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Why does my Base64 encoded hash only contain 28 characters?

The inverse question exists with no answer and a comment that I don't understand.
I am attempting to create a Base64 encoded HMAC-SHA1 signature for a pseudo OAuth authentication header for an API. I found a support document (requires authenticated access) that takes you through the evolution of creating the signature. I'm able to create the same data up until the final step which is to Base64 encode the hash.
The support document states that the HMAC-SHA1 signature is:
cb5acd2d3ef689a8fbec4d06c576371834689673
And I get:
CB5ACD2D3EF689A8FBEC4D06C576371834689673
The support document then states
From the hex result string in step 3, encode the value using Base64
and provides the following Base64 encoded result (58 characters):
Y2I1YWNkMmQzZWY2ODlhOGZiZWM0ZDA2YzU3NjM3MTgzNDY4OTY3Mw==
When I use Convert.ToBase64String() to convert my signature I get (28 characters):
y1rNLT72iaj77E0GxXY3GDRolnM=
I'm stumped, I don't know if the support document is incorrect or if I'm doing something wrong. The fact that I'm generating a string that is 28 characters and the example is 56 is too interesting to ignore.
The comment in the aforementioned semi-duplicate question also stumps me. I don't see how the string "MDY" translates to any ascii or unicode digits that make sense to me - I don't understand how the comment author came to that conclusion.
The hex value is being encoded as text ("062..." == 0x30, 0x36, 0x32,
...) rather than as the large number it represents.

Your signature is a 20 byte (160 bit) long byte array.
So it's basically a very long number.
When you show it, you are converting it into a hex string, so each byte is shown as 2 chars, so you get a 40 chars long string.
Base64 encoding gives you 4 chars every 3 bytes of payload to encode.
If you encode in base64 20 bytes of binary data you get 26.6 bytes, rounded up to 28 (you round up every 4 bytes).
If you encode your 40 characters long string (320 bits), you get 53,3 characters, again rounded up to 56.
I suppose you're doing the latter, and encoding a string instead of a byte[].

I came up with the same problem here. You have to convert sha1sum result to a Hex format. For example, when your sha1sum result is dfe5ec35f9c9f144d3814821a558bcfa23ab1a58, the console outputs it in string format.
You may use UltraEdit or another text editor, edit it as hex.
And then save it as a file.
The right answer 3+XsNfnJ8UTTgUghpVi8+iOrGlg= comes out after you base64 encode the file.
Have a try. Hope it helps.

The answer is correct, here is a small python example
import base64
import hashlib
# replace "hello" with your value from which you get sha1
sha = hashlib.sha1(b'hello')
sha_bytes = sha.digest() # 20 bytes
sha_str = sha.hexdigest() # 40 bytes - you are trying to b64encode this value
b64_sha_bytes = base64.b64encode(sha_bytes) # b'qvTGHdzF6KLavt4PO0gs2a6pQ00='
b64_sha_str = base64.b64encode(sha_str.encode()) # b'YWFmNGM2MWRkY2M1ZThhMmRhYmVkZTBmM2I0ODJjZDlhZWE5NDM0ZA=='

UTF-8 Encoding and Decoding in c#

I Searched for " How to Encode the data in utf-8 format". Regarding this I got the best result is following:
UTF8Encoding utf8 = new UTF8Encoding();
String unicodeString = "ABCD";
// Encode the string.
Byte[] encodedBytes = utf8.GetBytes(unicodeString);
// Decode bytes back to string.
String decodedString = utf8.GetString(encodedBytes);
But the Problem is when I see the encoded data I found that is not more than ASCII code.
can any one help me to improve my knowledge.
For example as I passed "ABCD " it gets converted into 65,66,67,68.... I think this is not utf-8

UTF-8 is backwards compatible with ASCII of course. You should test with some characters that are not included in ASCII.
If you program in C# the strings are already encoded in UTF-16. You will not see anything Special there. If you want to see something you should try to compare the LENGTH of the Byte[] when you encode the string into different Encodings.

