#Ascii And Utf 8 Codes Code Transformation Format

Ascii And Utf 8 Codes Code Transformation Format

UTF stands for Unicode Transformation Format, and the 8 at the end signifies that it’s an 8-bit variable encoding. UTF-8 is an encoding from the Unicode standard. The first half of the code points are identical to the ones defined in ASCII, while the second half encodes additional characters that do not exist in the ASCII character set.

The high-order bit of these codes is always 0. In this case the UTF-8 code has the same value as the ASCII code. It seems quite a common task, but its implementation isn’t obvious.One-byte codes are used only for the ASCII values 0 through 127. But what shall we do if we need to assign non-ASCII characters not in the message bodies, but in the header? Probably the most well-known case is setting a filename in 'Content-Disposition' header. At the same time, the message body can use another encoding assigned in 'Content-Type' header. HTTP messages are encoded with ISO-8859-1 (which can be nominally considered as an enhanced ASCII version, containing umlauts, diacritic and other characters of West European languages).

Here is a small intro to these encodings. , with UTF-8 Characters in.The article mentions US-ASCII (usually named just ASCII), ISO-8859-1 and UTF-8 encodings. Remove(x): x not in list exception. Introduction to encodingswe would like to remove diacritics and translate to string containing just ASCII characters.

The first half (128 characters) fully matches ASCII. It has French diacritic, German umlauts, etc.The encoding has 256 characters, thus, can be represented with 1 byte. The first bit of any character would always be 0, as the encoding has 128 characters, and a bite gives 2^8 = 256 variants.ISO-8859-1 is an encoding aimed for West European languages. The word 'test' in HEX representation would look as 0x74 0x65 0x73 0x74. If you’re not one of them, skip this part without hesitation.ASCII is a simple encoding with 128 characters, including Latin alphabet, digits, punctuation marks, and utility characters.7 bits is enough to represent any ASCII character.

110 - 2 bytes, 1110 - 3 bytes, 11110 - 4 bytes.Just like in case with ISO-8859-1, the first 128 well match ASCII. If an octet begins with 0, the character is represented by 1 byte. Each character size is varied from 1 to 4 bites ( previously the values could be up to 6 bites).The program processing this encoding checks the first bit and estimates the character size in bytes. It is capable of encoding 1,112,064 characters. If it’s 1 - we recognize a specific ISO-8859-1 character.UTF-8 is one of the most famous encodings alongside with ASCII.

So, if we transfer UTF-8 messages, but do not assign encoding in headers, they will be read as if they were encoded with ISO-8859-1.Entering an UTF-8 message in a header’s valueIn case of a message body, everything’s rather simple. A browser shouldn't try to guess the encoding, and it certainly shouldn’t ignore 'Content-Type'. For that purpose we use 'Content-Type' header.If 'Content-Type' isn’t assigned, a browser must process messages as if they were in ISO-8859-1. UTF-8 in message bodyBefore we proceed to headers, let’s see how UTF-8 is used in message bodies.

The only browser refusing to read such a header, of all the browsers I had when writing this article, was Edge.Such behavior is not described in specifications. Such cases include old iPhones, IE11, Firefox, Google Chrome. In fact, for most, maybe even for all, cases such a solution will work out. Therefore, we can assume that the same will happen to a header. It’s not possible to assign an encoding for one header via another.What if we just write a UTF-8 value into a header? We saw that with such a trick, applied to a message body, the value will be read in ISO-8859-1. But what shall we do with headers? The specification claims directly, that the order of headers in messages doesn’t matter.

This encoding method doesn’t work for HTTP anymore. I tried to encode messages using this format - the browser didn’t get the idea. Is this feature mentioned in Javadocs for servlets? Yes, it is:It mentions RFC 2047. RFC 2047As I’ve already mentioned, Tomcat didn’t let me enter UTF-8 in a message header. For example, if an application is aimed to a Russian audience, the filename can contain Tatar letters ә and ң, which should be somehow handled, not just replaced with '?'.

The usage of above-mentioned encoding in HTTP was offered only in 2010. Besides 'Content-Disposition' this encoding is used, for example, in Web Linking and Digest Access Authentication.It should be taken into account that the standards of this area are constantly changing. Of course, you can enter a parameter with * prefix in a header, and it would probably work for some browsers, but specification dictates not to do so.Currently, in each case, where UTF-8 is supported in headers, there’s a direct mention of the relevant RFC. What should be done with other headers?Encoding described in RFC 8187 isn’t generic. Other characters are just written in hex representation with '%' before each octet. Observed characters of ASCII don’t require encoding.

So, we only need to follow the news about the HTTP standards and their support level on the browser side. It is quite possible that new standards are going to appear, the standards that will allow us to work with various encodings in headers more consistently. Despite the fact that these standards are only on 'Proposed Standard' stage, they are supported everywhere.