Cryptography Reference
In-Depth Information
Diagram 8.4 LZ77 Compression
Source Data
Sliding-History
Buffer
Look-Ahead
Buffer
Input
←−−−−−−−−−−
First Word
Discard
←−−−−−−−−−
Last Word
Output
Compressed
Data
E-Mail Compatibility
Typically, PGP sends a stream of bytes of data. However, there are certain
e-mail networks allowingonly ASCII data to be transmitted. PGP satisfies
this requirement by transformingthe stream of bytes into a stream of print-
able ASCII characters, usingan encodingtechnique called radix 64, which we
describe in Appendix D; see page 541. This inflates the message by 33%, but
the aforementioned compression stage offsets this message expansion. In fact,
a standard analysis of the PGP mechanism shows that, even with the message
expansion, the net compression is approximately one-third.
From a security viewpoint, the aforementioned conversion to ASCII provides
a camouflage of the data since it blindly converts to ASCII (even in the case
that the original data is already ASCII text). Therefore, it will be unreadable to
Eve, even if the message is not enciphered. Optionally, PGP may be formatted
so that only the signature part is converted to ASCII, which permits Bob to
read the message without conversion, (albeit, PGP should still be used to verify
Alice's signature).
In summary, once h ( m ) is formed, the concatenation, ( m,h ( m )), is signed
by Alice to get d A ( m,h ( m )) = D A , which is compressed via Z . Then she
enciphers via k using E to get, E k ( Z ( D A )), then e B ( k ) is appended to get
( e B ( k ) ,E k ( Z ( D A )), which is converted to ASCII. Upon receipt, Bob reconverts
to binary; recovers k via d B , which he uses to get Z ( D A ). This is passed through
Z 1 , after which he uses e A to get ( m,h ( m )). He applies h to m; and compares
his h ( m ) to the version sent by Alice.
Segmentation
Anyone who has tried to send a very large e-mail attachment knows that
certain e-mail sites will “bounce back” the message stating that it exceeds the
maximum message length allowable (which typically over the Internet is 5
10 4
bytes). Hence, segmentation, the splitting up of the message into smaller pieces
or segments, is necessary. PGP meets this requirement by segmenting a message
into manageable, acceptable blocks for easy transmission. Segmentation is done
after all of the above processingis completed.
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