Resolve SSL certificate error with Git¶

Git uses cURL internally for transfering files. Unfortunately cURL uses its own certificate-store
and the certificate-chain (Telekom-CA-->DFN-CA-->TU-Clausthal) isn't included by default.

Now, when you try to access a Git-repository you will get an error similar to this:
Windows

git clone https://scm.in.tu-clausthal.de/git/testgit
Cloning into 'testgit'...
fatal: unable to access 'https://scm.in.tu-clausthal.de/git/testgit/': 
SSL certificate problem: unable to get local issuer certificate

Linux

git clone https://scm.in.tu-clausthal.de/git/testgit
Cloning into 'testgit'...
error: server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt 
CRLfile: none while accessing https://scm.in.tu-clausthal.de/git/testgit/info/refs

This is because by default cURL does not know the used certificate chain and rejects the GIT operation, which is a good thing to do so.
The solution is to tell Git/cURL about this chain.

1. the good and secure method which requires to install the certificate chain used here.
2. the bad and insecure method which is based on the fact that GIT/cURL bypasses the SSL certificate verification. The bad and insecure method allows a man-in-the-middle-attack should NOT be used. You have been warned! Don't even think about using it. If you are really, really, really ... sure about what you going to to, follow the bad and insecure method.

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The good and secure method to use Git over HTTPS¶

Git uses cURL for transfering files. Unfortunately the root certificate of the certificate chain used here at the Clausthal University of Technologoy is not included in the default cURL installation. (See this page for further information about TU Claustha-CA)

The certificate chain looks like this:

"Deutsche Telekom Root CA 2" 
+--"DFV-Verein PCA Global -G01" 
   +--"TU Clausthal CA - G02"@
      +--"scm.in.tu-clausthal.de" 

In order to securely work with Git you need to import the DFN certificate chain.
We will provide the step-by-step-manual for most operating systems.

1. Download the DFN-certificate-chain from the DFN (Deutsches Forschungs Netz)
   Right-click on "Zertifikatkette anzeigen" and select "Save target on"
   https://pki.pca.dfn.de/tu-clausthal-ca/pub/cacert/chain.txt
2. Locate the folder where the git executable is installed.
   1. On Windows this file is called curl-ca-bundle.crt and its located most likely at c:\Programs (x86)\Git\bin
   2. On Debian-Linux it is is located at /etc/ssl/certs/ca-certificates.crt
3. Decide if the certificate-chain should be installed system-wide or just user- or account-wide.
   1. For a system-wide installation just append the certificate-chain from above to this file-
      1. On Windows open a administrator console. (Windows-Key->Type "cmd"->right-click on it and select "Run as administrator" from the pop-up)
         cd to the download location and type type chain.txt >> "c:\Program Files (x86)\Git\bin\curl-ca-bundle.crt"
         
      2. On Debian-Linux (as root or sudo)
         cd to the doenload location and type cat chain.txt >> /etc/ssl/certs/ca-certificates.crt
         
   2. For user-wide installation make a copy of the *.crt file above and place it in your home directory. Then append the DFN certiticate-chain to the cURL-certificate-store just like above.
      A last step is needed. Tell Git to use this newly edited certificate-store.
      This can easily be done using git config --global https.sslCAInfo /path/to/ca-certificates.crt

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Note: cURL provides an automatic extract of all root-certificated from Mozilla. This ca-file can be downloaded here. For more information about cURL certificates head over to this page

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