yEnc Format Files

News and Mail transfer require that a binary attachment is "encoded" before it is sent. And they are "decoded" after they have been received. Normally all this is done by your newsreader (or mail-program). You don’t see it. Most don’t even know it.

The encoding is necessary because the special methods for the transfer of news & mail protocols require it. A message with a binary which is not encoded is corrupted during transmission - or transmission is denied at all.

Transport of messages by News and Mail was restricted to US-ASCII characters when the protocols were written (20 years ago). These services have been created to transport only plain US-text. Special characters (control-characters, symbols, non-US-characters) were forbidden - and used for special purposes. But because people wanted to send also binary attachments by News and Mail some 'tricks' were implemented: The binary was changed to "allowed US-ASCII-characters" before transmission (encoding) - and back to a binary after transmission (decoding). The usual encoding methods are still respecting these old limitations - and are used everywhere. 

Unfortunately there is a price for this 'trick': Encoding makes a message longer. And not just a little, but 33%-40% longer than the original attachments. This results in 33%-40% more bytes for a message - 33%-40% more time for the transmission - 33-40% more disk space on the hard disk where there messages are stored (on news- and mail-servers).

yEnc files are formatted in a coding system that is up to 40% smaller than UUencode and Base64, and also include error checking and multipart file support. This means that posts are smaller, and take less time to download.  If you are using a metered Usenet service, such as Giganews, you would be able to download more for your money.  The built-in error correction also offers a way to detect corrupted files, or corrupted portions of multi-part files.