Base64 encoding wikipedia Patch#

Converting from binary to base 64 only regroups bits and does not change them or their order (a high bit in binary does not affect the low bits in the base64 representation). The reverse is applied when decoding: The last block is padded to 5 bytes with the Ascii85 character u, and as many bytes as were added as padding are omitted from the end of the output (see example). After encoding, as many bytes as were added as padding are removed from the end of the output. The original specification only allows a stream that is a multiple of 4 bytes to be encoded.Įncoded data may contain characters that have special meaning in many programming languages and in some text-based protocols, such as left-angle-bracket ' to mark the end of an Ascii85-encoded string and represents the length by truncating the final group: If the last block of source bytes contains fewer than 4 bytes, the block is padded with up to 3 null bytes before encoding. White space between the characters is ignored and may occur anywhere to accommodate line-length limitations. Groups of characters that decode to a value greater than 2 32 − 1 (encoded as s8W-!) will cause a decoding error, as will z characters in the middle of a group. 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).īecause all-zero data is quite common, an exception is made for the sake of data compression, and an all-zero group is encoded as a single character z instead of !!!!!. This is converted, by repeatedly dividing by 85 and taking the remainder, into 5 radix-85 digits. 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). (Five radix-85 digits can represent the integers from 0 to 4 437 053 124 inclusive, which suffice to represent all 4 294 967 296 possible 4-byte sequences.) Thus, only the 94 printable ASCII characters are 'safe' to use to convey data.Įighty-five is the minimum integral value of n such that n 5 ≥ 256 4 so any sequence of 4 bytes can be encoded as 5 symbols, as long as at least 85 distinct symbols are available. 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.

The basic need for a binary-to-text encoding comes from a need to communicate arbitrary binary data over preexisting communications protocols that were designed to carry only English language human-readable text.

Base64 encoding wikipedia Patch#

Its main modern uses are in Adobe's PostScript and Portable Document Format file formats, as well as in the patch encoding for binary files used by Git. 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). RutterĪscii85, also called Base85, is a form of binary-to-text encoding developed by Paul E. Form of binary-to-text encoding developed by Paul E.