Digital Signatures: how it's done in PDF
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This document summarizes how digital signatures are implemented in PDF documents. It discusses the need for integrity, authenticity and non-repudiation that digital signatures provide. It covers basic concepts like hashing, encryption and digital signatures. It then explains how these concepts are applied specifically to PDF, following standards like ISO 32000 and PAdES. It discusses server-side signing, client-side signing and deferred signing architectures. It also covers how digital signatures can be used in document workflows and long-term validation strategies like revocation, timestamps and document security stores.
![© 2015, iText Group NV, iText Software Corp., iText Software BVBA
Concept 1 + Concept 2
Producer
Provides data as-is: [A]
Provides hash of data, encrypted using private key: [B]
Provides public key
Consumer
Creates hash from data [A]: hash1
Decrypts hash [B] using public key: hash2
If (hash1 == hash2) document OK!
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Digital Signatures: how it's done in PDF
- 1. © 2015, iText Group NV, iText Software Corp., iText Software BVBA© 2015, iText Group NV, iText Software Corp., iText Software BVBA Digital Signatures: How It’s Done in PDF Zetes eaZySign event (Kasteel van Ruisbroek: April 2, 2015)
- 2. © 2015, iText Group NV, iText Software Corp., iText Software BVBA© 2015, iText Group NV, iText Software Corp., iText Software BVBA Agenda • Why do we need digital signatures? • Basic concepts… • … applied to PDF • Architectures: server-side vs. client-side • Digital signatures and document workflow • Long term validation
- 3. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Digital signatures: How It's Done in PDF3 Why do we need digital signatures? Integrity Authenticity Non-repudiation
- 4. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Integrity Digital signatures: How It's Done in PDF4 I paid a forged invoice and lost 30K€ !
- 5. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Authenticity Digital signatures: How It's Done in PDF5 Why am I, Emperor Constantine I, in this picture? I never transferred authority to the Pope!
- 6. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Non-repudiation Digital signatures: How It's Done in PDF6 I didn’t do it!
- 7. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Three goals Integrity — we want assurance that the document hasn’t been changed somewhere in the workflow. Authenticity — we want assurance that the author of the document is who we think it is (and not somebody else). Non-repudiation — we want assurance that the author can’t deny his authorship. Digital signatures: How It's Done in PDF7
- 8. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Digital signatures: How It's Done in PDF8 Basic Concepts… Hashing algorithms Encryption algorithms Digital signatures
- 9. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Concept 1: Hashing Hashing algorithm a cryptographic hash function to turn an arbitrary block of data into a fixed-size bit string. Available algorithms MD5: Ron Rivest (deprecated) SHA: SHA-1: NSA (phased out!) SHA-2: NSA / NIST SHA-3: Keccak (made in Belgium!) RIPEMD: KULeuven Digital signatures: How It's Done in PDF9
- 10. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Concept 1: Integrity check using hash Digital signatures: How It's Done in PDF10 Document Generate Hash AF1B4C...D34E Secure Server / Website Retrieve Hash AF1B4C...D34E Compare!
