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HPKP Best Practices for Let’s Encrypt

This post describes how to generate a few backup public key hashes to add to your HTTP Public Key Pinning (HPKP) config that might save you from bricking your domain if Let’s Encrypt ever gets untrusted like StartCom did.

If you have a healthy distrust of the X.509 PKI trust model, then you’ve probably heard of HPKP (and probably also HSTS & CAA). Website certificate pinning was a trend first started by google, who hard-coded a pin of their certificates in their Chrome browser. Eventually, google helped build a more standardized pinning method under RFC 7469. And today, it’s supported by Chrome, Firefox, and Opera.

Pinning is a great TOFU improvement to https, but–if misconfigured–you could “brick” your domain–making it so that your client’s browsers will refuse to let them access your site for months or years (interestingly, this has also caused some security experts to think of how HPKP could be abused in ransom-ware). Therefore, it’s a good idea to follow a few HPKP Best Practices.

Calculate Public Key Pins

This section will show the commands used to calculate the Public Key Pins that you’ll need to add to your web server’s config. In these examples, I’m using certificates that I recently created for a website that I’m securing: openbuildinginstitute.org

This guide assumes you’ve already generated your certificates with Let’s Encrypt, and that they’ve been stored to ‘/etc/letsencrypt/live/<your_domain>/’.

Leaf Certificate

First, get the pin of your leaf certificate issued by Let’s Encrypt.

[root@centos7 ~]# openssl x509 -in /etc/letsencrypt/live/openbuildinginstitute.org/cert.pem -pubkey | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | base64
UbSbHFsFhuCrSv9GNsqnGv4CbaVh5UV5/zzgjLgHh9c=

Note that this hash may change every 90 days when your certificate is renewed.

Let’s Encrypt Intermediate Certificate

Next, get the pin of the intermediate Let’s Encrypt certificate that signed your certificate.

[root@centos7 ~]# openssl x509 -in /etc/letsencrypt/live/openbuildinginstitute.org/chain.pem -pubkey | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | base64
YLh1dUR9y6Kja30RrAn7JKnbQG/uEtLMkBgFF2Fuihg=

Note that this hash may change when Let’s Encrypt rotates their Intermediate certificates. At the time of writing, they’ve already rotated to their third intermediate certificate.

Let’s Encrypt Root Certificate

Next, get the pin of the Root Let’s Encrypt certificate (The Internet Security Research Group (ISRG) is a California public benefit corporation that benefits Let’s Encrypt).

[root@centos7 ~]# mkdir /var/tmp/letsencrypt
[root@centos7 ~]# cd /var/tmp/letsencrypt/
[root@centos7 letsencrypt]# wget --quiet https://letsencrypt.org/certs/isrgrootx1.pem
[root@centos7 letsencrypt]# cat isrgrootx1.pem 
-----BEGIN CERTIFICATE-----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=
-----END CERTIFICATE-----
[root@centos7 letsencrypt]# openssl x509 -in isrgrootx1.pem -pubkey | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | base64
C5+lpZ7tcVwmwQIMcRtPbsQtWLABXhQzejna0wHFr8M=

IdenTrust Root Certificate

Next, get the pin of the IdenTrust Root certificate, which was used to cross-sign the Let’s Encrypt Root Certificate above.

Unfortunately, there’s no easy way to script this. You have to go to IdenTrust’s website and copy & paste the contents of their <textarea> into a text editor. Then you have to add ‘—–BEGIN CERTIFICATE—–‘ to the top of the file and ‘—–END CERTIFICATE—–‘ to the end of the file.

[root@centos7 ~]# mkdir /var/tmp/letsencrypt
[root@centos7 ~]# cd /var/tmp/letsencrypt/

# type the contents of the <textarea> from this site https://www.identrust.com/certificates/trustid/root-download-x3.html
# then add '-----BEGIN CERTIFICATE-----' to the top of the file and
# then add '-----END CERTIFICATE-----' to the end of the file
[root@centos7 letsencrypt]# vim identrust.dst.root.x3.pem
[root@centos7 letsencrypt]# cat /var/tmp/identrust.dst.root.x3.pem
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
[root@centos7 letsencrypt]# openssl x509 -in identrust.dst.root.x3.pem -pubkey | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | base64
Vjs8r4z+80wjNcr1YKepWQboSIRi63WsWXhIMN+eWys=

Backup Key CSRs

It’s a good idea to pregenerate keys & Certificate Signing Requests (CSRs). In fact, this is required–though no tool can actually validate this.

