I know the main answer I am likely to get is why the hell would you want to do that?!
Unfortunately despite my protests I have to do it, even though I know it makes little sense.
I have functions written in .Net to decrypt using a private key, encrypt using a public key.
I also RSA sign and verify and have a reasonable understanding of how this all work I think.
I am now being sent a value that is RSA encrypted using a private key which I am supposed to derive a usable value by decrypting using the public key.
I can’t seem to figure out how to do this. Am I being an idiot? Is this a normal thing to do?
I am told by the person sending me the value that this is no problem in PHP. I don’t know and haven’t used PHP yet. I can’t find a library to do it in any of the main languages I know i.e. C++, Java, C#. The server I am working on uses .Net.
I am hoping someone might be able help me.
It would be great if there is some kind of reasonable solution besides begging them to change what they are doing.
This is my method (updated from my previous bad one as pointed out by Iridium) but when I try to decrypt the value I get an exception
“Error occurred while decoding OAEP padding.”
If I use rsa.Decrypt(bytes, false) I get a bad key exception.
public static string DecryptUsingPublic(string dataEncrypted, string publicKey)
{
if (dataEncrypted == null) throw new ArgumentNullException("dataEncrypted");
if (publicKey == null) throw new ArgumentNullException("publicKey");
try
{
RSAParameters _publicKey = LoadRsaPublicKey(publicKey, false);
RSACryptoServiceProvider rsa = InitRSAProvider(_publicKey);
byte[] bytes = Convert.FromBase64String(dataEncrypted);
byte[] decryptedBytes = rsa.Decrypt(bytes, true);
ArrayList arrayList = new ArrayList();
arrayList.AddRange(decryptedBytes);
return Encoding.UTF8.GetString(decryptedBytes);
}
catch
{
return null;
}
}
private static RSAParameters LoadRsaPublicKey(String publicKeyFilePath, Boolean isFile)
{
RSAParameters RSAKeyInfo = new RSAParameters();
byte[] pubkey = ReadFileKey(publicKeyFilePath, "PUBLIC KEY", isFile);
byte[] SeqOID = { 0x30, 0x0D, 0x06, 0x09, 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x01, 0x01, 0x01, 0x05, 0x00 };
byte[] seq = new byte[15];
// --------- Set up stream to read the asn.1 encoded SubjectPublicKeyInfo blob ------
MemoryStream mem = new MemoryStream(pubkey);
BinaryReader binr = new BinaryReader(mem); //wrap Memory Stream with BinaryReader for easy reading
byte bt = 0;
ushort twobytes = 0;
try
{
twobytes = binr.ReadUInt16();
if (twobytes == 0x8130) //data read as little endian order (actual data order for Sequence is 30 81)
binr.ReadByte(); //advance 1 byte
else if (twobytes == 0x8230)
binr.ReadInt16(); //advance 2 bytes
else
return RSAKeyInfo;
seq = binr.ReadBytes(15); //read the Sequence OID
if (!CompareBytearrays(seq, SeqOID)) //make sure Sequence for OID is correct
return RSAKeyInfo;
twobytes = binr.ReadUInt16();
if (twobytes == 0x8103) //data read as little endian order (actual data order for Bit String is 03 81)
binr.ReadByte(); //advance 1 byte
else if (twobytes == 0x8203)
binr.ReadInt16(); //advance 2 bytes
else
return RSAKeyInfo;
bt = binr.ReadByte();
if (bt != 0x00) //expect null byte next
return RSAKeyInfo;
twobytes = binr.ReadUInt16();
if (twobytes == 0x8130) //data read as little endian order (actual data order for Sequence is 30 81)
binr.ReadByte(); //advance 1 byte
else if (twobytes == 0x8230)
binr.ReadInt16(); //advance 2 bytes
else
return RSAKeyInfo;
twobytes = binr.ReadUInt16();
byte lowbyte = 0x00;
byte highbyte = 0x00;
if (twobytes == 0x8102) //data read as little endian order (actual data order for Integer is 02 81)
