![]() ![]() When you need to search for content that’s been Base-64 encoded, then, the solution is to generate the text at all possible three-byte offsets, and remove the characters that might be influenced by the context: content either preceding what you are looking for, or the content that follows. This is why the Base-64 encoding of Unicode content representing ASCII text has so many of the ‘A’ character: that is the Base-64 representation of a NULL byte. Unicode usually represents characters as two bytes (16 bits). All of these points still apply – but the bit patterns change. If additional content is added to the end of your string (i.e.: “Hello World!”), that additional content will influence both the padding bytes, as well as the character before them.Īnother major challenge is when the content is Unicode rather than ASCII. When final padding is added, you can’t just remove those “=” characters. It will use the “=” character to denote how many extra padding blocks were used: If your content isn’t evenly divisible by 24 bits, Base-64 encoding will pad the remainder with null bytes. So when we add a character to the beginning, we shift the whole bit pattern to the right and change the encoding of everything that follows!Īnother feature of Base-64 is padding. Here’s a graphical example from Wikipedia: It then encodes each of those 6 bits into the 64 characters that you know and love. When we’re encoding characters, Base-64 takes 3 characters (24 bits) and re-interprets them as 4 segments of 6 bits each. The main problem here is the way that Base-64 works. Adding a single character to the beginning changes almost everything: But what if “Hello World” is in the middle of a longer string? Can you still use ‘SGVobG8gV29fbGQ=’? It turns out, no. You might use PowerShell’s handy Base-64 classes to tell you what to search for: Pretend you’re looking for the string, “Hello World” in a log file or SIEM system, but you know that it’s been Base-64 encoded. Some tooling supports decoding of Base-64 automatically, but that requires some pretty detailed knowledge of where the Base-64 starts and stops. ![]() Wikipedia goes into the full details here. The basic idea behind Base-64 is that it takes arbitrary binary data and encodes it into 64 (naturally) ASCII characters that can be transmitted safely over any normal transmission channel. Base-64 is an incredibly common encoding format in malware, and all kinds of binary obfuscation tools alike. You might have run into situations in the past where you’re looking for some specific text or binary sequence, but that content is encoded with Base-64.
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