Chipertext

I visited Department of Mathematics booth in UBD just now, and saw a ciphertext challenge (i think got prize). This is the second question of the challenge, convert the following message into English by substitution method.

XHJCYZOHVCMJ WU YMG UXWGTXG ZB CHZYGXYWTO WTBZHSVYWZT DJ YHVTUBZHSWTO ZH GTXHJCYWTO WY WTYZ ITHGVFVDAG BZHSVY PTZNT VU XWCMGHYGLY. ZTAJ YMZUG NRZ CZUUGUU YMG UGXHGY PGJ XVT FGXWCMGH ZH FGXHJCY YMG SGUUVOG WTYZ CAVWT YGLY. GTXHJCVGF SGUUVDGU XVT UZSGYWSGU DG DHZDGT DJ XHJCYVTVAJUWU VAUZ XVAAGF XZFGDHGVPWTO, VAYMZIOM SZFGHT XHJCYZOHVCMJ YGXMTWEIGU VHG RWHYIVAAJ ITDHGVPVDAG.

So here is what I have solved, with the help of my friends who help me in English vocabulary + the spelling.. since I'm poor in English language!!!

CRYPTOGRAPHY IS THE SCIENCE OF PROTECTING INFORMATION BY TRANSFORMING OR ENCRYPTING IT INTO UNREADABLE FORMAT KNOWN AS CIPHERTEXT. ONLY WHO POSSESS THE SECRET KEY CAN DECIPHER OR DECRYPT THE MESSAGE INTO PLAIN TEXT. ENCRYPTED MESSAGES CAN SOMETIMES BE BROKEN BY CRYPTANALYSIS, ALSO CALLED CODEBREAKING, ALTHOUGH MODERN CRYPTOGRAPHY TECHNIQUES ARE VIRTUALLY UNBREAKABLE.

known substitution key - (from chipertext to plaintext)
A=L, B=F, C=P, D=B, E=Q, F=D, G=E, H=R, I=U, J=Y, K=?, L=X, M=H, N=W, O=G, P=K, Q=?, R=V, S=M, T=N, U=S, V=A, W=I, X=C, Y=T, Z=O

I forgot the last question because I was not allowed to bring back the question paper!!! (but i managed to copy the second question secretly... :D). I think the prize is still there tomorrow...