decrypt.html

<html>
<head>
<style>
input, textarea {
    display: block;
    width: 100%;
}
#private {background: palevioletred;}
#plaintext {background: lightyellow;}
#secret {background: lightblue;}
</style>
</head>
<body>
Paste the PRIVATE key that you generated at <a href="generate.html">generate.html</a>:
<textarea id="private" onfocus="this.select();" oninput='autosize(this)'></textarea>
Paste the message that sender ENCRYPTED at <a href="encrypt.html">encrypt.html</a>::
<textarea id="secret" onfocus="this.select();" oninput='autosize(this)'></textarea>
<br>
<input type="button" id="decrypt" value="decrypt">
Here is your secret message:
<textarea id="plaintext" readonly onfocus="this.select();"></textarea>
<p id="err-msg"></p>
<script>
function $(s){return document.querySelector(s)};
function text2bin(text){try {return new Uint8Array(atob(text).split('').map(function(a){return a.charCodeAt(0)}))}catch(err){displayError();}};
function autosize(textarea){textarea.style.height = "";textarea.style.height = textarea.scrollHeight +3 + "px"};
function displayError() {$('#plaintext').value="";$("#err-msg").innerHTML = "Sorry, an error occurred";}

$('#decrypt').onclick=async function(){

	privateKey = await crypto.subtle.importKey(
		"pkcs8", //can be "jwk" (public or private), "spki" (public only), or "pkcs8" (private only)
		text2bin($('#private').value),
		{   //these are the algorithm options
			name: "RSA-OAEP",
			hash: {name: "SHA-256"}, //can be "SHA-1", "SHA-256", "SHA-384", or "SHA-512"
		},
		true, //whether the key is extractable (i.e. can be used in exportKey)
		["decrypt"] //"encrypt" or "wrapKey" for public key import or
	);

		const [iv, ciphertext, encryptedKey] = $('#secret').value.split(",").map(text2bin);

		// Decrypt AES key
		const rawKey = await crypto.subtle.decrypt(
			{ name: "RSA-OAEP" },
			privateKey,
			encryptedKey
		);

		const aesKey = await crypto.subtle.importKey(
			"raw",
			rawKey,
			{ name: "AES-GCM" },
			false,
			["decrypt"]
		);

		// Decrypt data
		const decrypted = await crypto.subtle.decrypt(
			{ name: "AES-GCM", iv },
			aesKey,
			ciphertext
		);

			var enc = new TextDecoder();
			// console.log(enc.decode(decrypted));
			$("#err-msg").innerHTML = "";
			$('#plaintext').value=enc.decode(decrypted);
			autosize($('#plaintext'));

}

</script>
</body>
</html>