Procedure RSADecrypt

Code (Delphi)

procedure RSADecrypt(

const AKey: TRSAKey;

const AEncryptedData: Pointer;

out AData: Pointer;

var ADataSize: Cardinal

); overload;

function RSADecrypt(

const AKey: TRSAKey;

const AEncryptedData: RawByteString

): RawByteString; overload;

procedure RSADecrypt(

const AKey: TRSAKey;

var AInPlaceData: TEncryptBuffer

); overload;

Important!

Asymmetric chipers are slow. Therefore, they are not designed to encrypt/decrypt large amount of data. The usual approach is to use asymmetric chiper to encrypt/decrypt another key - a key for some symmetric cipher, and then use (fast) symmetric cipher to encrypt/decrypt large amounts of data.

Tip

You always use public key for encryption and private key for decryption with asymmetric cipher - and not the other way around. You should use digital signing and verification for reversed order. I.e. you sign data by using your private key and verify signature by using your public key.

Code (Delphi)

// This example decrypts file encrypted in code example for RSAEncrypt

// The source file contains encrypted (random) symmetric key in

// the beginning of the file, followed by encrypted data.

var

// (Random) symmetric key to decrypt file

TEAKey: TTEAKey;

// Asymmetric key to encrypt symmetric key

RSAKey: TRSAKey;

// Encrypted symmetric key

EncryptedKey: TEncryptBuffer;

// File/data to decrypt

Data: TMemoryStream;

// Wrapper for "in place" decryption

DataToDecrypt: TEncryptBuffer;

// Stream to read encrypted data

FS: TFileStream;

begin

FS := TFileStream.Create('C:\Test\Text.bin', fmOpenRead or fmShareDenyWrite);

try

try

// First, read encrypted (random) symmetric key from source

FS.ReadBuffer(EncryptedKey.cbData, SizeOf(EncryptedKey.cbData));

EncryptedKey.pbData := AllocMem(EncryptedKey.cbData);

try

 FS.ReadBuffer(EncryptedKey.pbData^, EncryptedKey.cbData);

 // Decrypt symmetric key with asymmetric (private) key

 RSAKey := RSALoadPrivateKey('C:\Private.pem', rsText, 'Private key password (if any set)');

 try

 EEncrypt.RSADecrypt(RSAKey, EncryptedKey);

 finally

 RSADestroyKey(RSAKey);

 end;

 // Copy unencrypted buffer into actual key

 Assert(EncryptedKey.cbData = SizeOf(TEAKey));

 Move(EncryptedKey.pbData^, TEAKey, SizeOf(TEAKey));

finally

 SecureFree(EncryptedKey);

end;

// Now we have symmetric key to decrypt actual file's content (data)

Data := TMemoryStream.Create;

try

 // The rest of the file is the actual data to decrypt

 Data.CopyFrom(FS, FS.Size - FS.Position);

 // Decrypt Data in place

 DataToDecrypt.pbData := Data.Memory;

 DataToDecrypt.cbData := Data.Size;

 // This does not alter Data.Memory and Data.Size

 // It simply changes content of Data.Memory^

 TEADecrypt(TEAKey, DataToDecrypt);

 // Symmetric key is no longer needed

 TEADestroyKey(TEAKey); // - same as SecureFree(TEAKey);

 // File/data is decrypted - save final result into file

 Data.SaveToFile('C:\Text.txt');

finally

 SecureFree(Data);

end;

finally

TEADestroyKey(TEAKey); // - same as SecureFree(TEAKey);

end;

finally

FreeAndNil(FS);

end;

end;