...
CryptGenKey(FProvider, AT_KEYEXCHANGE, 1024 shl 16, FUserKeyPair); 
CryptExportKey(FUserKeyPair, 0, PUBLICKEYBLOB, 0, @Key[0], BlobSize);
...

  procedure ExtractPublicKey(Blob: PByte; BlobSize: Integer; E, N: TBigNum);
  var
    PubKeyHdr: TPublicKeyStruc;
    RsaPubKey: TRsaPubKey;
    ModSize: Integer;
  begin
    if BlobSize < SizeOf(RsaPubKey) + SizeOf(PubKeyHdr) then
      raise Exception.Create('Invalid size of PUBLICKEYBLOB.');
    Move(Blob^, PubKeyHdr, SizeOf(PubKeyHdr));
    Inc(Blob, SizeOf(PubKeyHdr));
    Move(Blob^, RsaPubKey, SizeOf(RsaPubKey));
    Inc(Blob, SizeOf(RsaPubKey));
    ModSize := RsaPubKey.bitlen shr 3;
    if ModSize > BlobSize - SizeOf(RsaPubKey) - SizeOf(PubKeyHdr) then
      raise Exception.Create('Invalid size of PUBLICKEYBLOB.');
    E.CopyDigit(RsaPubKey.pubexp);
    N.CopyBytes(Blob, ModSize);
  end;

  function PKCS1Padding(Data: Pointer; Size: Integer; BitLen: Integer): TBytes;
  var
    Bound: Integer;
    Index: Integer;
  begin
    if (BitLen mod 8 > 0) or (Size + 11 > BitLen shr 3) then
      raise Exception.Create('Invalid bit length or data is too large.');
    Bound := BitLen shr 3;
    SetLength(Result, Bound);
    PWord(Result)^ := $0200;
    Dec(Bound, Size);
    for Index := 2 to Bound - 1 do
      Result[Index] := 1 + Byte(Random(255));
    Result[Bound] := $00;
    Move(Data^, Result[Bound + 1], Size);
  end;

var
  Message: string;
  PubKey: TBytes;
  Binary: TBytes;
  MsgNum: TBigNum;
...
  CAPI.PublicKeyExport(PubKey); // CAPI - самописная обёртка над CryptoAPI
  // RSA - класс RSA из библиотеки uCrypto: https://github.com/arkusuma/uCrypto
  ExtractPublicKey(PByte(PubKey), Length(PubKey), RSA.E, RSA.N); 
  Binary := PKCS1Padding(PByte(Message), Length(Message) * SizeOf(Char), 1024);
  MsgNum := TBigNum.CreateBytes(PByte(Binary), Length(Binary));
  try
    Assert(MsgNum.Compare(RSA.N) < 0, 'M must be in range of 0..N-1');
    RSA.Encrypt(MsgNum); // inline-функция для вызова MsgNum.PowMod(E, N);
    ... 

  ...
  if not CryptDecrypt(FUserKeyPair, 0, TRUE, 0, Data, DataSize) then
    RaiseLastOSErrorEx(sFailedDecryption);
  ...

MsgNum := TBigNum.CreateBytesBE(PByte(Binary), Length(Binary));