[root@centos7 ~]# mkdir /etc/pki/tls/hpkpBackupKeys
[root@centos7 ~]# chown root:root /etc/pki/tls/hpkpBackupKeys
[root@centos7 ~]# chmod 0700 /etc/pki/tls/hpkpBackupKeys/
[root@centos7 ~]# cd /etc/pki/tls/hpkpBackupKeys/
[root@centos7 hpkpBackupKeys]# openssl genrsa -out first.key 4096
Generating RSA private key, 4096 bit long modulus
.................................................................++
..............................................................................................................................................................................................................................++
e is 65537 (0x10001)
[root@centos7 hpkpBackupKeys]# openssl req -new -key first.key -sha256 -out first.csr
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [XX]:US
State or Province Name (full name) []:
Locality Name (eg, city) [Default City]:
Organization Name (eg, company) [Default Company Ltd]:
Organizational Unit Name (eg, section) []:
Common Name (eg, your name or your server's hostname) []:
Email Address []:

Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:

[root@centos7 hpkpBackupKeys]# openssl genrsa -out second.key 4096
Generating RSA private key, 4096 bit long modulus
...............................................................................................................++
...............................................................................................................................................++
e is 65537 (0x10001)
[root@centos7 hpkpBackupKeys]# openssl req -new -key second.key -sha256 -out second.csr
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [XX]:US
State or Province Name (full name) []:Missouri
Locality Name (eg, city) [Default City]:Maysville
Organization Name (eg, company) [Default Company Ltd]:Open Source Ecology
Organizational Unit Name (eg, section) []:
Common Name (eg, your name or your server's hostname) []:opensourceecology.org
Email Address []:letsencrypt@opensourceecology.org

Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:

[root@centos7 hpkpBackupKeys]# chown -R root:root /etc/pki/tls/hpkpBackupKeys
[root@centos7 hpkpBackupKeys]# chmod 0400 /etc/pki/tls/hpkpBackupKeys/*

[root@centos7 hpkpBackupKeys]# openssl req -pubkey < first.csr | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | base64
MDhNnV1cmaPdDDONbiVionUHH2QIf2aHJwq/lshMWfA=
[root@centos7 hpkpBackupKeys]# openssl req -pubkey < second.csr | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | base64
OIZP7FgTBf7hUpWHIA7OaPVO2WrsGzTl9vdOHLPZmJU=

In the above example, I locked-down the permissions of these backup key files to root:root 0400. However, it would be a good idea to store them in an encrypted backup in a safe & securely delete them from the server entirely.

Two Backup CAs

Finally, it’s a good idea to pin the root certificate for two distinct CAs in the event that Let’s Encrypt is unusable in the future. In this example, I’m using

  1. CloudFlare (because they offer free SSL certificates) and
  2. ssl.com (because they offer free 90-day certificates)

Note that CloudFlare’s certificates themselves are cross-signed by digicert, addtrust, globalsign, and gtecybertrust (now digicert), so we include those as well.

####################################################################
# pin cloudflare root CA certs if Let's Encrypt disappears one day #
####################################################################
# https://support.cloudflare.com/hc/en-us/articles/115001186052-What-intermediates-and-roots-are-Cloudflare-issued-certs-signed-against-

[root@centos7 ~]# mkdir /var/tmp/cloudflare
[root@centos7 ~]# cd /var/tmp/cloudflare

# CloudFlare Universal SSL Certs are signed by Comodo or GlobalSign
# https://www.crt.sh/?d=1
# https://www.crt.sh/?d=88

[root@centos7 cloudflare]# wget --quiet -O addtrust.pem https://www.crt.sh/?d=1
[root@centos7 cloudflare]# cat addtrust.pem 
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
[root@centos7 cloudflare]# openssl x509 -in /var/tmp/cloudflare/addtrust.pem -pubkey | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | base64
lCppFqbkrlJ3EcVFAkeip0+44VaoJUymbnOaEUk7tEU=

