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Cache AES one-shot cipher handles #129996

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c2a2fde
Cache AES one-shot cipher handles
vcsjones Jun 29, 2026
b0fcfea
Zero key when in stack buffer
vcsjones Jun 29, 2026
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233 changes: 153 additions & 80 deletions ...raries/System.Security.Cryptography/src/System/Security/Cryptography/AesImplementation.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -9,13 +9,28 @@ namespace System.Security.Cryptography
internal sealed partial class AesImplementation : Aes
{
private FixedMemoryKeyBox? _keyBox;
private ILiteSymmetricCipher? _encryptEcbCipher;
private ILiteSymmetricCipher? _decryptEcbCipher;
private ILiteSymmetricCipher? _encryptCbcCipher;
private ILiteSymmetricCipher? _decryptCbcCipher;
private ConcurrencyBlock _block;

@vcsjones vcsjones Jun 29, 2026

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We use a single ConcurrencyBlock for everything. In theory mix-and-matching concurrent calls to EncryptCbc and DecryptCbc would "work" if we had a per-handle block but that is just making the intentions less clear. I want to avoid "Sometimes concurrency is okay, sometimes it isn't" and just go for "it isn't".


private FixedMemoryKeyBox GetKey()
{
if (_keyBox is null)
{
GenerateKey();
Debug.Assert(_keyBox is not null);
Span<byte> key = stackalloc byte[KeySize / BitsPerByte];

@vcsjones vcsjones Jun 29, 2026

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We can't use GenerateKey() here because ConcurrencyBlock is not re-entrant. GetKey can be called while a ConcurrencyBlock is already being held and GenerateKey is also under a ConcurrencyBlock. We could have a GenerateKeyUnchecked or similar but given that's is 2 lines of code seemed not worth it.


try
{
RandomNumberGenerator.Fill(key);
SetKeyCoreUnchecked(key);
Debug.Assert(_keyBox is not null);
}
finally
{
CryptographicOperations.ZeroMemory(key);
}
}

return _keyBox;
Expand All @@ -32,9 +47,13 @@ public override int KeySize
get => base.KeySize;
set
{
base.KeySize = value;
_keyBox?.Dispose();
_keyBox = null;
using (ConcurrencyBlock.Enter(ref _block))
{
base.KeySize = value;
ClearCachedCiphers();
_keyBox?.Dispose();
_keyBox = null;
}
}
}

Expand Down Expand Up @@ -70,14 +89,23 @@ public sealed override void GenerateIV()
public sealed override unsafe void GenerateKey()
{
Span<byte> key = stackalloc byte[KeySize / BitsPerByte];
RandomNumberGenerator.Fill(key);
SetKeyCore(key);

try
{
RandomNumberGenerator.Fill(key);
SetKeyCore(key);
}
finally
{
CryptographicOperations.ZeroMemory(key);
}
}

protected sealed override void Dispose(bool disposing)
{
if (disposing)
{
ClearCachedCiphers();
_keyBox?.Dispose();
_keyBox = null;
}
Expand All @@ -87,9 +115,10 @@ protected sealed override void Dispose(bool disposing)

protected override void SetKeyCore(ReadOnlySpan<byte> key)
{
KeySizeValue = checked(BitsPerByte * key.Length);
_keyBox?.Dispose();
_keyBox = new FixedMemoryKeyBox(key);
using (ConcurrencyBlock.Enter(ref _block))
{
SetKeyCoreUnchecked(key);
}
}

protected override bool TryDecryptEcbCore(
Expand All @@ -98,19 +127,14 @@ protected override bool TryDecryptEcbCore(
PaddingMode paddingMode,
out int bytesWritten)
{
ILiteSymmetricCipher cipher = GetKey().UseKey(
BlockSize / BitsPerByte,
static (blockSizeBytes, key) => CreateLiteCipher(
using (ConcurrencyBlock.Enter(ref _block))
{
ILiteSymmetricCipher cipher = GetOrCreateCachedLiteCipher(
ref _decryptEcbCipher,
CipherMode.ECB,
key,
iv: default,
blockSize: blockSizeBytes,
paddingSize: blockSizeBytes,
0, /*feedback size */
encrypting: false));
encrypting: false);

