Codman Academy Visit, January 22, 2010
Idea: Replace the plaintex letters according to a permutation of the alphabet.
Examples:
Plaintext: the quick brown fox jumps over the lazy dog Ciphertext: PYC DJFEH BWZSX UZG RJATV ZNCW PYC OMKI LZQ Plaintext: She said, "Hi! How are you?" Ciphertext: VYC VMFL, "YF! YZS MWC IZJ?"
Often, capitalization is ignored and punctuation (including spaces) is omitted (but they could also be substituted like letters).
Plaintext: thequickbrownfoxjumpsoverthelazydog
Ciphertext: PYCDJFEHBWZSXUZGRJATVZNCWPYCOMKILZQ
Plaintext: shesaidhihowareyou
Ciphertext: VYCVMFLYFYZSMWCIZJ
Challenge 1: Decrypt the following message encrypted with the above substitution cipher.
SCOOCVOCI EZOOCQC
Below is some ciphertext that's been encrypted with a substitution cipher. Punctuation has been kept to make decryption easier. How could we decrypt it? (Just think about this for now; you'll actually do it later.)
FERV BV GER SMKCXR MZ EKYOV RARVGX BG HRSMYRX VRSRXXOCL ZMC MVR DRMDJR GM IBXXMJAR GER DMJBGBSOJ HOVIX FEBSE EOAR SMVVRSGRI GERY FBGE OVMGERC OVI GM OXXKYR OYMVQ GER DMFRCX MZ GER ROCGE, GER XRDOCOGR OVI RUKOJ XGOGBMV GM FEBSE GER JOFX MZ VOGKCR OVI MZ VOGKCR'X QMI RVGBGJR GERY, O IRSRVG CRXDRSG GM GER MDBVBMVX MZ YOVNBVI CRUKBCRX GEOG GERL XEMKJI IRSJOCR GER SOKXRX FEBSE BYDRJ GERY GM GER XRDOCOGBMV. FR EMJI GERXR GCKGEX GM HR XRJZ-RABIRVG, GEOG OJJ YRV OCR SCROGRI RUKOJ, GEOG GERL OCR RVIMFRI HL GERBC SCROGMC FBGE SRCGOBV KVOJBRVOHJR CBQEGX, GEOG OYMVQ GERXR OCR JBZR, JBHRCGL OVI GER DKCXKBG MZ EODDBVRXX. -- GEOG GM XRSKCR GERXR CBQEGX, QMARCVYRVGX OCR BVXGBGKGRI OYMVQ YRV, IRCBABVQ GERBC WKXG DMFRCX ZCMY GER SMVXRVG MZ GER QMARCVRI, -- GEOG FERVRARC OVL ZMCY MZ QMARCVYRVG HRSMYRX IRXGCKSGBAR MZ GERXR RVIX, BG BX GER CBQEG MZ GER DRMDJR GM OJGRC MC GM OHMJBXE BG, OVI GM BVXGBGKGR VRF QMARCVYRVG, JOLBVQ BGX ZMKVIOGBMV MV XKSE DCBVSBDJRX OVI MCQOVBPBVQ BGX DMFRCX BV XKSE ZMCY, OX GM GERY XEOJJ XRRY YMXG JBNRJL GM RZZRSG GERBC XOZRGL OVI EODDBVRXX. DCKIRVSR, BVIRRI, FBJJ IBSGOGR GEOG QMARCVYRVGX JMVQ RXGOHJBXERI XEMKJI VMG HR SEOVQRI ZMC JBQEG OVI GCOVXBRVG SOKXRX; OVI OSSMCIBVQJL OJJ RTDRCBRVSR EOGE XERFV GEOG YOVNBVI OCR YMCR IBXDMXRI GM XKZZRC, FEBJR RABJX OCR XKZZRCOHJR GEOV GM CBQEG GERYXRJARX HL OHMJBXEBVQ GER ZMCYX GM FEBSE GERL OCR OSSKXGMYRI. HKG FERV O JMVQ GCOBV MZ OHKXRX OVI KXKCDOGBMVX, DKCXKBVQ BVAOCBOHJL GER XOYR MHWRSG RABVSRX O IRXBQV GM CRIKSR GERY KVIRC OHXMJKGR IRXDMGBXY, BG BX GERBC CBQEG, BG BX GERBC IKGL, GM GECMF MZZ XKSE QMARCVYRVG, OVI GM DCMABIR VRF QKOCIX ZMC GERBC ZKGKCR XRSKCBGL. -- XKSE EOX HRRV GER DOGBRVG XKZZRCOVSR MZ GERXR SMJMVBRX; OVI XKSE BX VMF GER VRSRXXBGL FEBSE SMVXGCOBVX GERY GM OJGRC GERBC ZMCYRC XLXGRYX MZ QMARCVYRVG. GER EBXGMCL MZ GER DCRXRVG NBVQ MZ QCROG HCBGOBV BX O EBXGMCL MZ CRDROGRI BVWKCBRX OVI KXKCDOGBMVX, OJJ EOABVQ BV IBCRSG MHWRSG GER RXGOHJBXEYRVG MZ OV OHXMJKGR GLCOVVL MARC GERXR XGOGRX. GM DCMAR GEBX, JRG ZOSGX HR XKHYBGGRI GM O SOVIBI FMCJI.
Idea: Knowing the frequencies of letters in English text can help us break many ciphers, including substitution ciphers.
