Application Level Encryption for Azure Durable Entities

The code within this repository demonstrates a means by which durable entity state can be encrypted at the application layer. A key is configured for the function app. That key is then used during the serialization process to encrypt any properties annotated with the Encrypted attribute.

Dependencies

• .NET Core 3.1
• Azure Storage Emulator
• Azure Storage Explorer

Bootstrapping Encryption

As part of the Configure method in the FunctionsStartup class register the dependencies necessary to enable encryption

Register how to resolve the encryption options/configuration. The key is expected to come from a key management solution such as Azure Key Vault. See Azure Key Vault Configuration Provider in ASP.NET Core for how to retrieve secrets from Azure Key Vault.

builder.Services.AddSingleton(sp => new EncryptionOptions(config["ENCRYPTIONKey"]));

Register the implementation of IEncryptionService to be used

builder.Services.AddSingleton<IEncryptionService, EncryptionService>();

Register customization to IMessageSerializerSettingsFactory to customize how durable functions serializes the state

//necessary to engage custom settings during serialization
builder.Services.AddSingleton<IContractResolver, EncryptedContractResolver>();
builder.Services.AddSingleton(sp => new JsonSerializerSettings { ContractResolver = sp.GetService<IContractResolver>() });
builder.Services.AddSingleton<IMessageSerializerSettingsFactory, SerializerSettingsFactory>();

Register customization to Json.NET's default serialization settings to customize how durable functions deserializes the state

//necessary to engage custom settings during deserialization
JsonConvert.DefaultSettings = () => builder.Services.BuildServiceProvider().GetService<JsonSerializerSettings>();

Annotate Properties for Encryption

To indicate which entity properties should be encrypted, annotate the properties with the Encrypted attribute. The EncryptedContractResolver will scan for this attribute to determine which properties need to be encrypted/decrypted.

[JsonObject(MemberSerialization = MemberSerialization.OptOut)]
public class AccountEntity : IAccountEntity
{
    // ensure account number is encrypted when saved
    [Encrypted]
    public string AccountNumber { get; set; }

    public void Set(string accountNumber) => AccountNumber = accountNumber;

    [FunctionName(nameof(AccountEntity))]
    public static Task Run([EntityTrigger] IDurableEntityContext ctx) => ctx.DispatchAsync<AccountEntity>();
}

Testing The Encryption

To test the encryption/decryption in this sample, the following two endpoints can be used.

PUT /accounts/{accountId}

This will set the account number to a value for the account of the provided accountId.

The value when saved to Azure Table Storage should be encrypted. The following is from the Hub Instances table; Input column for the row that represents the Durable Entity.

{"exists":true,"state":"{\"AccountNumber\":\"A1k9ufGmHFMgM83sE/8EdCMls/TLeXbWDEU32ZXrhE4=\"}","sorter":{}}

GET /accounts/{accountId}

This will retrieve the account entity for the account of the provided accountId.

The returned value should be decrypted

{ "accountNumber": "123ABC" }

Customize the Encryption

To customize the specific encryption algorithm used, a new implementation of IEncryptionService can be created. This will not change what properties are selected for encryption. It will only change how the properties annotated are encrypted & decrypted. If for example, something other than AesManaged is needed, a new implementation of IEncryptionService could be created to use a different cryptography implementation.

public interface IEncryptionService
{
    string Decrypt(string value);
    string Encrypt(string value);
}

public class MyEncryptionService
{
    public string Encrypt(string value)
    {
        //todo: return encrypted value
    }

    public string Decrypt(string value)
    {
        //todo: return the decrypted value
    }
}

Then, register the new implementation with the IServiceProvider

builder.Services.AddSingleton<IEncryptionService, EncryptionService>();