/// <summary>
/// Removes padding that was added to the unencrypted data prior to encryption.
/// </summary>
/// <param name="dataBytes">Data to have padding removed</param>
/// <param name="params">RSA Parameters used for padding computation</param>
/// <returns>Unpadded message</returns>
public byte[] DecodeMessage(byte[] dataBytes, RSAParameters @params)
{
m_k = @params.D.Length;
if (!(m_k == dataBytes.Length))
{
throw new CryptographicException("Bad Data.");
}
//Length of the datablock
int iDBLen = dataBytes.Length - m_hLen - 1;
//Starting index for the data block. This will be equal to m_hLen + 1. The
//index is zero based, and the dataBytes should start with a single leading byte,
//plus the maskedSeed (equal to hash length m_hLen).
int iDBidx = m_hLen + 1;
//Single byte for leading byte
byte bytY = 0;
//Byte array matching the length of the hashing algorithm.
//This array will hold the masked seed.
byte[] maskedSeed = new byte[m_hLen];
//Byte array matching the length of the following:
//Private Exponent D minus Hash Length, minus 1 (for the leading byte)
byte[] maskedDB = new byte[iDBLen];
//Copy the leading byte
bytY = dataBytes[0];
//Copy the mask
Array.Copy(dataBytes, 1, maskedSeed, 0, m_hLen);
//Copy the data block
Array.Copy(dataBytes, iDBidx, maskedDB, 0, iDBLen);
//Reproduce the seed mask from the masked data block using the mask generation function
byte[] seedMask = Mathematics.OAEPMGF(maskedDB, m_hLen, m_hLen, m_hashProvider);
//Reproduce the Seed from the Seed Mask.
byte[] seed = Mathematics.BitwiseXOR(maskedSeed, seedMask);
//Reproduce the data block bask from the seed using the mask generation function
byte[] dbMask = Mathematics.OAEPMGF(seed, m_k - m_hLen - 1, m_hLen, m_hashProvider);
//Reproduce the data block from the masked data block and the seed
byte[] dataBlock = Mathematics.BitwiseXOR(maskedDB, dbMask);
//Pull the message from the data block. First m_hLen bytes are the lHash,
//followed by padding of 0x00's, followed by a single 0x01, then the message.
//So we're going to start and index m_hLen and work forward.
if (!(dataBlock[m_hLen] == 0x00))
{
throw new CryptographicException("Decryption Error. Bad Data.");
}
//If we passed the 0x00 first byte test, iterate through the
//data block and find the terminating character.
int iDataIdx = 0;
for (int i = m_hLen; i <= dataBlock.Length - 1; i++)
{
if (dataBlock[i] == 0x01)
{
iDataIdx = i + 1;
break;
}
}
//Now find the length of the data and copy it to a byte array.
int iDataLen = dataBlock.Length - iDataIdx;
byte[] result = new byte[iDataLen];
Array.Copy(dataBlock, iDataIdx, result, 0, iDataLen);
return(result);
}
/// <summary>
/// Adds padding to the input data and returns the padded data.
/// </summary>
/// <param name="dataBytes">Data to be padded prior to encryption</param>
/// <param name="params">RSA Parameters used for padding computation</param>
/// <returns>Padded message</returns>
public byte[] EncodeMessage(byte[] dataBytes, RSAParameters @params)
{
//Iterator
int i = 0;
//Get the size of the data to be encrypted
m_mLen = dataBytes.Length;
//Get the size of the public modulus (will serve as max length for cipher text)
m_k = @params.N.Length;
if (m_mLen > GetMaxMessageLength(@params))
{
throw new CryptographicException("Bad Data.");
}
//Generate the random octet seed (same length as hash)
BigInteger biSeed = new BigInteger();
biSeed.genRandomBits(m_hLen * 8, new Random());
byte[] bytSeed = biSeed.getBytesRaw();
//Make sure all of the bytes are greater than 0.
for (i = 0; i <= bytSeed.Length - 1; i++)
{
if (bytSeed[i] == 0x00)
{
//Replacing with the prime byte 17, no real reason...just picked at random.
bytSeed[i] = 0x17;
}
}
//Mask the seed with MFG Function(SHA1 Hash)
//This is the mask to be XOR'd with the DataBlock below.
byte[] dbMask = Mathematics.OAEPMGF(bytSeed, m_k - m_hLen - 1, m_hLen, m_hashProvider);
//Compute the length needed for PS (zero padding) and
//fill a byte array to the computed length
int psLen = GetMaxMessageLength(@params) - m_mLen;
//Generate the SHA1 hash of an empty L (Label). Label is not used for this
//application of padding in the RSA specification.
byte[] lHash = m_hashProvider.ComputeHash(System.Text.Encoding.UTF8.GetBytes(string.Empty.ToCharArray()));
//Create a dataBlock which will later be masked. The
//data block includes the concatenated hash(L), PS,
//a 0x01 byte, and the message.
int dbLen = m_hLen + psLen + 1 + m_mLen;
byte[] dataBlock = new byte[dbLen];
int cPos = 0;
//Current position
//Add the L Hash to the data blcok
for (i = 0; i <= lHash.Length - 1; i++)
{
dataBlock[cPos] = lHash[i];
cPos += 1;
}
//Add the zero padding
for (i = 0; i <= psLen - 1; i++)
{
dataBlock[cPos] = 0x00;
cPos += 1;
}
//Add the 0x01 byte
dataBlock[cPos] = 0x01;
cPos += 1;
//Add the message
for (i = 0; i <= dataBytes.Length - 1; i++)
{
dataBlock[cPos] = dataBytes[i];
cPos += 1;
}
//Create the masked data block.
byte[] maskedDB = Mathematics.BitwiseXOR(dbMask, dataBlock);
//Create the seed mask
byte[] seedMask = Mathematics.OAEPMGF(maskedDB, m_hLen, m_hLen, m_hashProvider);
//Create the masked seed
byte[] maskedSeed = Mathematics.BitwiseXOR(bytSeed, seedMask);
//Create the resulting cipher - starting with a 0 byte.
byte[] result = new byte[@params.N.Length];
result[0] = 0x00;
//Add the masked seed
maskedSeed.CopyTo(result, 1);
//Add the masked data block
maskedDB.CopyTo(result, maskedSeed.Length + 1);
return(result);
}