encrypt.html

<label> Encrypt <input type="file" id="encrypt"> </label>
<label> Decrypt <input type="file" id="decrypt"> </label>
<a id="link">Download</a>
<script>
	async function main() {
		const initial = Uint8Array.from([
			55, 44, 146, 89,
			30, 93, 68, 30,
			209, 23, 56, 140,
			88, 149, 55, 221
		])
		const key = await crypto.subtle.importKey("jwk", {
				"alg": "A256CBC",
				"ext": true,
				"k": prompt(
					"AES-CBC JSON Web Key"),
				"key_ops": ["encrypt", "decrypt"],
				"kty": "oct"
			}, {
				"name": "AES-CBC"
			},
			false,
			["encrypt", "decrypt"]
		)
		/*
			const key = await crypto.subtle.generateKey({
				'name': 'AES-CBC',
				'length': 256
			}, false, ['encrypt', 'decrypt'])
			const exported = JSON.stringify(await crypto.subtle.exportKey("jwk", key))
			*/

		const encrypt = document.getElementById("encrypt")
		const decrypt = document.getElementById("decrypt")
		encrypt.addEventListener("change", () => {
			const file = encrypt.files[0]
			const reader = new FileReader()

			reader.onload = function(event) {
				const data = event.target.result

				crypto.subtle.encrypt({
						'name': 'AES-CBC',
						'iv': initial
					}, key, data)
					.then(encrypted => {
						const blob = new Blob([encrypted], {
							type: "application/octet-stream"
						})
						const url = URL.createObjectURL(blob)
						const link = document.getElementById(
							"link")
						link.href = url
					})
					.catch(console.error)
			}

			reader.readAsArrayBuffer(file)
		})

		decrypt.addEventListener("change", () => {
			const file = decrypt.files[0]
			const reader = new FileReader()

			reader.onload = function(event) {
				const data = event.target.result

				crypto.subtle.decrypt({
						'name': 'AES-CBC',
						'iv': initial
					}, key, data)
					.then(decrypted => {
						console.log(decrypted)
						const blob = new Blob([decrypted], {
							type: "image/png"
						})
						const url = URL.createObjectURL(blob)
						const link = document.getElementById(
							"link")
						link.href = url
					})
					.catch(console.error)
			}

			reader.readAsArrayBuffer(file)
		})
	}
	main()
</script>