dozernz / clone-cert

Simple shell script to "clone" X.509 certificates

clone-cert.sh

This is a simple shell script that retrieves the X.509 certificate associated with an SSL wrapped TCP port and uses openssl to create a similar certificate by replacing the RSA modulus and fixing the signature. This is particularly useful if you want to demonstrate why self-signed certificates cannot provide any security. Of course, the fingerprint of the cloned certificate will be different... but who checks the fingerprint of a self-signed certificate by hand?

Example

Run the script:

$ ./clone-cert.sh www.example.com:443
/tmp/www.example.com:443.key
/tmp/www.example.com:443.cert

The new certificate is in /tmp/www.example.com:443.cert and the corresponding private key is in /tmp/www.example.com:443.key. Their difference is only in the RSA modulus and the signature:

$ diff <(openssl x509 -in /tmp/www.example.com:443.cert -noout -text) \
       <(openssl s_client  -connect www.example.com:443 < /dev/null 2> /dev/null \
         | openssl x509 -noout -text)
16,33c16,33
<                     00:c3:59:26:a5:ed:1c:2b:75:3a:0c:a2:ab:49:43:
<                     e8:1a:cd:24:64:4a:11:5a:fe:94:10:57:2a:af:f6:
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<                     25:2f
---
>                     00:b3:40:96:2f:61:63:3e:25:c1:97:ad:65:45:fb:
>                     ef:13:42:b3:2c:99:86:f4:b5:80:0b:76:dc:06:38:
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>                     55:44:93:ab:78:a1:47:4d:a5:dc:07:5a:9c:67:f4:
>                     11:68:12:2f:d3:28:71:bc:ad:72:05:3c:16:75:d4:
>                     f8:72:58:ba:19:f1:dc:09:ed:f1:18:c6:92:2f:7d:
>                     bc:16:0b:37:8d:8a:ef:1b:6f:4f:b9:e0:7a:54:98:
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>                     e1:db:d5:39:d8:18:5b:d1:f2:64:33:d0:d6:c4:23:
>                     ff:09:ab:6d:71:ce:da:cf:c1:17:9c:23:be:2c:af:
>                     2f:92:1c:3f:90:08:89:58:f2:b1:e1:10:6f:83:2e:
>                     f7:9f
67,81c67,81
<          66:9e:dc:08:c6:81:2e:91:80:d7:7a:27:a9:0f:fb:72:89:53:
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<          e3:2b:97:3b
---
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