Develop Reference

How do I read exactly one char from a Stream?

I have a Stream with some text data (can be ASCII, UTF-8, Unicode; encoding is known). I need to read exactly one char from the stream, without advancing stream position any longer. StreamReader is inappropriate, as it aggressively prefetches data from the stream.
Ideas?

If you want to read and decode the text one byte at a time, the most convenient approach I know of is to use the System.Text.Decoder class.
Here's a simple example:
class Program
{
static void Main(string[] args)
{
Console.OutputEncoding = Encoding.Unicode;
string originalText = "Hello world! ブ䥺ぎょズィ穃 槞こ廤樊稧 ひゃご禺 壪";
byte[] rgb = Encoding.UTF8.GetBytes(originalText);
MemoryStream dataStream = new MemoryStream(rgb);
string result = DecodeOneByteAtATimeFromStream(dataStream);
Console.WriteLine("Result string: \"" + result + "\"");
if (originalText == result)
{
Console.WriteLine("Original and result strings are equal");
}
}
static string DecodeOneByteAtATimeFromStream(MemoryStream dataStream)
{
Decoder decoder = Encoding.UTF8.GetDecoder();
StringBuilder sb = new StringBuilder();
int inputByteCount;
byte[] inputBuffer = new byte[1];
while ((inputByteCount = dataStream.Read(inputBuffer, 0, 1)) > 0)
{
int charCount = decoder.GetCharCount(inputBuffer, 0, 1);
char[] rgch = new char[charCount];
decoder.GetChars(inputBuffer, 0, 1, rgch, 0);
sb.Append(rgch);
}
return sb.ToString();
}
}
Presumably you are already aware of the drawbacks of processing data of any sort just one byte at a time. :) Suffice to say, this is not a very efficient way to do things.

