Verifying a Digital Signature using message digest

Question

Hi, i am new to java.
I have a Digitally signed document, i wanna verify this signed document against the original one.
i got the idea from this link:
http://help.sap.com/saphelp_45b/helpdata/en/8d/517619da7d11d1a5ab0000e835363f/content.htm
i signed a pdf doc with my SmartCard. the third party signing tool passed me the PKCS7 digital signature and i stored it in database. the problem arose when i retrieved this digital signature from DB and verified against the original doc using the message digest method. the base64 result strings are always not equal.
I am sure about this:
-the retrieved digital signature was GOOD.
-the original doc was GOOD.
but why i can't get the same 2 message digests? can somebody please help?
below is part of my code:
while (rsetDs.next())
     InputStream DSName2 = rsetDs.getBinaryStream(1);
     ByteArrayOutputStream byteStream = new ByteArrayOutputStream();
     byte[] myByte = Base64.decode(byteStream.toString());
     ByteArrayInputStream newStream = new ByteArrayInputStream(myByte);
     CertificateFactory cf = CertificateFactory.getInstance("X.509");
     Collection c = cf.generateCertificates(newStream2);
     Iterator i = c.iterator();
     while (i.hasNext())
          Certificate cert = (Certificate)i.next();
          X509Certificate cert1 = (X509Certificate)cert;
          try
               java.security.MessageDigest md = java.security.MessageDigest.getInstance("MD5");
               /*=============DB MD (BEGIN)==================*/
               byte [] pubkeyByte = cert1.getPublicKey().getEncoded();
               md.update(myByte);
               md.update(pubkeyByte);
               byte[] raw = md.digest();
               String db_md = Base64.encode(raw);
               /*============DB MD (end)============*/
               /*=============PDF MD (BEGIN)==================*/
               DataInputStream m_disFile = new DataInputStream(new FileInputStream("C:\\" + "original_doc.pdf"));
               int m_iNum = m_disFile.available();
               byte[] msgBytes = new byte[m_iNum];
               m_iNum = m_disFile.read(msgBytes, 0, m_iNum);
               md.update(msgBytes);
               byte[] digestMd = md.digest();
               md.reset();
               String pdf_md = Base64.encode(digestMd);
               /*=============PDF MD (END)==================*/
..thanks in advance.

Answer 1

PKCS#7 SignedData objects are far more complex then it looks like you are taking them. First the PKCS#7 SignedData object will contain the OID for the message digest algorithm used and for the encryption algorithm used. From the looks of your code you are simply assuming MD5.
It also contains all of the data that was signed which is typically much more than just the document. It also of course contains the public keys and signatures which singed the document. In your case it will probably only have one public certificate and one signature.
Also note that a signature is an encrypted hash. Looking at your code I do not see you use encryption at all or rather for verification decryption.
Here is the basic process a signature takes.
MessageDigest md = MessageDigest.getInstance(algOID);
byte[] digest = md.digest(message.getBytes(charEncoding));
Cipher c = Cipher.getInstance("RSA/2/PKCS1Padding");
c.init(Cipher.ENCRYPT_MODE, priKey);
byte[] signature = c.doFinal(digest);Note that the resulting byte array is not the message digest but the encrypted message digest. You must use the corresponding public key to decrypt the signature to get the message digest value. It is because the trusted public key can decrypt the correct message digest that we know it was encrypted by the holder of the private key. It is because the decrypted message digest value is equal to my computed message digest value that we know the document has not be altered...
Now PKCS#7 SignedData does not take the message digest of the document, in your case your PDF. It creates a message digest on an ASN.1 object which includes the bytes of your document plus a bunch of meta data.
For more info on the exact format of a PKCS#7 signature file check out
http://www.rsasecurity.com/rsalabs/pkcs/pkcs-7/index.html
Look through this doucment for SignedData as a starting place and follow through all of the sub objects that make up a SignedData object. This will give you an idea of what is involved.