Check out these resources on letter frequencies:
Some things to think about --- does letter frequency depend on:
Here is an applet for letter frequency analysis.
Challenge 2: Perform a letter frequency analysis on some of the following examples:
VSP OKEEKVF KE DV NPLNYPL EDSTLFDE ZVP ZSWW NYPDKUKNYDKVF KF DBL KFDLWWLUDSYW, LUVFVOKU YFT UKMKU WKZL VZ VSP EVUKLDR, CR LFESPKFA DBLKP ZSWW NPLNYPYDKVF YFT YUULEE DV ZSPDBLP LTSUYDKVF, DBL EHKWWE YFT MKEKVF DV SFTLPDYHL Y PLXYPTKFA UYPLLP, YFT DBL OVDKMYDKVF YFT UBYPYUDLP FLLTLT DV LFAYAL TLLNWR YFT NPVTSUDKMLWR KF UVOOSFKDR WKZL. XL MKLX NYPLFDE YFT UVOOSFKDR OLOCLPE YE KFDLAPYW NYPDFLPE KF DBKE LFTLYMVP.
Now you're ready to do one on your own:
Challenge 3: Use the substitution cipher program to decrypt the following ciphertext:
FERV BV GER SMKCXR MZ EKYOV RARVGX BG HRSMYRX VRSRXXOCL ZMC MVR DRMDJR GM IBXXMJAR GER DMJBGBSOJ HOVIX FEBSE EOAR SMVVRSGRI GERY FBGE OVMGERC OVI GM OXXKYR OYMVQ GER DMFRCX MZ GER ROCGE, GER XRDOCOGR OVI RUKOJ XGOGBMV GM FEBSE GER JOFX MZ VOGKCR OVI MZ VOGKCR'X QMI RVGBGJR GERY, O IRSRVG CRXDRSG GM GER MDBVBMVX MZ YOVNBVI CRUKBCRX GEOG GERL XEMKJI IRSJOCR GER SOKXRX FEBSE BYDRJ GERY GM GER XRDOCOGBMV. FR EMJI GERXR GCKGEX GM HR XRJZ-RABIRVG, GEOG OJJ YRV OCR SCROGRI RUKOJ, GEOG GERL OCR RVIMFRI HL GERBC SCROGMC FBGE SRCGOBV KVOJBRVOHJR CBQEGX, GEOG OYMVQ GERXR OCR JBZR, JBHRCGL OVI GER DKCXKBG MZ EODDBVRXX. -- GEOG GM XRSKCR GERXR CBQEGX, QMARCVYRVGX OCR BVXGBGKGRI OYMVQ YRV, IRCBABVQ GERBC WKXG DMFRCX ZCMY GER SMVXRVG MZ GER QMARCVRI, -- GEOG FERVRARC OVL ZMCY MZ QMARCVYRVG HRSMYRX IRXGCKSGBAR MZ GERXR RVIX, BG BX GER CBQEG MZ GER DRMDJR GM OJGRC MC GM OHMJBXE BG, OVI GM BVXGBGKGR VRF QMARCVYRVG, JOLBVQ BGX ZMKVIOGBMV MV XKSE DCBVSBDJRX OVI MCQOVBPBVQ BGX DMFRCX BV XKSE ZMCY, OX GM GERY XEOJJ XRRY YMXG JBNRJL GM RZZRSG GERBC XOZRGL OVI EODDBVRXX. DCKIRVSR, BVIRRI, FBJJ IBSGOGR GEOG QMARCVYRVGX JMVQ RXGOHJBXERI XEMKJI VMG HR SEOVQRI ZMC JBQEG OVI GCOVXBRVG SOKXRX; OVI OSSMCIBVQJL OJJ RTDRCBRVSR EOGE XERFV GEOG YOVNBVI OCR YMCR IBXDMXRI GM XKZZRC, FEBJR RABJX OCR XKZZRCOHJR GEOV GM CBQEG GERYXRJARX HL OHMJBXEBVQ GER ZMCYX GM FEBSE GERL OCR OSSKXGMYRI. HKG FERV O JMVQ GCOBV MZ OHKXRX OVI KXKCDOGBMVX, DKCXKBVQ BVAOCBOHJL GER XOYR MHWRSG RABVSRX O IRXBQV GM CRIKSR GERY KVIRC OHXMJKGR IRXDMGBXY, BG BX GERBC CBQEG, BG BX GERBC IKGL, GM GECMF MZZ XKSE QMARCVYRVG, OVI GM DCMABIR VRF QKOCIX ZMC GERBC ZKGKCR XRSKCBGL. -- XKSE EOX HRRV GER DOGBRVG XKZZRCOVSR MZ GERXR SMJMVBRX; OVI XKSE BX VMF GER VRSRXXBGL FEBSE SMVXGCOBVX GERY GM OJGRC GERBC ZMCYRC XLXGRYX MZ QMARCVYRVG. GER EBXGMCL MZ GER DCRXRVG NBVQ MZ QCROG HCBGOBV BX O EBXGMCL MZ CRDROGRI BVWKCBRX OVI KXKCDOGBMVX, OJJ EOABVQ BV IBCRSG MHWRSG GER RXGOHJBXEYRVG MZ OV OHXMJKGR GLCOVVL MARC GERXR XGOGRX. GM DCMAR GEBX, JRG ZOSGX HR XKHYBGGRI GM O SOVIBI FMCJI.