PDF 签名验证失败

Question

I'm using iTextSharp.LGPLv2 to sign PDF documents (The only available option for.Net Core right now). I'm able to produce a signature but most readers reject it. You can download the file: https://www.dropbox.com/s/ns35fdi5tyq52cw/blank.bs.pdf?dl=0

Few important issues:

• Both Adobe Reader and Foxit reject the signature saying that "the document has been altered..."
• The signature uses subfilter of "adbe.pkcs7.detached". If changed to "ETSI.CAdES.detached", Foxit decides that the signature is fine . Reader does not.
• SHA256 is being used for all hashing purposes.
• Looks like the ByteRange is correctly set.
• By using openssl, I was able to verify that the signature is properly formatted and contains a valid PKCS1.5 padded buffer.

I'm kind of puzzled here and I'll be glad to hear any suggestions/hints/directions.

Answer 1

There are (at least) two errors in the signature:

• Mismatch in document hash:

  The SHA256 hash of the signed byte ranges is

  69CF153087670FC5D450FF23675C207BB9659A235A465202F5802ABED7BA7C21

  but your signature container claims it is

  93D5804C1A9A31B17F041DE79FB1CD598E1FCC7CDC2A37885CD0502D4FCBB5A6

  which explains why validators assume that the document has been altered.

• Mismatch in hash of signed attributes:

  The SHA256 hash of the signed attributes of the SignerInfo in your CMS signature container is

  C84C6600F5E11291AAD8240239B7DC09FC00BD6A9056B6BBF584E8D46C8CEBE5

  but the hash value in you SHA256withRSA signature value is

  93D5804C1A9A31B17F041DE79FB1CD598E1FCC7CDC2A37885CD0502D4FCBB5A6

  which would make validation fail even if the document hash (see above) would match.

  Incidentally this hash in the signature value is identical to your claimed (incorrect) document hash value.

The latter mismatch probably is caused by you signing the wrong data.

The former mismatch may be due because of some error in your code calculating the hash of the signed byte ranges, or it might be due to the signed data actually being manipulated.

To further analyze the problem, your signing code would be required.

PS: There also is at least one error in your certificate, it has a negative serial number.