Hi,
> I'm having problem with unwarpping/decoding/deciphering a file.
>
> Here's how the file is encrypted:
>
> CBC_Mode<AES>::Encryption encrAES;
> byte aesKey[32], aesIV[16];
> rng.GenerateBlock(aesKey, 32); rng.GenerateBlock(aesIV, 16);
> encrAES.SetKeyWithIV(aesKey, 32, aesIV);
>
> Base64Encoder *baseEnc = new Base64Encoder(new FileSink("encoded.txt"));
> StreamTransformationFilter aesEnc(encrAES, baseEnc);
> aesEnc.Put(plainText, sizeof(plainText));
> aesEnc.MessageEnd();
> ............repeat the last two multiple times...........
>
> Then I feed the data to aesEnc and it works fine.
>
> Now I'm trying to reverse this process, and unwrap the contents of that file
> record by record. So ideally I need something like
> FileSource(file)->Base64Decoder->Decryptor, from which I
> can Get() my data back.
>
> However I fail miserably in attempts to do something like:
>
> Base64Decoder *b6d = new
> Base64Decoder(new FileSource("encoded.txt", true));
> StreamTransformationFilter aesDec(b6d, decrAES);
>
> I'm sure you understand what it is that I'm trying to accomplish. It is
> clear that I haven't gotten the hang of filters and piping in Crypto++, so
> I'm not connecting/using the right pieces. What is the best way to do what
> I'm trying to achieve?
>
> In the above code, decrAES is decryptor that reverses encrAES (of course
> :-).
Think of a source as something that pulls from one place and puts data
into its attachment. Each attachment can then have an attachment that
it puts it output into. So suppose I have a file containing base64
encoded ciphertext. First, I set up a decryptor with the key and IV
needed to decrypt that file:
CBC_Mode<AES>::Decryption decryptor(key,AES::DEFAULT_KEYLENGTH,iv);
Then I can set up a filesource that reads from the encrypted file and
pumps data into its attachment, a base64 decoder. That pumps the
decoded data into *its* attachment, a StreamTransformationFilter
wrapping the decryptor above. The stream transformation filter pumps
*its* output into the buffered transformation of its choosing, which
might be a file sink, a string sink, another encoder/decoder, etc.
Here's a block that generates an AES key and IV, encrypts a string,
encodes the encrypted string, writes that to a file, reads the file
containing the encoded encrypted string, decodes and decrypts it, and
writes the recovered plaintext to a file. All error handling is
omitted here, and you (obviously) need some mechanism to transmit the
key and IV.
{
string plaintext("I think Smithers picked me because of my
motivational skills. Everyone says they have to work a lot harder when
I'm around.\n");
string encfile("encrypted.txt");
string decfile("decrypted.txt");
SecByteBlock key,iv;
key.New(AES::DEFAULT_KEYLENGTH);
iv.New(AES::BLOCKSIZE);
AutoSeededRandomPool rng;
rng.GenerateBlock(key,AES::DEFAULT_KEYLENGTH);
rng.GenerateBlock(iv,AES::BLOCKSIZE);
cout << "Plaintext: " << plaintext << endl;
CBC_Mode<AES>::Encryption encryptor(key,AES::DEFAULT_KEYLENGTH,iv);
StringSource incoming(plaintext, true,
new StreamTransformationFilter(
encryptor, new Base64Encoder(new
FileSink(encfile.c_str()))));
cout << "Encrypted data written to: " << encfile << endl;
CBC_Mode<AES>::Decryption decryptor(key,AES::DEFAULT_KEYLENGTH,iv);
FileSource outgoing(encfile.c_str(), true,
new Base64Decoder(new
StreamTransformationFilter(decryptor, new
FileSink(decfile.c_str()))));
cout << "Decrypted data written to: " << decfile << endl;
}
It's not intuitive at first, but once you get your mind around it it's
very easy to use. Anyone who thinks this is a decent example is free
to use it, post it to the FAQ or wiki, etc.
HTH,
Geoff
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