An End-to-End Measurement of Certificate Revocation in the Webs PKI - PowerPoint PPT Presentation

An End-to-End Measurement of Certificate Revocation in the Web’s PKI Yabing Liu*, Will Tome*, Liang Zhang*, David Choffnes*, Dave Levin † , Bruce Maggs ‡ , Alan Mislove*, Aaron Schulman § , Christo Wilson* † University of Maryland *Northeastern University ‡ Duke University and Akamai Technologies § Stanford University

Public Key Infrastructures (PKIs) How can users truly know with whom they are communicating? Browser Website 2

Public Key Infrastructures (PKIs) How can users truly know with whom they are communicating? Browser Website 2

Public Key Infrastructures (PKIs) How can users truly know with whom they are communicating? Browser Website 2

Public Key Infrastructures (PKIs) How can users truly know with whom they are communicating? Browser Website Certificate Authority 2

Public Key Infrastructures (PKIs) How can users truly know with whom they are communicating? Browser Website Vetting Certificate Authority 2


 Public Key Infrastructures (PKIs) How can users truly know with whom they are communicating? Browser Website Certificate Authority The owner of is indeed BoA Certificate 2

Public Key Infrastructures (PKIs) How can users truly know with whom they are communicating? Browser Website Certificate Certificate Certificate Authority 2

Certificate revocation What happens when a certificate is no longer valid? Browser Website Certificate Certificate Authority 3

Certificate revocation What happens when a certificate is no longer valid? Browser Website Certificate Attacker Certificate Certificate Authority 3

Certificate revocation What happens when a certificate is no longer valid? Attacker Browser Certificate Certificate Authority 3

Certificate revocation What happens when a certificate is no longer valid? Attacker Browser Certificate Certificate Certificate Authority 3

Certificate revocation What happens when a certificate is no longer valid? Attacker Browser Certificate Certificate Please 
 Certificate Authority revoke ✗ Certificate Certificate 
 Revocation 3

Certificate revocation What happens when a certificate is no longer valid? Attacker Browser Certificate Certificate Please 
 Certificate Authority revoke ✗ ✗ ✗ Certificate Certificate Certificate ✗ ✗ ✗ Certificate Certificate Certificate 3

Certificate revocation What happens when a certificate is no longer valid? Attacker Browser Certificate Certificate Periodically pull / query Please 
 Certificate Authority revoke (CRL) (OCSP) ✗ ✗ ✗ Certificate Certificate Certificate ✗ ✗ ✗ Certificate Certificate Certificate 3

Certificate revocation What happens when a certificate is no longer valid? ✗ Attacker Browser ✗ Certificate Certificate Certificate Periodically pull / query Please 
 Certificate Authority revoke (CRL) (OCSP) ✗ ✗ ✗ Certificate Certificate Certificate ✗ ✗ ✗ Certificate Certificate Certificate 3

Certificate revocation responsibilities ✗ Administrators must revoke certificates 
 Certificate when keys are compromised Certificate authorities must publish revocations 
 as quickly as possible Browsers must check revocation status 
 on each connection This talk: 
 Do these entities do what they need to do? 4

Outline ✗ Website admin behavior Certificate e.g., what is the frequency of revocation? Certificate authorities behavior e.g., how do CAs serve revocations? Client behavior e.g., do browsers check revocations? 5

Dataset Rapid7 IPv4 scans 38M certs (~ 1 /wk for 18mos) 6

Dataset 38M certs Non-CA Rapid7 IPv4 classify scans CA 38M certs (~ 1 /wk for 18mos) 1,946 certs 6

Dataset 38M certs Non-CA Rapid7 IPv4 classify Leaf Set validate scans CA 5M valid certs 38M certs (~ 1 /wk for 18mos) 1,946 certs 6

Dataset 38M certs Non-CA Rapid7 IPv4 classify Leaf Set validate scans CA 5M valid certs 38M certs (~ 1 /wk for 18mos) 1,946 certs Download revocation information daily 6

