using C programming language

First I have a csv file and contain follows:

a,y b,w c,n d,e e,s f,r g,u h,k i,p j,z k,x l,t m,v n,f o,o p,j q,i r,b s,m t,a u,d v,h w,g x,q y,l z,c where the plaintext alphabet appears before the comma, and the ciphertext alphabet appears after the comma.

I also have a txt file contains: p shot ufpadjvon I want to DECRYPT the above words in txt file using the above csv file, which mean replace the letters in txt file with the corresponding letter from the plaintext alphabets. So p should replace with i, etc. And output the result in reverse order.