##Convex, 4 6 5 bytes
Convex, 4 6 5 bytes
Crossed out 4 is still regular 4 :(
Note: As @Dennis pointed out, the input format I was using was not up to spec. This version should comply with the new rules, though.
lqT@Ë
Explanation:
l Read a line from input
q Read the rest of the input
T Push lowercase alphabet ("abcdefghijklmnopqrstuvwxyz")
@ Rotate the top 3 items on the stack.
er Transliterate
Implied output
Thanks to @LuisMendo for pointing out an error in the program