Windows Crypto Api bad length

Question

I found a simple unit for generating a RSA KeyPair and encrypting / decrypting. It works fine on small strings but on larger text I get error : Bad Length.

What could be the cause , here is the unit, the unit wcrypt2 you can download here

Tested on Delphi 2010

unit Crypt_RSA;

interface

uses
  Windows, Classes, SysUtils, WCrypt2;

function RSAGenerateKeys(var PrivateKey, PublicKey: String): Boolean;
function RSAEncrypt(Source, Key: String): String;
function RSADecrypt(Source, Key: String): String;

implementation

function RSAGenerateKeys(var PrivateKey, PublicKey: String): Boolean;
const
  RSA1024BIT_KEY = $04000000;

var
  RSA: HCRYPTPROV;
  HKeyPair: HCRYPTKEY;
  Pair: TStringStream;
  buflen: DWORD;

  function SetKey(BlobDef: Cardinal; var Key: String): Boolean;
  begin
    Result := Bool(CryptExportKey(HKeyPair, 0, BlobDef, 0, nil, @buflen));
    if Result then
    begin
      Pair.SetSize(buflen);
      Result := Bool(CryptExportKey(HKeyPair, 0, BlobDef, 0, PByte(Pair.Memory), @buflen));
    end;

    Key := Pair.ReadString(buflen);
    Pair.Seek(0, soBeginning);
  end;

begin
  Pair := TStringStream.Create;

  Result := Bool(CryptAcquireContext(@RSA, nil, nil, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT));
  if Result then
    Result := Bool(CryptGenKey(RSA, AT_KEYEXCHANGE, RSA1024BIT_KEY or CRYPT_EXPORTABLE, @HKeyPair));

  if Result then
    Result := SetKey(PRIVATEKEYBLOB, PrivateKey);
  if Result then
    Result := SetKey(PUBLICKEYBLOB, PublicKey);

  CryptDestroyKey(HKeyPair);
  CryptReleaseContext(RSA, 0);
  FreeAndNil(Pair);
end;

function RSAEncrypt(Source, Key: String): String;
var
  KeyPair: TStringStream;
  RSA: HCRYPTPROV;
  HPair: HCRYPTKEY;
  DDataSize, EDataSize: DWORD;

begin
  Result := '';
  KeyPair := TStringStream.Create(Key);
  if CryptAcquireContext(@RSA, nil, nil, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT) then
  try
    if CryptImportKey(RSA, PByte(KeyPair.Memory), KeyPair.Size, 0, 0, @HPair) then
    try
      EDataSize := SizeOf(Source);
      if CryptEncrypt(HPair, 0, true, 0, nil, @EDataSize, 0) then
      begin
        Result := Source;
        SetLength(Result, EDataSize);
        DDataSize := Length(Source) * SizeOf(Char);
        if not(CryptEncrypt(HPair, 0, True, 0, PByte(PChar(Result)), @DDataSize, EDataSize)) then
          Result := '';
      end;
    finally
      CryptDestroyKey(HPair);
    end;
  finally
  CryptReleaseContext(RSA, 0);
  end;
  FreeAndNil(KeyPair);
end;

function RSADecrypt(Source, Key: String): String;
var
  KeyPair: TStringStream;
  RSA: HCRYPTPROV;
  HPair: HCRYPTKEY;
  EDataSize: DWORD;

begin
  KeyPair := TStringStream.Create(Key);
  Result := '';
  if CryptAcquireContext(@RSA, nil, nil, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT) then
  try
    if CryptImportKey(RSA, PByte(KeyPair.Memory), KeyPair.Size, 0, 0, @HPair) then
    try
      Result := Source;
      EDataSize := Length(Result);
      if not Bool(CryptDecrypt(HPair, 0, True, 0, PByte(PChar(Result)), @EDataSize)) then
        EDataSize := 0;
      SetLength(Result, EDataSize div SizeOf(Char));
    finally
      CryptDestroyKey(HPair);
    end;
  finally
    CryptReleaseContext(RSA, 0);
  end;
  FreeAndNil(KeyPair);
end;

end.

Answer 1

C# in a Nutshell says that plaintext much longer than 1/2 the key size will cause an error. It's likely the same for most languages for security purposes. For longer messages, use RSA to pass a key for symmetrical encryption such as AES.