Encrypted message to John Pemberton, 1863-07-04

Ciphertext
SEAN WIEUIIUZH DTG CNP LBNEncoding error, should be 'H'. [sic]XGK OZ BJQB FEQT FEQTEncoding error, duplicate word. [sic] XZBW JJOY TK FHR TPZWK PVEncoding error, should be 'B'. [sic]U RYSQ VOWEncoding error, should be 'U' [sic]PZXGG OEPH BEncoding error, should be 'C' [sic]K WEncoding error, should be 'U' [sic]ASFKIPW PLVO JKEncoding error, should be 'I' [sic]Z HMN NVAEUD XYF DWEncoding error, should be 'U' [sic]RJ BOVPA SF MLV FYYRDE LVPL MFYSIN XY FQEO NPK M OBPC FYXJFHOHT AS ETOV B OCAJOEncoding error, should be 'D' [sic]SVQU M ZTZV TPIEncoding error, should be 'H' [sic]Y DAWEncoding error, should be 'U' [sic] FQTI WEncoding error, should be 'U' [sic]TTJ J DOGOAIA FLWHTXTI QMEncoding error, should be 'L' [sic]TR SEA LVLFLXFO.

Key
The key is "MANCHESTERBLUFF", repeated until the end of the message.

Cleartext
GENL PEMBERTON YOU CAN EXPECT NO HELP FROM THIS SIDE OF THE RIVER LET GENL JOHNSTON KNOW IF POSSIBLE WHEN YOU CAN ATTACK THE SAME POINT ON THE ENEMYS LINE INFORM ME ALSO AND I WILL ENDEAVOUR TO MAKE A DIVERSION I HAVE SENT SOME CAPS I SUBJOIN DESPATCH FROM GEN JOHNSTON.