lowbyte = binr.ReadByte(); // read next bytes which is bytes in modulus
else if (twobytes == 0x8202)
{
highbyte = binr.ReadByte(); //advance 2 bytes
lowbyte = binr.ReadByte();
}
else
return RSAKeyInfo;
byte[] modint = { lowbyte, highbyte, 0x00, 0x00 }; //reverse byte order since asn.1 key uses big endian order
int modsize = BitConverter.ToInt32(modint, 0);
byte firstbyte = binr.ReadByte();
binr.BaseStream.Seek(-1, SeekOrigin.Current);
if (firstbyte == 0x00)
{ //if first byte (highest order) of modulus is zero, don't include it
binr.ReadByte(); //skip this null byte
modsize -= 1; //reduce modulus buffer size by 1
}
byte[] modulus = binr.ReadBytes(modsize); //read the modulus bytes
if (binr.ReadByte() != 0x02) //expect an Integer for the exponent data
return RSAKeyInfo;
int expbytes = (int)binr.ReadByte(); // should only need one byte for actual exponent data (for all useful values)
byte[] exponent = binr.ReadBytes(expbytes);
RSAKeyInfo.Modulus = modulus;
RSAKeyInfo.Exponent = exponent;
return RSAKeyInfo;
}
catch (Exception)
{
return RSAKeyInfo;
}
finally { binr.Close(); }
//return RSAparams;
}
private static RSACryptoServiceProvider InitRSAProvider(RSAParameters rsaParam)
{
//
// Initailize the CSP
// Supresses creation of a new key
//
CspParameters csp = new CspParameters();
//csp.KeyContainerName = "RSA Test (OK to Delete)";
const int PROV_RSA_FULL = 1;
csp.ProviderType = PROV_RSA_FULL;
const int AT_KEYEXCHANGE = 1;
// const int AT_SIGNATURE = 2;
csp.KeyNumber = AT_KEYEXCHANGE;
//
// Initialize the Provider
//
RSACryptoServiceProvider rsa =
new RSACryptoServiceProvider(csp);
rsa.PersistKeyInCsp = false;
//
// The moment of truth...
//
rsa.ImportParameters(rsaParam);
return rsa;
}
private static int GetIntegerSize(BinaryReader binr)
{
byte bt = 0;
byte lowbyte = 0x00;
byte highbyte = 0x00;
int count = 0;
bt = binr.ReadByte();
if (bt != 0x02) //expect integer
return 0;
bt = binr.ReadByte();
if (bt == 0x81)
count = binr.ReadByte(); // data size in next byte
else
if (bt == 0x82)
{
highbyte = binr.ReadByte(); // data size in next 2 bytes
lowbyte = binr.ReadByte();
byte[] modint = { lowbyte, highbyte, 0x00, 0x00 };
count = BitConverter.ToInt32(modint, 0);
}
else
{
count = bt; // we already have the data size
}
while (binr.ReadByte() == 0x00)
{ //remove high order zeros in data
count -= 1;
}
binr.BaseStream.Seek(-1, SeekOrigin.Current); //last ReadByte wasn't a removed zero, so back up a byte
return count;
}
private static bool CompareBytearrays(byte[] a, byte[] b)
{
if (a.Length != b.Length)
return false;
int i = 0;
foreach (byte c in a)
{
if (c != b[i])
return false;
i++;
}
return true;
}
The two methods above InitRSAProvider and LoadRsaPublicKey were gotten out of tutorials to allow PEM keys as Strings to be used with .Net.
Having looked at some of the information on RSA encryption modes, it would appear that PKCS#1 v1.5 (which you’re using, because you’re calling
Decrypt(..., false))(RFC 3447, emphasis mine).
Based on the error message, which indicates that your key is 128 bytes, that means that you can’t perform RSA (en|de)cryption using PKCS#1 v1.5 on a message with more than 128 – 11 = 117 bytes.
Instead of encrypting your message directly using RSA, you should be using a symmetric algorithm to encrypt the body of the message, and encrypt only the symmetric encryption key using RSA. Only if your message is reasonably short (i.e. below 117 bytes for your key size) should you consider encrypting the message directly with RSA.
I have added the following, assuming that your input is Base64 encoded as you indicate in your comment below: