Transcript CRYPTOGRAPHY - Brown University

CRYPTOGRAPHY
lecture 3
Wednesday
Secure transmission
Steganography
cryptography
Transposition
Monoalphabetic
Substitution
The difference between
substitution and transposition is
that in:
Subtitution: each letter retains its
position but changes its identity,
Transposition: each letter retains
its identity but changes its
position.
Example 3
HW #2: Caesar shift problems
2. KENKMOC PYBDEXK TEFKD
3.MHILYLZAZBHLXBPZXBLMVYABUHLHW
WPBZJSHBKPBZJHLJBZKPJABTHYJHUB
TLZAULBAYVU
• QK MEJXNEJJ ZMEVTJ Q YVNGEZ
VNHKE KVMMHY JXMEEXJ HB
FHWWNEJXHKE WEKEVXP XPE
PVNH HB V JXMEEX NVTI Q XRMK
TA FHNNVM XH XPE FHNZ VKZ ZVTI
YPEK TA EAEJ YEME JXVWWEZ WA
XPE BNVJP HB V KEHK NQDPX
XPVX JINQX XPE KQDPX VKZ
XHRFPEZ XPE JHRKZ HB JQNEKFE
new ciphers
1. MPV DIZ DFF EPUQH D WZIM QUSZ
NPY EZSUBJZIUQH GZTTDHZT UT UC
KVQ
2. HGJUBQPU V HY H CVP AHT BA DHQG
ZVYBT V KB GVZJRT JB BJURW YQZVE
JBB CQJ ZVTER VJ VZ YBZJGS
EGHZZVEHG VJ KBRZ TBJ VTZDVWR
HTS EVDURWR
new ciphers
1. HGJUBQPU V HY H CVP AHT BA
DHQG ZVYBT V KB GVZJRT JB
BJURW YQZVE JBB CQJ ZVTER
VJ VZ YBZJGS EGHZZVEHG VJ
KBRZ TBJ VTZDVWR HTS
EVDURWR
Mono-alphabetic
Substitution Cipher
• Allow any permutation of the alphabet
• Each letter is replaced by a different letter
or symbol
• Key = permutation (still need to decide on a
key and exchange this information in a
secure way).
• 26! Possibilities
– What does this mean?
Frequency
Analysis
HW # 3: Reading
Review: The Code Book chapter 1 & read
chapter 2 p. 45-51
Find websites on substitution ciphers and
frequency analysis
Begin research for a 5-7 page essay on “Why
are substitution and transposition ciphers
obsolete, and when did they become so.” Due on
Monday in class.
(in class): Substitution cipher
SK BKGKC FBTKCHZWBT W ZXUBA HM SKOO,
WBT EWQK UZ MFC MSB, WH SXKB SK XWGK
TUHJMGKCKT UZ DMC MFCHKOGKH
(in class): Substitution cipher
CGXTOUNZL NQ UDOU HDNTD BCJONLQ
HDCL ZLC DOQ WZBMZUUCL CACBEUDNLM
DC KCOBLCG NL QTDZZK.
HW # 3: Substitution cipher
UWZEZ VTPUW ZBEIK WVRWT UPUZT UWPUV YZFZE PAIQB
SISVT RBFZE TZJPR UNIKW PUUWZ GAVFZ ETZVT YBEPA
SKWIV UVTWZ EZVUK VNNVA TUPAU NISVT PCCZP EPASQ
ZEZCN PRZSQ ITBOZ UWVAX ZFZAO BEZQV HPEEZ PASVA
ZJCNV RPQNZ UWZEZ VTPAB UWZEU WZBEI KWVRW TUPUZ
TUWPU UWVTW PTPNE ZPSIW PCCZA ZS
HW # 3: Substitution cipher
VBIPA
IJPIE
IGDLA
IRVAM
IFJDT
MADPM
FIJKD
IDGAP
BDQLN
MDMBI
PMBIE
IJJWF
MAEWL
BWPDM
MBIIW
DVBAE
NIPMA
ADPJD
NIELW
JIGWF
DQFJI
YTDFD
HWPNJ
BIFWP
FMBMB
BMBIL
MLIMB
TRWPO
FIMBI
WMADP
DTBQR
PIGID
VBAEB
NMDWJ
IJIGW
WVJDT
IRWNI
APNFI
EWQJI
WPIUI
GLIMD
BWUIE
JQRIW
FWMIW
PWMQF
EIPMF
SQAFI
JVBAE
PMJAM
NAJJD
DPPIE
RDPKM
PNISQ
IWPND
IJGIE
JMBWM
BARGI
HIEDR
LUIMB
MINMB
BIGDV
WLJMW
TPWMQ
MMDMB
MBIYJ
LMBIR
HW # 3: Substitution cipher
NBSQF
VLMVE
LSNRH
QSRMN
SHZMC
HLDMR
LFMCQ
RHDLF
HLHSR
FQSRM
GVZQS
VKLUV
LQSGV
HSGVQ
VPVVO
HVMHV
XGVQR
VXVRU
MCVCG
KKNVS
SGRHX
ZMCRG
GZSGN
LSGVQ
VMVUV
HNVZM
HTFRC
HGGRH
VCEQL
VZQSE
GLFTG
GZMTV
ZUVGR
BGVZQ
TRUVM
QDZHZ
CGRNR
VHGVK
AVXZF
NNBHR
LQZHE
SRMNV
HLFTG
H
SZMCR
RGLKC
AZQTZ
MLMVN
LUVHN
HVRMN
TGSNB
QLNNV
GRHGF
SLFQA
GZUVG
GRHCV
RMAVS
BGVZQ
BGVZQ
VRSAR
GVZQS
LMGRN
QSCRC
KRHHN
RHABI
ZQZMC
SVQCQ
SRMGR
SELQL
CVHGR
DZHDL
GRHGF
HNZQS
BSQFV
FHSVW
NRMVG
RUVMG
NGRHS
MXVRS
HGVZQ
FMCVC
QSCRC
ALSGV
KLUVG
XGZMT
VXZMM
RHGVZ
GLFTG
DZHGR
SGRHD
DRSGG
KRTGS
JFZKG
ZSGNB
HW # 3: Substitution cipher
826821526251162515231172625148114
14826232621111161411615142623722526
14826168262326172213262514826232621111
11221614231252212142221622161827182625
8262326116252026232671782152625148114
14826232621111161411615142623722526
252610711112512011
Websites which help
http://www.geocities.com/cryptogramcorner/
http://cs.colgate.edu/faculty/nevison/Core139Web/tools/substitution.html
What can we do to improve the
substitution cipher?
In 1460 Battista Alberti wrote an essay on what he
believed to be a new form of cipher: use two or more
cipher alphabets alternately to encrypt a message.
ABCDEFGHIJKLMNOPQRSTUVWXYZ
GHIJKLMNOPQRSTUVWXYZABCDEF
RSTUVWXYZABCDEFGHIJKLMNOPQ
plain text
cipher1
cipher2
Let’s encrypt the statement
IT IS WAY TOO EARLY IN THE DAY TO BE DOING THIS
OK OJ CRE KUF KRXCE ZT KNV JRE KU SK UUZTX ZYOJ
12 34 567 890 12345 67 890 123 45 67 89012 3456
What can we do to improve the
substitution cipher?
Alberti was followed by Johannes Trithemius (born
1462) and Giovanni Porta (born 1535) who developed
his ideas. Finally, Vigenere put all these ideas
together. Let’s take a whole table of Caesar shift
alphabets. The first row will have a Caesar shift of 1,
the second of 2, etc. Each letter in the plaintext
message can be enciphered by a different row.
Vigenere
cipher:
Start
with a
table of
Caesar
shift
alphabets.
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
SAVEMEPLEASE
CRYPTOGRAMCR
URTTFSVCEMUV
plain text
keyword
enciphered
The first letter, S is encrypted using
the row beginning with C
The second letter, A is encryted
using the row beginning with R
The third letter, V is encrypted using
the row beginning with Y
The fourth letter, E, is encrypted
using the row beginning with P.
And so on . . .
Z A B C D E F G H I J K L M N O P Q R S T U V W X Y
You can use http://www.simonsingh.net/The_Black_Chamber/v_square.html
Vigenere cipher
• Frequency analysis does not apply.
• Enormous number of possible keys
• The Vigenere cipher is a polyalphabetic
cipher.
• It was then neglected for 2 centuries –
it is hard to break but also hard to
encrypt.
Vigenere cipher:
the unbreakable code
• At first glance the Vigenère Cipher appears
to be unbreakable, due to its use of up to 26
different cipher alphabets. Ciphers like this,
which use more than one cipher alphabet are
known as Polyalphabetic Ciphers. These can be
incredibly difficult to decipher, because of
their resistance to letter frequency analysis.
Indeed, over time, the Vigenère cipher
became known as 'Le Chiffre Undechiffrable',
or 'The Unbreakable Cipher'.
This slide and the next few copied directly from Simon Singh’s website.
Ciphers
• Monoalphabetic ciphers: each letter in the
plaintext is encoded by only one letter from
the cipher alphabet, and each letter in the
cipher alphabet represents only one letter in
the plaintext.
• Polyalphabetic ciphers: each letter in the
plaintext can be encoded by any letter in the
cipher alphabet, and each letter in the cipher
alphabet may represent different letters from
the plaintext each time it appears.