Public Key Infrastructure

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Public-Key-Infrastrukturen

Public-Key-Infrastrukturen versuchen das Problem zu lösen, zu überprüfen, ob ein öffentlicher Schlüssel zu einer bestimmten Einheit gehört, um Man-in-the-Middle-Angriffe zu verhindern.

Ansätze
  1. Certificate Authorities
  2. Web of Trust
Zertifizierungsstellen (CA)
  • signieren die Zertifikate von Endteilnehmern und verknüpfen so eine Art von Identität (z. B. einen Domänennamen oder eine E-Mail-Adresse) mit einem öffentlichen Schlüssel.
  • CAs werden mit TLS- und S/MIME-Zertifikaten verwendet, und das CA-System hat eine lange Liste möglicher und tatsächlicher Probleme, die im Abschnitt Hardening PKI und (Durumeric, Kasten, Bailey, & Halderman, 2013) zusammengefasst sind.
Web of Trust

Das Web of Trust ist ein dezentralisiertes System, bei dem Menschen gegenseitig ihre Schlüssel signieren, so dass die Wahrscheinlichkeit hoch ist, dass es einen "Vertrauenspfad" von einem Schlüssel zum anderen gibt.

  • Es wird mit PGP-Schlüsseln verwendet, und obwohl es die meisten Probleme des CA-Systems vermeidet, ist es umständlicher.
Alternativen

Als Alternativen zu diesen öffentlichen Systemen gibt es zwei weitere Möglichkeiten

  • Die Verwendung einer privaten Zertifizierungsstelle und das manuelle Vertrauen in Schlüssel (wie es bei SSH-Schlüsseln oder manuell vertrauenswürdigen Schlüsseln in Webbrowsern verwendet wird).
    • Der erste Teil dieses Abschnitts befasst sich damit, wie man ein Zertifikat im CA-System erhält.
  • Der zweite Teil enthält Empfehlungen, wie Sie die Sicherheit Ihrer PKI verbessern können.

Certificate Authorities

In order to get a certificate, you can find an external CA willing to issue a certificate for you, run your own CA, or use self-signed certificates.

  • As always, there are advantages and disadvantages for every one of these options; a balance of security versus usability needs to be found.

Certificates From an External Certificate Authority

There is a fairly large number of commercial CAs that will issue certificates for money.

  • Some of the most ubiquitous commercial CAs are Verisign, GoDaddy, and Teletrust.
  • However, there are also CAs that offer certificates for free.
  • The most notable examples are StartSSL, which is a company that offers some types of certificates for free, and CAcert, which is a non-profit volunteer-based organization that does not charge at all for issuing certificates.
  • Finally, in the research and education field, a number of CAs exist that are generally well-known and well-accepted within the higher-education community.

A large number of CAs is pre-installed in client software’s or operating system’s`‘trust stores’'; depending on your application, you have to select your CA according to this, or have a mechanism to distribute the chosen CA’s root certificate to the clients. When requesting a certificate from a CA, it is vital that you generate the key pair yourself.

  • In particular, the private key should never be known to the CA.
  • If a CA offers to generate the key pair for you, you should not trust that CA.

Generating a key pair and a certificate request can be done with a number of tools.

  • On Unix-like systems, it is likely that the OpenSSL suite is available to you.

In this case, you can generate a private key and a corresponding certificate request as follows: 

$ openssl req -new -nodes -keyout <servername>.key -out <servername>.csr -newkey rsa:<keysize> -sha256
Country Name (2 letter code) [AU]:DE
State or Province Name (full name) [Some-State]:Bavaria
Locality Name (eg, city) []:Munich
Organization Name (eg, company) [Internet Widgits Pty Ltd]:Example
Organizational Unit Name (eg, section) []:Example Section
Common Name (e.g. server FQDN or YOUR name) []:example.com
Email Address []:admin@example.com
Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:

Setting Up Your Own Certificate Authority

In some situations it is advisable to run your own certificate authority.

  • Whether this is a good idea depends on the exact circumstances.
  • Generally speaking, the more centralized the control of the systems in your environment, the fewer pains you will have to go through to deploy your own CA.
  • On the other hand, running your own CA maximizes the trust level that you can achieve because it minimizes external trust dependencies.