using (cipher)
{
return UniversalCryptoOneShot.OneShotDecrypt(cipher, paddingMode, ciphertext, destination, out bytesWritten);
}
}
Expand All @@ -121,19 +145,14 @@ protected override bool TryEncryptEcbCore(
PaddingMode paddingMode,
out int bytesWritten)
{
ILiteSymmetricCipher cipher = GetKey().UseKey(
BlockSize / BitsPerByte,
static (blockSizeBytes, key) => CreateLiteCipher(
using (ConcurrencyBlock.Enter(ref _block))
{
ILiteSymmetricCipher cipher = GetOrCreateCachedLiteCipher(
ref _encryptEcbCipher,
CipherMode.ECB,
key,
iv: default,
blockSize: blockSizeBytes,
paddingSize: blockSizeBytes,
0, /*feedback size */
encrypting: true));
encrypting: true);

using (cipher)
{
return UniversalCryptoOneShot.OneShotEncrypt(cipher, paddingMode, plaintext, destination, out bytesWritten);
}
}
Expand All @@ -145,20 +164,14 @@ protected override bool TryEncryptCbcCore(
PaddingMode paddingMode,
out int bytesWritten)
{
ILiteSymmetricCipher cipher = GetKey().UseKey(
iv,
BlockSize / BitsPerByte,
static (iv, blockSizeBytes, key) => CreateLiteCipher(
using (ConcurrencyBlock.Enter(ref _block))
{
ILiteSymmetricCipher cipher = GetOrCreateCachedLiteCipher(
ref _encryptCbcCipher,
CipherMode.CBC,
key,
iv,
blockSize: blockSizeBytes,
paddingSize: blockSizeBytes,
0, /*feedback size */
encrypting: true));
encrypting: true);

using (cipher)
{
return UniversalCryptoOneShot.OneShotEncrypt(cipher, paddingMode, plaintext, destination, out bytesWritten);
}
}
Expand All @@ -170,20 +183,14 @@ protected override bool TryDecryptCbcCore(
PaddingMode paddingMode,
out int bytesWritten)
{
ILiteSymmetricCipher cipher = GetKey().UseKey(
iv,
BlockSize / BitsPerByte,
static (iv, blockSizeBytes, key) => CreateLiteCipher(
using (ConcurrencyBlock.Enter(ref _block))
{
ILiteSymmetricCipher cipher = GetOrCreateCachedLiteCipher(
ref _decryptCbcCipher,
CipherMode.CBC,
key,
iv,
blockSize: blockSizeBytes,
paddingSize: blockSizeBytes,
0, /*feedback size */
encrypting: false));
encrypting: false);

using (cipher)
{
return UniversalCryptoOneShot.OneShotDecrypt(cipher, paddingMode, ciphertext, destination, out bytesWritten);
}
}
Expand All @@ -198,21 +205,24 @@ protected override bool TryDecryptCfbCore(
{
ValidateCFBFeedbackSize(feedbackSizeInBits);

ILiteSymmetricCipher cipher = GetKey().UseKey(
iv,
(BlockSizeBytes: BlockSize / BitsPerByte, FeedbackSizeBytes: feedbackSizeInBits / BitsPerByte),
static (iv, state, key) => CreateLiteCipher(
CipherMode.CFB,
key,
iv: iv,
blockSize: state.BlockSizeBytes,
paddingSize: state.FeedbackSizeBytes,
state.FeedbackSizeBytes,
encrypting: false));

using (cipher)
using (ConcurrencyBlock.Enter(ref _block))

@vcsjones vcsjones Jun 29, 2026

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Even though CFB is not cached it felt odd to leave it out of the ConcurrencyBlock. We can take it out but I want to more or less make it official: don't call these things concurrently.

