I'm using python's base64 module and I get a string that can be encoded or not encoded. I would like to do something like:
if isEncoded(s):
output = base64.decodestring(s)
else:
output = s
ideas?
views:
153answers:
3
+3
A:
You could just try it, and see what happens:
import base64
def decode_if_necessary(s):
try:
return base64.decodestring(s)
except:
return s
But you have to ask yourself: what if the original message was in fact a syntactically valid base64 string, but not meant to be one? Then "decoding" it will succeed, but the result is not the required output. So I have to ask: is this really what you want?
Edit: Note that decodestring is deprecated.
Stephan202
2009-10-07 16:04:39
you're saying that if s isn't decoded, decodestring() raises an exception?
Guy
2009-10-07 16:05:51
He's saying that the chances of a string you want to use being validly base64 encoded are slim, and when you call `decodestring` on an invalidly base64 encoded string, `decodestring` raises an exception. This looks to me like a reasonable, simple approach. +1
Dominic Rodger
2009-10-07 16:08:07
I actually tried something like that and when the string, that was not decoded, did not throw an exception, I got gibrish.
Guy
2009-10-07 16:08:10
Then the input you supplied was in fact a valid base64 encoding. This demonstrates the issue at hand.
Stephan202
2009-10-07 16:09:49
+10
A:
In general, it's impossible; if you receive string 'MjMj', for example, how could you possibly know whether it's already decoded and needs to be used as is, or decoded into '23#'?
Alex Martelli
2009-10-07 16:07:00
exactly: you can only test that there aren't forbidden chars and that the length is divisible by four.
giorgian
2009-10-07 16:09:44
it's also worth noting that because it's impossible, attempting to decode it "when necessary" can be used as an attack vector for XSS and similar attacks by crafting seemingly-encoded data that your system does bad things with after its decoded.
rmeador
2009-10-07 16:54:50
A:
You could check to see if a string may be base64 encoded. In general, the function can predict with 75%+ accuracy is the data is encoded.
def isBase64(s):
return (len(s) % 4 == 0) and re.match('^[A-Za-z0-9+/]+[=]{0,2}$', s)
brianegge
2009-10-19 02:26:22