Now the roles are swapped. Everybody can decrypt the
signature using Bob??™s public key, having the proof that it was
really Bob (as the sole owner for his private key) who performed
the signature. Operatively, once Alice receives the data fragment,
she reapplies the same thumbprint function used by Bob
for signing; then she decrypts the signature, using Bob??™s public
key and compares the decrypted value with the thumbprint
value she calculated. If the two values match, Alice has the
proof that the data fragment is actually the one that Bob originally
signed. Figure 1-6 summarizes the process.
Alice
Bob
Data H H(Data) H(Data)
H(Data)
Signature
Data
H H(Data)
H(Data)
=?
Bob
Bob
Bob
E
E
Bob
E E
Figure 1-6 How a digital signature works. In the top half of the picture,
Bob applies his signature to a data fragment. In the lower half,
Alice veri?¬?es Bob??™s signature.
44 The Problem
If anybody modi?¬?es the data fragment after it has been signed,
the signature will break. Applying the thumbprint function to the
modi?¬?ed statement will give a certain result that will differ from
the result recorded inside the digital signature, hence exposing
the modi?¬?cation.
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