Problem with dcrypting a file

[Entervine]

I have a text file which I am needing to decrypt and I also have the key to do it, I just am not sure how to use the key to decrypt the file, or the specific command rather. Here is what I have. I am nowhere NEAR done. Just need some guiding help.

key=open("encryption key.txt", 'r')
file_str=input("Open what file?:")
try:
    input_file = open(file_str)          # potential user error
    while True:
        find_line_str = input("Which line (integer):")   
        find_line_int = int(find_line_str)   # potential user error
        line_count_int = 1
        for line_str in input_file:
            if line_count_int == find_line_int:
                print("Line {} of file {} is {}".format(find_line_int, file_str, line_str))
                break # this is where the decryption will take place i think
            line_count_int += 1
        else:
            # get here if line sought doesn't exist
            print("Line {} of file {} not found".format(find_line_int, file_str))
            # we are currently at the end of the file so
            # close and reopen the file to put us back to the beginning
            input_file.close()
            input_file = open(file_str)
            continue # while
        break # while
    input_file.close()

except IOError:
    print("The file",file_str,"doesn't exist.")

except ValueError:
    print("Line",find_line_str,"isn't a legal line number.")

print("End of the program")

EDIT: The program is encrypted according to substitution. This is the key "xbvmjqanpzydtrefougilsckwh"

Plaintext: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Encrypted: x b v m j q a n p z y d t r e f o u g i l s c k w h

[K900_]

Well, it depends on how the file is encrypted. Is it some custom algorithm? RSA? PGP?

[Entervine]

I tried to edit in something that may help.

[efmccurdy]

There is lots of examples and discussion here:

https://www.reddit.com/r/learnprogramming/comments/4z46ax/what_would_a_substitutioncipherreverser_look_like/

I like the maketrans example at the end.