Cryptography Reference
In-Depth Information
the details of the algorithm in action.
Pretty Good Privacy (PGP)
PGP embodies five protocols for the secure transmission of e-mail messages.
PGP Protocols
1. Authentication.
2. Compression.
3. Confidentiality.
4. E-mail compatibility.
5. Segmentation.
Now we look at each of these in detail. We assume that Alice is communi-
catingwith Bob.
Authentication (Digital Signature)
Protocol Steps
1. Alice creates a message,
m
, to be used for the purpose of authenticating
herself to Bob.
2. SHA-1 (see page 255) is used on
m
to create a 160-bit message digest,
h
(
m
).
3. Alice enciphers
h
(
m
) with her private RSA key
d
A
. She sends
D
A
=
(
d
A
(
h
(
m
))
,m
) to Bob. On the network,
D
A
passes through a ZIP com-
pression operation, denoted by
Z
. (We will learn more about ZIP later.)
4. After decompression, denoted by
Z
−
1
, Bob uses Alice's public RSA key
e
A
to decipher and recover
h
(
m
).
5. Bob applies
h
to the value of
m
sent by Alice and compares the result to
the value of
h
(
m
) he deciphered in step 4.
Diagram 8.1 PGP Authentication
−−−−−−−−→
h
(
m
)
d
A
←−−−−
h
(
m
)
h
m
d
A
(
h
(
m
))
Alice
m
(
d
A
(
h
(
m
))
,
m
)
=
D
A
−−−−→
m
D
A
−−−−→
D
A
−−−−→
Z
−
1
Z
D
A
h
←−−−−−−−−−−−−−−−−−−
←−−−−
e
A
m
↓
h
(
m
)
↓
h
(
m
)
←
Compare
→
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