Calculating HMACSHA256 using c# to match payment provider example

For a payment provider, I need to calculate a hash-based message authentication code, using HMAC-SHA256. That is causing me quite a bit of trouble.
The payment provider gives two examples of orrectly calculated authentication code in pseudo-code. All keys are in hex.
Method 1
key = 57617b5d2349434b34734345635073433835777e2d244c31715535255a366773755a4d70532a5879793238235f707c4f7865753f3f446e633a21575643303f66
message = "amount=100&currency=EUR"
MAC = HMAC-SHA256( hexDecode(key), message )
result = b436e3e86cb3800b3864aeecc8d06c126f005e7645803461717a8e4b2de3a905
Method 2
message = "amount=100&currency=EUR"
Ki = 61574d6b157f757d02457573556645750e0341481b127a07476303136c005145436c7b46651c6e4f4f040e1569464a794e534309097258550c17616075060950
Ko = 0b3d27017f151f17682f1f193f0c2f1f64692b227178106d2d096979066a3b2f2906112c0f760425256e647f032c2013243929636318323f667d0b0a1f6c633a
MAC = SHA256( hexDecode(Ko) + SHA256( hexDecode(Ki) + message ) )
result = b436e3e86cb3800b3864aeecc8d06c126f005e7645803461717a8e4b2de3a905
I tried to write the code to do this, after doing some research, but I keep coming up with different results.
private static void Main(string[] args)
{
var key = "57617b5d2349434b34734345635073433835777e2d244c31715535255a366773755a4d70532a5879793238235f707c4f7865753f3f446e633a21575643303f66";
var ki = "61574d6b157f757d02457573556645750e0341481b127a07476303136c005145436c7b46651c6e4f4f040e1569464a794e534309097258550c17616075060950";
var ko = "0b3d27017f151f17682f1f193f0c2f1f64692b227178106d2d096979066a3b2f2906112c0f760425256e647f032c2013243929636318323f667d0b0a1f6c633a";
var mm = "amount=100&currency=EUR";
var result1 = CalcHMACSHA256Hash(HexDecode(key), mm);
var result2 = CalcSha256Hash(string.Format("{0}{1}", HexDecode(ko), CalcSha256Hash(HexDecode(ki) + mm)));
Console.WriteLine("Expected: b436e3e86cb3800b3864aeecc8d06c126f005e7645803461717a8e4b2de3a905");
Console.WriteLine("Actual 1: " + result1);
Console.WriteLine("Actual 2: " + result2);
Console.WriteLine("------------------------------");
Console.ReadKey();
}
private static string HexDecode(string hex)
{
var sb = new StringBuilder();
for (int i = 0; i <= hex.Length - 2; i += 2)
{
sb.Append(Convert.ToString(Convert.ToChar(Int32.Parse(hex.Substring(i, 2), System.Globalization.NumberStyles.HexNumber))));
}
return sb.ToString();
}
private static string CalcHMACSHA256Hash(string plaintext, string salt)
{
string result = "";
var enc = Encoding.Default;
byte[]
baText2BeHashed = enc.GetBytes(plaintext),
baSalt = enc.GetBytes(salt);
System.Security.Cryptography.HMACSHA256 hasher = new HMACSHA256(baSalt);
byte[] baHashedText = hasher.ComputeHash(baText2BeHashed);
result = string.Join("", baHashedText.ToList().Select(b => b.ToString("x2")).ToArray());
return result;
}
public static string CalcSha256Hash(string input)
{
SHA256 sha256 = new SHA256Managed();
byte[] sha256Bytes = Encoding.Default.GetBytes(input);
byte[] cryString = sha256.ComputeHash(sha256Bytes);
string sha256Str = string.Empty;
for (int i = 0; i < cryString.Length; i++)
{
sha256Str += cryString[i].ToString("x2");
}
return sha256Str;
}
And this is the result I get:
Expected: b436e3e86cb3800b3864aeecc8d06c126f005e7645803461717a8e4b2de3a905
Actual 1: 421ce16f2036bb9f2a3770c16f01e9220f0232d45580584ca41768fd16c15fe6
Actual 2: 290f14398bf8c0959dfc963e2fd9c377534c6fec1983025d2ab192382f132b92
So with none of the two methods, I can get the result the provider example wants.
What am I missing here? Is it encoding? Is my hexDecode screwed up?
Test tool from payment provider: http://tech.dibs.dk/dibs_api/other_features/hmac_tool/
PHP sample code: http://tech.dibspayment.com/dibs_api/other_features/mac_calculation/

Edit: You likely are looking for a quick and simple way to do HMAC-SHA256 and not get into the finer details. The original question asks of those finer details which are explained further below.
I want to perform a HMAC-SHA256 on a byte[] message input
using System.Security.Cryptography;
...
private static byte[] HashHMAC(byte[] key, byte[] message)
{
var hash = new HMACSHA256(key);
return hash.ComputeHash(message);
}
I want to perform HMAC-SHA256 but I have a hex string input
In .NET 5 and above, use System.Convert.FromHexString like so, (thanks #proximab). If you're on pre-.NET 5, scroll to "Helper functions" which has alternative solutions.
using System;
using System.Security.Cryptography;
...
private static byte[] HashHMACHex(string keyHex, string messageHex)
{
var key = Convert.FromHexString(hexKey);
var message = Convert.FromHexString(messageHex);
var hash = new HMACSHA256(key);
return hash.ComputeHash(message);
}
I'm using a strange API service that sort of does HMAC, but it's something custom
Continue reading. You likely want to use "Method 2" below as a reference point and adjust it to however your service wants you to implement HMAC for message anti-tampering.
How HMAC-SHA256 Works (should you need to know how...)
Here we will compute an HMAC-SHA256 manually (this answers "Method 2" from the original question).
Assume outerKey, innerKey, and message are already byte arrays, we perform the following:
Notation: Assume A + B concatenates byte array A and B. You may
alternatively see A || B notation used in more academic settings.
HMAC = SHA256( outerKey + SHA256( innerKey + message ) )
. . `------------------´ . .
\ \ `innerData` / /
\ `------------------------´ /
\ `innerHash` /
`----------------------------------´
`data`
So the code can be broken down into these steps (using the above as a guide):
Create an empty buffer byte[] innerData the length of innerKey.Length + message.Length (again assuming byte arrays)
Copy the innerKey and the message into the byte[] innerData
Compute SHA256 of innerData and store it in byte[] innerHash
Create an empty buffer byte[] data the length of outerKey.Length + innerHash.Length
Copy the outerKey and innerHash (from step #3)
Compute the final hash of data and store it in byte[] result and return it.
To do the byte copying I'm using the Buffer.BlockCopy() function since it apparently faster than some other ways (source).
n.b. There is likely (read: most certainly) a better way to do this using the the new ReadOnlySpan<T> API.
We can translate those steps into the following:
using System;
using System.Security.Cryptography;
...
private static byte[] HashSHA(byte[] innerKey, byte[] outerKey, byte[] message)
{
var hash = new SHA256Managed();
// Compute the hash for the inner data first
byte[] innerData = new byte[innerKey.Length + message.Length];
Buffer.BlockCopy(innerKey, 0, innerData, 0, innerKey.Length);
Buffer.BlockCopy(message, 0, innerData, innerKey.Length, message.Length);
byte[] innerHash = hash.ComputeHash(innerData);
// Compute the entire hash
byte[] data = new byte[outerKey.Length + innerHash.Length];
Buffer.BlockCopy(outerKey, 0, data, 0, outerKey.Length);
Buffer.BlockCopy(innerHash, 0, data, outerKey.Length, innerHash.Length);
byte[] result = hash.ComputeHash(data);
return result;
}
Helper functions
string -> byte[]
You have plain ASCII or UTF8 text, but need it to be a byte[].
Use ASCIIEncoding or UTF8Encoding or whichever exotic encoding you're using.
private static byte[] StringEncode(string text)
{
var encoding = new System.Text.ASCIIEncoding();
return encoding.GetBytes(text);
}
byte[] -> hex string
You have a byte[], but you need it to be a hex string.
private static string HashEncode(byte[] hash)
{
return BitConverter.ToString(hash).Replace("-", "").ToLower();
}
hex string -> byte[]
You have a hex string, but you need it to be a byte[]`.
.NET 5 and above
private static byte[] HexDecode(string hex) =>
System.Convert.FromHexString(hex);
Before .NET 5 (thanks #bobince)
private static byte[] HexDecode(string hex)
{
var bytes = new byte[hex.Length / 2];
for (int i = 0; i < bytes.Length; i++)
{
bytes[i] = byte.Parse(hex.Substring(i * 2, 2), NumberStyles.HexNumber);
}
return bytes;
}
n.b. If you need a performance tuned version on .NET Framework 4.x, you can alternatively backport the .NET 5+ version (by replacing ReadOnlySpan<byte> with byte[]). It uses proper lookup tables and conscious about hot-code paths. You can reference the .NET 5 (MIT licensed) System.Convert code on Github.
For completeness, here are the final methods answering the question using both "Method 1" and "Method 2"
"Method 1" (using .NET libraries)
private static string HashHMACHex(string keyHex, string message)
{
byte[] hash = HashHMAC(HexDecode(keyHex), StringEncode(message));
return HashEncode(hash);
}
"Method 2" (manually computed)
private static string HashSHAHex(string innerKeyHex, string outerKeyHex, string message)
{
byte[] hash = HashSHA(HexDecode(innerKeyHex), HexDecode(outerKeyHex), StringEncode(message));
return HashEncode(hash);
}
We can perform a quick sanity check with a console app:
static void Main(string[] args)
{
string message = "amount=100&currency=EUR";
string expectedHex = "b436e3e86cb3800b3864aeecc8d06c126f005e7645803461717a8e4b2de3a905";
Console.WriteLine("Expected: " + expectedHex);
// Test out the HMAC hash method
string key = "57617b5d2349434b34734345635073433835777e2d244c31715535255a366773755a4d70532a5879793238235f707c4f7865753f3f446e633a21575643303f66";
string hashHMACHex = HashHMACHex(key, message);
Console.WriteLine("Method 1: " + hashHMACHex);
// Test out the SHA hash method
string innerKey = "61574d6b157f757d02457573556645750e0341481b127a07476303136c005145436c7b46651c6e4f4f040e1569464a794e534309097258550c17616075060950";
string outerKey = "0b3d27017f151f17682f1f193f0c2f1f64692b227178106d2d096979066a3b2f2906112c0f760425256e647f032c2013243929636318323f667d0b0a1f6c633a";
string hashSHAHex = HashSHAHex(innerKey, outerKey, message);
Console.WriteLine("Method 2: " + hashSHAHex);
}
You should have all the hashes line up correctly:
Expected: b436e3e86cb3800b3864aeecc8d06c126f005e7645803461717a8e4b2de3a905
Method 1: b436e3e86cb3800b3864aeecc8d06c126f005e7645803461717a8e4b2de3a905
Method 2: b436e3e86cb3800b3864aeecc8d06c126f005e7645803461717a8e4b2de3a905
The original code for this answer can be accessed at:
http://pastebin.com/xAAuZrJX

Here's a string extension method for getting a fairly standard HMAC SHA 256 token for a given string:
usage:
myMessageString.HmacSha256Digest(mySecret)
string extension method:
public static string HmacSha256Digest(this string message, string secret)
{
ASCIIEncoding encoding = new ASCIIEncoding();
byte[] keyBytes = encoding.GetBytes(secret);
byte[] messageBytes = encoding.GetBytes(message);
System.Security.Cryptography.HMACSHA256 cryptographer = new System.Security.Cryptography.HMACSHA256(keyBytes);
byte[] bytes = cryptographer.ComputeHash(messageBytes);
return BitConverter.ToString(bytes).Replace("-", "").ToLower();
}

You can use this method for HMACSHA256.
string key = "your key";
string message = "your message";
System.Text.ASCIIEncoding encoding = new System.Text.ASCIIEncoding();
byte[] keyByte = encoding.GetBytes(key);
HMACSHA256 hmacsha256 = new HMACSHA256(keyByte);
byte[] messageBytes = encoding.GetBytes(message);
byte[] hashmessage = hmacsha256.ComputeHash(messageBytes);
return ByteToString(hashmessage);
Here is the ByteToString method:
public static string ByteToString(byte[] buff)
{
string sbinary = "";
for (int i = 0; i < buff.Length; i++)
{
sbinary += buff[i].ToString("X2"); // hex format
}
return (sbinary);
}

A SHA hash is calculated on a sequence of bytes. Bytes are a profoundly different datatype to characters. You should not use character Strings to store binary data such as hashes.
sb.Append(Convert.ToString(Convert.ToChar(Int32.Parse(hex.Substring(i, 2)...
This creates a character string by reading each encoded byte and turning into a character of the same Unicode code point number. This is equivalent to decoding the bytes 0-255 using the ISO-8859-1 (Latin1) encoding, due to that encoding's property of matching the first 256 code points in Unicode.
var enc = Encoding.Default; [...]
baSalt = enc.GetBytes(salt);
byte[] sha256Bytes = Encoding.Default.GetBytes(input);
These both convert the characters back to bytes using the system default encoding. This encoding varies between installs, but it will never be ISO-8859-1 - even the similar Western European code page 1252 has different characters in the range 0x80-0x9F.
Consequently the byte array you are using doesn't contain the bytes implied by the example hex sequences. A cheap fix would be to use Encoding.GetEncoding("ISO-8859-1") instead of the default encoding, but really you should be using a bytes array to store data in the first place instead of a String, eg:
byte[] key= new byte[] { 0x57, 0x61, 0x7b, 0x5d, 0x23, 0x49, ... };
and pass that directly into ComputeHash.
If you must initialise data from a hex string, parse it directly into a byte array, eg:
private static byte[] HexDecode(string hex) {
var bytes= new byte[hex.Length/2];
for (int i= 0; i<bytes.Length; i++) {
bytes[i]= byte.Parse(hex.Substring(i*2, 2), NumberStyles.HexNumber);
}
return bytes;
}

