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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
using System;
using System.IO;
using System.Security.Cryptography;
using Microsoft.AspNetCore.Cryptography;
using Microsoft.AspNetCore.DataProtection.AuthenticatedEncryption;
using Microsoft.AspNetCore.DataProtection.SP800_108;
namespace Microsoft.AspNetCore.DataProtection.Managed;
// An encryptor which does Encrypt(CBC) + HMAC using SymmetricAlgorithm and HashAlgorithm.
// The payloads produced by this encryptor should be compatible with the payloads
// produced by the CNG-based Encrypt(CBC) + HMAC authenticated encryptor.
internal sealed unsafe class ManagedAuthenticatedEncryptor : IAuthenticatedEncryptor, IDisposable
{
// Even when IVs are chosen randomly, CBC is susceptible to IV collisions within a single
// key. For a 64-bit block cipher (like 3DES), we'd expect a collision after 2^32 block
// encryption operations, which a high-traffic web server might perform in mere hours.
// AES and other 128-bit block ciphers are less susceptible to this due to the larger IV
// space, but unfortunately some organizations require older 64-bit block ciphers. To address
// the collision issue, we'll feed 128 bits of entropy to the KDF when performing subkey
// generation. This creates >= 192 bits total entropy for each operation, so we shouldn't
// expect a collision until >= 2^96 operations. Even 2^80 operations still maintains a <= 2^-32
// probability of collision, and this is acceptable for the expected KDK lifetime.
private const int KEY_MODIFIER_SIZE_IN_BYTES = 128 / 8;
private static readonly Func<byte[], HashAlgorithm> _kdkPrfFactory = key => new HMACSHA512(key); // currently hardcoded to SHA512
private readonly byte[] _contextHeader;
private readonly IManagedGenRandom _genRandom;
private readonly Secret _keyDerivationKey;
private readonly Func<SymmetricAlgorithm> _symmetricAlgorithmFactory;
private readonly int _symmetricAlgorithmBlockSizeInBytes;
private readonly int _symmetricAlgorithmSubkeyLengthInBytes;
private readonly int _validationAlgorithmDigestLengthInBytes;
private readonly int _validationAlgorithmSubkeyLengthInBytes;
private readonly Func<KeyedHashAlgorithm> _validationAlgorithmFactory;
public ManagedAuthenticatedEncryptor(Secret keyDerivationKey, Func<SymmetricAlgorithm> symmetricAlgorithmFactory, int symmetricAlgorithmKeySizeInBytes, Func<KeyedHashAlgorithm> validationAlgorithmFactory, IManagedGenRandom? genRandom = null)
{
_genRandom = genRandom ?? ManagedGenRandomImpl.Instance;
_keyDerivationKey = keyDerivationKey;
// Validate that the symmetric algorithm has the properties we require
using (var symmetricAlgorithm = symmetricAlgorithmFactory())
{
_symmetricAlgorithmFactory = symmetricAlgorithmFactory;
_symmetricAlgorithmBlockSizeInBytes = symmetricAlgorithm.GetBlockSizeInBytes();
_symmetricAlgorithmSubkeyLengthInBytes = symmetricAlgorithmKeySizeInBytes;
}
// Validate that the MAC algorithm has the properties we require
using (var validationAlgorithm = validationAlgorithmFactory())
{
_validationAlgorithmFactory = validationAlgorithmFactory;
_validationAlgorithmDigestLengthInBytes = validationAlgorithm.GetDigestSizeInBytes();
_validationAlgorithmSubkeyLengthInBytes = _validationAlgorithmDigestLengthInBytes; // for simplicity we'll generate MAC subkeys with a length equal to the digest length
}
// Argument checking on the algorithms and lengths passed in to us
AlgorithmAssert.IsAllowableSymmetricAlgorithmBlockSize(checked((uint)_symmetricAlgorithmBlockSizeInBytes * 8));
AlgorithmAssert.IsAllowableSymmetricAlgorithmKeySize(checked((uint)_symmetricAlgorithmSubkeyLengthInBytes * 8));
AlgorithmAssert.IsAllowableValidationAlgorithmDigestSize(checked((uint)_validationAlgorithmDigestLengthInBytes * 8));
_contextHeader = CreateContextHeader();
}
private byte[] CreateContextHeader()
{
var EMPTY_ARRAY = Array.Empty<byte>();
var EMPTY_ARRAY_SEGMENT = new ArraySegment<byte>(EMPTY_ARRAY);
var retVal = new byte[checked(
1 /* KDF alg */
+ 1 /* chaining mode */
+ sizeof(uint) /* sym alg key size */
+ sizeof(uint) /* sym alg block size */
+ sizeof(uint) /* hmac alg key size */
+ sizeof(uint) /* hmac alg digest size */
+ _symmetricAlgorithmBlockSizeInBytes /* ciphertext of encrypted empty string */
+ _validationAlgorithmDigestLengthInBytes /* digest of HMACed empty string */)];
var idx = 0;
// First is the two-byte header
retVal[idx++] = 0; // 0x00 = SP800-108 CTR KDF w/ HMACSHA512 PRF
retVal[idx++] = 0; // 0x00 = CBC encryption + HMAC authentication
// Next is information about the symmetric algorithm (key size followed by block size)
BitHelpers.WriteTo(retVal, ref idx, _symmetricAlgorithmSubkeyLengthInBytes);
BitHelpers.WriteTo(retVal, ref idx, _symmetricAlgorithmBlockSizeInBytes);
// Next is information about the keyed hash algorithm (key size followed by digest size)
BitHelpers.WriteTo(retVal, ref idx, _validationAlgorithmSubkeyLengthInBytes);
BitHelpers.WriteTo(retVal, ref idx, _validationAlgorithmDigestLengthInBytes);
// See the design document for an explanation of the following code.
