Ascii85 - Adobe Version

Adobe Version

Adobe adopted the basic btoa encoding, but with slight changes, and gave it the name Ascii85. In particular, Adobe uses the delimiter "~>" 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 three null bytes before encoding. After encoding, as many bytes as were added as padding are removed from the end of the output.

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).

NOTE: The padding is not arbitrary. 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). In converting a binary number to base85 (85 is not a power of two) high bits do affect the low order base85 digits and conversely. Padding the binary low (with zero bits) while encoding and padding the base85 value high (with 'u's) in decoding assures that the high order bits are preserved (the zero padding in the binary gives enough room so that a small addition is trapped and there is no "carry" to the high bits).

In Ascii85-encoded blocks, whitespace and line-break characters may be present anywhere, including in the middle of a 5-character block, but they must be silently ignored.

Adobe's specification does not support the "y" exception.