Java Security

Hi everybody,

I'am getting stuck around a decryption problem. It is my first time (since one month) I work in java cryptography - and even in cryptography at all.
Well, I made two programs (2 java applet) :
- The first one compute a MD5 hash code of a specified file. Then this hash code is signed with a private key thanks to a X509 certificate, ensuring data integrity (MD5 hash code) and authenticity (signature). This signature is stored in a file with extension .sig.
- The second one simply read the .sig. Then it decrypts the signature thanks to the same certificate (providing the public key). Hence I will be able to read the crypted MD5 hash code in clear.

The problem is that I don't manage to decrypt the signature successfully. Indeed, I sign the MD5 hash code with the algorithm "MD5withRSA", the only one which seems suitable for my case (I want to crypt/decrypt with RSA).
On the other side, when I try to decrypt the signature, this latter is too long : the .sig file has 217 bytes ! So when I pass all this content in a buffer, this one is too long for decryption (with RSA algorithm which works only with blocks of 128 bytes). I managed to have a look at how "block cipher" works including padding. I even found a bit of java code to crypt a long message in multipart. But in my case, I am interested for decryption in multi block. The signature was made in one block (called stream cipher).
My question is : How to do a decryption of a long message with RSA in multi block ?

I try a "debug" code (see below) and I have always the same error : "too much data for RSA block". And yet, I read only 1 byte !!

byte[] decryptedBytes;
			for (int i = 0, j = 0; i < sig.length; i++) {
				if ((decryptedBytes = cipher.update(sig, i, 1)) == null)
					continue;
				else {
					System.out.println(new String(decryptedBytes));
					System.arraycopy(decryptedBytes, 0, plainText, j, decryptedBytes.length);
					j += decryptedBytes.length;
				}		
			}

Thanks for your explanations.