Generate private key:
openssl genrsa -out privkey.pem -3des 2048

Extract private key:
openssl rsa -in privkey.pem -pubout -out pubkey.pem

Calculate sha1 sum and sign it with the priv key:
sha1 file | openssl rsautl -sign -inkey privkey.pem -out file.sum

Verify the sum and compare with sum of file:
openssl rsautl -verify -pubin -inkey pubkey.pem -in file.sum | sha1 -c