var tempKeys = new byte[_symmetricAlgorithmSubkeyLengthInBytes + _validationAlgorithmSubkeyLengthInBytes];
ManagedSP800_108_CTR_HMACSHA512.DeriveKeys(
kdk: EMPTY_ARRAY,
label: EMPTY_ARRAY_SEGMENT,
context: EMPTY_ARRAY_SEGMENT,
prfFactory: _kdkPrfFactory,
output: new ArraySegment<byte>(tempKeys));
// At this point, tempKeys := { K_E || K_H }.
// Encrypt a zero-length input string with an all-zero IV and copy the ciphertext to the return buffer.
using (var symmetricAlg = CreateSymmetricAlgorithm())
{
using (var cryptoTransform = symmetricAlg.CreateEncryptor(
rgbKey: new ArraySegment<byte>(tempKeys, 0, _symmetricAlgorithmSubkeyLengthInBytes).AsStandaloneArray(),
rgbIV: new byte[_symmetricAlgorithmBlockSizeInBytes]))
{
var ciphertext = cryptoTransform.TransformFinalBlock(EMPTY_ARRAY, 0, 0);
CryptoUtil.Assert(ciphertext != null && ciphertext.Length == _symmetricAlgorithmBlockSizeInBytes, "ciphertext != null && ciphertext.Length == _symmetricAlgorithmBlockSizeInBytes");
Buffer.BlockCopy(ciphertext, 0, retVal, idx, ciphertext.Length);
}
}
idx += _symmetricAlgorithmBlockSizeInBytes;
// MAC a zero-length input string and copy the digest to the return buffer.
using (var hashAlg = CreateValidationAlgorithm(new ArraySegment<byte>(tempKeys, _symmetricAlgorithmSubkeyLengthInBytes, _validationAlgorithmSubkeyLengthInBytes).AsStandaloneArray()))
{
var digest = hashAlg.ComputeHash(EMPTY_ARRAY);
CryptoUtil.Assert(digest != null && digest.Length == _validationAlgorithmDigestLengthInBytes, "digest != null && digest.Length == _validationAlgorithmDigestLengthInBytes");
Buffer.BlockCopy(digest, 0, retVal, idx, digest.Length);
}
idx += _validationAlgorithmDigestLengthInBytes;
CryptoUtil.Assert(idx == retVal.Length, "idx == retVal.Length");
// retVal := { version || chainingMode || symAlgKeySize || symAlgBlockSize || macAlgKeySize || macAlgDigestSize || E("") || MAC("") }.
return retVal;
}
private SymmetricAlgorithm CreateSymmetricAlgorithm()
{
var retVal = _symmetricAlgorithmFactory();
CryptoUtil.Assert(retVal != null, "retVal != null");
retVal.Mode = CipherMode.CBC;
retVal.Padding = PaddingMode.PKCS7;
return retVal;
}
private KeyedHashAlgorithm CreateValidationAlgorithm(byte[] key)
{
var retVal = _validationAlgorithmFactory();
CryptoUtil.Assert(retVal != null, "retVal != null");
retVal.Key = key;
return retVal;
}
public byte[] Decrypt(ArraySegment<byte> protectedPayload, ArraySegment<byte> additionalAuthenticatedData)
{
protectedPayload.Validate();
additionalAuthenticatedData.Validate();
// Argument checking - input must at the absolute minimum contain a key modifier, IV, and MAC
if (protectedPayload.Count < checked(KEY_MODIFIER_SIZE_IN_BYTES + _symmetricAlgorithmBlockSizeInBytes + _validationAlgorithmDigestLengthInBytes))
{
throw Error.CryptCommon_PayloadInvalid();
}
// Assumption: protectedPayload := { keyModifier | IV | encryptedData | MAC(IV | encryptedPayload) }
try
{
// Step 1: Extract the key modifier and IV from the payload.
