2. Public Key Encryption Using Digital

Signatures and Hash Digests
Hash function:
Mathematical algorithm that produces fixed-length number called
message or hash digest
 A HAS FUNCTION CEN BE SIMPLE, AND COUNT THE NUMBER OF
DIGITAL 1S IN A MESSAGE, OR It can be more complex, and produce a
128 bit number that reflects the number of 0s and 1s , the number 00s
, 11s, and so on.
 Example: MD4, MD5, SHA-1, SHA-2
Hash digest of message sent to recipient along with message to verify
integrity
(Hash digest and message) encrypted with recipient’s public key
Entire cipher text then encrypted with Sender private key—creating
digital signature (e - signature)—for authenticity, nonrepudiation
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Slide 5-1
Public Key Encryption Using Digital Signatures and
Hash Digests
A digital signature or “signed” cipher text that
can be sent over the Internet.
 A digital signature is a close parallel to a
handwritten signature. Like a handwritten
signature, a digital signature is unique.
 When used with a hash function, the digital
signature is even more unique than a
handwritten signature. And changes for every
document.

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MD5 Overview (Just concept)
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Public Key Cryptography with Digital Signatures
HASH
+
MESSAGE
HASH
+
MESSAGE
Figure 5.9, Page 291
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Slide 5-4
Public Key Encryption Using Digital Signatures and
Hash Digests

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The receiver of this signed cipher( message + message digest)
first uses the sender’s public key to authenticate the
message.
Once Authenticate, the recipient uses his or her private key to
obtain the hash result (message digest) and original message.
As a final step, the recipient applies the same hash function to
the original text (original message) and compares the result
with the result sent by the sender.
If the results are same (Send message digest = receiver
message digest) the receiver now knows the message has not
been changed during transmission The message has integrity.
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