3. 2008 Kutztown University 3
Steganography
Merriam-Webster: The art or practice of concealing a
message, image, or file within another message, image, or
file
from Greek
» steganos = covered
» grafo = write
Histiaeus – tyrant of Miletus
shaved head of most trusted slave
tattooed a message
hair grew back covering message
Advantage – does not draw attention to
itself
messenger
recipient
Often combined with cryptography
4. 2008 Kutztown University 4
Steganography Example
You may have seen the TV show – In Plain Sight –which is
based entirely on the federal witness protection program.
The show is about people who have testified or will be
testifying soon as witnesses in criminal cases but whose
lives are in danger as a result. For their protection they
are given new identities and are moved to a new
community. Ergo they are all hidden “in plain sight”. And
if you think this would not work, according to the U.S.
Marshalls extant website, no program participant who
follows security guidelines has ever been harmed while
under the active protection of the Marshals Service.
5. 2008 Kutztown University 5
Caesar Cipher
Example of a shift cipher
Encryption – forward shift by 3
Decryption – backward shift by 3
Shift ciphers
Private key
Symmetric key
Key = shift amount
Keyspace = 25
Plain text – IHAVEASECRET
Cipher text – LKDYHDVHFUHW
6. 2008 Kutztown University 6
Caesar Cipher – Example
L KDYH D GUHDP WKDW RQH GDB WKLV QDWLRQ ZLOO ULVH XS DQG OLYH
RXW WKH WUXH PHDQLQJ RI LWV FUHHG: "ZH KROG WKHVH WUXWKV WR
EH VHOI-HYLGHQW: WKDW DOO PHQ DUH FUHDWHG HTXDO."
L KDYH D GUHDP WKDW RQH GDB RQ WKH UHG KLOOV RI JHRUJLD WKH VRQV
RI IRUPHU VODYHV DQG WKH VRQV RI IRUPHU VODYH RZQHUV ZLOO EH
DEOH WR VLW GRZQ WRJHWKHU DW WKH WDEOH RI EURWKHUKRRG.
L KDYH D GUHDP WKDW RQH GDB HYHQ WKH VWDWH RI PLVVLVVLSSL, D
VWDWH VZHOWHULQJ ZLWK WKH KHDW RI LQMXVWLFH, VZHOWHULQJ
ZLWK WKH KHDW RI RSSUHVVLRQ, ZLOO EH WUDQVIRUPHG LQWR DQ
RDVLV RI IUHHGRP DQG MXVWLFH.
L KDYH D GUHDP WKDW PB IRXU OLWWOH FKLOGUHQ ZLOO RQH GDB OLYH LQ
D QDWLRQ ZKHUH WKHB ZLOO QRW EH MXGJHG EB WKH FRORU RI WKHLU
VNLQ EXW EB WKH FRQWHQW RI WKHLU FKDUDFWHU.
L KDYH D GUHDP WRGDB.
7. 2008 Kutztown University 7
Substitution Cipher
Randomly generated substitution
Example
A F
B K
C D
D J
etc.
Characteristics
Private & symmetric key
Monoalphabetic
Key = alphabet of substitutions
Keyspace = 26!
8. 2008 Kutztown University 8
Substitution Cipher – Analysis
Keyspace = 26! =
403291461126605635584000000 = 4.03 x
1026
But other factors make it insecure
Letter frequency
N-grams
Strong elimination coefficient
With patience, can be decoded by hand
Plain text – BOOKKEEPINGROCKS
Cipher text – JXXTTZZDOYBEXATU
11. 2008 Kutztown University 11
Transposition Cipher – Example
Cipher text
TYTSNHOAGTGERLUSHATEUAGNTIHVLBEAURRYTHHAOH
UUCGLGOATHYTNSUSGHTGREGNHLATUEEATAIHLVBEOT
LUAHHNERDWTAANRODESUHIEVNETAAMINYNFENNOTOR
TSTIHFLAEAHAINSNTDEHBGAEVSYTREHEENFIHMNOIARS
EPWDEEEEAUSRFPEALSYIBMMSAAIYTROINBNSYEOKNME
CCOOLUEDRYADMECRSAOEAECSNEHEWFNTLHONRDISBA
EYFOUOURTSSSPSTEOLFDHIFEELWEOHTIRETDHIWREAEIA
SNVVABKLRIEYMSHNEEEGANIRONPECLHFITNUFAAOIRNG
HCRBKOTAEHEUCKRGNNLSDEIAIMNTAGKSSMICELSOOTO
EFDLNTGHIIENNNIDMNNAAABSOYTETNNDEWOIRYOWWN
HLSLDIEAGYNECSHOCMTNETOSHIMTIIEGNHLTOHFNETRI
NMSCBLUHLOSOWWSYOBSWULLWEATSRTWTOHDOEKLS
NBLDRIARHEITMIGSBIEETRMTNHTGOAIEIAMNDREAGBIA
OSNSFTYLEIOONTNGHTIOAIEDTIRESRWAYVAMDEBTFIAL
EAONNGRENSDTEHIONRCDLWOIANDRSWWUNRTCCHOAL
SHLRWIGDAEIPNAYMNOSOAETHRTEUSTALUGEGNGIALHV
EATRBTTYHAHUCGULHORNRACDLIACRMCMUAOCHORYN
OPCRONYNOO
12. 2008 Kutztown University 12
Vigenere Cipher
Polyalphabetic substitution
Use n randomly generated substitutions
1st letter is encoded by 1st substitution alphabet
2nd letter is encoded by 2nd substitution alphabet
. . .
nth letter is encoded by nth substitution alphabet
n+1st letter is encoded by 1st substitution alphabet
etc.
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Vigenere – Simple Example
Key = 3752
Successive letters are shifted by 3, 7, 5, 2
Plain text – BOOKKEEPINGROCKS
Cipher text – EVTMNLJRLULTRJPU
Eliminates double letters
Scatters N-grams
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Vigenere Cipher
Advantages
Creates confusion
Same letter can be encoded n different ways
Pretty much eliminates n-grams
Keyspace > 26!
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Vigenere Cipher
le chiffre indéchiffrable
Named for Blaise de Vigenère
Invented by Giovan Battista Bellaso
ca. 1550
Broken by
Charles Babbage in 1854 (unpublished)
Major Friedrich Kasiski in 1863
»Prussian infantry office
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Vigenere Cipher – Example
T KRCS L GILOX WYHH ZQV KOJ WYPG YDKPCY ZZSZ CLJL IA DEK ZTYV VIE WYL HCXV
TSLQZUU ZI ZAG NUVLR: "HH YVZO WYLGP WIBHSV KV PP VVST-PYZKSYW: KOOE DCS
APQ RYS NUVHHPG VXILO."
Z OOGH R KFPDD AVLW FUS ODP VB EKV YSO KZSZD RW NSZUXPO EKV ZCYV FM TZUDLF
DORCSD DEK HSH JVBD RW MCCPVY GWDML CHQVYG HLCS PP DSSS ER JPH ORNU
HZJVAVPU RA HSH KHPWH FM PCRKOSCKFVR.
T KRCS L GILOX WYHH ZQV KOJ HMLB EKV ZHLWV VT XLJZWDVZWDT, D JAOEH
JDSWWVYWYJ NPHS WYL VPDK VT TQABGELTL, GHHCASCLEN KTWY AVP KVHH ZI
FWDCHJZWZQ, NPZW EV AFLQJMCCPVK WYWF HB ZDJPG ZI WYSPGFT OYG ABGELTL.
W SDML O OUVHA EKRA AJ IFBF WLKAZP FYPZOUVU KTOC VBP GRF ZTYV PB L QRAWZQ
NOSCH KOSJ ZZSZ YRK IS UXUNSO EP AVP FFSCC RW AVPLI ZYTQ SBH MB KOS
NREASYW FM HSHZY QSDIHQEHI.
P VLYV H RCHRT HZGRF.
W SDML O OUVHA EKRA CYH UHM, ORNU WY DCHPLPR, DWEK ZAG GLTPCFV IHQTVKZ,
KTWY PHD JFCSCQFY VLYZUU SLJ SWAV UYWASZUU HLKO HSH NVFOV FM
WYWVYDZVZAWZQ RUR YXCSWQLTHHTRE; VBP GRF FTJYA HSHIL WY DCHPLPR,
SWEWCL PWDTR PZBJ HBO ECHQV JZYZD ZZSZ MH RIZP WF QCTQ YHBOV NPHS
OZAHWH NOWEH SVMD DEK KSLKL UTUCZ OD VZZHPUJ HBO EIVHSHIZ.