Check out the Wikipedia article on UTF8: Wikipedia.
From there:
Backward compatibility: One-byte codes are used only for the ASCII
values 0 through 127. In this case the UTF-8 code has the same value
as the ASCII code. The high-order bit of these codes is always 0. This
means that UTF-8 can be used for parsers expecting 8-bit extended
ASCII even if they are not designed for UTF-8.
The point here is that for anything that would be ASCII 0-127 in UTF8 it's the same. You need to try more extended characters (an example in the article is the Euro symbol) to see how it's different. Or try an ASCII value greater than 127 and you'll see it different.

A weird thing in c# Encoding

I convert a byte array to a string , and I convert this string to byte array.
these two byte arrays are different.
As below:
byte[] tmp = Encoding.ASCII.GetBytes(Encoding.ASCII.GetString(b));
Suppose b is a byte array.
b[0]=3, b[1]=188, b[2]=2 //decimal system
Result:
tmp[0]=3, tmp[1]=63, tmp[2]=2
So that's my problem, what's wrong with it?

188 is out of range for ASCII. Characters that are not in the corresponding character set are transposed to '?' by design (would you prefer transposing to "1/4"?)

ASCII is 7-bit only, so others are invalid. By default it uses ? to replace any invalid bytes and that's why you get a ?.
For 8-bit character sets, you should be looking for either the Extended ASCII (which is later defined "ISO 8859-1") or the code page 437 (which is often confused with Extended ASCII, but in fact it's not).
You can use the following code:
Encoding enc = Encoding.GetEncoding("iso-8859-1");
// For CP437, use Encoding.GetEncoding(437)
byte[] tmp = enc.GetBytes(enc.GetString(b));

The character 188 is not defined for ASCII. Instead, you're getting 63, which is a question mark.

The ASCII character set has a range from 1 to 127. You can see 188 is not in this range and is converted to ? (= ASC 63).

Not every sequence of bytes is necessarily a valid sequence of encoded values for a particular encoding.
So the result of Encoding.ASCII.GetString(b) on an arbitrary array of bytes, b, is poorly defined. (And could be, for any other encoding also).
If you need to take an arbitrary byte array and obtain a sequence of characters, you might want to look into the Convert classes ToBase64String and FromBase64String. If that's not what you're trying to do, maybe explain the original problem to us.

188 isn't in the range of ASCII (7 bit), you should use Encoding.Default to get the ANSI encoding:
byte[] b = new byte[3]{ 3, 188, 2 };
byte[] tmp = Encoding.Default.GetBytes(Encoding.Default.GetString(b));

"Unable to translate Unicode character" error when saving to txt file

Additional information: Unable to
translate Unicode character \uDFFF at
index 195 to specified code page.
I made an algorithm, who's result are binary values (different lengths). I transformed it into uint, and then into chars and saved into stringbuilder, as you can see below:
uint n = Convert.ToUInt16(tmp_chars, 2);
_koded_text.Append(Convert.ToChar(n));
My problem is, that when i try to save those values into .txt i get the previously mentioned error.
StreamWriter file = new StreamWriter(filename);
file.WriteLine(_koded_text);
file.Close();
What i am saving is this: "忿췾᷿］볯褟ﶞ痢ﳻ��伞ﳴ㿯ﹽ翼蛿㐻ﰻ筹��﷿￦ﾏ랿鳿⏟麞펿"... which are some weird signs.
What i need is to convert those binary values into some kind of string of chars and save it to txt. I saw somewhere that converting to UTF8 should help, but i don't know how to. Would changing files encoding help too?

You cannot transform binary data to a string directly. The Unicode characters in a string are encoded using utf16 in .NET. That encoding uses two bytes per character, providing 65536 distinct values. Unicode however has over one million codepoints. To make that work, the Unicode codepoints above \uffff (above the BMP, Basic Multilingual Plane) are encoded with a surrogate pair. The first one has a value between 0xd800 and 0xdbff, the second between 0xdc00 and 0xdfff. That provides 2 ^ (10 + 10) = 1 million additional codes.
You can perhaps see where this leads, in your case the code detects a high surrogate value (0xdfff) that isn't paired with a low surrogate. That's illegal. Lots more possible mishaps, several codepoints are unassigned, several are diacritics that get mangled when the string is normalized.
You just can't make this work. Base64 encoding is the standard way to carry binary data across a text stream. It uses 6 bits per character, 3 bytes require 4 characters. The character set is ASCII so the odds of the receiving program decoding the character back to binary incorrectly are minimal. Only a decades old IBM mainframe that uses EBCDIC could get you into trouble. Or just plain avoid encoding to text and keep it binary.