I realize the question is answered, but I am posting this in case others need it. Here is a snippet of code created by the payment provider (DIBS):
/**
* calculateMac
* Calculates the MAC key from a Dictionary<string, string> and a secret key
* #param params_dict The Dictionary<string, string> object containing all keys and their values for MAC calculation
* #param K_hexEnc String containing the hex encoded secret key from DIBS Admin
* #return String containig the hex encoded MAC key calculated
**/
public static string calculateMac(Dictionary<string, string> paramsDict, string kHexEnc)
{
//Create the message for MAC calculation sorted by the key
var keys = paramsDict.Keys.ToList();
keys.Sort();
var msg = "";
foreach (var key in keys)
{
if (key != keys[0]) msg += "&";
msg += key + "=" + paramsDict[key];
}
//Decoding the secret Hex encoded key and getting the bytes for MAC calculation
var kBytes = new byte[kHexEnc.Length / 2];
for (var i = 0; i < kBytes.Length; i++)
{
kBytes[i] = byte.Parse(kHexEnc.Substring(i * 2, 2), NumberStyles.HexNumber);
}
//Getting bytes from message
var msgBytes = Encoding.Default.GetBytes(msg);
//Calculate MAC key
var hash = new HMACSHA256(kBytes);
var macBytes = hash.ComputeHash(msgBytes);
var mac = BitConverter.ToString(macBytes).Replace("-", "").ToLower();
return mac;
}
http://tech.dibspayment.com/DX/Hosted/HMAC

Thanks you saved my time.
request.Method = "GET";
string signature = "";
string strtime = DateTime.UtcNow.ToString("yyyy-MM-ddTHH\\:mm\\:ssZ");
string secret = "xxxx";
string message = "sellerid:email:" + strtime;
var encoding = new System.Text.ASCIIEncoding();
byte[] keyByte = encoding.GetBytes(secret);
byte[] messageBytes = encoding.GetBytes(message);
using (var hmacsha256 = new HMACSHA256(keyByte))
{
var hash = new HMACSHA256(keyByte);
byte[] signature1 = hash.ComputeHash(messageBytes);
signature = BitConverter.ToString(signature1).Replace("-", "").ToLower();
}
request.Headers.Add("authorization", "HMAC-SHA256" + " " +
"emailaddress=xxx#xx.com,timestamp=" + strtime + ",signature=" + signature);
HttpWebResponse response = request.GetResponse() as HttpWebResponse;

private static string GenerateSignature(string data, string signatureKey)
{
var keyByte = Encoding.UTF8.GetBytes(signatureKey);
using (var hmacsha256 = new HMACSHA256(keyByte))
{
hmacsha256.ComputeHash(Encoding.UTF8.GetBytes(data));
return hmacsha256.Hash.Aggregate("", (current, t) => current + t.ToString("X2")).ToLower();
}
}

How can I SHA512 a string in C#?

I am trying to write a function to take a string and sha512 it like so?
public string SHA512(string input)
{
string hash;
~magic~
return hash;
}
What should the magic be?

Your code is correct, but you should dispose of the SHA512Managed instance:
using (SHA512 shaM = new SHA512Managed())
{
hash = shaM.ComputeHash(data);
}
512 bits are 64 bytes.
To convert a string to a byte array, you need to specify an encoding. UTF8 is okay if you want to create a hash code:
var data = Encoding.UTF8.GetBytes("text");
using (...

This is from one of my projects:
public static string SHA512(string input)
{
var bytes = System.Text.Encoding.UTF8.GetBytes(input);
using (var hash = System.Security.Cryptography.SHA512.Create())
{
var hashedInputBytes = hash.ComputeHash(bytes);
// Convert to text
// StringBuilder Capacity is 128, because 512 bits / 8 bits in byte * 2 symbols for byte
var hashedInputStringBuilder = new System.Text.StringBuilder(128);
foreach (var b in hashedInputBytes)
hashedInputStringBuilder.Append(b.ToString("X2"));
return hashedInputStringBuilder.ToString();
}
}
Please, note:
SHA512 object is disposed ('using' section), so we do not have any resource leaks.
StringBuilder is used for efficient hex string building.

