On the Importance of Encrypted-SNI (ESNI) to

Censorship Circumvention

Zimo Chai, Amirhossein Ghafari, Amir Houmansadr

University of Massachusetts Amherst

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2

What is SNI?

3

Server Name Indication (SNI) allows web-hosts to render the correct certificate

a.comb.com

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Server Name Indication (SNI) allows web-hosts to render the correct certificate

ClientHello,SNI="a.com"

a.comb.com

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Server Name Indication (SNI) allows web-hosts to render the correct certificate

Website A'sDigital

Certificate

ClientHello,SNI="a.com"

a.comb.com

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Server Name Indication (SNI) allows web-hosts to render the correct certificate

Website A'sDigital

Certificate

ClientHello,SNI="a.com"

ClientHello,

SNI='b.com'

a.comb.com

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Server Name Indication (SNI) allows web-hosts to render the correct certificate

Website A'sDigital

Certificate

Website B'sDigital

Certificate

ClientHello,SNI="a.com"

ClientHello,

SNI='b.com'

a.comb.com

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TLS 1.2: SNI and CA are NOT encrypted...

Website A'sDigital

Certificate

Website B'sDigital

Certificate

ClientHello,SNI="a.com"

ClientHello,

SNI='b.com'

unencryptedencrypted TLS 1.2

a.comb.com

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TLS 1.2: SNI and CA are NOT encrypted...

Website A'sDigital

Certificate

ClientHello,SNI="a.com"

unencryptedencrypted

a.comb.com

SNI=a.comCA=a.com

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TLS 1.2: SNI and CA are NOT encrypted...

Website A'sDigital

Certificate

ClientHello,SNI="a.com"

unencryptedencrypted

a.comb.com

SNI=a.comCA=a.com

RST

RST

Timeline: TLS 1.3 is finalized

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3/21/2018

TLS 1.3 finalized

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TLS 1.3: SNI is still NOT encrypted...

Website A'sDigital

Certificate

Website B'sDigital

Certificate

ClientHello,SNI="a.com"

ClientHello,

SNI='b.com'

unencryptedencrypted TLS 1.3

Censors exploit SNI for censorship

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Circumvention: Domain Fronting

ServerClient

TLS ServerHello with Certificate

TLS 1.2

TLS ClientHello with SNI

server_name: unblocked.comcertificate: unblocked.com

CensorEncrypted

Unencrypted

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Circumvention: Domain Fronting

ServerClient

TLS ServerHello with Certificate

GET / HTTP/1.1Host: blocked.com

TLS 1.2

TLS ClientHello with SNI

server_name: unblocked.comcertificate: unblocked.com

CensorEncrypted

Unencrypted

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HTTP Response

Timeline: CDNs Cease Domain Fronting

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DNS-over-HTTPS added to Firefox 60.0

3/21/2018 4/13/2018

4/2/2018

TLS 1.3 finalized

Amazon announced an end to Domain Fronting

Timeline: CDNs cease domain fronting

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DNS-over-HTTPS added to Firefox 60.0

3/21/2018

4/30/2018

4/13/2018

4/2/2018

TLS 1.3 finalized

Amazon announced an end to Domain Fronting

Google announced an end to Domain Fronting

Timeline: ESNI is proposed for TLS 1.3

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DNS-over-HTTPS added to Firefox 60.0

3/21/2018

4/30/2018

4/13/2018 7/2/2018

4/2/2018

TLS 1.3 finalized

Amazon announced an end to Domain Fronting

Google announced an end to Domain Fronting

First ESNI Internet Draft

Timeline: Cloudflare supports ESNI

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DNS-over-HTTPS added to Firefox 60.0

3/21/2018

4/30/2018 9/24/2018

4/13/2018 7/2/2018

4/2/2018

TLS 1.3 finalized

Amazon announced an end to Domain Fronting

Google announced an end to Domain Fronting

First ESNI Internet Draft

Cloudflare started supporting ESNI

Timeline: Firefox supports ESNI

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DNS-over-HTTPS added to Firefox 60.0

3/21/2018

4/30/2018 9/24/2018

4/13/2018 7/2/2018 10/18/2018

4/2/2018

TLS 1.3 finalized

Amazon announced an end to Domain Fronting

Google announced an end to Domain Fronting

First ESNI Internet Draft

Cloudflare started supporting ESNI

ESNI added to Firefox Nightly

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How ESNI works?