Since you're trying to encode binary data to a text stream this SO question already contains an answer to the question: "How do I encode something as base64?" From there plain ASCII/ANSI text is fine for the output encoding.

Can we simplify this string encoding code

Is it possible to simplify this code into a cleaner/faster form?
StringBuilder builder = new StringBuilder();
var encoding = Encoding.GetEncoding(936);
// convert the text into a byte array
byte[] source = Encoding.Unicode.GetBytes(text);
// convert that byte array to the new codepage.
byte[] converted = Encoding.Convert(Encoding.Unicode, encoding, source);
// take multi-byte characters and encode them as separate ascii characters
foreach (byte b in converted)
builder.Append((char)b);
// return the result
string result = builder.ToString();
Simply put, it takes a string with Chinese characters such as 鄆 and converts them to ài.
For example, that Chinese character in decimal is 37126 or 0x9106 in hex.
See http://unicodelookup.com/#0x9106/1
Converted to a byte array, we get [145, 6] (145 * 256 + 6 = 37126). When encoded in CodePage 936 (simplified chinese), we get [224, 105]. If we break this byte array down into individual characters, we 224=e0=à and 105=69=i in unicode.
See http://unicodelookup.com/#0x00e0/1
and
http://unicodelookup.com/#0x0069/1
Thus, we're doing an encoding conversion and ensuring that all characters in our output Unicode string can be represented using at most two bytes.
Update: I need this final representation because this is the format my receipt printer is accepting. Took me forever to figure it out! :) Since I'm not an encoding expert, I'm looking for simpler or faster code, but the output must remain the same.
Update (Cleaner version):
return Encoding.GetEncoding("ISO-8859-1").GetString(Encoding.GetEncoding(936).GetBytes(text));

Well, for one, you don't need to convert the "built-in" string representation to a byte array before calling Encoding.Convert.
You could just do:
byte[] converted = Encoding.GetEncoding(936).GetBytes(text);
To then reconstruct a string from that byte array whereby the char values directly map to the bytes, you could do...
static string MangleTextForReceiptPrinter(string text) {
return new string(
Encoding.GetEncoding(936)
.GetBytes(text)
.Select(b => (char) b)
.ToArray());
}
I wouldn't worry too much about efficiency; how many MB/sec are you going to print on a receipt printer anyhow?
Joe pointed out that there's an encoding that directly maps byte values 0-255 to code points, and it's age-old Latin1, which allows us to shorten the function to...
return Encoding.GetEncoding("Latin1").GetString(
Encoding.GetEncoding(936).GetBytes(text)
);
By the way, if this is a buggy windows-only API (which it is, by the looks of it), you might be dealing with codepage 1252 instead (which is almost identical). You might try reflector to see what it's doing with your System.String before it sends it over the wire.

Almost anything would be cleaner than this - you're really abusing text here, IMO. You're trying to represent effectively opaque binary data (the encoded text) as text data... so you'll potentially get things like bell characters, escapes etc.
The normal way of encoding opaque binary data in text is base64, so you could use:
return Convert.ToBase64String(Encoding.GetEncoding(936).GetBytes(text));
The resulting text will be entirely ASCII, which is much less likely to cause you hassle.
EDIT: If you need that output, I would strongly recommend that you represent it as a byte array instead of as a string... pass it around as a byte array from that point onwards, so you're not tempted to perform string operations on it.

Does your receipt printer have an API that accepts a byte array rather than a string?
If so you may be able to simplify the code to a single conversion, from a Unicode string to a byte array using the encoding used by the receipt printer.
Also, if you want to convert an array of bytes to a string whose character values correspond 1-1 to the values of the bytes, you can use the code page 28591 aka Latin1 aka ISO-8859-1.
I.e., the following
foreach (byte b in converted)
builder.Append((char)b);
string result = builder.ToString();
can be replaced by:
// All three of the following are equivalent
// string result = Encoding.GetEncoding(28591).GetString(converted);
// string result = Encoding.GetEncoding("ISO-8859-1").GetString(converted);
string result = Encoding.GetEncoding("Latin1").GetString(converted);
Latin1 is a useful encoding when you want to encode binary data in a string, e.g. to send through a serial port.