGOAIA FLWHTXTI QLTR SEA LVLFLXFO.If you repeatedly write the key phrase "Manchester Bluff" above the letters it tells you the offset from the actual letter. A is an offset of zero (the same letter in the plain text). B is an offset of 1, C is 2, etc. The decoded message is:
Gen'l Pemberton, You can expect no help from this side of the river. Let Gen'l Johnston know, if possible, when you can attack the same point on the enemy's line. Inform me also and I will endeavour to make a diversion. I have sent you some caps. I subjoin despatch from Gen Johnston.After I figured out the key phrase I noticed in the Wikipedia article that the Confederate leadership primarily relied upon three key phrases, "Manchester Bluff", "Complete Victory" and, as the war came to a close, "Come Retribution".