How frequently are certificates revoked? Percentage of Fresh Certs 12.0 that are Revoked 10.0 8.0 6.0 4.0 2.0 0.0 01/14 03/14 05/14 07/14 09/14 11/14 01/15 03/15 Date 7

How frequently are certificates revoked? Percentage of Fresh Certs 12.0 that are Revoked 10.0 8.0 6.0 4.0 2.0 0.0 01/14 03/14 05/14 07/14 09/14 11/14 01/15 03/15 Date Significant fraction of certificates revoked 
 1% in steady state; more than 8% after Heartbleed 7

Are there revoked certificates being used? 0.006 Fraction of Alive Certs 0.005 that are Revoked 0.004 0.003 0.002 0.001 0.000 01/14 03/14 05/14 07/14 09/14 11/14 01/15 03/15 Date Over 0.5% advertised certificates are revoked 
 Website admins failed to update their servers 8

Outline ✗ Website admin behavior Certificate e.g., revocation is common ~8% Certificate authorities behavior e.g., how do CAs serve revocations? Client behavior e.g., do browsers check revocations? 9

CRLs, OCSP , and OCSP Stapling Browser Website Certificate Certificate Certificate Certificate Authority ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate Certificate Certificate Certificate Certificate Certificate ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate Certificate Certificate Certificate Certificate Certificate 10

CRLs, OCSP , and OCSP Stapling Browser Website Certificate Certificate Certificate Certificate Authority ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate Certificate Certificate Certificate Certificate Certificate ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate Certificate Certificate Certificate Certificate Certificate 10

CRLs, OCSP , and OCSP Stapling Browser Website Certificate Certificate Certificate Certificate Authority ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate Certificate Certificate Certificate Certificate Certificate ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate Certificate Certificate Certificate Certificate Certificate 10

CRLs, OCSP , and OCSP Stapling Browser Website Certificate Certificate Certificate ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate Certificate Authority ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate 10

Cost of obtaining CRLs 1 0.9 0.8 0.7 0.6 CDF 0.5 0.4 0.3 0.2 0.1 0 0.1 1 10 100 1000 10000 CRL Size (KB) 11

Cost of obtaining CRLs 1 0.9 0.8 76MB Apple CRL 0.7 0.6 CDF 0.5 0.4 0.3 0.2 0.1 0 0.1 1 10 100 1000 10000 CRL Size (KB) 11

Cost of obtaining CRLs 1 0.9 0.8 76MB Apple CRL 0.7 0.6 CDF 0.5 0.4 0.3 0.2 Raw 0.1 Weighted 0 0.1 1 10 100 1000 10000 CRL Size (KB) Most CRLs small, but large CRLs downloaded more often 
 Result: 50% of certs have CRLs larger than 45KB 11

CRLs from different CAs Certificates Unique Avg. CRL CA CRLs size (KB) Total Revoked GoDaddy 322 1,050,014 277,500 1,184.0 RapidSSL 5 626,774 2,153 34.5 Comodo 30 447,506 7,169 517.6 PositiveSSL 3 415,075 8,177 441.3 Verisign 37 311,788 15,438 205.2 CAs use only a small number of CRLs 12

CRLs, OCSP , and OCSP Stapling Browser Website Certificate Certificate Authority ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate 13

CRLs, OCSP , and OCSP Stapling Browser Website Certificate Certificate Certificate Authority ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate 13

CRLs, OCSP , and OCSP Stapling Browser Website Certificate Certificate Certificate Certificate Authority ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate 13

CRLs, OCSP , and OCSP Stapling Browser Website Certificate Certificate Certificate Certificate Authority ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate 13

CRLs, OCSP , and OCSP Stapling Browser Website Certificate Certificate Certific / ✗ ✔ Certific Certificate Authority ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate ✗ ✗ ✗ ✗ ✗ Certificate Certificate Certificate Certificate Certificate 13