Code i'm using (XE):

// Encrypt a string and return the Base64 encoded result
function Encrypt(DataToEncrypt: ansistring):ansistring;
const Key: Ansistring = 'keykey';
  KeySize = 32; // 32 bytes = 256 bits
  BlockSize = 16; // 16 bytes = 128 bits
var
  Cipher : TDCP_rijndael;
  Data: ansistring;
  IV: array[0..15] of byte;      // the initialization vector
  i:Integer;
begin
  // Pad Key, IV and Data with zeros as appropriate
  FillChar(IV,Sizeof(IV),0);            // make the IV all zeros

  Data := PadWithZeros(DataToEncrypt,BlockSize);

  for i := 0 to (Length(IV) - 1) do   //just random values for the IV
    IV[i] := Random(256);

  Cipher := TDCP_rijndael.Create(nil);

  if Length(Key) <= 16 then
    Cipher.Init(Key[1],128,@IV[1])
  else if Length(Key) <= 24 then
    Cipher.Init(开发者_运维百科Key[1],192,@IV[1])
  else
    Cipher.Init(Key[1],256,@IV[1]);
  // Encrypt the data
  Cipher.EncryptCBC(Data[1],Data[1],Length(Data));
  // Free the cipher and clear sensitive information
  Cipher.Free;

  SetString(InitializationVector,PAnsiChar(@IV[1]),Length(IV));  //Save IV
  InitializationVector := Base64EncodeStr(InitializationVector);

  //Base64 encoded result
  Result := Base64EncodeStr(Data);
end;

function Decrypt(IV,Cryptogram:ansistring):ansistring;
const Key: Ansistring = 'keykey';
  KeySize = 32; // 32 bytes = 256 bits
  BlockSize = 16; // 16 bytes = 128 bits
var
  Cipher : TDCP_rijndael;
begin
  if IV='' then
    IV := InitializationVector;

  Cryptogram := Base64DecodeStr(cryptogram);
  // Create the cipher and initialise according to the key length
  cipher := tdcp_rijndael.Create(nil);
  if Length(Key) <= 16 then
    Cipher.Init(Key[1],128,@IV[1])
  else if Length(Key) <= 24 then
    Cipher.Init(Key[1],192,@IV[1])
  else
    Cipher.Init(Key[1],256,@IV[1]);
  // Decrypt the data
  Cipher.DecryptCBC(cryptogram[1],cryptogram[1],Length(cryptogram));
  // Free the cipher and clear sensitive information
  Cipher.Free;
  // Display the result
  Result := cryptogram;
end;

It works pretty well, except when i try to decrypt the string, i get:

$C#$C'Ç'#$B'ÛW'#$1F'Ø‹™Ç'#$8D'Ž'#$8D'!‘mydata

so first few letters get very weird characters. rest of it is decrypted just fine! Found similar problem here, but no solution. Thanks in advance!

The first thing that stands out to me is that you are reading/writing past the end of IV. You declare it as [0..15] but access everything from element 1(!) onwards, both in Cipher.Init and SetString.