The following text was encrypted with the Affine cipher algorithm. The alphabet used was:

ABCDEFGHIJKLMNOPQRSTUVWXYZ ,.

The numeric value of each character is it's index in the alphabet. So 'A' is mapped to 0, 'B' to 1, etc, all the way up to '.' which is mapped to 28. The size of the alphabet was chosen to be prime to allow for more possible keys.

Cipher Text:
BOHHIKBI,OZ,REI,WZRIKZIR,EX.,BOHI,RO,KISU,XSHO.R,ICBSG.WYISU,OZ, WZXZBWXS,WZ.RWRGRWOZ.,.IKYWZP,X.RKG.RIT,REWKT,DXKRWI.,RO,DKOBI..,ISIBRKOZWB,DXUHIZR.F,NEWSI,REI,.U.RIH,NOKA.,NISS,IZOGPE, OKHO.R,RKXZ.XBRWOZ.Q,WR,.RWSS,.G IK., KOH,REI,WZEIKIZR,NIXAZI..I.,O ,REI,RKG.R,MX.IT,HOTISF,BOHDSIRISU,ZOZKIYIK.WMSI,RKXZ.XBRWOZ.,XKI,ZOR,KIXSSU,DO..WMSIQ,.WZBI, WZXZBWXS,WZ.RWRGRWOZ.,BXZZORXYOWT,HITWXRWZP,TW.DGRI.F,REI,BO.R,O ,HITWXRWOZ,WZBKIX.I.,RKXZ.XBRWOZ,BO.R.Q,SWHWRWZP,REIHWZWHGH,DKXBRWBXS,RKXZ.XBRWOZ,.WJI,XZT,BGRRWZP,O ,REI,DO..WMWSWRU, OK,.HXSS,BX.GXS,RKXZ.XBRWOZ.QXZT,REIKI,W.,X,MKOXTIK,BO.R,WZ,REI,SO..,O ,XMWSWRU,RO,HXAI,ZOZKIYIK.WMSI,DXUHIZR., OK,ZOZKIYIK.WMSI.IKYWBI.F,NWRE,REI,DO..WMWSWRU,O ,KIYIK.XSQ,REI,ZIIT, OK,RKG.R,.DKIXT.F,HIKBEXZR.,HG.RMI,NXKU,O ,REIWK,BG.ROHIK.Q,EX..SWZP,REIH, OK,HOKI,WZ OKHXRWOZ,REXZ,REIU,NOGST,OREIKNW.I,ZIITF,X,BIKRXWZ,DIKBIZRXPI,O , KXGT,W.,XBBIDRIT,X.,GZXYOWTXMSIF,REI.I,BO.R.,XZT,DXUHIZR,GZBIKRXWZRWI.BXZ,MI,XYOWTIT,WZ,DIK.OZ,MU,G.WZP,DEU.WBXS,BGKKIZBUQ,MGR,ZO,HIBEXZW.H,ICW.R.,RO,HXAI,DXUHIZR.OYIK,X,BOHHGZWBXRWOZ.,BEXZZIS,NWREOGR,X,RKG.RIT,DXKRUF

Submit the (lowercase hex) md5 hash of the plaintext. Note the plaintext contains no newline characters, even at the end of the file.

Test Vector

This ciphertext

RLZZC SCIZJB

Was encrypted with the same alphabet and the parameters a=2, b=3 (remember the decryption parameters are the inverses of the encryption parameters). The decrypted message is:

HELLO WORLD.

The md5 hash of this plaintext is

8ca106826bc593c20811b9c466ce9c77

Solution