How to get the original raw commit message without removing extra leading/trailing line breaks

Hi Folks, 

I am using command "git show --format=raw <commit hash>" to get raw commit object in order to verify the GPG signature in my program to customize public key loading. However in the raw commit object, extra line breaks before and after the commit message is removed, which cause the signature verification failure as git generates the signature without removing those line breaks. 

So is there any way to get the exact, original commit message? I also tried format option %B in no vail. 

Thanks for your time. Robin