512/8 = 64, so 64 is indeed the correct size. Perhaps you want to convert it to hexadecimal after the SHA512 algorithm.
See also: How do you convert Byte Array to Hexadecimal String, and vice versa?

You might try these lines:
public static string GenSHA512(string s, bool l = false)
{
string r = "";
try
{
byte[] d = Encoding.UTF8.GetBytes(s);
using (SHA512 a = new SHA512Managed())
{
byte[] h = a.ComputeHash(d);
r = BitConverter.ToString(h).Replace("-", "");
}
r = (l ? r.ToLowerInvariant() : r);
}
catch
{
}
return r;
}
It is disposed at the end
It's safe
Supports lower case

Instead of WinCrypt-API using System.Security.Cryptography, you can also use BouncyCastle:
public static byte[] SHA512(string text)
{
byte[] bytes = System.Text.Encoding.UTF8.GetBytes(text);
Org.BouncyCastle.Crypto.Digests.Sha512Digest digester = new Org.BouncyCastle.Crypto.Digests.Sha512Digest();
byte[] retValue = new byte[digester.GetDigestSize()];
digester.BlockUpdate(bytes, 0, bytes.Length);
digester.DoFinal(retValue, 0);
return retValue;
}
If you need the HMAC-version (to add authentication to the hash)
public static byte[] HmacSha512(string text, string key)
{
byte[] bytes = Encoding.UTF8.GetBytes(text);
var hmac = new Org.BouncyCastle.Crypto.Macs.HMac(new Org.BouncyCastle.Crypto.Digests.Sha512Digest());
hmac.Init(new Org.BouncyCastle.Crypto.Parameters.KeyParameter(System.Text.Encoding.UTF8.GetBytes(key)));
byte[] result = new byte[hmac.GetMacSize()];
hmac.BlockUpdate(bytes, 0, bytes.Length);
hmac.DoFinal(result, 0);
return result;
}

Keeping it simple:
using (SHA512 sha512 = new SHA512Managed())
{
password = Encoding.UTF8.GetString(sha512.ComputeHash(Encoding.UTF8.GetBytes(password)));
}

I'm not sure why you are expecting 128.
8 bits in a byte. 64 bytes. 8 * 64 = 512 bit hash.

From the MSDN Documentation:
The hash size for the SHA512Managed algorithm is 512 bits.

You could use the System.Security.Cryptography.SHA512 class
MSDN on SHA512
Here is an example, straigt from the MSDN
byte[] data = new byte[DATA_SIZE];
byte[] result;
SHA512 shaM = new SHA512Managed();
result = shaM.ComputeHash(data);

UnicodeEncoding UE = new UnicodeEncoding();
byte[] message = UE.GetBytes(password);
SHA512Managed hashString = new SHA512Managed();
string hexNumber = "";
byte[] hashValue = hashString.ComputeHash(message);
foreach (byte x in hashValue)
{
hexNumber += String.Format("{0:x2}", x);
}
string hashData = hexNumber;

I used the following
public static string ToSha512(this string inputString)
{
if (string.IsNullOrWhiteSpace(inputString)) return string.Empty;
using (SHA512 shaM = new SHA512Managed())
{
return Convert.ToBase64String(shaM.ComputeHash(Encoding.UTF8.GetBytes(inputString)));
}
}

Made it into an extension method in my ExtensionUtility.cs class
public static string SHA512(this string plainText)
{
using (SHA512 shaM = new SHA512Managed())
{
var buffer = Encoding.UTF8.GetBytes(plainText);
var hashedInputBytes = shaM.ComputeHash(buffer);
return BitConverter.ToString(hashedInputBytes).Replace("-", "");
}
}

How to convert a string to UTF8?

I have a string that contains some unicode, how do I convert it to UTF-8 encoding?