How ESNI works?

CDN Edge ServerClient

TLS 1.3

CensorDNS over HTTPS/TLS

DNS Server

Encrypted

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How ESNI works?

CDN Edge ServerClient

TLS 1.3

DNS query: saf3241@vasdf3213ff....

Censor

DNS TXT QueryDNS TXT Result

DNS over HTTPS/TLS

DNS Server

Encrypted

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How ESNI works?

CDN Edge ServerClient

TLS 1.3

DNS query: saf3241@vasdf3213ff....

Censor

DNS TXT QueryDNS TXT Result

DNS over HTTPS/TLS

DNS Server

Encrypted

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How ESNI works?

CDN Edge ServerClientTLS ServerHello

TLS 1.3

TLS ClientHello with ESNI

DNS query: saf3241@vasdf3213ff....encrypted_server_name: f9jkls3zq....

Censor

DNS TXT QueryDNS TXT Result

DNS over HTTPS/TLS

DNS Server

Encrypted

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Research Questions

Research Questions

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● How many websites are supporting ESNI?

● How many currently censored websites in Chinacan be unblocked with the help of ESNI?

● Is there any censor already censoring ESNI traffic?

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How many websites are supporting ESNI?

How many sites are supporting ESNI?

As of August 2019, Cloudflare is the only CDN provider supporting ESNI.

Cloudflare provides an informative debugging page for every site using its CDN service.

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How to know if a site supports ESNI?

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Cloudflare debugging page

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Cloudflare debugging page

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How many sites are supporting ESNI?

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Location: On a VPS located in US

ESNI Enabled

How many sites are supporting ESNI?

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Location: On a VPS located in US

Alexa Top 1 Million Sites

ESNI Enabled

Websites Supporting ESNI

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More than

10% of Alexa Top 1 Million sites are supporting ESNI!

Result: SNI Status and TLS Version

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ESNI adoption with Sites Popularity

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9Websites Grouped by Rank

0

2.5

5.0

7.5

10.0

12.5

15.0

% W

ebsi

tes S

uppo

rting

ESN

I

Ranking (Millions)1

Research Questions

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● How many websites are supporting ESNI?

● How many currently censored websites in China can be unblocked with the help of ESNI?

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How websites are censored in China?

Major Censorship techniques in China

● DNS Hijacking

● IP Blocking

● SNI Filtering

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Detect DNS Hijacking - Result

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located in China Alexa Top 1 Million Sites

Detect DNS Hijacking - Result

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24,128 domains under DNS-Hijacking

located in China

Detect SNI Filtering - Setup

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located in China Alexa Top 1 Million Sites

Detect SNI Filtering - Result

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21,446 domains under SNI-Filtering

located in China

Detect IP Blocking - IP List

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Alexa Top 1 Million Sites

DNS

Resolve from Hong Kong

Select the first IP in an answer

Detect IP Blocking - IP List

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Alexa Top 1 Million Sites

DNS

Resolve from Hong Kong

Select the first IP in an answer

539,456 unique IPs

Detect IP Blocking - Setup

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Detect IP Blocking - Result

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39,787 domains under

IP-Blocking

47,069 sites censored among Alexa Top 1M

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39,787 domains under

IP-Blocking (84.5%)

24,128 domains under DNS-Hijacking

(51.2%)

21,446 domains under SNI-Filtering (45.6%)