[root@centos7 cloudflare]# wget --quiet -O globalsign.pem https://www.crt.sh/?d=88
[root@centos7 cloudflare]# cat globalsign.pem 
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
[root@centos7 cloudflare]# openssl x509 -in /var/tmp/cloudflare/globalsign.pem -pubkey | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | base64
K87oWBWM9UZfyddvDfoxL+8lpNyoUB2ptGtn0fv6G2Q=

# CloudFlare Dedicated Certificates are signed by DigiCert
# https://www.crt.sh/?d=76

[root@centos7 cloudflare]# wget --quiet -O digicert.pem https://www.crt.sh/?d=76
[root@centos7 cloudflare]# cat digicert.pem 
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
[root@centos7 cloudflare]# openssl x509 -in /var/tmp/cloudflare/digicert.pem -pubkey | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | base64 
Y9mvm0exBk1JoQ57f9Vm28jKo5lFm/woKcVxrYxu80o=

# GTE CyberTrust was acquired by DigiCert
[root@centos7 cloudflare]# wget --quiet -O gtecybertrust.pem https://www.crt.sh/?d=10
[root@centos7 cloudflare]# cat gtecybertrust.pem 
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
[root@centos7 cloudflare]# openssl x509 -in /var/tmp/cloudflare/gtecybertrust.pem -pubkey | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | base64
EGn6R6CqT4z3ERscrqNl7q7RC//zJmDe9uBhS/rnCHU=

#################################################################
# pin ssl.com root CA certs if Let's Encrypt disappears one day #
#################################################################
# https://www.ssl.com/repository/

[root@centos7 cloudflare]# mkdir /var/tmp/ssl.com
[root@centos7 cloudflare]# cd /var/tmp/ssl.com

[root@centos7 ssl.com]# wget --quiet https://www.ssl.com/repository/SSLcom-RootCA.zip
[root@centos7 ssl.com]# unzip SSLcom-RootCA.zip Archive:  SSLcom-RootCA.zip
  inflating: SSLcom-RootCA-EV-RSA-4096-R2.pem  
  inflating: SSLcomEVRootCertificationAuthorityECC.pem  
  inflating: SSLcomRootCertificationAuthorityECC.pem  
  inflating: SSLcomRootCertificationAuthorityRSA.pem  
[root@centos7 ssl.com]# cat SSLcom-RootCA-EV-RSA-4096-R2.pem
Subject: CN=SSL.com EV Root Certification Authority RSA R2,O=SSL Corporation,L=Houston,ST=Texas,C=US
Issuer: CN=SSL.com EV Root Certification Authority RSA R2,O=SSL Corporation,L=Houston,ST=Texas,C=US
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
[root@centos7 ssl.com]# cat SSLcomEVRootCertificationAuthorityECC.pem
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
[root@centos7 ssl.com]# cat SSLcomRootCertificationAuthorityECC.pem 
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
[root@centos7 ssl.com]# cat SSLcomRootCertificationAuthorityRSA.pem 
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
[root@centos7 ssl.com]# for file in $(ls *.pem); do echo /var/tmp/ssl.com/$file; openssl x509 -in /var/tmp/ssl.com/$file -pubkey | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | base64; done
/var/tmp/ssl.com/SSLcomEVRootCertificationAuthorityECC.pem
NIdnza073SiyuN1TUa7DDGjOxc1p0nbfOCfbxPWAZGQ=
/var/tmp/ssl.com/SSLcom-RootCA-EV-RSA-4096-R2.pem
fNZ8JI9p2D/C+bsB3LH3rWejY9BGBDeW0JhMOiMfa7A=
/var/tmp/ssl.com/SSLcomRootCertificationAuthorityECC.pem
oyD01TTXvpfBro3QSZc1vIlcMjrdLTiL/M9mLCPX+Zo=
/var/tmp/ssl.com/SSLcomRootCertificationAuthorityRSA.pem
0cRTd+vc1hjNFlHcLgLCHXUeWqn80bNDH/bs9qMTSPo=

Put it all together

Extracting just the public key pins from above, that gives us 14 hashes:

