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Public Key
Infrastructures
Andreas Hülsing
Public Key Infrastructures
… a public key infrastructure (PKI) is designed to
facilitate the use of public key cryptography.
Source: Housley, R. and Polk, T.: Planning for PKI; Wiley 2001
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Tasks of a PKI
• Assure that the public key is available
• Assure that the public key is authentic
• Assure that the public key is valid
• Enforce security and interoperability
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Authenticate Public Keys
• Bind public key to electronic identity
• Seal the binding
• Answer for the binding
Public key certificates
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Public Key Certificate
Public key certificates are data structures that bind
public key values to subjects. The binding is
asserted by having a trusted CA digitally sign each
certificate …
[From RFC 5280]
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Public Key Certificate
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Public Key Certificate
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Digital Signature
Subject (Name)
Public-key Binding eID public key
protection of authenticity
Certificate Properties
• Protected binding of a key to the key holder
• Its authenticity is independent of the means of
transportation
• It can be used online and offline
• It is a proof of the binding
• It can be used for key servers
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Certificate Standards
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• X.509 • X.509 (ITU-T)
• PKIX (RFC 5280)
• Pretty Good Privacy (PGP) • OpenPGP (RFC 4880)
• GNU Privacy Guard (GnuPG or GPG)
• WAP certificates • Like X.509 certificates but smaller
• Card Verifiable Certificates (CVC) • Even smaller than WAP certificates
• Simple PKI / Simple Distributed Security Infrastructure • SPKI, pronounced spoo-key
• SDSI, pronounced sudsy
Validity of Public Keys
• Monitor binding public key electronic identity
key owner
• Establish time constraints
• Provide means to revoke binding
Certificate revocation
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Certificate Revocation
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• Abortive ending of the binding between
• subject and key (public key certificate)
OR
• subject and attributes (attribute certificate)
• The revocation is initiated by
• the subject
OR
• the issuer
• Typical frequency (assumption):
• 10% of the issued certificates will be revoked (See: “Selecting
Revocation Solutions for PKI” by Årnes, Just, Knapskog, Lloyd and Meijer)
Certificate Revocation List
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Publish Public Key Information
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• Directories • (L)DAP
• Active Directory
• Web pages • HTTP
• File transfer • FTP
• Services
• OCSP
• SCVP
LDAP
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Security of Key Pairs
Select suitable algorithms and key sizes
Monitor possible security threads and react adequately
Provide suitable means to generate key pairs
Provide suitable formats and media to store private keys
Provide suitable means of delivering private keys
Personal security environments
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PSE: Smartcard
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Interoperability
• Comply to accepted (international) standards
• Certificates / revocations
− X.509, PGP, SPKI/SDSI, …
• Directory services
− (L)DAP, Active Directory, …
• Cryptographic algorithms / protocols / formats
− PKCS, RFC, …
• Constraints on content and processing
− PKIX, ISIS-MTT, …
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Policy Enforcement
• Certificate policy (CP)
• States what to comply to
• Certificate practice statement (CPS)
• States how to comply
• Policies are enforced by the PKI through:
• Selecting standards, parameters, hardware, …
• Monitor behavior of involved parties
• Reacting on infringement of the policy
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Trust Models
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Trust
The perhaps most important part of a PKI is to
establish trust in the binding between an entity and a
certificate
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Direct Trust
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• User receives public key directly from owner
OR
• User verifies public key directly with owner
Most Common: Fingerprint comparison
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Fingerprint = hash value of the certificate (incl. Signature) (e.g. SHA1)
Face-to-Face Verification
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Phone Verification
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Web Page Verification
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http://www.cacert.org/index.php?id=3
Printed Media Verification
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BNetzA publishes the public key
…and more
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~# gpg --list-public-keys
/root/.gnupg/pubring.gpg
------------------------
pub 2048R/3D25D3D9 1999-03-06 SuSE Security Team
<security@suse.de>
pub 1024D/9C800ACA 2000-10-19 SuSE Package Signing Key
<build@suse.de>
sub 2048g/8495160C 2000-10-19 [expires: 2006-02-12]
e.g. public keys on software CD/DVD
Summary: Direct Trust
• Establishes • Which keys are authentic
• Why they are considered authentic
• Bad scalability • n * (n-1) = O(n2) verifications
• Worse complexity than secret key exchange!
• Basis for all other trust models • To be seen
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PGP (Pretty Good Privacy)
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Web of Trust
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[From PGP-Pretty Good Privacy by Simon Garfinkel]
Web of Trust
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A web of trust is a concept used in PGP, GnuPG, and
other OpenPGP-compatible systems to establish the
authenticity of the binding between a public key and a
user.
Its decentralized trust model is an alternative to the
centralized trust model of a public key infrastructure
(PKI), which relies exclusively on a certificate authority
(or a hierarchy of such).