int keyModifierOffset; // position in protectedPayload.Array where key modifier begins
int ivOffset; // position in protectedPayload.Array where key modifier ends / IV begins
int ciphertextOffset; // position in protectedPayload.Array where IV ends / ciphertext begins
int macOffset; // position in protectedPayload.Array where ciphertext ends / MAC begins
int eofOffset; // position in protectedPayload.Array where MAC ends
checked
{
keyModifierOffset = protectedPayload.Offset;
ivOffset = keyModifierOffset + KEY_MODIFIER_SIZE_IN_BYTES;
ciphertextOffset = ivOffset + _symmetricAlgorithmBlockSizeInBytes;
}
ArraySegment<byte> keyModifier = new ArraySegment<byte>(protectedPayload.Array!, keyModifierOffset, ivOffset - keyModifierOffset);
var iv = new byte[_symmetricAlgorithmBlockSizeInBytes];
Buffer.BlockCopy(protectedPayload.Array!, ivOffset, iv, 0, iv.Length);
// Step 2: Decrypt the KDK and use it to restore the original encryption and MAC keys.
// We pin all unencrypted keys to limit their exposure via GC relocation.
var decryptedKdk = new byte[_keyDerivationKey.Length];
var decryptionSubkey = new byte[_symmetricAlgorithmSubkeyLengthInBytes];
var validationSubkey = new byte[_validationAlgorithmSubkeyLengthInBytes];
var derivedKeysBuffer = new byte[checked(decryptionSubkey.Length + validationSubkey.Length)];
fixed (byte* __unused__1 = decryptedKdk)
fixed (byte* __unused__2 = decryptionSubkey)
fixed (byte* __unused__3 = validationSubkey)
fixed (byte* __unused__4 = derivedKeysBuffer)
{
try
{
_keyDerivationKey.WriteSecretIntoBuffer(new ArraySegment<byte>(decryptedKdk));
ManagedSP800_108_CTR_HMACSHA512.DeriveKeysWithContextHeader(
kdk: decryptedKdk,
label: additionalAuthenticatedData,
contextHeader: _contextHeader,
context: keyModifier,
prfFactory: _kdkPrfFactory,
output: new ArraySegment<byte>(derivedKeysBuffer));
Buffer.BlockCopy(derivedKeysBuffer, 0, decryptionSubkey, 0, decryptionSubkey.Length);
Buffer.BlockCopy(derivedKeysBuffer, decryptionSubkey.Length, validationSubkey, 0, validationSubkey.Length);
// Step 3: Calculate the correct MAC for this payload.
// correctHash := MAC(IV || ciphertext)
byte[] correctHash;
using (var hashAlgorithm = CreateValidationAlgorithm(validationSubkey))
{
checked
{
eofOffset = protectedPayload.Offset + protectedPayload.Count;
macOffset = eofOffset - _validationAlgorithmDigestLengthInBytes;
}
correctHash = hashAlgorithm.ComputeHash(protectedPayload.Array!, ivOffset, macOffset - ivOffset);
}
// Step 4: Validate the MAC provided as part of the payload.
if (!CryptoUtil.TimeConstantBuffersAreEqual(correctHash, 0, correctHash.Length, protectedPayload.Array!, macOffset, eofOffset - macOffset))
{
throw Error.CryptCommon_PayloadInvalid(); // integrity check failure
}
// Step 5: Decipher the ciphertext and return it to the caller.
using (var symmetricAlgorithm = CreateSymmetricAlgorithm())
using (var cryptoTransform = symmetricAlgorithm.CreateDecryptor(decryptionSubkey, iv))
{
var outputStream = new MemoryStream();
using (var cryptoStream = new CryptoStream(outputStream, cryptoTransform, CryptoStreamMode.Write))
{
cryptoStream.Write(protectedPayload.Array!, ciphertextOffset, macOffset - ciphertextOffset);
cryptoStream.FlushFinalBlock();
// At this point, outputStream := { plaintext }, and we're done!
return outputStream.ToArray();
}
}
}
finally
{
// delete since these contain secret material
Array.Clear(decryptedKdk, 0, decryptedKdk.Length);
Array.Clear(decryptionSubkey, 0, decryptionSubkey.Length);
Array.Clear(validationSubkey, 0, validationSubkey.Length);
Array.Clear(derivedKeysBuffer, 0, derivedKeysBuffer.Length);
}
}
}
catch (Exception ex) when (ex.RequiresHomogenization())
{
// Homogenize all exceptions to CryptographicException.
throw Error.CryptCommon_GenericError(ex);
}
}
public void Dispose()
{
_keyDerivationKey.Dispose();
}
public byte[] Encrypt(ArraySegment<byte> plaintext, ArraySegment<byte> additionalAuthenticatedData)
{
plaintext.Validate();
additionalAuthenticatedData.Validate();
try
{
var outputStream = new MemoryStream();
// Step 1: Generate a random key modifier and IV for this operation.