UbSbHFsFhuCrSv9GNsqnGv4CbaVh5UV5/zzgjLgHh9c=
YLh1dUR9y6Kja30RrAn7JKnbQG/uEtLMkBgFF2Fuihg=
C5+lpZ7tcVwmwQIMcRtPbsQtWLABXhQzejna0wHFr8M=
Vjs8r4z+80wjNcr1YKepWQboSIRi63WsWXhIMN+eWys=
lCppFqbkrlJ3EcVFAkeip0+44VaoJUymbnOaEUk7tEU=
K87oWBWM9UZfyddvDfoxL+8lpNyoUB2ptGtn0fv6G2Q=
Y9mvm0exBk1JoQ57f9Vm28jKo5lFm/woKcVxrYxu80o=
EGn6R6CqT4z3ERscrqNl7q7RC//zJmDe9uBhS/rnCHU=
NIdnza073SiyuN1TUa7DDGjOxc1p0nbfOCfbxPWAZGQ=
fNZ8JI9p2D/C+bsB3LH3rWejY9BGBDeW0JhMOiMfa7A=
oyD01TTXvpfBro3QSZc1vIlcMjrdLTiL/M9mLCPX+Zo=
0cRTd+vc1hjNFlHcLgLCHXUeWqn80bNDH/bs9qMTSPo=
MDhNnV1cmaPdDDONbiVionUHH2QIf2aHJwq/lshMWfA=
OIZP7FgTBf7hUpWHIA7OaPVO2WrsGzTl9vdOHLPZmJU=

We’ll use these in our web server config below.

Add to Webserver config

To enable HPKP with the backup public key pins calculated above, add the following to your cooresponding web server’s configuration file & reload your web server.

Be sure to test this on a disposable domain first. After you’ve validated everything (I’d wait 6 months–or through a few Let’s Encrypt certificate renewals at least), you should consider increasing the “max-age” to something like 6 months = “15552001” seconds.

Note that you should change the “report-uri” argument as needed to your domain. This can be very useful in troubleshooting issues with client-side HPKP issues which would otherwise be invisible to a System Administrator.

Apache

If you’re using apache, you need to add this line to your <VirtualHost> block.

Header set Public-Key-Pins "pin-sha256=\"UbSbHFsFhuCrSv9GNsqnGv4CbaVh5UV5/zzgjLgHh9c=\"; pin-sha256=\"YLh1dUR9y6Kja30RrAn7JKnbQG/uEtLMkBgFF2Fuihg=\"; pin-sha256=\"C5+lpZ7tcVwmwQIMcRtPbsQtWLABXhQzejna0wHFr8M=\"; pin-sha256=\"Vjs8r4z+80wjNcr1YKepWQboSIRi63WsWXhIMN+eWys=\"; pin-sha256=\"lCppFqbkrlJ3EcVFAkeip0+44VaoJUymbnOaEUk7tEU=\"; pin-sha256=\"K87oWBWM9UZfyddvDfoxL+8lpNyoUB2ptGtn0fv6G2Q=\"; pin-sha256=\"Y9mvm0exBk1JoQ57f9Vm28jKo5lFm/woKcVxrYxu80o=\"; pin-sha256=\"EGn6R6CqT4z3ERscrqNl7q7RC//zJmDe9uBhS/rnCHU=\"; pin-sha256=\"NIdnza073SiyuN1TUa7DDGjOxc1p0nbfOCfbxPWAZGQ=\"; pin-sha256=\"fNZ8JI9p2D/C+bsB3LH3rWejY9BGBDeW0JhMOiMfa7A=\"; pin-sha256=\"oyD01TTXvpfBro3QSZc1vIlcMjrdLTiL/M9mLCPX+Zo=\"; pin-sha256=\"0cRTd+vc1hjNFlHcLgLCHXUeWqn80bNDH/bs9qMTSPo=\"; pin-sha256=\"MDhNnV1cmaPdDDONbiVionUHH2QIf2aHJwq/lshMWfA=\"; pin-sha256=\"OIZP7FgTBf7hUpWHIA7OaPVO2WrsGzTl9vdOHLPZmJU=\"; max-age=3600; includeSubDomains; report-uri=\"http://opensourceecology.org/hpkp-report\""

Nginx

If you’re using nginx, you need to add this line to your http{ } block.

