Ciphers and Encryption Worksheet
Caesar Cipher[1] (Transposition Code) SOLUTIONS Recoded 26 Apr 2008
A. Encode/encipher or decode/decipher the messages shown using the transposition code of length 4 given here.
Encode Decode
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 / ZW / 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
Find ltr→ Look
Look ¯ ←Find ltr
¯Encode CROSS THE RUBICON
ynkoo pda nqxeykj
______
Decode SDKW IWPD ZQZA
whoa math dude
______
B. Use the Caesar cipher of ciphers with code word CRYPT to encode or decode the messages shown.
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 / ZC / 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
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
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
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
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
¯Encode CROSS THE RUBICON NO DEATH AND TAXES
CRYPT CRY PTCRYPT CR YPTCR YPT CRYPT
ehkeh vxa djdyyac
Encode¯ YONDER DEAD FROM THE NECK UP GRADUATE
aejptt.tams hhky iju jqrm kl sgctqmig
surrender antwon to your right brain
Decode
UKNDTPTAD PPJSAC VE UAJT HESWV RNMXP
CRYPTCRYP TCRYPT CR YPTC RYPTC RYPTC
Decode
VXA UCVUNZTV YO ZDV IAOJTU
the internet is not secure
jas::C:\WWW\cs435\CipherWorkSheet-Slant.doc 30 April 2000
Ciphers and Encryption Worksheet
Public Key Encryption[2]
ER Public Keys ES
Apply R’s public key ER ES Apply S’s public key
Sender, DS, S Receiver, R, DR Apply private
¯
M -> DS(M) -> ER(S.DS(M)) --Send--> DR(ER(S.DS(M)))
Attach signature of S S.DS(M) Sender S known
Apply public key of S ES(DS(M)) --> M
With signature:
M → S.DS(M) → ER(S.DS(M)) → DR(ER(S.DS(M))) → S.DS(M) → ES(DS(M)) → M
E and D are inverse functions such that: ED = I = DE = identity
E is public, and D is secret.
Example
User / 1 / 2 / 3 / 4E¯ / ¯Encode / ABCD1234 / ABCD1234 / ABCD1234 / ABCD1234
D / Decode / BCDA2134 / CDAB1432 / DABC4321 / DBAC4123
Message, M: BADCAB from User-1 to User-2
Message, M
/BADCAB
Comment / Operation /Result
User 1 encodes message with their private key / D1(M) /User-3 Sees
User 1 adds return address / 1 / User 3 decodes w / private keyEncode w/User 2’s public key / E2(1.D1(M)) / D3(E2(1.D1(M)))
User 2 decodes w / private key / D2(E2(1.D1(M)))
User 2 applies User 1’s public key / E1(______) / User 3 applies 4’s public key
E4(______)
jas::E:\CS101\SocialIssues\Privacy\CipherWorkSheet-Slant.doc 30 April 2000
[1] Based on a talk by Dr. Gordon Pritchett, Babson College, "Cryptology: From Caesar Cipher to Trap Door Functions," R.I.C., 27 February 1985.
Luciano, Dennis M. "Cryptology: From Caesar Ciphers to Public-Key Cryptosystems," The College Mathematics Journal (Jan 1987)
[2] See Martin E. Hellman, "The Mathematics of Public-Key Cryptography," Scientific American, 241, (August 1979), 146-157 (130-139?).