// Both will be equal to the block size of the block cipher algorithm.
var keyModifier = _genRandom.GenRandom(KEY_MODIFIER_SIZE_IN_BYTES);
var iv = _genRandom.GenRandom(_symmetricAlgorithmBlockSizeInBytes);
// Step 2: Copy the key modifier and the IV to the output stream since they'll act as a header.
outputStream.Write(keyModifier, 0, keyModifier.Length);
outputStream.Write(iv, 0, iv.Length);
// At this point, outputStream := { keyModifier || IV }.
// Step 3: Decrypt the KDK, and use it to generate new encryption and HMAC keys.
// We pin all unencrypted keys to limit their exposure via GC relocation.
var decryptedKdk = new byte[_keyDerivationKey.Length];
var encryptionSubkey = new byte[_symmetricAlgorithmSubkeyLengthInBytes];
var validationSubkey = new byte[_validationAlgorithmSubkeyLengthInBytes];
var derivedKeysBuffer = new byte[checked(encryptionSubkey.Length + validationSubkey.Length)];
fixed (byte* __unused__1 = decryptedKdk)
fixed (byte* __unused__2 = encryptionSubkey)
fixed (byte* __unused__3 = validationSubkey)
fixed (byte* __unused__4 = derivedKeysBuffer)
{
try
{
_keyDerivationKey.WriteSecretIntoBuffer(new ArraySegment<byte>(decryptedKdk));
ManagedSP800_108_CTR_HMACSHA512.DeriveKeysWithContextHeader(
kdk: decryptedKdk,
label: additionalAuthenticatedData,
contextHeader: _contextHeader,
context: new ArraySegment<byte>(keyModifier),
prfFactory: _kdkPrfFactory,
output: new ArraySegment<byte>(derivedKeysBuffer));
Buffer.BlockCopy(derivedKeysBuffer, 0, encryptionSubkey, 0, encryptionSubkey.Length);
Buffer.BlockCopy(derivedKeysBuffer, encryptionSubkey.Length, validationSubkey, 0, validationSubkey.Length);
// Step 4: Perform the encryption operation.
using (var symmetricAlgorithm = CreateSymmetricAlgorithm())
using (var cryptoTransform = symmetricAlgorithm.CreateEncryptor(encryptionSubkey, iv))
using (var cryptoStream = new CryptoStream(outputStream, cryptoTransform, CryptoStreamMode.Write))
{
cryptoStream.Write(plaintext.Array!, plaintext.Offset, plaintext.Count);
cryptoStream.FlushFinalBlock();
// At this point, outputStream := { keyModifier || IV || ciphertext }
// Step 5: Calculate the digest over the IV and ciphertext.
// We don't need to calculate the digest over the key modifier since that
// value has already been mixed into the KDF used to generate the MAC key.
using (var validationAlgorithm = CreateValidationAlgorithm(validationSubkey))
{
// As an optimization, avoid duplicating the underlying buffer
var underlyingBuffer = outputStream.GetBuffer();
var mac = validationAlgorithm.ComputeHash(underlyingBuffer, KEY_MODIFIER_SIZE_IN_BYTES, checked((int)outputStream.Length - KEY_MODIFIER_SIZE_IN_BYTES));
outputStream.Write(mac, 0, mac.Length);
// At this point, outputStream := { keyModifier || IV || ciphertext || MAC(IV || ciphertext) }
// And we're done!
return outputStream.ToArray();
}
}
}
finally
{
// delete since these contain secret material
Array.Clear(decryptedKdk, 0, decryptedKdk.Length);
Array.Clear(encryptionSubkey, 0, encryptionSubkey.Length);
Array.Clear(validationSubkey, 0, validationSubkey.Length);
Array.Clear(derivedKeysBuffer, 0, derivedKeysBuffer.Length);
}
}
}
catch (Exception ex) when (ex.RequiresHomogenization())
{
// Homogenize all exceptions to CryptographicException.
throw Error.CryptCommon_GenericError(ex);
}
}
}
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