Blocked by DNS HijackingFiltered by SNIBlocked by IP

Domains under different censorship

2514 704698

22,859

250 12

16,666

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Blocked by DNS HijackingFiltered by SNIBlocked by IP

2514 704698

22,859

250 12

16,666

70 sites are exclusively under SNI - Filtering

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Domains under different censorship

Blocked by DNS HijackingFiltered by SNIBlocked by IP

84.5% censored websites remain blocked in China

2514 704698

22859

250 12

16666

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Domains under different censorship

Blocked by DNS HijackingFiltered by SNIBlocked by IP

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Effectiveness of ESNI

DNS HijackingSNI FilteringIP Blocking

Effectiveness of ESNI

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101,200 domains

supporting ESNI

24,128 domains under DNS-Hijacking

39,787 domains under

IP-Blocking

ESNI Supported

21,446 domains under SNI-Filtering

DNS HijackingSNI FilteringIP Blocking

Assume DNS-based censorship evaded

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101,200 domains

supporting ESNI

0 domains under DNS-Hijacking

39,787 domains under

IP-Blocking

ESNI Supported

21,446 domains under SNI-Filtering

66 sites can be unblocked by ESNI

101,049 4,70266

23,024

85 16,678

0

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Current Effectiveness of ESNI

Support ESNIFiltered by SNIBlocked by IP

Censored websites VS. ESNI supporting websites

66 sites can be unblocked by ESNI

101,049 4,70266

23,024

85 16,678

0

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Current Effectiveness of ESNI

medium.comboxun.comchinadigitaltimes.netbannedbook.orgrsf.orgamnesty.org.au….

Support ESNIFiltered by SNIBlocked by IP

Censored websites VS. ESNI supporting websites

ESNI increases the cost of blocking 101k sites

101,049 4,70266

23,024

85 16,678

0

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Support ESNIFiltered by SNIBlocked by IP

Censored websites VS. ESNI supporting websites

IPs belong to CDN edge server are blocked

101,049 4,70266

23,024

85 16,678

0

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Support ESNIFiltered by SNIBlocked by IP

Censored websites VS. ESNI supporting websites

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Monitoring ESNI-based Censorship

Any area already censoring ESNI traffic?

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Monitoring ESNI-based Censorship from 14 Different Areas

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Monitoring ESNI-based Censorship - Setup

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Sites supporting ESNI

ESNI Enabled

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Monitoring from 14 Different Areas

No ESNI-based Censorship Detected!

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Conclusions

Conclusions

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● 10% websites among Alexa Top 1M are supporting ESNI.

Conclusions

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● 10% websites among Alexa Top 1M are supporting ESNI.

● 84.5% currently censored websites will remain blocked in China even if DNS- and SNI-based censorship are evaded.

Conclusions

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● 10% websites among Alexa Top 1M are supporting ESNI.

● 84.5% currently censored websites will remain blocked in China even if DNS- and SNI-based censorship are evaded.

● Only 66 websites currently censored in China can be unblocked by ESNI.

Conclusions

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● 10% websites among Alexa Top 1M are supporting ESNI.

● 84.5% currently censored websites will remain blocked in China even if DNS- and SNI-based censorship are evaded.

● Only 66 websites currently censored in China can be unblocked by ESNI.

● No ESNI-based censorship is detected in our experiment across 14 different areas.

ContactsZimo Chai - CS MS/PhD Studentzchai@cs.umass.edu

SPIN Lab, with Amir Houmansadrhttps://people.cs.umass.edu/~amir/Research.html

We have released all our probing tools and datasets at http://traces.cs.umass.edu/index.php/Network, to maintain reproducibility and to benefit future research works.

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Let’s Enable ESNI Now!1. Open about:config in Firefox2. Set network.security.esni.enabled to true3. Set network.trr.mode to 34. Set network.trr.uri to

https://1.1.1.1/dns-query5. Check if it works:

https://www.cloudflare.com/ssl/encrypted-sni

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