JPK - CryptoAPI RSA 256/ECB/PKCS#1 one time secret password encryption with public key from certificate (solution)

Polish government (MF - Ministry of Finance) recently implemented SAF-T (Standard Audit File-Tax / pl: JPK - Jednolity Plik Kontrolny). I had much trouble to implement one of the key parts of this solution in the right way. This part is to encrypt user generated password used to encrypt files, sended to Azure Cloud Storage, with RSA 256/ECB/PKCS#1 algorithm from MS CryptoAPI, using public key loaded from certificate file served by the MF.

My working solution to this is (uses JEDI API Library & Security Code Library):

unit CryptoAPI_RSA;
// The MIT License (MIT)
// 
// Copyright (c) 2016 Grzegorz Molenda
// 
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
interface

uses
  SysUtils,
  Classes;

function CryptoAPI_Encrypt_RSA(const Input: TBytes; const cert: TMemoryStream): String;

implementation

uses
  Windows,
  StrUtils,
  JwaWinCrypt,
  JwaWinError,
  EncdDecd;

type
  ERSAEncryptionError = class(Exception);

function WinError(const RetVal: BOOL; const FuncName: String): BOOL;
var
  dwResult: Integer;
begin
  Result:=RetVal;
  if not RetVal then begin
    dwResult:=GetLastError();
    raise ERSAEncryptionError.CreateFmt('Error [x%x]: %s failed.'#13#10'%s', [dwResult, FuncName, SysErrorMessage(dwResult)]);
  end;
end;

procedure reverse(var p: TBytes; len: Integer);
var
  i, j: Integer;
  temp: Byte;
begin
  i:=0;
  j:=len - 1;
  while i < j do begin
    temp:=p[i];
    p[i]:=p[j];
    p[j]:=temp;
    Inc(i);
    Dec(j);
  end;
end;

function CryptoAPI_Encrypt_RSA(const Input: TBytes; const cert: TMemoryStream): String;
var
  derCert: AnsiString;
  derCertLen: Cardinal;
  hProv: HCRYPTPROV;
  certContext: PCCERT_CONTEXT;
  certPubKey: HCRYPTKEY;
  len: LongWord;
  rsa: TBytes;
  ins: TMemoryStream;
  ous: TStringStream;
begin
  Result:='';
  if (cert <> Nil) and (cert.Size > 0) then begin
    SetLength(derCert, 4096);
    FillChar(derCert[1], 4096, 0);
    // Convert from PEM format to DER format - removes header and footer and decodes from base64
    WinError(CryptStringToBinaryA(PAnsiChar(cert.Memory), cert.Size, CRYPT_STRING_BASE64HEADER, @derCert[1], derCertLen, Nil), 'CryptStringToBinaryA');
    SetLength(derCert, derCertLen);
    try
      // Get the certificate context structure from a certificate.
      certContext:=CertCreateCertificateContext(X509_ASN_ENCODING or PKCS_7_ASN_ENCODING, @derCert[1], derCertLen);
      WinError(certContext <> Nil, 'CertCreateCertificateContext');
      try
        hProv:=0;
        WinError(CryptAcquireContext(hProv, Nil, Nil, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT), 'CryptAcquireContext'); // flag CRYPT_VERIFYCONTEXT - for backward compatibility with win2003server (and probably with win10pro+)
        try
          // Get the public key information for the certificate.
          certPubKey:=0;
          WinError(CryptImportPublicKeyInfo(hProv, X509_ASN_ENCODING or PKCS_7_ASN_ENCODING,
                                            @certContext.pCertInfo.SubjectPublicKeyInfo, certPubKey), 'CryptImportPublicKeyInfo');
          len:=Length(Input);
          if len > 0 then begin
            SetLength(rsa, len + 512);
            FillChar(rsa, len + 512, 0);
            try
              CopyMemory(@rsa[0], @Input[0], len);
              // encrypt our Input buffer
              WinError(CryptEncrypt(certPubKey, 0, True, 0, @rsa[0], len, len + 512), 'CryptEncrypt');
              SetLength(rsa, len);
              // IMPORTANT !!!
              // .Net RSA algorithm is BIG-ENDIAN,
              // CryptoAPI is LITTLE-ENDIAN, 
              // so reverse output before sending to Azure Cloud Storage
              reverse(rsa, len);
              ins:=TMemoryStream.Create;
              try
                ins.Write(rsa[0], len);
                ins.Position:=0;
                ous:=TStringStream.Create;
                try
                  EncodeStream(ins, ous);
                  ous.Position:=0;
                  Result:=ous.DataString;
                  Result:=ReplaceStr(Result, #13#10, '');
                finally
                  ous.Free;
                end;
              finally
                ins.Free;
              end;
            finally
              SetLength(rsa, 0);
            end;
          end;
        finally
          WinError(CryptReleaseContext(hProv, 0), 'CryptReleaseContext');
        end;
      finally
        CertFreeCertificateContext(certContext);
      end;
    finally
      SetLength(derCert, 0);
    end;
  end;
end;

end.

Usage is:

var
  cf: TMemoryStream;
  input: TBytes;
  output: String;
begin
  if Edit1.Text = '' then
    Exit;
  Memo1.Clear;
  cf:=TMemoryStream.Create;
  try
    cf.LoadFromFile('cert.pem'); // certificate with public key
    input:=TEncoding.Default.GetBytes(Edit1.Text);
    try
      output:=CryptoAPI_Encrypt_RSA(input, cf);
    finally
      SetLength(input, 0);
    end;
    Memo1.Lines.Text:=output;
  finally
    cf.Free;
  end;
end;

Hope that this might help someone, see ya.