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PHP Server Side Scripting Forum

Check if something is base64_encoded?

 10:13 am on Apr 26, 2006 (gmt 0)

How can I check is a string is base64_encoded?



 10:54 am on Apr 26, 2006 (gmt 0)

probably you can't. you can check if a string might be base64 encoded.

A 65-character subset of US-ASCII is used, enabling 6 bits to be represented per printable character. (The extra 65th character, "=", is used to signify a special processing function.)

from RFC 2045, Section 6.8 [faqs.org]

Table 1: The Base64 Alphabet
Value Encoding Value Encoding Value Encoding Value Encoding
0 A 17 R 34 i 51 z
1 B 18 S 35 j 52 0
2 C 19 T 36 k 53 1
3 D 20 U 37 l 54 2
4 E 21 V 38 m 55 3
5 F 22 W 39 n 56 4
6 G 23 X 40 o 57 5
7 H 24 Y 41 p 58 6
8 I 25 Z 42 q 59 7
9 J 26 a 43 r 60 8
10 K 27 b 44 s 61 9
11 L 28 c 45 t 62 +
12 M 29 d 46 u 63 /
13 N 30 e 47 v
14 O 31 f 48 w (pad) =
15 P 32 g 49 x
16 Q 33 h 50 y

as this table (taken from the RFC) shows, a base64 encoded string only contains A-Z, a-z. 0-9 and +, /, =, so you can express that with a regular expression quite easy: [A-Za-z0-9+/=]

if this expression matches, it means, that the provided string is base64 encoded, but it might be no guarantee. example for a non base64 encoded string:Hello - okay, just kidding a bit ;)



 11:02 am on Apr 26, 2006 (gmt 0)

Hmmm superlame. What about a function that maybe decodes an 'encoded' string? If you decode a non-encoded string, it comes up with all TM and C and R (trademark, copyright, registered logos) and stuff, not just normal text. So maybe, if we can check that the decoded data is a-z or 0-9 or -_ etc..., maybe?


 11:06 am on Apr 26, 2006 (gmt 0)

decode is no problem at all, take this: base64_decode() [php.net]. you can add some more decisions on the decoded text, that's right. if you even know what should be within the decoded data, then even better.


 11:16 am on Apr 26, 2006 (gmt 0)

Yeah I get what you mean, ok, I'll go ahead and build a function.


 11:27 am on Apr 26, 2006 (gmt 0)

beacause of the "might" be base64 encoded nature of such a string i have no general function in my libraries, sorry. i think there could be a general not_base64_encoded(string) function available maybe ;)


 7:38 pm on Jan 24, 2008 (gmt 0)

System: The following message was spliced on to this thread from: http://www.webmasterworld.com/php/3557138.htm [webmasterworld.com] by coopster - 7:30 pm on Jan. 24, 2008 (utc -6)

I would like to make an ammendment to the topic:


although hakre looks like he knows his stuff on the topic, I had to do some simple base64 checks and the proposed regular expression did not work. I think he missed a - somewhere in the list of accepted characters (which to be fair could easily be a typo ;) ).

below is a revised solution in PHP

preg_match('/^([A-Za-z0-9]\+\/\-\=)+$/', $string);

As hakre said though, this doesn't guarantee that it is base64 encoded, but it can check that JPEG image data has been encoded, for example.


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