- 11. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Concept 2: Encryption Assymetric key algorithms Encryption Digital signing Digital signatures: How It's Done in PDF11
- 12. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Concept 2: Some name dropping Public Key Cryptography Standards PKCS#1: RSA Cryptography Standard (Rivest, Shamir, Adleman) PKCS#7: Cryptographic Message Standard (CMS) PKCS#11: Cryptographic Token Interface PKCS#12: Personal Information Exchange Syntax Standard PKCS#13: Elliptic Curve Cryptography Standard (ECDSA) Federal Information Processing Standards (FIPS) DSA: Digital Signature Algorithm (DSA) European Telecommunications Standards Institute (ETSI) CMS Advanced Electronic Signatures (CAdES) Digital signatures: How It's Done in PDF12
- 13. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Concept 1 + Concept 2 Producer Provides data as-is: [A] Provides hash of data, encrypted using private key: [B] Provides public key Consumer Creates hash from data [A]: hash1 Decrypts hash [B] using public key: hash2 If (hash1 == hash2) document OK! Digital signatures: How It's Done in PDF13
- 14. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Goals met? Integrity Hashes are identical Authenticity Identity is stored in public key A time-stamp can be added Non-repudiation If hash can be decrypted with public key, the document was signed with the corresponding private key Digital signatures: How It's Done in PDF14
- 15. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Differences between EU and US In the US, we make a distinction Electronic signatures don’t necessarily involve PKI Digital signatures when a PKI infrastructure is involved In Europe, we speak of electronic signatures As a synonym for digital signatures All laws and regulations take this wording There’s no sharp distinction between electronic and digital signatures (which leads to confusion) I always speak of digital signatures Digital signatures: How It's Done in PDF15
- 16. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Digital signatures: How It's Done in PDF16 … Applied to PDF ISO 32000-1 ETSI TS 102 778 (PAdES) ISO 32000-2
- 17. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Standards ISO ISO-32000-1 (2008) based on PDF 1.7 (2006) ISO-32000-2 will define PDF 2.0 (2016) ETSI: TS 102 778 (2009 - 2010) PAdES 1: Overview PAdES 2: Basic – CMS based (ISO-32000-1) PAdES 3: Enhanced – CAdES based (ISO-32000-2) PAdES 4: LTV – Long Term Validation PAdES 5: XAdES based (XML content) PAdES 6: Visual representation guidelines ETSI: TS 103 172 (2011 - 2013) PAdES Baseline Profile Digital signatures: How It's Done in PDF17
- 18. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Signatures in PDF There are no bytes in the PDF that aren’t covered, other than the PDF signature itself. (*) The digital signature isn’t part of the ByteRange. The concept “to initial a document” doesn’t exist; you sign the complete document at once, not on a page per page basis. (*) Digital signatures: How It's Done in PDF18
- 19. © 2015, iText Group NV, iText Software Corp., iText Software BVBA What’s inside a signature? Digital signatures: How It's Done in PDF19 ISO-32000: At minimum the PKCS#7 object shall include the signer’s X.509 signing certificate. This certificate shall be used to verify the signature value in /Contents. Best practices (“should” also have): • Full certificate chain • Revocation information (CRL / OCSP) • Timestamp %PDF-1.x ... /ByteRange ... /Contents< >... %%EOF DIGITAL SIGNATURE • Signed Message Digest • Certificate chain • Revocation information • Timestamp
- 20. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Digital signatures: How It's Done in PDF20 Architectures Server-side signing Client-side signing Deferred signing
- 21. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Server-side signing Digital signatures: How It's Done in PDF21 <</Type/Sig/ /Contents < %PDF-1.x ... ... %%EOF >>> Signed Message Digest Application DeviceSERVER
- 22. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Use cases server-side signing Company signature Invoices Contracts … Signing services in the Cloud Docusign Echosign … Security management responsibilities! Digital signatures: How It's Done in PDF22
- 23. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Client-side signing Digital signatures: How It's Done in PDF23 <</Type/Sig/ /Contents < %PDF-1.x ... ... %%EOF >>> Signed Message Digest ApplicationDevice CLIENT
- 24. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Use cases client-side signing Desktop applications Adobe Acrobat Pro Adobe Reaer (for Reader-enabled documents) Home-made, e.g. using iText In a web context The PDF software runs on the client, e.g. using Java Web Start Access to the token or smart card through MSCAPI PKCS#11 Custom smart card library Security User has smart card and PIN or USB token and passphrase Digital signatures: How It's Done in PDF24
- 25. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Deferred signing Digital signatures: How It's Done in PDF25 Signed Message Digest AppDevice CLIENT <</Type/Sig/ /Contents < %PDF-1.x ... ... %%EOF >>> Application SERVER
