用私钥签名，生成签名文件：

openssl smime -sign -inkey private.key -signer public.crt -in test.txt -out test.signed.txt

用公钥验证签名文件：

openssl smime -verify -in test.signed.txt -signer public.crt -out test.txt -CAfile RootCA.crt
#成功显示 Verification successful
#失败显示 Verification failure