Decrypt a PDF - Encryption Key Algorithm

Hi to All,

I'm italian, so I want to apologize if my english isn't perfect.

I'm a young developer and I've some problem with decrypting PDF. I try to explain my problem telling you what I'm trying to do.

Using this parameter:

• << /Filter /Standard
  /V 1/R 2/Length 40/P -44
  /O <2055C756C72E1AD702608E8196ACAD447AD32D17CFF583235F6DD15FED7DAB67>
  /U <7C1EB4017D43EA47D4590D3A1EC87C61A95F3AB02DEB3E823668F7BFCA1FB313>
  >>
• Padding String: < 28 BF 4E 5E 4E 75 8A 41 64 00 4E 56 FF FA 01 08 2E 2E 00 B6 D0 68 3E 80 2F 0C A9 FE 64 53 69 7A >
• <<
  /Size 18
  /Root 2 0 R
  /Info 4 0 R
  /Encrypt 5 0 R
  /ID[<777E149019263FC69355C55EFBCE3F18><5138934AF72E21B909B0BC3DFB52774 5>]
  >>

This is what I do in order to get the encryption key:

• Create a HEX string by appending (in order)
  
  • Padding string: 28BF4E5E4E758A4164004E56FFFA01082E2E00B6D0683E802F0CA9FE6453697A
  • Owner Pwd: 2055C756C72E1AD702608E8196ACAD447AD32D17CFF583235F6DD15FED7DAB67
  • P entry: 4DFFFFFF (I'm not sure that the conversion is Ok)
  • First element of ID array: 777E149019263FC69355C55EFBCE3F18
• Trasform all to an ASCII string
• MD5 this string
• The first 5 bytes, so the first 5 characters, are the Encryption key

Is this correct?

Thanks for your help and best regards

Mattia