How to detect if a string is Base64Encoded or not? - delphi

Question

Which is the best method to detect if a string is Base64Encoded or not (using Delphi)?

Answer 1

Best you can do is try to decode it. If the decode fails then the input was not base64 encoded. It the string successfully decodes then the input might have been base64 encoded.

Answer 2

You can check if the string only contains Base64 valids chars
function StringIsBase64(const InputString : String ) : Boolean;
const
Base64Chars: Set of AnsiChar = ['A'..'Z','a'..'z','0'..'9','+','/','='];
var
i : integer;
begin
Result:=True;
for i:=1 to Length(InputString) do
{$IFDEF UNICODE}
if not CharInSet(InputString[i],Base64Chars) then
{$ELSE}
if not (InputString[i] in Base64Chars) then
{$ENDIF}
begin
Result:=False;
break;
end;
end;
The = char is used for padding so you can add an aditional valiation to the function for padded base64 strings checking if the length of the string is mod 4

Answer 3

In addition to RRUZ answer you can also check the length of the string (is it a multiple of 4).
function IsValidBase64(const aValue: string): Boolean;
var
i: Integer;
lValidChars: set of Char;
begin
Result := aValue <> '';
lValidChars := ['a'..'z', 'A'..'Z', '0'..'9', '/', '+'];
//length of string should be multiple of 4
if Length(aValue) mod 4 > 0 then
Result := False
else
for i := 1 to Length(aValue) do
begin
if aValue[i] = '=' then
begin
if i < Length(aValue) - 1 then
begin
Result := False;
Exit;
end
else
lValidChars := ['='];
end
else if not (aValue[i] in lValidChars) then
begin
Result := False;
Break;
end;
end;
end;
Please note that this code is Delphi 7 code and not adjusted for Unicode use.

Answer 4

As was already told here, there is no reliable verification if a certain string is Base64 encoded or not, so even when you consider the input as a valid Base64 encoded string, it doesn't mean the string is actually encoded that way. I'm posting here just another version of a validation function, which according to RFC 4648 verifies:
if the input string is not empty and its length is multiple of 4
if the input string contains at most two padding characters and only at the end of the string
if the input string contains only characters from the Base64 alphabet (see the Page 5, Table 1)
function IsValidBase64EncodedString(const AValue: string): Boolean;
const
Base64Alphabet = ['A'..'Z', 'a'..'z', '0'..'9', '+', '/'];
var
I: Integer;
ValLen: Integer;
begin
ValLen := Length(AValue);
Result := (ValLen > 0) and (ValLen mod 4 = 0);
if Result then
begin
while (AValue[ValLen] = '=') and (ValLen > Length(AValue) - 2) do
Dec(ValLen);
for I := ValLen downto 1 do
if not (AValue[I] in Base64Alphabet) then
begin
Result := False;
Break;
end;
end;
end;