ASCII85 Encoding

ASCII85 is a character encoding using 85 ASCII symbols. Popularized by the PDF format, it allows a little lighter encoding than Base64.

Encryption uses the binary code of the text (which depend on the encoding used: ASCII, Unicode, etc.)

The binary string is then split every 32 bits (4 bytes)

Numbers obtained are then converted to base 85.

Each value in base 85 is replaced by an ASCII character of code (value+33)

The decryption start by splitting the text into groups of 5 characters (if the tuple is not 5-character long, it is filled with u)

For each group of 5 characters, get the ASCII code and subtract 33.

Every tuple of 5 numbers is then considered as a unique number written in base 85, and converted to base 2.

The plain text is then this binary code encoded with initial encoding (ASCII, Unicode, etc.), n last characters are ignored when n characters have been added in initial completion step.

The encoding table used by ASCII-85 is the ASCII table (starting at the first printable character)

With the variant IPv6, the alphabet used is 0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz!#$%&()*+-;<=>?@^_`{|}~

With the variant ZeroMQ, the alphabet used is 0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ.-:+=^!/*?&<>()[]{}@%$#

The message uses only ASCII characters of codes 33 (!) to 117 (u) with sometimes spaces, carriage return, but also sometimes characters y and z.

The message begins with <~ and ends with ~> (variant Adobe)

the message begins with xbtoa Begin and ends with xbtoa End (variant BTOA)

With IPv6 or ZeroMQ variants the characters are different (but there is no prefix)

Adobe Base85 is used in PDF files, it begins with <~ and ends with ~> and authorizes z exception.

In order to store shorter IPv6s, RFC 1924 proposed a compact representation using the mathematical base 85 and the symbols 0–9, A–Z, a–z, !#$%&()*+-;<=>?@^_`{|}~.

ZeroMQ is a messaging tool that uses Base85 in a function named zmq_z85_encode(), the 85 symbols used are: 0-9, a-z, A-Z, .-:+=^!/*?&<>()[]{}@%$#

The character z is used (even if it is not in the character list) to indicate a group of 0, this trick allows reducing the size of the message. (used in Adobe and BTOA variants)

The character y is used to indicate a group of spaces. (not compatible with Adobe)

ASCII85 code was created around 1990 by Paul E. Rutter.