//------------------------------------------------------------------------------
// <copyright file="CryptographicKey.cs" company="Microsoft">
//     Copyright (c) Microsoft Corporation.  All rights reserved.
// </copyright>                                                                
//------------------------------------------------------------------------------

namespace System.Web.Security.Cryptography {
    using System;
    using System.Web.Util;

    // Represents a key that can be used for a cryptographic operation.

    internal sealed class CryptographicKey {

        private readonly byte[] _keyMaterial;

        public CryptographicKey(byte[] keyMaterial) {
            _keyMaterial = keyMaterial;
        }

        // Returns the length of the key (in bits).
        public int KeyLength {
            get {
                return checked(_keyMaterial.Length * 8);
            }
        }

        // Extracts the specified number of bits at the specified offset
        // and returns a new CryptographicKey. This is not appropriate
        // for subkey derivation, but it can be used if this cryptographic
        // key is actually two keys (like encryption + validation)
        // concatenated together. Inputs are specified as bit lengths.
        public CryptographicKey ExtractBits(int offset, int count) {
            Debug.Assert(offset % 8 == 0, "Offset must be divisible by 8.");
            Debug.Assert(count % 8 == 0, "Count must be divisible by 8.");

            int offsetBytes = offset / 8;
            int countBytes = count / 8;

            byte[] newKey = new byte[countBytes];
            Buffer.BlockCopy(_keyMaterial, offsetBytes, newKey, 0, countBytes);
            return new CryptographicKey(newKey);
        }

        // Returns the raw key material as a byte array.
        public byte[] GetKeyMaterial() {
            return _keyMaterial;
        }

    }
}