Public Key Infrastructures PKI

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Public Key Infrastructures(PKI)
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Paper on PKI and Digital Signatures
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Symmetric Key versus Public Key Encryption

• Key distribution for symmetric keys
• secure channel for distribution
• Distributing keys in a group
• one key per group
• one key per pair of (group) members
• one key per (group) member
• Compare solutions!

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Symmetric Key versus Public Key Encryption

• Key distribution for symmetric keys
• Number of key distributions depends on
  communication structure (see figure).
• One key per pair

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Symmetric Key versus Public Key Encryption

• Key distribution for symmetric keys by a central
  server (KDC)
• fixed number of distributions (for given n)

• However, need security protocol
• public key encryption for distribution
• Needham Schroeder
• Kerberos

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Symmetric Key versus Public Key Encryption

• Efficiency
• symmetric key long messages can be processed
  fast
• 7 megabytes per second (DES)
• public key slower, for short messages
• digital signatures
• (symmetric) key distribution
• Problem with public key encryption
• binding between public keys and names
  (identities)

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Problems with Public Keys
look up of Aichas key
But Alice has replaced Aichas key by her own!
sends encr. message
What does Alice do?
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Signatures

• signing
• authenticity of identity
• expressing (free) will of signer (by an action)
• fixing the content (position of signatures)
• confirming integrity (according to
  interpretation of signer)
• checking
• recognizing identities
• checking authenticity of signature (individual
  signature)
• non repudiation (individual signature)
• integrity of content
• duplication

• Content
• Identities
• Signatures

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Signature Scheme Key Generation

• key generation
• RSA, El-Gamal
• secure environment

key pair
key pair

• key pair
• RSA
• El-Gamal

public
publication of keys
signature component uses private key
signature component uses private key
signature generation secret key on device from
the beginning
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Signature Scheme Signature Components

• Signature Component
• stores secret key
• computation of signature
• Smart Card, PDA,
• security access control/information flow
  policy may be necessary

• key pair
• RSA
• El-Gamal

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Signature Scheme Public Keys
key pair
key pair
Have to kept in trusted public repository.
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Signature Scheme Trust Ccenter
Public Repository for certificates
Trust Centre (TC)
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Signature Scheme Registration and Repository

• Registration
• checking the identity of users

Identity Card,
Identity Card,

• Repository
• certificates bind identities to public keys
• certificates are signed by TC
• requests
• revocation
• security of server

Public Repository
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Trust Center Activities
Registration
Key Generation
TC Cert. Auth.
Personali- zation
Repository
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Signature Scheme Signing
Principle of Digital Signature
hash
private key
encr
hash value fingerprint
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Signature Scheme Signing
d

• Computing hash value
• fixed (small) length)
• no collisions
• trusted viewer problem

hash
hash(d)
hash value fingerprint
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Signature Scheme Signing

• Signature
• encryption with secret key
• critical will of signer
• protection against manipulation necessary

secret key sk
hash(d)
encr
sig(d,sk) encr(hash(d),sk)
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Signature Scheme Checking
result
hash(d) decr(sig,pk) ??
sig
d
pk
check
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Signature Scheme Verification
Validity of Certificates
result
hash
public key of Alice
check
fingerprint
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Infrastructure
Root Authority
Trust Center
Trust Center
User
User
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Signature Scheme Verification
Certificates
DN-U(ser)
DOC
DN-U DN-TC CERT
DN-TC DN-ROOT CERT
?
hash
decr.
?
?
?
?
hash
decr.
hash
decr.

• All keys public
• DN distinguished name

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Signature Scheme Verification

• Online Verification
• retrieve certificates and root key or
• send/store certificates and root key and
  check revocation online
• online certificate check (OCR)

• Offline Verification
• send/store certificates
• store root key (securely)
• store revocation list
• update intervals

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Basic Notions (revisited)

• Hash Functions
• fixed length digest of (arbitrary) messages
• one way function
• 160 bit
• Message Authentication Code (MAC)
• encrypting a digest
• Signatures
• encrypted hash value (MAC) can be checked using
  public key

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Basic Notions (revisited)

• Certificates
• binding of public key to identification and
  authorization data
• Certification Authorities
• sign certificates (digitally)

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Trust Center (German Signature Law)

• Key Generation (KG)
• for Trust Center
• for participants
• Certification Authority (CA)
• generation of certificates
• signing certificates
• Personalization (PS)
• transferring keys (secret key) and certificates
  
• Registration Authority (RA)
• registration of participants
• checking their identity

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Trust Center

• Directory Service (DS)
• keeping certificates in a repository
• answer requests (checking certificates)
• revocation of certificates
• Time Stamping Service
• attaching time stamps
• digitally signed

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Naming (X.500)
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Naming (X.500)
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Certificates (X.509)