Cryptography Reference
In-Depth Information
2.7. In this exercise use the key
IRONCLAD
.
DXXAXAF VXDFDAA AFAAXVG FFXDFAX
GXAGFVF GAFVFAX FXDXXDA XFXAVGF
This is a quote from
Ben Jonson (1573-1637)
taken from
“To the Memory
of My Beloved, the Author, Mr. William Shakespeare” (1623)
. Jonson
was an English actor, poet, and dramatist.
2.8. In this exercise use the key
FRANCISKEY
.
AXFAA AFDXA FFFXX DFXDV VGAFX
FGFFA VAXXG AGFFX XVXXX XFFXF
This is a quote from
Adolph S. Ochs (1858-1935)
that has become the slo-
gan for the New York Times. Ochs was an American newspaper publisher.
2.9. In this exercise use the key
FLAUBERT
.
XAXXAGFF GFFXXAAF FFVFAXGF DFFAXXXA
AXGFXXXG XAAXXFXX FAGFVFFF AXXXXFVV
This was written by
Francis Scott Key (1729-1843)
and is part of
“The
Star-Spangled Banner”
. Key was an American poet.
2.10. This exercise refers to Friedman's index of coincidence discussed on pages
85-87.
Calculate the index of coincidence of the following ciphertext using the
formula displayed on page 86.
BAETKBESZMZIOMVWSSWYFEUKP
YEBHLNOBIQAMSXAOQFGBDPAE
2.11. Assuming the ciphertext in Exercise 2.10 was produced via the Vigenere
cipher with keyword
XANADU
, find the plaintext.
This is a quote from
Gustave Flaubert (1821-1880)
. Flaubert was a French
novelist. Compare with the comment on page 334 in parentheses at the end
of part 2 in the discussion of “Token Applications”.
2.12. Show that the probability of choosing identical letters from a text in
which there are equal numbers of each letter in English is
0
.
0385.
(
Hint: The probability of choosing two of any letter is
(1
/
26)
2
.
)
≈
2.13. If you are told that the following ciphertext was created by a modular
shift given by
c
m
+ 7 (mod 26), find the plaintext.
TVKBSHYHYPAOTLAPJPGLHZF
≡
In Exercises 2.14-2.20, solve the given congruence for
x
.
Hint: To find
the multiplicative inverse of the coeGcient
a
of
x
in each case, find that
integer
y
such that
ay
≡
1 (mod
n
). For instance, in Exercise 2.15,
6
·
6
≡
1 (mod 7)
,
so 6 is the inverse of itself modulo 7. For the reader requiring more back-
ground on modular arithmetic, see pages 475-482 in Appendix A.
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