问题
I need to communicate from a C# application to another application via encrypted messages in OFB mode. I know that RijndaelManaged
does not have support for AES OFB mode. Is there anybody more experienced than me aware of any other way to encrypt/decrypt using OFB mode?
回答1:
The following stream implements OFB by using a key stream generated by a zero-fed CBC cipher stream.
public class OFBStream : Stream
{
private const int BLOCKS = 16;
private const int EOS = 0; // the goddess of dawn is found at the end of the stream
private Stream parent;
private CryptoStream cbcStream;
private CryptoStreamMode mode;
private byte[] keyStreamBuffer;
private int keyStreamBufferOffset;
private byte[] readWriteBuffer;
public OFBStream (Stream parent, SymmetricAlgorithm algo, CryptoStreamMode mode)
{
if (algo.Mode != CipherMode.CBC)
algo.Mode = CipherMode.CBC;
if (algo.Padding != PaddingMode.None)
algo.Padding = PaddingMode.None;
this.parent = parent;
this.cbcStream = new CryptoStream (new ZeroStream (), algo.CreateEncryptor (), CryptoStreamMode.Read);
this.mode = mode;
keyStreamBuffer = new byte[algo.BlockSize * BLOCKS];
readWriteBuffer = new byte[keyStreamBuffer.Length];
}
public override int Read (byte[] buffer, int offset, int count)
{
if (!CanRead) {
throw new NotSupportedException ();
}
int toRead = Math.Min (count, readWriteBuffer.Length);
int read = parent.Read (readWriteBuffer, 0, toRead);
if (read == EOS)
return EOS;
for (int i = 0; i < read; i++) {
// NOTE could be optimized (branches for each byte)
if (keyStreamBufferOffset % keyStreamBuffer.Length == 0) {
FillKeyStreamBuffer ();
keyStreamBufferOffset = 0;
}
buffer [offset + i] = (byte)(readWriteBuffer [i]
^ keyStreamBuffer [keyStreamBufferOffset++]);
}
return read;
}
public override void Write (byte[] buffer, int offset, int count)
{
if (!CanWrite) {
throw new NotSupportedException ();
}
int readWriteBufferOffset = 0;
for (int i = 0; i < count; i++) {
if (keyStreamBufferOffset % keyStreamBuffer.Length == 0) {
FillKeyStreamBuffer ();
keyStreamBufferOffset = 0;
}
if (readWriteBufferOffset % readWriteBuffer.Length == 0) {
parent.Write (readWriteBuffer, 0, readWriteBufferOffset);
readWriteBufferOffset = 0;
}
readWriteBuffer [readWriteBufferOffset++] = (byte)(buffer [offset + i]
^ keyStreamBuffer [keyStreamBufferOffset++]);
}
parent.Write (readWriteBuffer, 0, readWriteBufferOffset);
}
private void FillKeyStreamBuffer ()
{
int read = cbcStream.Read (keyStreamBuffer, 0, keyStreamBuffer.Length);
// NOTE undocumented feature
// only works if keyStreamBuffer.Length % blockSize == 0
if (read != keyStreamBuffer.Length)
throw new InvalidOperationException ("Implementation error: could not read all bytes from CBC stream");
}
public override bool CanRead {
get { return mode == CryptoStreamMode.Read; }
}
public override bool CanWrite {
get { return mode == CryptoStreamMode.Write; }
}
public override void Flush ()
{
// should never have to be flushed, implementation empty
}
public override bool CanSeek {
get { return false; }
}
public override long Seek (long offset, System.IO.SeekOrigin origin)
{
throw new NotSupportedException ();
}
public override long Position {
get { throw new NotSupportedException (); }
set { throw new NotSupportedException (); }
}
public override long Length {
get { throw new NotSupportedException (); }
}
public override void SetLength (long value)
{
throw new NotSupportedException ();
}
}
Additional class ZeroStream
required by OFBStream
class ZeroStream : System.IO.Stream
{
public override int Read (byte[] buffer, int offset, int count)
{
for (int i = 0; i < count; i++) {
buffer [offset + i] = 0;
}
return count;
}
public override bool CanRead {
get { return true; }
}
... the rest is not implemented
}
And you can use it as I do for a test vector:
// NIST CAVP test vector F.4.1: OFB-AES128.Encrypt from NIST SP 800-38A
RijndaelManaged aes = new RijndaelManaged ();
aes.Key = FromHex ("2b7e151628aed2a6abf7158809cf4f3c");
aes.IV = FromHex ("000102030405060708090A0B0C0D0E0F");
MemoryStream testVectorStream = new MemoryStream (FromHex (
"6bc1bee22e409f96e93d7e117393172aae2d8a571e03ac9c9eb76fac45af8e5130c81c46a35ce411e5fbc1191a0a52eff69f2445df4f9b17ad2b417be66c3710"));
OFBStream testOFBStream = new OFBStream (testVectorStream, aes, CryptoStreamMode.Read);
MemoryStream cipherTextStream = new MemoryStream ();
testOFBStream.CopyTo (cipherTextStream);
Console.WriteLine (ToHex (cipherTextStream.ToArray ()));
Note that the stream handling has not been fully tested (yet).
来源:https://stackoverflow.com/questions/24515521/aes-ofb-encryption-for-rijndaelmanaged