Again using OpenSSL as an example, you can set up your own CA with the following commands on a Debian system: 

$ cd /usr/lib/ssl/misc
$ sudo ./CA.pl -newca
Answer the questions according to your setup.
  • Now that you have configured your basic settings and issued a new root certificate, you can issue new certificates as follows:
$ cd /usr/lib/ssl/misc
$ sudo ./CA.pl -newreq

Alternatively, software such as TinyCA that acts as a "wrapper" around OpenSSL and tries to make life easier is available.

Creating a Self-Signed Certificate

If the desired trust level is very high and the number of systems involved is limited, the easiest way to set up a secure environment may be to use self-signed certificates.

  • A self-signed certificate is not issued by any CA at all, but is signed by the entity that it is issued to.
  • Thus, the organizational overhead of running a CA is eliminated at the expense of having to establish all trust relationships between entities manually.

With OpenSSL, you can self-sign a previously created certificate with this command: 

$ openssl req -new -x509 -key privkey.pem -out cacert.pem -days 1095

You can also create a self-signed certificate in just one command:
$ openssl req -new -x509 -keyout privkey.pem -out cacert.pem -days 1095 -nodes -newkey rsa:<keysize> -sha256

The resulting certificate will by default not be trusted by anyone at all, so in order to be useful, the certificate will have to be made known a priori to all parties that may encounter it.

Hardening PKI

In recent years several CAs were compromised by attackers in order to get a hold of trusted certificates for malicious activities.

  • In 2011 the Dutch CA Diginotar was hacked and all certificates were revoked (Elinor Mills, 2011).
  • Recently Google found certificates issued to them, which were not used by the company (Damon Poeter, 2011).
  • The concept of PKIs heavily depends on the security of CAs.
  • If they get compromised the whole PKI system will fail.
  • Some CAs tend to incorrectly issue certificates that were designated to do a different job than what they were intended to by the CA (Adam Langley, et. al., 2013).

Therefore several security enhancements were introduced by different organizations and vendors (H. Tschofenig and E. Lear, 2013).

Certification Authorization Records

RFC 6844 describes Certification Authorization Records, a mechanism for domain name owners to signal which Certificate Authorities are authorized to issue certificates for their domain. When a CAA record is defined for a particular domain, it specifies that the domain owner requests Certificate Authorities to validate any request against the CAA record.

  • If the certificate issuer is not listed in the CAA record, it should not issue the certificate.

The RFC also permits Certificate Evaluators to test an issued certificate against the CAA record, but should exercise caution, as the CAA record may change during the lifetime of a certificate, without affecting its validity. CAA also supports an iodef property type which can be requested by a Certificate Authority to report certificate issue requests which are inconsistent with the issuer’s Certificate Policy.

Configuration of CAA records

BIND supports CAA records as of version 9.9.6. A CAA record can be configured by adding it to the zone file: $ORIGIN example.com 

      CAA 0 issue "ca1.example"
      CAA 0 iodef "mailto:security@example.com"

If your organization uses multiple CA’s, you can configure multiple records: 

     CAA 0 issue "ca1.example"
     CAA 0 issue "ca2.example"

"ca1.example" and "ca2.example" are unique identifiers for the CA you plan on using.

  • These strings can be obtained from your Certificate Authority, and typically are its top level domain.
  • An example is "letsencrypt.org" for the Let’s Encrypt CA operated by the Internet Security Research Group.

Knot-DNS supports CAA records as of version 2.2.0.

Validation of CAA records

Once a CAA record is deployed, it can be validated using the following dig query: 

$ dig CAA google.com
; <<>> DiG 9.10.3-P4-Debian <<>> CAA google.com
;; ANSWER SECTION:
google.com.          3600 IN   CAA  0 issue "symantec.com"

On older versions of Dig, which do not support CAA records, you can query the record type manually: 

$ dig +short -t TYPE257 google.com
\# 19 0005697373756573796D616E7465632E636F6D

Weblinks

  1. https://bettercrypto.org/
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