Encryption.cs

public class Encryption(IJSRuntime jsRuntime)
{
    public async Task<string> GetEncryptionKeyAsync() =>
        await jsRuntime.InvokeAsync<string>("getEncryptionKey") ?? string.Empty;
        
    public async Task<string> EncryptAsync(string value) =>
        await jsRuntime.InvokeAsync<string>("encrypt", value) ?? string.Empty;
        
    public async Task<string> DecryptAsync(string value) =>
        await jsRuntime.InvokeAsync<string>("decrypt", value) ?? string.Empty;
}

encryption.js

document.addEventListener('DOMContentLoaded', async () => {
    const objectKey = window.location.hash.slice("#key=".length);
    if (objectKey.length > 0) {
        window.encryptionKey = await window.crypto.subtle.importKey(
            "jwk",
            {
                k: objectKey,
                alg: "A128GCM",
                ext: true,
                key_ops: ["encrypt", "decrypt"],
                kty: "oct",
            },
            { name: "AES-GCM", length: 128 },
            true,
            ["encrypt", "decrypt"]
        );
    }
});

async function getEncryptionKey() {
    if (!window.encryptionKey) {
        window.encryptionKey = await window.crypto.subtle.generateKey(
            { name: "AES-GCM", length: 128 },
            true,
            ["encrypt", "decrypt"],
        );
    }

    return (await window.crypto.subtle.exportKey("jwk", window.encryptionKey)).k
}

async function encrypt(value) {
    return window.btoa(String.fromCharCode.apply(null, new Uint8Array(
        await window.crypto.subtle.encrypt(
            { name: "AES-GCM", iv: new Uint8Array(12) },
            window.encryptionKey,
            new TextEncoder().encode(value)
        )
    )));
}

async function decrypt(value) {
    const bValue = window.atob(value)
    const buffer = new ArrayBuffer(bValue.length)
    const bufferView = new Uint8Array(buffer)

    for (let i = 0; i < bValue.length; i++) {
        bufferView[i] = bValue.charCodeAt(i)
    }

    return new TextDecoder().decode(new Uint8Array(
        await window.crypto.subtle.decrypt(
            { name: "AES-GCM", iv: new Uint8Array(12) },
            window.encryptionKey,
            buffer
        )
    ));
}