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Original : https://stackoverflow.com/a/38974483
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// original : https://stackoverflow.com/a/38974483 | |
public class SeekableAesStream : Stream | |
{ | |
private Stream baseStream; | |
private AesManaged aes; | |
private ICryptoTransform encryptor; | |
public bool autoDisposeBaseStream { get; set; } = true; | |
/// <param name="salt">//** WARNING **: MUST be unique for each stream otherwise there is NO security</param> | |
public SeekableAesStream(Stream baseStream, string password, byte[] salt) | |
{ | |
this.baseStream = baseStream; | |
using (var key = new PasswordDeriveBytes(password, salt)) | |
{ | |
aes = new AesManaged(); | |
aes.KeySize = 128; | |
aes.Mode = CipherMode.ECB; | |
aes.Padding = PaddingMode.None; | |
aes.Key = key.GetBytes(aes.KeySize / 8); | |
aes.IV = new byte[16]; //zero buffer is adequate since we have to use new salt for each stream | |
encryptor = aes.CreateEncryptor(aes.Key, aes.IV); | |
} | |
} | |
private void cipher(byte[] buffer, int offset, int count, long streamPos) | |
{ | |
//find block number | |
var blockSizeInByte = aes.BlockSize / 8; | |
var blockNumber = (streamPos / blockSizeInByte) + 1; | |
var keyPos = streamPos % blockSizeInByte; | |
//buffer | |
var outBuffer = new byte[blockSizeInByte]; | |
var nonce = new byte[blockSizeInByte]; | |
var init = false; | |
for (int i = offset; i < count; i++) | |
{ | |
//encrypt the nonce to form next xor buffer (unique key) | |
if (!init || (keyPos % blockSizeInByte) == 0) | |
{ | |
BitConverter.GetBytes(blockNumber).CopyTo(nonce, 0); | |
encryptor.TransformBlock(nonce, 0, nonce.Length, outBuffer, 0); | |
if (init) keyPos = 0; | |
init = true; | |
blockNumber++; | |
} | |
buffer[i] ^= outBuffer[keyPos]; //simple XOR with generated unique key | |
keyPos++; | |
} | |
} | |
public override bool CanRead { get { return baseStream.CanRead; } } | |
public override bool CanSeek { get { return baseStream.CanSeek; } } | |
public override bool CanWrite { get { return baseStream.CanWrite; } } | |
public override long Length { get { return baseStream.Length; } } | |
public override long Position { get { return baseStream.Position; } set { baseStream.Position = value; } } | |
public override void Flush() { baseStream.Flush(); } | |
public override void SetLength(long value) { baseStream.SetLength(value); } | |
public override long Seek(long offset, SeekOrigin origin) { return baseStream.Seek(offset, origin); } | |
public override int Read(byte[] buffer, int offset, int count) | |
{ | |
var streamPos = Position; | |
var ret = baseStream.Read(buffer, offset, count); | |
cipher(buffer, offset, count, streamPos); | |
return ret; | |
} | |
public override void Write(byte[] buffer, int offset, int count) | |
{ | |
cipher(buffer, offset, count, Position); | |
baseStream.Write(buffer, offset, count); | |
} | |
protected override void Dispose(bool disposing) | |
{ | |
if (disposing) | |
{ | |
encryptor?.Dispose(); | |
aes?.Dispose(); | |
if (autoDisposeBaseStream) | |
baseStream?.Dispose(); | |
} | |
base.Dispose(disposing); | |
} | |
} |
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