W SDML O OUVHA ERUHM.
T KRCS L GILOX WYHH ZQV KOJ HMLFJ YRSZPB JOOWO SL SIDCASO, HMLFJ KZSZ LQU
TCFQKHWY VYHZW EV TOOH
20. 2008 Kutztown University 20
Deciphering Vigenere
Determine the number of alphabets
Compute distances between matching sequences
Compute GCD of distances
Treat cipher text as n separate texts
For each separate text & each of 25 possible shifts
Compute Index of Coincidence
based on frequencies found in cipher text
using table of frequencies of letters in English
Index of Coincidence formula
fee = S {relFreqTab(k, ciphLet) *
charFreq(ciphLet)}
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Frequency Table – English
A 0.08; B 0.015; C 0.03; D 0.04; E 0.13;
F 0.02; G 0.015; H 0.06; I 0.065; J 0.005
K 0.005; L 0.035; M 0.03; N 0.07; O 0.08
P 0.02; Q 0.002; R 0.065; S 0.06; T 0.09
U 0.03; V 0.01; W 0.015; X 0.005; Y 0.02
Z 0.002
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Vigenere – Final Step
Produce possible plain texts
using combination of
highest ranking fee table values
Choose best plain text
This step can be automated
Rate each possible plain text
using n-gram information
or list of 5 letter words in English
23. 2008 Kutztown University 23
Vernam Cipher
Gilbert Sandford Vernam – inventor
Also known as one-time pad
Invented ca. 1919
Proven unbreakable by Claude Shannon
Communication Theory of Secrecy Systems
1949
Unbreakable if and only if
Key is same length as plain text
Key is never re-used
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Vernam Cipher
Basic operation – bitwise XOR
XOR table
0 xor 0 = 0
0 xor 1 = 1
1 xor 0 = 1
1 xor 1 = 0
Plain text is represented as bit stream
Key is random bit stream of same length
Cipher text is produced via bitwise XOR of
plain bit stream and key bit stream.
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Vernam Cipher – Example
Plain text :: Grade = A – Great!
Plain text in ASCII
71 114 97 100 101 32 61 32 65 32 45 32
71 114 101 97 116 33
Plain text as bit stream
01000111 01110010 01100001 01100100
01100101 00100000 00111101 00100000
01000110 00100000 10010110 00100000
01010011 01101111 01110010 01110010
01111001 00100001
26. 2008 Kutztown University 26
Vernam Cipher – Example
Key as bit stream
11000001 01110000 11011110 10111001 01100001
10001000 01101100 11111010 00110011
01001110 01111001 00011110 00001000 10010001
10100100 01000000 10000000 01000010
Cipher text as bit stream
10000110 00000010 10111111 11011101 00000100
10101000 01010001 11011010 01110010
01101110 01010100 00111110 01001111 11100011
11000001 00100001 11110100 01100011
27. 2008 Kutztown University 27
Vernam Cipher – Why Unbreakable
Try attack by exhaustive search
Among possible keys
11000001 01110000 11011110 10111001
01100001 10001000 01101100 11111010
00110100 01001110 01111001 00011110
00011100 10001100 10110011 01010011
10001101 01000010
Produces this recovered plain text:
Grade = F – Sorry!
28. 2008 Kutztown University 28
Vernam Cipher – Why Unbreakable
Exhaustive search will produce every
possible combination of 18 characters.
And there is no way to distinguish between
them
Among the possible recovered texts:
Tickle me Elmo now
Jabberwocky Rocks!
Attack tomorrow am
Attack tomorrow pm
Grade = C++ & Java
29. 2008 Kutztown University 29
Vernam Cipher – Why Look
Elsewhere?
Key distribution problem
Every sender/recipient must have same pad
N sender recipient pairs require O(N2) pads
Pad distribution is security risk
Key coordination problem
Sheets on pad must match exactly
Messages must arrive in order sent
Key generation problem
High quality random numbers hard to generate
Bottom line – has some limited use
