When you use the sui keytool generate command, the output is indeed in a Base64 format. The .key file generated contains Base64 encoded information, including the keypair. You typically don't need to extract Base64 from it separately unless you have specific needs for the Base64 string itself.

If your key file was created using a tool like sui keytool, the entire content should be Base64 encoded. However, if you want to confirm or use it as a Base64 string elsewhere, you can read the file content as you would any text file, as it's already in a Base64 formatted string.