Cesar cypher with wrap around for characters

Question

Ajai Kannan 2019-2-14

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https://ww2.mathworks.cn/matlabcentral/answers/444981-cesar-cypher-with-wrap-around-for-characters

关闭： Walter Roberson 2019-4-23

My question is as follows:

Write a function called caesar that accepts two arguments: the first is the character vector to be encrypted, while the second is the shift amount. The function returns the output argument coded, the encrypted text. The function needs to work with all the visible ASCII characters from space to ~. The ASCII codes of these are 32 through 126. If the shifted code goes outside of this range, it should wrap around. For example, if we shift ~ by 1, the result should be space. If we shift space by -1, the result should be ~

I tried many solutions and they turn out to be partially correct and the code fails when the character's ascii value exceeds 126 or falls below 32. I tried so hard and i have given up at this point. I will really appreciate it if anyone can give their version of the code so that i can interpret both and try to figure out where i went wrong.

the following is the code i tried:

function coded = caesar(stringg,shift_amt)
    char_num = double(stringg);
    encrypted_num = char_num + shift_amt;
    for i = 1:length(encrypted_num)
        if encrypted_num(1,i)>126
            while encrypted_num(1,i)>126, encrypted_num(1,i) = abs(mod(encrypted_num(1,i),126)); end,
            encrypted_num(1,i)=31+encrypted_num(1,i); 
        end
        if encrypted_num(1,i)<32, 
            while encrypted_num(1,i)<32, encrypted_num(1,i) = abs(mod(encrypted_num(1,i),126)); 
            end, 
            encrypted_num(1,i) = 127 - encrypted_num(1,i) ;
        end
    end
    
        coded = char(encrypted_num);
        

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Walter Roberson 2019-4-23

Jan:

Caeser Cipher is definitely out of bounds for open discussion under US law, which is based upon key size, not upon the fact that there are known techniques to solve the cipher.

Jan 2019-4-23

@Walter: I think, it is not correct to close the question based on this reason:

"Closure Reason: Not appropriate for MATLAB Answers - Due to the laws of the United States, it is not permitted to discuss Caeser Cypher without advance permission from the USA Department of Commerce."

The publishing of code for a strong encryption is not allowed, e.g. a 128 bit AES encryption. A Caesar cypher is not strong. As far as I remember I was asked, if I can limit the key length to 44 or 64 bit in my published AES code to be conform with the export restrictions. Of course a user could remove this restriction from the code easily. Nobody claimed, that these laws are useful.

此问题已关闭。

Answer 1

Jan 2019-2-26

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https://ww2.mathworks.cn/matlabcentral/answers/444981-cesar-cypher-with-wrap-around-for-characters#answer_362839

编辑：Jan 2019-2-26

在 MATLAB Online 中打开

See the mod() function.

    char_num = double(stringg);
    enc_num  = char_num + shift_amt;
    enc_num  = mod(enc_num - 32, 126 - 32 + 1) + 32

The idea: Convert the data [32:126] to the range [0:126-32]. Then apply the shifting and use mod for the wrap around. Afterwards reconvert the data to the range [32:126].