Edit: Actually you can use your browser's developer tools to decrypt the priv key.
Took some messing around to make this work on Chrome (Press F12 to get console)... It doesn't seem to have the CryptoJS stuff auto imported?
I had to first of all copy/paste this code snippet into the console to get access to the CryptoJS library... the source code for which is no longer available on code.google.com
So, I'm using some random link I found while googling for it
var script = document.createElement('script');
script.type = 'text/javascript';
script.src = 'https://cdnjs.cloudflare.com/ajax/libs/crypto-js/3.1.2/rollups/aes.js';
document.head.appendChild(script);
Then, I could copy/paste the following code into the console and it would pop up the alert showing the private key.
var mencrypted = "enter encrypted priv key here";
var mpassword = "enter password here";
var decrypted = CryptoJS.AES.decrypt(mencrypted, mpassword);
alert(decrypted.toString(CryptoJS.enc.Utf8));
If you try and copy/paste the whole lot in one go... it'll complain that CryptoJS doesn't exist:
Uncaught ReferenceError: CryptoJS is not defined
at <anonymous>:8:17
But, if you just copy/paste it again, it'll work...
What is interesting, is that I attempted to use the OPs example:
Here is an example of an encrypted wif key and it's corresponding wif key and password that i have generated here (https://coinb.in/#newAddress) for testing purpose
WIF key : "KyYEjVKDcbUrgfrpdpzaZBm1szWbzTmrBvxsQzDWgCAAZnd6uw8S"
AES-256 Encrypted WIF key : "U2FsdGVkX187FiUTQ0lMNeO5AkYQS9xU874h0g4AWch3ZfLuKEiUndejRQ0uRGw03sY5tOlTiVyiLf4oax+6IGF7hJUAjOMEomCGOxd8jvg="
Password : "123"
And I just get a blank alert... but when I use a randomly generated one from coinb.in... it works!
WIF Key : "L3pLNp7KgZAT2LSuue4RPkum6XGCgeZFE2Dk37xPvxnN8TAEtQX1"
AES-256 Encrypted WIF key : "U2FsdGVkX1/fN3CAnYeGfVPhdq8G/MM+u8U4K7nR1xm3iHwXQwOojQmzyF3Zp/e4N5l8T+iNFSw8OGxC5P67+Jil8HR/Tb64QJfX0qUDxSg="
Password : "123"
I'm guessing the OPs example is broken? bad AES encrypted key or bad password?
EDIT:
I tried as your suggestion it doesn't work in firefox it always shows "ReferenceError: CryptoJS is not defined"
I was getting the same... I think maybe Abdussamad already had the CryptoJS stuff loaded (browser extension maybe?)... try running my little code snippet to load the aes.js script and see if that helps!