Standard encryption algorithm for Bash script and Javascript?
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I have a directory in my mac which has files with following names -

    Directory images -
    1.jpg
    2.jpg
    3.jpg
    4.jpg

I would like to write a bash script to rename all the files in this folder, by using some standard encryption algorithm, so that the directory now looks something like this -

Lets assume that the encryption key is common_key = "mykey123"

    Directory images -
    U2FsdGVk.jpg
    X1O9Z6e.jpg
    ECdjybF.jpg
    U2FsdGVajpg

How can I achieve this ?

Furthur, I am writing some code in Nodejs (basically javascript), where I wish to pick a random image out of these four and display it, so I am using- 

    var random_image_number = Math.floor((Math.random()*4)+1); // Picking a random number out of 4

Suppose random_image comes out to be 3. Now I need to display image number 3. If I had not encrypted the image names, I would normally do this -

    <img src="/images/" + random_image_number + ".jpg" />

But now I need to use the standard encryption algorithm again on the random number that I have generated, before I can display the image. (I keep stressing on "standard" since the same encryption algorithm has to work on JS side as well as in the bash script and generate the same result)

so i need some thing like this 

    var random_image_number = Math.floor((Math.random()*4)+1); // Picking a random number out of 4
    var random_image_number_encrypted = some_encryption_function(common_key, random_image_number)
    <img src="/images/" + random_image_number_encrypted + ".jpg" />

Please tell me what algorithm I could use. It doesn't need to be much sophisticated, but must give the same result in the bash script as well as javascript side.

Also, I am not sure how to write the bash script for renaming all the files using the encryption algorithm so any code regarding that will be really useful. Thank you.