Presented by Gene Yang Compliance Defects in Publickey Cryptography By Don Davis. “ A public-key security system trusts its users to validate each others’s public keys rigorously and to manage their own private keys securely. Both tasks are hard to do well, but public-key security systems lack a centralized infrastructure for enforcing users' discipline. ”
Outline • Public-key infrastructure services • How public-key infrastructure (PKI) works • Compliance Defects • Conclusion
Public-key Infrastructure services • A public-key security system comprise three infrastructure services – Certification Authority (CA) • Signs users’ public keys – Directory • Public-access database of valid certificates – Certificate Revocation List (CRL) • Public-access database of invalid certificates
How PKI Works 1. Key Creation • The user create a new key pair. • Root CA’s Private key CA (root) The user proves his ID to CA (not electronically). • Root CA’s Public Key CA The CA signs a certificate that names the user as the bearer of his new public key. • The user chooses a secret pass phrase, and uses it to encrypt his asymmetric private key. Private key Public key The user also recerives the Root CA’s public key for later use. • CA 2. Single Sign On • • ID Public Key With his private key, the user participates in public-key protocols. CA Certificate User ID Info At login, the user types his pass phrase to decrypt his private key. CA User’s Public Key CA ID Info Digitally signed by CA’s private key
How PKI works cont. 3. Authenticating Others • • • The user either exchanges certificates directly with other users, or he get others' certificates from the Directory service. Before using a certificate, the user must check the CRL for notice of the certificate's revocation. Must validate the CA's signature. 4. Password-Change • The user should regularly change the pass phrase with which he decrypts his asymmetric private key. 5. Key-Revocation • • • Certificates are time stamped to expire after a few months or a year. If a user's pass phrase or his private key is compromised, then he must inform the CRL administrator, who disseminates a notice that the corresponding public-key certificate has been revoked The user should check the CRL every time he uses a certificate, because the CRL may be updated at any moment.
Compliance Defects • Authenticating the User. – CA signs public-key certificate. – Problem: CA cannot trust electronic assurances of new user. – Face to face identification checks are required. However, it becomes unrealistic. • Authenticating the CA. – A user must authenticate public key certificate by checking its certifying signature and the signature on each public key in its chain of CAs. – Problem: public-key crytography cannot afford the user any automatic procedure for validating the top-level CA key. – Keep root key in the smart card or under the pass phrase’s encryption.
Compliance Defects Cont. • Certificate Revocation Lists. – When a user's public key must be removed from use, the only way to enforce prompt revocation is to check every certificate before use against a Certificate Revocation List. – Problem: a rigorous check of a certificate's validity requires that the public key of each CRL in the chain to the Root has to be revocation-checked. – This extra performance burden makes it likely that publickey deployment is proceeding without a revocation infrastructure.
Compliance Defects Cont. • Private-Key Management. – The user must keep his private key in memory throughout his login session. – Problem: it exposes a long-lived secret, the private key. – Private key can be compromised by physical theft, viruses and Trojan-House programs. • Pass phrase Quality. – User don't share their pass phrases with any security service or administrator. – Problem: there is no way to enforce expiration or quality controls on pass phrases. – If the user find the controls of local pass phrase is inconvenient, he can just use a more lenient program to encrypt his private key.
Conclusion • Public-key's decentralized nature actually places a lot of trust on users, that properly belongs to the security infrastructure and its administrators. • Question: Is that public-key cryptography best suited to securing communications between servers or desktop applications?
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