Implement this key cipher
Python 3, 130 bytes
Thanks to @Rod for pointing out a bug
from random import*
def f(x):l=10**len(x);k=str(randint(0,l-1)+l)[1:];print(''.join(chr(ord(i)+int(j))for i,j in zip(x,k))+'\n'+k)

A function that takes input via argument as a string and prints to STDOUT.
How it works
from random import*  Import everything from the random module
def f(x):            Function with input string x
l=10**len(x)         Define l for later use as 10^length(x)
randint(0,l-1)+l     Generate a random integer in the range [0, l-1] and add l, giving a
                     number with l+1 digits...
k=str(...)[1:]       ...convert to a string and remove the first character, giving a key of
                     length l that can include leading zeroes, and store in k
for i,j in zip(x,k)  For each character pair i,j in x and k:
chr(ord(i)+int(j))    Find the UTF-8 code-point (same as ASCII for the ASCII characters),
                      add the relevant key digit and convert back to character
''.join(...)         Concatenate the characters of the ciphertext
print(...+'\n'+k)    Add newline and key, then print to STDOUT

Try it on Ideone