The function is performing a Caesar cipher with a shift of 20 on the input string.

[CODE]: 
```python
def caesar_cipher(input_string, shift=20):
    output_string = ""
    for char in input_string:
        if char.isalpha():
            shifted_code = ord(char) + shift
            if char.islower():
                new_char = chr((shifted_code - 97) % 26 + 97)
            else:
                new_char = chr((shifted_code - 65) % 26 + 65)
        else:
            new_char = char
        output_string += new_char
    return output_string
```