This is especially helpful if you run your own servers. If you are presented with an error message or warning that the signature has changed or does not match the IP/domain you are connecting to you always want to verify manually.
So your e-mail/web client will show you an SHA-1 fingerprint like this:
"Could not verify this certificate because the issuer is unkown" or other reasons such as a mismatch in IP/domain.
It will also show you the "SHA1 fingerprint". Copy this and compare below with the results of your actual server certificate.
How to verify it against the actual certificate on your server?:
openssl x509 -fingerprint -in /pathto/your-certificate.crt -noout
How to manually fetch the SHA1 certificate straight from the server to compare?
openssl s_client -showcerts -connect yourdomain.com:port 2>/dev/null|openssl x509 -fingerprint -noout
Automated Bash Script
#change servercertpath to your certificate
#change remote host/ip and port number as necessary
localfingerprint=`openssl x509 -fingerprint -in $servercertpath -noout`
#the echo -e \n prints a newline to the SSL client this is necessary or it will never exit so the script will halt and not complete
remotefingerprint=`echo -e "\n"|openssl s_client -showcerts -connect $remotehost 2>/dev/null|openssl x509 -fingerprint -noout`
if [ "$localfingerprint" == "$remotefingerprint" ]; then
echo "OK - Certs match: local=$localfingerprint remote=$remotefingerprint"
echo "BAD - Certs don't match could be man in the middle!: local=$localfingerprint remote=$remotefingerprint"
This is an important and good way to verify that you are actually talking to who you think you are and that there is no direct interception or Middleman attack.