This snippet makes an array of bytes with your string encoded in UTF-8:
UTF8Encoding utf8 = new UTF8Encoding();
string unicodeString = "Quick brown fox";
byte[] encodedBytes = utf8.GetBytes(unicodeString);

Try this function, this should fix it out-of-box. You may need to fix naming conventions though.
private string UnicodeToUTF8(string strFrom)
{
byte[] bytSrc;
byte[] bytDestination;
string strTo = String.Empty;
bytSrc = Encoding.Unicode.GetBytes(strFrom);
bytDestination = Encoding.Convert(Encoding.Unicode, Encoding.ASCII, bytSrc);
strTo = Encoding.ASCII.GetString(bytDestination);
return strTo;
}

This should be with the minimum code:
byte[] bytes = Encoding.Default.GetBytes(myString);
myString = Encoding.UTF8.GetString(bytes);

try to this code
string unicodeString = "Quick brown fox";
var bytes = new List<byte>(unicodeString);
foreach (var c in unicodeString)
bytes.Add((byte)c);
var retValue = Encoding.UTF8.GetString(bytes.ToArray());

C# Email subject parsing

I'm building a system for reading emails in C#. I've got a problem parsing the subject, a problem which I think is related to encoding.
The subject I'm reading is as follows: =?ISO-8859-1?Q?=E6=F8sd=E5f=F8sdf_sdfsdf?=, the original subject sent is æøsdåføsdf sdfsdf (Norwegian characters in there).
Any ideas how I can change encoding or parse this correctly? So far I've tried to use the C# encoding conversion techniques to encode the subject to utf8, but without any luck.
Here is one of the solutions I tried:
Encoding iso = Encoding.GetEncoding("iso-8859-1");
Encoding utf = Encoding.UTF8;
string decodedSubject =
utf.GetString(Encoding.Convert(utf, iso,
iso.GetBytes(m.Subject.Split('?')[3])));

The encoding is called quoted printable.
See the answers to this question.
Adapted from the accepted answer:
public string DecodeQuotedPrintable(string value)
{
Attachment attachment = Attachment.CreateAttachmentFromString("", value);
return attachment.Name;
}
When passed the string =?ISO-8859-1?Q?=E6=F8sd=E5f=F8sdf_sdfsdf?= this returns "æøsdåføsdf_sdfsdf".

public static string DecodeEncodedWordValue(string mimeString)
{
var regex = new Regex(#"=\?(?<charset>.*?)\?(?<encoding>[qQbB])\?(?<value>.*?)\?=");
var encodedString = mimeString;
var decodedString = string.Empty;
while (encodedString.Length > 0)
{
var match = regex.Match(encodedString);
if (match.Success)
{
// If the match isn't at the start of the string, copy the initial few chars to the output
decodedString += encodedString.Substring(0, match.Index);
var charset = match.Groups["charset"].Value;
var encoding = match.Groups["encoding"].Value.ToUpper();
var value = match.Groups["value"].Value;
if (encoding.Equals("B"))
{
// Encoded value is Base-64
var bytes = Convert.FromBase64String(value);
decodedString += Encoding.GetEncoding(charset).GetString(bytes);
}
else if (encoding.Equals("Q"))
{
// Encoded value is Quoted-Printable
// Parse looking for =XX where XX is hexadecimal
var regx = new Regex("(\\=([0-9A-F][0-9A-F]))", RegexOptions.IgnoreCase);
decodedString += regx.Replace(value, new MatchEvaluator(delegate(Match m)
{
var hex = m.Groups[2].Value;
var iHex = Convert.ToInt32(hex, 16);
// Return the string in the charset defined
var bytes = new byte[1];
bytes[0] = Convert.ToByte(iHex);
return Encoding.GetEncoding(charset).GetString(bytes);
}));
decodedString = decodedString.Replace('_', ' ');
}
else
{
// Encoded value not known, return original string
// (Match should not be successful in this case, so this code may never get hit)
decodedString += encodedString;
break;
}
// Trim off up to and including the match, then we'll loop and try matching again.
encodedString = encodedString.Substring(match.Index + match.Length);
}
else
{
// No match, not encoded, return original string
decodedString += encodedString;
break;
}
}
return decodedString;
}