add_header Public-Key-Pins 'pin-sha256="UbSbHFsFhuCrSv9GNsqnGv4CbaVh5UV5/zzgjLgHh9c="; pin-sha256="YLh1dUR9y6Kja30RrAn7JKnbQG/uEtLMkBgFF2Fuihg="; pin-sha256="C5+lpZ7tcVwmwQIMcRtPbsQtWLABXhQzejna0wHFr8M="; pin-sha256="Vjs8r4z+80wjNcr1YKepWQboSIRi63WsWXhIMN+eWys="; pin-sha256="lCppFqbkrlJ3EcVFAkeip0+44VaoJUymbnOaEUk7tEU="; pin-sha256="K87oWBWM9UZfyddvDfoxL+8lpNyoUB2ptGtn0fv6G2Q="; pin-sha256="Y9mvm0exBk1JoQ57f9Vm28jKo5lFm/woKcVxrYxu80o="; pin-sha256="EGn6R6CqT4z3ERscrqNl7q7RC//zJmDe9uBhS/rnCHU="; pin-sha256="NIdnza073SiyuN1TUa7DDGjOxc1p0nbfOCfbxPWAZGQ="; pin-sha256="fNZ8JI9p2D/C+bsB3LH3rWejY9BGBDeW0JhMOiMfa7A="; pin-sha256="oyD01TTXvpfBro3QSZc1vIlcMjrdLTiL/M9mLCPX+Zo="; pin-sha256="0cRTd+vc1hjNFlHcLgLCHXUeWqn80bNDH/bs9qMTSPo="; pin-sha256="MDhNnV1cmaPdDDONbiVionUHH2QIf2aHJwq/lshMWfA="; pin-sha256="OIZP7FgTBf7hUpWHIA7OaPVO2WrsGzTl9vdOHLPZmJU="; max-age=5256000; includeSubDomains'; report-uri="http://opensourceecology.org/hpkp-report"

Conclusion

HPKP can be dangerous if deployed incorrectly, but–if done properly–it can be a valuable way to reduce the risk of MITM attacks.

In this guide, we certainly pinned a large number of certificates, including many root CA’s certificates. With each pin, we increase the MITM attack surface area, but we also reduce the risk of “bricking” a domain.

I think the approach shown here is the best middleground for risk & security that applies to most SysAdmins using Let’s Encrypt on their webservers today.

Further Reading

If you like using TOFU to further lock-down https on your websites, you would probably also be interested in HSTS & CAA:

  1. HTTP Strict Transport Security
  2. Certification Authority Authorization

2 comments to HPKP Best Practices for Let’s Encrypt

  • You said that HPKP can be dangerous if deployed incorrectly, which is so true.

    What I am curious about is to know if you think that HPKP is worth the effort considering what little benefit it actually provides.

    In my opinion, there is little gain in security by using HPKP, but you add a hell of a lot of risk to your service if you decide to use it.

    I picture it like putting an explosive device on a hair trigger in order to protect against the risk that zombies from the Resident Evil will set up a lemonade stand in front of your fast food chain franchise. Isn’t it an overkill solution to an extremely unlikely threat? I’d like to know your opinion.

    • Michael

      @Tomas Thanks for your question [and for publishing your own awesome blog :) ]. I think you have to weigh the pros & cons. Your position definitely has valid concerns, and is generally accepted by many website administrators.

      Personally, I do think the pros outweigh the cons of using HPKP when you follow the above best-practices guide.

      The breadth of CAs whitelisted by popular browsers today is dangerous. If we use HPKP to limit this whiltelist to only a few popular & trusted CA’s root certificates, then we’ll be safely able to generate a new cert for our site in the future–signed by one of these CAs. While I think this pinning is more broad than what HPKP was probably intended for, I think it’s the best middle-ground to gaining the benefits of HPKP without the dangers of pinning only a few of our own pre-generated certs.

      In fact, as a customer, I’d greatly appreciate if the websites I send sensitive data to (CCNs, SSNs, or any PII really) would implement HPKP. I don’t think it’s an “extremely unlikely” threat that someone might use a rogue “trusted” CA in the browser’s whitelist to MITM an https connection. We have countless examples (Comodo, d-link, eDellroot, Startcom, Diginotar, MCS Holdings, etc) of such rogue CAs in the past, and I’m confident that there are still trusted CAs in that whilteist that are in the hands of someone wishing to preform malicious deeds.

      Rather than relying on the Browser’s reactive game of whack-a-mole, why not just explicitly limit the whitelist for your site to a few redundant, trusted CAs using HPKP? That seems like the best middle-ground to me.

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