Check if something is base64_encoded?

[ahmedtheking]

How can I check is a string is base64_encoded?

[hakre]

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 ;)

--hakre

[ahmedtheking]

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?

[hakre]

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.

[ahmedtheking]

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

[hakre]

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 ;)

[msykes]

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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)

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I would like to make an ammendment to the topic:

[webmasterworld.com...]

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.

Marcus