Introduction to Information Security

What Is A Digital Signature?

• A digital signature is an electronic signature that uses a special kind of encryption program. Here is a sample program.
• The message: “The British are coming!”
• The encryption instructions: replace every letter with the letter that follows it in the alphabet.
• This yields: “Uif Csjujti bsf dpnjoh!”

Asymmetric Encryption

• Digital signatures use a special kind of encryption called asymmetric encryption (or public key encryption).
• A “key” is just a sequence of numbers.
• You add it to the message you want to encrypt; then you then apply the encryption program to the message plus the key.

Example

Message
Key
+
Encrypted result
Message
Key
+
Encrypted result
Same message
Different key
+
Different encrypted result
Application of the encryption program
=

Private and Public Keys

• Asymmetric encryption uses two keys. The sender uses one to encrypt; the recipient uses one to decrypt.
• The keys are referred to as the private and public keys.
• Private key is private in the sense that the key owner makes sure the public does not have access to it.
• The public key is public in the sense the owner makes it freely available to the public.
• An example is helpful.

How Does A Digital Signature Work?

• Suppose Alice wants to digitally sign an e-mail.
• She runs a “hash function” on the message. This turns the message into a sequence of letters and numbers, called the message digest. Each message is associated with a unique message digest.
• Asymmetric encryption is slow. It is not ideal for encrypting a whole message.
• So what you encrypt is the much shorter message digest.
• The point is not secrecy, but signature.