Ascii85

Ascii85, also called Base85, is a form of binary-to-text encoding developed by Paul E. Rutter for the btoa utility. By using five ASCII characters to represent four bytes of binary data (making the encoded size ¹⁄₄ larger than the original, assuming eight bits per ASCII character), it is more efficient than uuencode or Base64, which use four characters to represent three bytes of data (¹⁄₃ increase, assuming eight bits per ASCII character).

Its main modern use is in Adobe's PostScript and Portable Document Format file formats.

The basic need for a binary-to-text encoding comes from a need to communicate arbitrary binary data over preexisting that were designed to carry only human-readable text. Those communication protocols may only be 7-bit safe (and within that avoid certain ASCII control codes), and may require line breaks at certain maximum intervals, and may not maintain whitespace. Thus, only the 95 printable ASCII characters are "safe" to use to convey data.

Four bytes can represent 2³² = 4,294,967,296 possible values. Five radix-85 digits provide 85⁵ = 4,437,053,125 possible values, enough to provide for a unique representation for each possible 32-bit value. Because five radix-84 digits only provide 84⁵ = 4,182,119,424 representable values, 85 is the minimum possible integral base that will represent four bytes in five characters, hence its choice.

When encoding, each group of 4 bytes is taken as a 32-bit binary number, most significant byte first (Ascii85 uses a big-endian convention). This is converted, by repeatedly dividing by 85 and taking the remainder, into 5 radix-85 digits. Then each digit (again, most significant first) is encoded as an ASCII printable character by adding 33 to it, giving the ASCII characters 33 ("!") through 117 ("u").

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