Making Digital Signatures

Remember public-key crypto:
Separate keys for locking and unlocking
Give everyone a copy of the locking key
Keep the unlocking key secret

For digital signatures:
Also separate keys for locking and unlocking
Keep the locking key secret
Give everyone a copy of the unlocking key

To make a digital signature:
Alice locks the message with her secret locking key.
To verify Alice's signature:
Bob unlocks the message with his copy of Alice's unlocking key.  If it unlocks correctly, the signature is valid.