- 26. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Use cases deferred signing Signing on an iPad/Tablet App on the device has a low footprint Easy to integrate into a document management system Disadvantage You need to trust the server that the hash you receive is actually the hash of the document you want to sign Common Criteria A framework in which computer system users can specify their security functional and assurance requirements (SFRs and SARs respectively) through the use of Protection Profiles (PPs), vendors can then implement and/or make claims about the security attributes of their products, and testing laboratories can evaluate the products to determine if they actually meet the claims. Digital signatures: How It's Done in PDF26
- 27. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Digital signatures: How It's Done in PDF27 Digital signatures and workflow Author signatures Recipient signatures Locking fields / documents
- 28. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Serial signatures 28 %PDF-1.x % Original document % Additional content 1 ... ... %%EOF DIGITAL SIGNATURE 1 ... %%EOF DIGITAL SIGNATURE 2 % Additional content 2 ... ... %%EOF DIGITAL SIGNATURE 3 Rev1 Rev2 Rev3 A PDF document can be signed more than once, but parallel signatures aren’t supported, only serial signatures: additional signatures sign all previous signatures. Digital signatures: How It's Done in PDF
- 29. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Digital signatures: types Certification (aka author) signature Only possible for the first revision Involves modification detection permissions: No changes allowed Form filling and signing allowed Form filling, signing and commenting allowed Approval (aka recipient) signature Workflow with subsequent signers New in PDF 2.0: modification detection permissions Digital signatures: How It's Done in PDF29
- 30. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Other possible icons Signer’s identity is unknown Document has been altered or corrupted Digital signatures: How It's Done in PDF30
- 31. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Certified by Alice Digital signatures: How It's Done in PDF31
- 32. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Read, approved and signed by Bob Digital signatures: How It's Done in PDF32
- 33. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Bob’s signature invalidated by Chuck Digital signatures: How It's Done in PDF33
- 34. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Read, approved and signed by Carol Digital signatures: How It's Done in PDF34
- 35. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Read, approved and signed by Dave Digital signatures: How It's Done in PDF35
- 36. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Signature and lock broken by Chuck Digital signatures: How It's Done in PDF36
- 37. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Digital signatures: How It's Done in PDF37 Long-term validation Revocation Timestamps LTV
- 38. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Certificates expire Digital signatures: How It's Done in PDF38 Expiration date 2013 2014 2015
- 39. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Certificates get revoked Digital signatures: How It's Done in PDF39 Expiration dateRevocation date 2013 2014 2015
- 40. © 2015, iText Group NV, iText Software Corp., iText Software BVBA How to survive revocation / expiration? Digital signatures: How It's Done in PDF40 Expiration dateRevocation date
- 41. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Timestamps Digital signatures: How It's Done in PDF41
- 42. © 2015, iText Group NV, iText Software Corp., iText Software BVBA What to do when: There’s no CRL/OCSP/TS in the document? The certificate is about to expire in one of your documents? The hashing/encryption algorithm is about to be deprecated? Digital signatures: How It's Done in PDF42
- 43. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Document Security Store (DSS) Digital signatures: How It's Done in PDF43 %PDF-1.x ... /ByteRange ... /Contents< >... %%EOF DIGITAL SIGNATURE • Signed Message Digest • Certificate %PDF-1.x ... /ByteRange ... /Contents< >... %%EOF DSS for DIGITAL SIGNATURE • VRI, Certs, OCSPs, CRLs DIGITAL SIGNATURE • Signed Message Digest • Certificate
- 44. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Document-level timestamp Digital signatures: How It's Done in PDF44 %PDF-1.x ... /ByteRange ... /Contents< >... %%EOF DSS for DIGITAL SIGNATURE • VRI, Certs, OCSPs, CRLs %PDF-1.x ... /ByteRange ... /Contents< >... %%EOF DSS for DIGITAL SIGNATURE • VRI, Certs, OCSPs, CRLs DOCUMENT TIMESTAMP TS1 ETSI.RFC3161 DIGITAL SIGNATURE • Signed Message Digest • Certificate DIGITAL SIGNATURE • Signed Message Digest • Certificate
- 45. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Digital signatures: How It's Done in PDF45 %PDF-1.x ... /ByteRange ... /Contents< >... %%EOF DSS for DIGITAL SIGNATURE • VRI, Certs, OCSPs, CRLs DOCUMENT TIMESTAMP TS1 %PDF-1.x ... /ByteRange ... /Contents< >... %%EOF DSS for DIGITAL SIGNATURE • VRI, Certs, OCSPs, CRLs DOCUMENT TIMESTAMP TS1 DSS for TS1 DOCUMENT TIMESTAMP TS2 DIGITAL SIGNATURE • Signed Message Digest • Certificate DIGITAL SIGNATURE • Signed Message Digest • Certificate
- 46. © 2015, iText Group NV, iText Software Corp., iText Software BVBA Digital signatures: How It's Done in PDF46 Questions? itextpdf.com SG: [email protected] +65 31 58 39 47 BE: [email protected] +32 92 98 02 31 US: [email protected] +1 617 982 646