Encrypting a message in matlab?

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Nick Haufler
Nick Haufler 2015-10-28
评论： Rik 2020-10-9
For the attached document, were supposed to encrypt a message, and I got the numbers to come out, but when I try to convert it back to a message it just displays the original message instead of the secret code. I'll attach my code, I have no idea whats wrong with it, and why it isnt working. Thanks.

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Geoff Hayes
Geoff Hayes 2015-10-28
Nick - look closely at your for loop
for k=1:1
k is only ever going to be one. You need to iterate over each element of the number_message array. Hint: use length to do so.
Also, take a look at your code to add the key to the number_message array. Remember that the array is an array of doubles. What is the data type for your key?
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Nick Haufler
Nick Haufler 2015-10-29
编辑：Geoff Hayes 2015-10-29
Ahh, so key=double(original_message). I have in my for loops
number_message(k)=number_message(k)+key,
wouldnt I want to multiply the two instead of make them equal? so change the equal sign to multiplication.
Geoff Hayes
Geoff Hayes 2015-10-29
Your code asks the user to enter a key. What is the range for this key? Is it one through twenty-six? Because that seems to be the case given the remainder of your code when you add the key and check the conditions
number_message(k)>=90
or
number_message(k)>=122
Now, your code asks the user to enter in the key and you save it to the variable key. But this is a string and so is not necessarily in the range of one through twenty six. You need to convert it to a double so that the (for example) '2' becomes 2 as
key = str2double(key);
Nick, you really need to step through the code using the debugger so that you can understand what is happening with each line of code that you have written.

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Thorsten
Thorsten 2015-10-29
编辑：Thorsten 2015-10-29
If you use input with the 's' argument, it will return a string. For the key you need a number, so use
key=input('What will be the encryption key you are using:')
and of course you have to run the loop for all values of the message:
for k=1:length(original_message)
That's it. Given that your key values are not larger then 26, as has been pointed out by Geoff. If you have larger numbers, you have to use the mod function. I've detailed this in a response to one of your colleagues http://www.mathworks.com/matlabcentral/answers/251101-writing-secret-codes.
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ahmed ibrahim
ahmed ibrahim 2020-10-9
please any one who know this
Write a computer program to encrypt the message “I Love Gondar” in the least significant bit of a gray scale image converted from your own picture. That is:
1. Take a picture of yourself
2. Convert it to grayscale
3. Apply bit plane slicing
4. Separate the lease significant bit plane
5. Encrypt the above message
6. Put all the bit planes back together
7. Show the picture with the encryption
Rik
Rik 2020-10-9
We generally discourage posting solutions to homework questions. You also have not shown any attempt and are hijacking someone else's question. Please post this as a separate question.
Also note that due to the US law that governs this site most discussions about cryptography are not allowed. It may be illegal to answer your question.

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John BG
John BG 2015-10-29
original_message=input('Please enter the message you want encrypted:', 's') % original message
% do not input key as string or you will have to write a program with a vector as key
key=input('What will be the encryption key you are using:')
number_message=double(original_message)
mask1=find(number_message>=65 & number_message<=90)
wrapup1=repmat(key,1,length(mask1))
number_message(mask1)
% number_message(mask1)=number_message(mask1)+key % without wrapup
% with wrapup
number_message(mask1)=...
mod(number_message(mask1)+wrapup1,13)+repmat(65,1,length(mask1))
mask2=find(number_message>=97 & number_message<=122)
wrapup1=repmat(key,1,length(mask1))
number_message(mask1)
number_message(mask1)=...
mod(number_message(mask1)+wrapup1,13)+repmat(97,1,length(mask1))
You finish off the fprintf, ok?
John
John BG
John BG 2015-10-29
The second mask should not have any reference to the first mask, sorry. Replace the last 5 lines of code with
mask2=find(number_message>=97 & number_message<=122)
wrapup1=repmat(key,1,length(mask2))
number_message(mask2)
number_message(mask2)=...
mod(number_message(mask2)+wrapup1,13)+repmat(97,1,length(mask2))
John

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