{
return UniversalCryptoOneShot.OneShotDecrypt(cipher, paddingMode, ciphertext, destination, out bytesWritten);
ILiteSymmetricCipher cipher = GetKey().UseKey(
iv,
(BlockSizeBytes: BlockSize / BitsPerByte, FeedbackSizeBytes: feedbackSizeInBits / BitsPerByte),
static (iv, state, key) => CreateLiteCipher(
CipherMode.CFB,
key,
iv: iv,
blockSize: state.BlockSizeBytes,
paddingSize: state.FeedbackSizeBytes,
state.FeedbackSizeBytes,
encrypting: false));

using (cipher)
{
return UniversalCryptoOneShot.OneShotDecrypt(cipher, paddingMode, ciphertext, destination, out bytesWritten);
}
}
}

Expand All @@ -226,21 +236,24 @@ protected override bool TryEncryptCfbCore(
{
ValidateCFBFeedbackSize(feedbackSizeInBits);

ILiteSymmetricCipher cipher = GetKey().UseKey(
iv,
(BlockSizeBytes: BlockSize / BitsPerByte, FeedbackSizeBytes: feedbackSizeInBits / BitsPerByte),
static (iv, state, key) => CreateLiteCipher(
CipherMode.CFB,
key,
iv,
blockSize: state.BlockSizeBytes,
paddingSize: state.FeedbackSizeBytes,
state.FeedbackSizeBytes,
encrypting: true));

using (cipher)
using (ConcurrencyBlock.Enter(ref _block))
{
return UniversalCryptoOneShot.OneShotEncrypt(cipher, paddingMode, plaintext, destination, out bytesWritten);
ILiteSymmetricCipher cipher = GetKey().UseKey(
iv,
(BlockSizeBytes: BlockSize / BitsPerByte, FeedbackSizeBytes: feedbackSizeInBits / BitsPerByte),
static (iv, state, key) => CreateLiteCipher(
CipherMode.CFB,
key,
iv,
blockSize: state.BlockSizeBytes,
paddingSize: state.FeedbackSizeBytes,
state.FeedbackSizeBytes,
encrypting: true));

using (cipher)
{
return UniversalCryptoOneShot.OneShotEncrypt(cipher, paddingMode, plaintext, destination, out bytesWritten);
}
}
}

Expand Down Expand Up @@ -291,6 +304,66 @@ private static void ValidateCFBFeedbackSize(int feedback)
}
}

private ILiteSymmetricCipher GetOrCreateCachedLiteCipher(
ref ILiteSymmetricCipher? cipher,
CipherMode cipherMode,
ReadOnlySpan<byte> iv,
bool encrypting)
{
Debug.Assert(cipherMode is CipherMode.ECB or CipherMode.CBC);

if (cipher is not null)
{
try
{
cipher.Reset(iv);
return cipher;
}
catch
{
cipher.Dispose();
cipher = null; // Null-out the cipher field passed by reference.
throw;
}
}

int blockSizeBytes = BlockSize / BitsPerByte;
cipher = GetKey().UseKey(
iv,
(BlockSizeBytes: blockSizeBytes, CipherMode: cipherMode, Encrypting: encrypting),
static (iv, state, key) => CreateLiteCipher(
state.CipherMode,
key,
iv,
blockSize: state.BlockSizeBytes,
paddingSize: state.BlockSizeBytes,
0, /* feedback size */
encrypting: state.Encrypting));

return cipher;
}

private void SetKeyCoreUnchecked(ReadOnlySpan<byte> key)
{
KeySizeValue = checked(BitsPerByte * key.Length);
FixedMemoryKeyBox keyBox = new FixedMemoryKeyBox(key);
ClearCachedCiphers();
_keyBox?.Dispose();
_keyBox = keyBox;
}

private void ClearCachedCiphers()
{
_encryptEcbCipher?.Dispose();
_encryptEcbCipher = null;
_decryptEcbCipher?.Dispose();
_decryptEcbCipher = null;
_encryptCbcCipher?.Dispose();
_encryptCbcCipher = null;
_decryptCbcCipher?.Dispose();
_decryptCbcCipher = null;
}

private const int BitsPerByte = 8;
}
}
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