Why I am getting the "error while loading serial number" error when running OpenSSL "ca" command?

If you are running the OpenSSL "ca" command installed with the slproweb binary package for Windows, you may get the "error while loading serial number" error as shown below:

C:\Users\fyicenter>\local\OpenSSL-Win32\bin\openssl.exe

OpenSSL> ca -in test.csr -keyfile my_ca.key -cert my_ca.crt
Using configuration from C:\local\OpenSSL-Win32\bin\openssl.cfg
Enter pass phrase for my_ca.key:
./demoCA/serial: No error
error while loading serial number
5016:error:02001002:system library:fopen:No such file or directory:
   .\crypto\bio\bss_file.c:398:fopen('./demoCA/serial','rb')
5016:error:20074002:BIO routines:FILE_CTRL:system lib:.\crypto\bio\bss_file.c:400:
error in ca

This error is caused by the "dir=./demoCA" and "serial=$dir/serial" options in the configuration file. These options requires you to have a file called "\demoCA\serial" under the current directory to be used as a serial number register. You have to set an initial value like "1000" in the file. After that OpenSSL will increment the value each time a new certificate is generated.

Fixing this error is easy. Just create the serial number file: ./demoCA/serial, as shown below:

C:\Users\fyicenter>copy CON demoCA\serial
1000
<Ctrl>-Z
        1 file(s) copied.

C:\Users\fyicenter>dir demoCA\serial
    10:27 PM                 6 index.txt

Note that the value 1000 is a hexadecimal format, which is 4096 in decimal format.

Also note that press <Ctrl>-Z is to end the input stream to finish the copy command.

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⇑ OpenSSL "ca" Command

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