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ASCII85 Encoding

Tool to decrypt/encrypt with ASCII 85. ASCII85 (or Base85) is a coding system created by Paul E. Rutter similar to base64 encoding, using 5 ASCII characters to code 4 bytes. ASCII 85 is used in PDF file format for example.

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ASCII85 Encoding -

Tag(s) : Character Encoding

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ASCII85 Encoding

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ASCII85 Decoder





ASCII85 Encoder




Tool to decrypt/encrypt with ASCII 85. ASCII85 (or Base85) is a coding system created by Paul E. Rutter similar to base64 encoding, using 5 ASCII characters to code 4 bytes. ASCII 85 is used in PDF file format for example.

Answers to Questions

How to encrypt using ASCII85 encoding

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

Example: Consider the message DCODE encoded 01000100 01000011 01001111 01000100 01000101 (ASCIIhref 8-bit encoding)

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

Example: 01000100010000110100111101000100 (base2) = 1145261892 (base10) and 01000101000000000000000000000000 (base2) = 1157627904 (base10) (filled with 3 null characters = 24 times 0 to get 32 bits)

Numbers obtained are then converted in base 85href.

Example: 1145261892 (base10) = [21,79,73,64,27] (base85) and 1157627904 (base10) = [22,15,0,26,69] (base85)

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

Example: 21 => ASCIIhref(54) = 6, 79 => ASCIIhref(112) = p, 76 => ASCIIhref(106) = j etc.

Example: The encrypted message is 6pja<70

There coulb be some final null characters, that can be deleted (it corresponds to the previously added null characters)

How to decrypt ASCII85 encoding

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

Example: Consider the message 6pja<70, split into 6pja< and 70 (that is treated as 70uuu)

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

Example: 6pja< = 54,112,106,97,60 (ASCIIhref codes), -33 => 21,79,73,64,27

Example: 70uuu = 55,48,117,117,117 (ASCIIhref codes), -33 => 22,15,84,84,84

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

Example: [21,79,73,64,27] (base85) = 01000100010000110100111101000100 (base2)

Example: [22,15,84,84,84] (base85) = 01000101000010010101011000000101 (base2)

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

Example: 01000100 01000011 01001111 01000100 01000101 (00001001 01010110 00000101) = DCODE (ASCIIhref 8bit encoding).

How to recognize an ASCII85 ciphertext?

The message uses only ASCIIhref characters of codes 33 to 117 with sometimes spaces, carriage return, but also 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)

What is the Adobe variant?

Adobe ASCII85 is used in PDF files, it begins with <~ and ends with ~> and authorizes z exception.

What is the 'z' exception?

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

What is the 'y' exception?

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

When ASCII85 have been invented?

Around 1990.

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