The receiver then performs the following three steps to verify that the
message was not changed during transmission.
This is the Title of the Book, eMatter Edition
Copyright ?© 2007 O??™Reilly & Associates, Inc. All rights reserved.
200 | Chapter 5: Authentication and Encryption
First, using the sender??™s public key, the recipient decrypts the digital signature to
obtain the message digest originally generated by the sender.
Second, using the same message digest algorithm originally used by the sender, the
recipient generates another message digest of the received message.
Third, the recipient compares both message digests (the one sent by the sender, and
the one generated by the recipient). If the two digests are not identical (exactly the
same), it means the message was modified during transmission and cannot be trusted.
The recipient can be sure that the digital signature was sent by the sender (and not by a
malicious user) because only the sender??™s public key can decrypt the digital signature
(which was encrypted by the sender??™s private key). If the recipient decrypts using the
wrong public key, that decrypting renders a faulty message digest, which means that
either the message or the message digest is not exactly what the sender sent.
Using public key cryptography in this manner ensures integrity, because the recipient
possesses the means to tell whether the message received was exactly what was
sent.
Pages:
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383