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I have studied about the mono-alphabetic substitution cipher. With Character frequency graph, the encrypted text can be decoded. I am wondering how can be happen?

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Monoalphabetic cipher is a substitution cipher in which for a given key, the cipher alphabet for each plain alphabet is fixed throughout the encryption process. For example, if ‘A’ is encrypted as ‘D’, for any number of occurrence in that plaintext, ‘A’ will always get encrypted to ‘D’.

source -> https://www.tutorialspoint.com/cryptography/traditional_ciphers.htm#:~:text=Monoalphabetic%20cipher%20is%20a%20substitution,get%20encrypted%20to%20'D'.

Here an example for how it can be decrypted with character frequency graph

Part of the plain text

Computer security, cybersecurity1 or information technology security (IT security) is the protection of computer systems from theft or damage to their hardware, software or electronic data, as well as from disruption or misdirection of the services they provide. The field is growing in importance due to increasing reliance on computer systems, the Internet2 and wireless networks such as Bluetooth and Wi-Fi, and due to the growth of "smart" devices, including smartphones, televisions and the various tiny devices that constitute the Internet of things. Due to its complexity, both in terms of politics and technology, it is also one of the major challenges of the contemporary world.[3]

Part of the cipher text

XLNKFGVI HVXFIRGB, XBYVIHVXFIRGB1 LI RMULINZGRLM GVXSMLOLTB HVXFIRGB (RG HVXFIRGB) RH GSV KILGVXGRLM LU XLNKFGVI HBHGVNH UILN GSVUG LI WZNZTV GL GSVRI SZIWDZIV, HLUGDZIV LI VOVXGILMRX WZGZ, ZH DVOO ZH UILN WRHIFKGRLM LI NRHWRIVXGRLM LU GSV HVIERXVH GSVB KILERWV. GSV URVOW RH TILDRMT RM RNKLIGZMXV WFV GL RMXIVZHRMT IVORZMXV LM XLNKFGVI HBHGVNH, GSV RMGVIMVG2 ZMW DRIVOVHH MVGDLIPH HFXS ZH YOFVGLLGS ZMW DR-UR, ZMW WFV GL GSV TILDGS LU "HNZIG" WVERXVH, RMXOFWRMT HNZIGKSLMVH, GVOVERHRLMH ZMW GSV EZIRLFH GRMB WVERXVH GSZG XLMHGRGFGV GSV RMGVIMVG LU GSRMTH. WFV GL RGH XLNKOVCRGB, YLGS RM GVINH LU KLORGRXH ZMW GVXSMLOLTB, RG RH ZOHL LMV LU GSV NZQLI XSZOOVMTVH LU GSV XLMGVNKLIZIB DLIOW.[3] character frequncy graph for actual text character frequncy graph for cipher text

Five cipher text characters with the highest frequency and speculate on the mapping to the plaintext characters in the alphabet Ciphertext plaintext V E G T Z A R I I R

Basically this encryption is the reverse order of A-Z to Z-A A-Z B-Y C-X

For this method, we need original text and ciphertext.

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In each language there is an expected distribution of characters so you can use this to make reasonable estimate as to characters.

For example, E is by far the most common letter in English words, followed by T, then A then O etc. So in your substitution ciphertext, the most common letter is probably E.

From pi.math.cornell.edu:

enter image description here

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