Determine if a string contains a base64 string inside of it
I'm trying to figure out a way to parse out a base64 string from with a larger string.
I have the string 开发者_运维问答"Hello <base64 content> World"
and I want to be able to parse out the base64 content and convert it back to a string. "Hello Awesome World"
Answers in C# preferred.
Edit: Updated with a more real example.
--abcdef
\n
Content-Type: Text/Plain;
Content-Transfer-Encoding: base64
\n
<base64 content>
\n
--abcdef--
This is taken from 1 sample. The problem is that the Content.... vary quite a bit from one record to the next.
There is no reliable way to do it. How would you know that, for instance, "Hello" is not a base64 string ? OK, it's a bad example because base64 is supposed to be padded so that the length is a multiple of 4, but what about "overflow" ? It's 8-character long, it is a valid base64 string (it would decode to "¢÷«~Z0"), even though it's obviously a normal word to a human reader. There's just no way you can tell for sure whether a word is a normal word or base64 encoded text.
The fact that you have base64 encoded text embedded in normal text is clearly a design mistake, I suggest you do something about it rather that trying to do something impossible...
In short form you could:
- split the string on any chars that are not valid base64 data or padding
- try to convert each token
- if the conversion succeeds, call replace on the original string to switch the token with the converted value
In code:
var delimiters = new char[] { /* non-base64 ASCII chars */ };
var possibles = value.Split(delimiters, StringSplitOptions.RemoveEmptyEntries);
//need to tweak to include padding chars in matches, but still split on padding?
//maybe better off creating a regex to match base64 + padding
//and using Regex.Split?
foreach(var match in possibles)
{
try
{
var converted = Convert.FromBase64String(match);
var text = System.Text.Encoding.UTF8.GetString(converted);
if(!string.IsNullOrEmpty(text))
{
value = value.Replace(match, text);
}
}
catch (System.ArgumentNullException)
{
//handle it
}
catch (System.FormatException)
{
//handle it
}
}
Without a delimiter though, you can end up converting non-base64 text that happens to be also be valid as base64 encoded text.
Looking at your example of trying to convert "Hello QXdlc29tZQ== World"
to "Hello Awesome World"
the above algorithm could easily generate something like "ée¡Ý•Í½µ”¢¹]"
by trying to convert the whole string from base64 since there is no delimiter between plain and encoded text.
Update (based on comments):
If there are no '\n'
s in the base64 content and it is always preceded by "Content-Transfer-Encoding: base64\n"
, then there is a way:
- split the string on
'\n'
- iterate over all the tokens until a token ends in
"Content-Transfer-Encoding: base64"
- the next token (if there are any) should be decoded (if possible) and then the replacement should be made in the original string
- return to iterating until out of tokens
In code:
private string ConvertMixedUpTextAndBase64(string value)
{
var delimiters = new char[] { '\n' };
var possibles = value.Split(delimiters,
StringSplitOptions.RemoveEmptyEntries);
for (int i = 0; i < possibles.Length - 1; i++)
{
if (possibles[i].EndsWith("Content-Transfer-Encoding: base64"))
{
var nextTokenPlain = DecodeBase64(possibles[i + 1]);
if (!string.IsNullOrEmpty(nextTokenPlain))
{
value = value.Replace(possibles[i + 1], nextTokenPlain);
i++;
}
}
}
return value;
}
private string DecodeBase64(string text)
{
string result = null;
try
{
var converted = Convert.FromBase64String(text);
result = System.Text.Encoding.UTF8.GetString(converted);
}
catch (System.ArgumentNullException)
{
//handle it
}
catch (System.FormatException)
{
//handle it
}
return result;
}
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