aspnetcore/src/Microsoft.AspNetCore.DataProtection/XmlEncryption/EncryptedXmlDecryptor.cs

// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System;
using System.Security.Cryptography.Xml;
using System.Xml;
using System.Xml.Linq;
using Microsoft.Extensions.DependencyInjection;

namespace Microsoft.AspNetCore.DataProtection.XmlEncryption
{
    /// <summary>
    /// An <see cref="IXmlDecryptor"/> that decrypts XML elements by using the <see cref="EncryptedXml"/> class.
    /// </summary>
    public sealed class EncryptedXmlDecryptor : IInternalEncryptedXmlDecryptor, IXmlDecryptor
    {
        private readonly IInternalEncryptedXmlDecryptor _decryptor;

        /// <summary>
        /// Creates a new instance of an <see cref="EncryptedXmlDecryptor"/>.
        /// </summary>
        public EncryptedXmlDecryptor()
            : this(services: null)
        {
        }

        /// <summary>
        /// Creates a new instance of an <see cref="EncryptedXmlDecryptor"/>.
        /// </summary>
        /// <param name="services">An optional <see cref="IServiceProvider"/> to provide ancillary services.</param>
        public EncryptedXmlDecryptor(IServiceProvider services)
        {
            _decryptor = services?.GetService<IInternalEncryptedXmlDecryptor>() ?? this;
        }

        /// <summary>
        /// Decrypts the specified XML element.
        /// </summary>
        /// <param name="encryptedElement">An encrypted XML element.</param>
        /// <returns>The decrypted form of <paramref name="encryptedElement"/>.</returns>
        public XElement Decrypt(XElement encryptedElement)
        {
            if (encryptedElement == null)
            {
                throw new ArgumentNullException(nameof(encryptedElement));
            }

            // <EncryptedData Type="http://www.w3.org/2001/04/xmlenc#Element" xmlns="http://www.w3.org/2001/04/xmlenc#">
            //   ...
            // </EncryptedData>

            // EncryptedXml works with XmlDocument, not XLinq. When we perform the conversion
            // we'll wrap the incoming element in a dummy <root /> element since encrypted XML
            // doesn't handle encrypting the root element all that well.
            var xmlDocument = new XmlDocument();
            xmlDocument.Load(new XElement("root", encryptedElement).CreateReader());
            var elementToDecrypt = (XmlElement)xmlDocument.DocumentElement.FirstChild;

            // Perform the decryption and update the document in-place.
            var encryptedXml = new EncryptedXml(xmlDocument);
            _decryptor.PerformPreDecryptionSetup(encryptedXml);
            encryptedXml.DecryptDocument();

            // Strip the <root /> element back off and convert the XmlDocument to an XElement.
            return XElement.Load(xmlDocument.DocumentElement.FirstChild.CreateNavigator().ReadSubtree());
        }

        void IInternalEncryptedXmlDecryptor.PerformPreDecryptionSetup(EncryptedXml encryptedXml)
        {
            // no-op
        }
    }
}