Source: http://en.wikipedia.org/wiki/Web_of_trust
Key Validity
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• Alice computes key validity using Bob’s signatures
Carl
Dorian
Bob Alice
Chaining Key Validity
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• Alice computes key validity using Bob’s and Carl’s
signatures
Alice Bob Carl
Dorian
Eve
Public Keyring
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Public Keyring
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Alice’s public keyring
Key Validity vs. Owner Trust
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• Key Validity:
• Is the key owner who he claims to be?
• Levels: no answer; unknown; marginal; complete;
ultimate
• Owner trust:
• Is the key owner reliable? (in respect to signing keys of others)
• Levels: unknown; none; marginal; complete; ultimate
Key Validity: Levels
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• no answer
• Nothing is said about this key.
• unknown
• Nothing is known about this key.
• marginal
• The key probably belongs to the name.
• complete
• The key definitely belongs to the name.
• (ultimate)
• (Own keys).
Owner Trust: Levels
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• unknown
• Nothing can be said about the owner's judgment in key signing.
• none
• The owner is known to improperly sign keys.
• marginal
• The owner is known to properly sign keys.
• complete
• The owner is known to put great care in key signing.
• ultimate
• The owner is known to put great care in key signing, and is allowed to make trust decisions for you.
Assigning Key Validity
• Manually (Key Signing)
OR
• computed from the trust in the corresponding
signers, only considering signers with key validity
“complete” (or better).
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Assigning Key Validity
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Alice signs the public key of other users.
Key Signing: Direct Trust
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Bob’s key validity is complete for Alice because she decided it when signing the key after verifying the fingerprint.
Key Validity Computation: “complete” (1)
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If the key is signed by at least one user with owner trust complete.
Key Validity Computation: “complete” (2)
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If the key is signed by at least x (here x=2) names with owner trust marginal.
Key Validity Computation: “marginal”
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If the key is signed by less than x (here x=2) names with owner trust marginal.
Key Validity Computation: “unknown”
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If the key is signed by no name with at least owner trust marginal
Assigning Owner Trust
• Manually (Trust Setting)
OR
• computed from the owner trust of signers only using
“ultimate” valid keys.
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Trust Anchor: Owner Trust
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Alice assigns owner trust to users.
“Simple” PGP
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Alice signs Bob’s key (level 0) and trusts him. Alice uses Bob’s signatures on Dorian’s and Frank’s
keys.
Trusted Introducers
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Alice signs Bob’s key (level 1) and trusts him. Bob signs Carl’s key (level 0) and trusts him. Alice uses Carl’s signatures on Dorian’s and Frank’s
keys. Bob = Trusted Introducer
By allowing more intermediate signers (level >1), Bob becomes a Meta Introducer
PGP Certificates
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PGP Certificates: Content
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[From http://www.ece.cmu.edu/~adrian/630-f04/PGP-intro.html]
A Simple PGP Certificate - Example
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One UserID with one signature
Legend
Public Key Packet
User ID Packet
Signature Packet
Example, cont’d
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Legend
Public Key Packet
User ID Packet
Signature Packet
One UserID with one signature and
a second UserID without signature
Example, cont’d
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One UserID with four signatures
Legend
Public Key Packet
User ID Packet
Signature Packet
A More Complicated Example
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One UserID with one signature and
a second UserID with one signature and
a second key (subkey) with one signature
Legend
Public Key Packet
User ID Packet
Signature Packet
Public Key Packet
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Creation Time Version
Public Key Algorithm
Public Key
(RSA case)
User ID Packet
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A User ID packet consists of UTF-8 text that is intended to
represent the name and email address of the key holder. By
convention, it includes an RFC 2822 mail name-addr, but
there are no restrictions on its content. The packet length in
the header specifies the length of the User ID.
[From RFC 4880]
Example:
Andreas Hülsing <a.t.huelsing@tue.nl>
Signature Package
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…
…
Version Signature Type Public Key Algorithm Hash Algorithm Counter
Hashed Subpackets Unhashed Subpackets 16 bits of signed hash value Signature (RSA Case)
Subpacket Content
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• signature creation time
• signature expiration time
• exportable certification
• trust signature
• regular expression
• revocable
• key expiration time
• placeholder for backward compatibility
• preferred symmetric algorithms
• revocation key
• issuer key ID
• notation data
• preferred hash algorithms
• preferred compression algorithms
• key server preferences
• preferred key server
• primary user id
• policy URL
• key flags
• signer's user id
• reason for revocation
PGP Revocation
• Uses Key Revocation Certificate
• generated during KeyGen using private key
• Uploading Key Revocation Certificate to one of the
public key servers revokes key pair.
• Key Revocation Certificate can contain new UserID
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