RE: [dev-crypto-csharp] Problems with the RSA decrypt: input data too large

[Herbert Falk]

Jim,

Thank you for your response.  I have gone back to the drawing board.

The code is now self-contained and is attached.

                        decrypted = cipherDec.ProcessBlock(adjusted, 0, 
adjusted.Length-1);

Is still throwing an exception even though I adjusted.  I have tried it with 
adjusted.Length and adjusted.Length-1.

So, to summarize, I have generated an RSA key pair, encrypted with the private 
key, and attempted to decrypt with the public key. What am I doing wrong.  This 
is BC 1.8.5 based.

The other question, for the future, is that I believe the Pkcs1Encoding Is NOT 
PCKS1 padding.  How should I approach an encrypt/decrypt with PKCS1 padding?

From: Jim Schaad <i...@augustcellars.com>
Sent: Saturday, September 5, 2020 4:08 PM
To: Herbert Falk <herb.f...@otb-consultingservices.com>; 
dev-crypto-csharp@bouncycastle.org
Subject: RE: [dev-crypto-csharp] Problems with the RSA decrypt: input data too 
large

Check to see if one of the items is zero prefixed.  Msft requires that all of 
the parameters be of the correct length but other libraries want to have 
unsigned numbers, thus the zero prefix.

From: Herbert Falk 
<herb.f...@otb-consultingservices.com<mailto:herb.f...@otb-consultingservices.com>>
Sent: Saturday, September 5, 2020 12:30 PM
To: 
dev-crypto-csharp@bouncycastle.org<mailto:dev-crypto-csharp@bouncycastle.org>
Subject: [dev-crypto-csharp] Problems with the RSA decrypt: input data too large

I have public and private key information from a self-signed certificate. I 
extract the RSA parameters using Microsoft cypto and then pass it to the code 
in the following function:

byte[] retValue = null;
            byte[] encrypted = null;

            IAsymmetricBlockCipher cipher = new Pkcs1Encoding(new RsaEngine());

            BigInteger modI = new BigInteger(1, rsaPubParmsMicrosoft.Modulus);
            BigInteger expI = new BigInteger(rsaPubParmsMicrosoft.Exponent);
            RsaKeyParameters rsaPublic = new RsaKeyParameters(false, modI, 
expI);

            modI = new BigInteger(1, rsaPrivateParmMicrosoft.Modulus);
            expI = new BigInteger(rsaPrivateParmMicrosoft.Exponent);
            RsaKeyParameters rsaPrivate = new RsaKeyParameters(true, modI, 
expI);
            byte[] data = { 0x01, 0x02, 0x03 };

            cipher.Init(true, rsaPrivate);
            try
            {
               encrypted = cipher.ProcessBlock(data, 0, data.Length);
            }
            catch(Exception ex)
            {
                string emsg = ex.ToString();
            }

            IAsymmetricBlockCipher cipher1 = new Pkcs1Encoding(new RsaEngine());
            cipher1.Init(false, rsaPublic);
            try
            {
                int blockLength = cipher1.GetInputBlockSize();
                retValue = cipher.ProcessBlock(encrypted, 0, blockLength);
            }
            catch(Exception ex)
            {
                string emsg = ex.ToString();
            }

The encryption works, however the decrypt throws the exception {"input data too 
large (Parameter 'inLen')"}.  The blocksize is: 0x00000100 And the size of 
encrypted is: {byte[0x00000100]}

Any ideas about why the exception is being thrown

            byte[] retValue = null;
            byte[] encrypted = null;

            RsaKeyPairGenerator rsaKeyPairGnr = new RsaKeyPairGenerator();
            rsaKeyPairGnr.Init(new 
Org.BouncyCastle.Crypto.KeyGenerationParameters(new SecureRandom(), 2048));
            Org.BouncyCastle.Crypto.AsymmetricCipherKeyPair keyPair = 
rsaKeyPairGnr.GenerateKeyPair();

            byte[] inputBytes = { 0x01, 0x02, 0x03 };
            RsaKeyParameters publicKey = (RsaKeyParameters)keyPair.Public;
            RsaKeyParameters privateKey = (RsaKeyParameters)keyPair.Private;

            try
            {
                //encrypt
                IAsymmetricBlockCipher cipherEnc = new Pkcs1Encoding(new 
RsaEngine());
                cipherEnc.Init(true, privateKey);
                //byte[] ciphered = cipher.ProcessBlock(inputBytes, 0, 
inputMessage.Length);
                byte[] ciphered = cipherEnc.ProcessBlock(inputBytes, 0, 
inputBytes.Length);

                //decrypte
                IAsymmetricBlockCipher cipherDec = new Pkcs1Encoding(new 
RsaEngine());
                cipherDec.Init(false, publicKey);
                //byte[] ciphered = cipher.ProcessBlock(inputBytes, 0, 
inputMessage.Length);
                byte[] decrypted = null;
                if (ciphered[0] != 0x0)
                {
                    byte[] adjusted = new byte[ciphered.Length + 1];
                    adjusted[0] = 0x0;
                    Buffer.BlockCopy(ciphered, 0, adjusted, 1, ciphered.Length);
                    decrypted = cipherDec.ProcessBlock(adjusted, 0, 
adjusted.Length-1);
                }
                else
                {
                    decrypted = cipherDec.ProcessBlock(inputBytes, 0, 
inputBytes.Length);
                }
            }
            catch(Exception ex)
            {
                string emsg = ex.ToString();
            }