Fixed-digit base64 encode and decode in Python

Question

I'm trying to encode and decode a base64 string. It works fine normally, but if I try to restrict the hash to 6 digits, I get an error on decoding:

from base64 import b64encode
from base64 import b64decode

s="something"

base 64 encode/decode:

# Encode:
hash = b64encode(s)
# Decode:
dehash = b64decode(hash)
print dehash

(works)

6-digit base 64 encode/decode:

# Encode:
hash = b64encode(s)[:6]
# Decode:
dehash = b64decode(hash)
print dehash

TypeError: Incorrect padding

What am I doing wrong?

UPDATE:

Based on Mark's answer, I added padding to the 6-digit hash to make it divisible by 4:

hash = hash += "=="

But now the decode result = "some"

UPDATE 2

Wow that was stupid ..

Why does the updated result surprise you? Are you expecting to get the entire original string somehow magically/fractally encoded to 6 bytes? — Wooble
– Wooble, Commented Jan 13, 2012 at 21:02
oh man- im an idiot - trying to encode a longer string into a shorter hash... sorry for wasting your guys time — Yarin
– Yarin, Commented Jan 13, 2012 at 21:12

Mark Ransom · Accepted Answer · 2012-01-13 20:35:41Z

4

Base64 by definition requires padding on the input if it does not decode into an integral number of bytes on the output. Every 4 base64 characters gets turned into 3 bytes. Your input length does not divide evenly by 4, thus there's an error.

Wikipedia has a good description of the specifics of Base64.

answered Jan 13, 2012 at 20:35

Mark Ransom

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