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Unit 14: Authentication

z z Hash value is encrypted with the sender's private key. Notes
z z Encrypted hash value is appended to the message as a digital signature.
z z Message is sent.
The following figure 14.13 shows the sequence of verifying with the addition of the supporting
elements of public key cryptography.

Figure 14.12: Sequence of signing Public Key Cryptography

Source: http://technet.microsoft.com/en-us/library/aa998077(v=exchg.65).aspx
z z Message is received.
z z Digital signature containing encrypted hash value is retrieved from the message.
z z Message is retrieved.
z z Hash value of the message is calculated.
z z Sender's public key is retrieved.
z z Encrypted hash value is decrypted with the sender's public key.

z z Decrypted hash value is compared against the hash value produced on receipt.
z z If the values match, the message is valid.
The sequence shows how public key cryptography provides the capabilities that give a digital
signature its core security services: authentication, nonrepudiation, and data integrity.

14.4.4 Public Key Cryptography and Message Encryption

Unlike digital signatures, the relationship between public key cryptography and message
encryption is generally more straightforward, because encryption is a core function of public
key cryptography. However, message encryption is not accomplished by only encrypting and
decrypting the message using the key pair. The key pair is used in message encryption, but not
for the entire message.

Because the goal of message encryption is to ensure that only authorized recipients can view the
message, the private key of each recipient is suited to provide that service. Because the private
key can only be successfully used by its owner, the use of the key during the reading of a message
ensures that the owner of that key, and only the owner of that key, can read the message. This
capability provides the confidentiality that underlies message encryption. Further, because the
public key can be distributed widely, it allows any number of people to send information to a
single private key holder.

However, the key pair is not used on the entire message. This is because the encryption and
decryption operation using a key pair is an expensive process, due to the necessary complexity
of the keys' algorithms. Although a key pair needs to be used, it does not necessarily have to

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