Cannot perform RSA Encrption / Decryption using Crypto ++ (isValidCoding is false)
I am using Crypto ++ to encrypt a byte array using RSA. I have followed the Crypto ++ wiki examples and you have no luck getting them to work. Encryption and decryption in all samples is done in the same process, but I am trying to decrypt content that is already encrypted in another process. Here is my code:
class FixedRNG : public CryptoPP::RandomNumberGenerator
{
public:
FixedRNG(CryptoPP::BufferedTransformation &source) : m_source(source) {}
void GenerateBlock(byte *output, size_t size)
{
m_source.Get(output, size);
}
private:
CryptoPP::BufferedTransformation &m_source;
};
uint16_t Encrypt()
{
byte *oaepSeed = new byte[2048];
for (int i = 0; i < 2048; i++)
{
oaepSeed[i] = (byte)i;
}
CryptoPP::ByteQueue bq;
bq.Put(oaepSeed, 2048);
FixedRNG prng(bq);
Integer n("Value of N"),
e("11H"),
d("Value of D");
RSA::PrivateKey privKey;
privKey.Initialize(n, e, d);
RSA::PublicKey pubKey(privKey);
CryptoPP::RSAES_OAEP_SHA_Encryptor encryptor( pubKey );
assert( 0 != encryptor.FixedMaxPlaintextLength() );
byte blockSize = encryptor.FixedMaxPlaintextLength();
int divisionCount = fileSize / blockSize;
int proccessedBytes = 0;
// Create cipher text space
uint16_t cipherSize = encryptor.CiphertextLength( blockSize );
assert( 0 != cipherSize );
encryptor.Encrypt(prng, (byte*)plaintext, blockSize, (byte*)output);
return cipherSize;
}
void Decrypt(uint16_t cipherSize)
{
byte *oaepSeed = new byte[2048];
for (int i = 0; i < 2048; i++)
{
oaepSeed[i] = (byte)i;
}
CryptoPP::ByteQueue bq;
bq.Put(oaepSeed, 2048);
FixedRNG prng(bq);
Integer n("Value of N"),
e("11H"),
d("Value of D");
RSA::PrivateKey privKey;
privKey.Initialize(n, e, d);
//RSA::PublicKey pubKey(privKey);
CryptoPP::RSAES_OAEP_SHA_Decryptor decryptor( privKey );
byte blockSize = decryptor.FixedMaxPlaintextLength();
assert(blockSize != 0);
size_t maxPlainTextSize = decryptor.MaxPlaintextLength( cipherSize );
assert( 0 != maxPlainTextSize );
void* subBuffer = malloc(maxPlainTextSize);
CryptoPP::DecodingResult result = decryptor.Decrypt(prng, (byte*)cipherText, cipherSize, (byte*)subBuffer);
assert( result.isValidCoding );
assert( result.messageLength <= maxPlainTextSize );
}
Unfortunately, the isValidCoding value is false. I think I don't understand something about RSA encryption / decryption !!
Note that privKey and pubKey were validated using KEY.Validate (prng, 3). I've also tried using RAW RSA instead of OAEP and SHA with no luck. I tried to debug the crypto ++ code, which I am suspicious of the prng variable . I think there is something wrong with that. I also used AutoSeededRandomPool instead of FixedRNG, but that didn't help.
It's worth knowing that if I copy the decryption code right after the encryption code and execute it in the Encrypt () method, everything is fine and isValidCoding is correct !!
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This may not be correct:
byte blockSize = encryptor.FixedMaxPlaintextLength();
...
encryptor.Encrypt(prng, (byte*)plaintext, blockSize, (byte*)output);
return cipherSize;
Try:
size_t maxLength = encryptor.FixedMaxPlaintextLength();
size_t cipherLength = encryptor.CiphertextLength( blockSize );
...
SecureByteBlock secBlock(cipherLength);
cipherLength = encryptor.Encrypt(prng, (byte*)plaintext, blockSize, secBlock);
secBlock.resize(cipherLength);
FixedMaxPlaintextLength
returns a size_t
, not byte
.
You should probably be calling CiphertextLength
on plaintext
.
I'm not sure how you just return uint_t
from encrypt()
.
You can do it better by starting from scratch and using the example from Crypto ++ as a starting point. I'm not sure if this design is worth it.
If you're starting over, the Shoup Elliptic Curve (ECIES) integrated encryption scheme is a good choice as it combines the public key with symmetric ciphers and authentication tags.
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