By Steven J Tucker - firstname.lastname@example.org
APE Home Page: http://www.atarimax.com/
October 20, 1996
This specification was created for the benefit of the Atari 8-bit community and the information contained within may be used as part of any project, wither for profit or not, and without notice or payment to the author. This document must be distributed only in its original form. -- * Why do we need authentication? * Most major Atari archives make ATR images available for download in UNCOMPRESSED form to allow compatibility with all platforms including the Atari itself. Unlike compression envelopes like PKZIP, the ATR format provides no protection for its contents. If an ATR image becomes damaged at the archive, or in transit there is no way to tell other than the image might not work right away. (even worse, it might bug out after you spent 4 hours playing that game of Spellbreaker) This specification provides a 32-Bit CRC like that used in professional compression programs such as PKZIP. As of version 1.10 APE fully supports validation of images marked as authenticated. -- ** How does ATR Authentication work? ** The SIGNATR program included with this archive (or any program that follows this specification) is used by your Archivist to sign known-good disk images with a 32-Bit CRC that covers the total contents and the fixed header information. These ATR images are now considered to be 'authentic in their unmodified form. When you download an image, APE (or your favorite emulator that supports authentication) takes special steps when the image is loaded: Step 1: Check to see if image is a SEALED & authenticated image Step 2: Read the CRC32 stored in the ATR header by SIGNATR Step 3: Calculate the actual CRC32 of the Image If the CRC32 is good, the program quietly breaks the authentication seal (by setting a bit in the ATR header) and goes about its business. Since the authentication seal is now broken, any program loading the file need never take the time to authenticate it again, its loaded as quickly as any other non-authenticated image. If the CRC is good, the entire process is transparent to the user, and takes less than 1/2 a second on a 486/50. If the file was damaged, the program can take appropriate action to let you know. Because this method uses only extra bytes/bits in the ATR header, it is transparent to utilities that are unaware of authentication. -- * Ok, How do I authenticate images for download? * The program included, SIGNATR will do all the work for you. Just run it with no parameters for help. Once authenticated, you can let users having trouble with your images use the same freeware program to check to see if the image was corrupted.Download the free Sign_ATR package, including this document and free authentication utility.
-- * Technical stuff * The 'authenticated image' flag is stored in BIT 1 of BYTE 15 of the ATR header. This is the last user byte in the header. Note: This byte should NOT be clobbered, the other bits are in use or reserved for future use. The CRC32 is a 32 bit signed integer stored in bytes 8, 9, 10, and 11 of the ATR header. A signed CRC with the hex representation of $ABCDEF01 is stored in the header as: 8 9 10 11 ----------- 01 EF CD AB The CRC is generated using all bytes of the ATR including the header. Bytes 8-15 of the header are evaluated as 0, and NOT skipped. The steps for signing a unsigned file are: 1) Calculate the CRC32 using the code below 2) Store the CRC32 in the header as described above 3) Set the Authentication Envelope bit as described above. The steps for handling a authenticated file the first time it is accessed by a user program: 1) Check to see if its an authenticated ATR (if byte 15/bit 1 = 1) 2) If not load as normal ATR and exit 3) Read CRC stored in header 4) Calculate the actual crc using the method below 5) If the values match CLEAR THE ENVELOPE BIT and load ATR 6) If CRC is wrong do whatever you want, warn user, etc. The header is generated using a method common to many programs. Similar source code snippets are available for all languages on the net. Complete pascal source for creating a valid CRC32 of an ATR file is included below. If you write a C version of the code please send me a copy so I can include it in this document. This code is not optimal, just quick stuff and it does not check for memory, disk errors, etc. Check it over if you use it. const crc32tab:array[0..255] of longint=( $00000000, $77073096, $ee0e612c, $990951ba, $076dc419, $706af48f, $e963a535, $9e6495a3, $0edb8832, $79dcb8a4, $e0d5e91e, $97d2d988, $09b64c2b, $7eb17cbd, $e7b82d07, $90bf1d91, $1db71064, $6ab020f2, $f3b97148, $84be41de, $1adad47d, $6ddde4eb, $f4d4b551, $83d385c7, $136c9856, $646ba8c0, $fd62f97a, $8a65c9ec, $14015c4f, $63066cd9, $fa0f3d63, $8d080df5, $3b6e20c8, $4c69105e, $d56041e4, $a2677172, $3c03e4d1, $4b04d447, $d20d85fd, $a50ab56b, $35b5a8fa, $42b2986c, $dbbbc9d6, $acbcf940, $32d86ce3, $45df5c75, $dcd60dcf, $abd13d59, $26d930ac, $51de003a, $c8d75180, $bfd06116, $21b4f4b5, $56b3c423, $cfba9599, $b8bda50f, $2802b89e, $5f058808, $c60cd9b2, $b10be924, $2f6f7c87, $58684c11, $c1611dab, $b6662d3d, $76dc4190, $01db7106, $98d220bc, $efd5102a, $71b18589, $06b6b51f, $9fbfe4a5, $e8b8d433, $7807c9a2, $0f00f934, $9609a88e, $e10e9818, $7f6a0dbb, $086d3d2d, $91646c97, $e6635c01, $6b6b51f4, $1c6c6162, $856530d8, $f262004e, $6c0695ed, $1b01a57b, $8208f4c1, $f50fc457, $65b0d9c6, $12b7e950, $8bbeb8ea, $fcb9887c, $62dd1ddf, $15da2d49, $8cd37cf3, $fbd44c65, $4db26158, $3ab551ce, $a3bc0074, $d4bb30e2, $4adfa541, $3dd895d7, $a4d1c46d, $d3d6f4fb, $4369e96a, $346ed9fc, $ad678846, $da60b8d0, $44042d73, $33031de5, $aa0a4c5f, $dd0d7cc9, $5005713c, $270241aa, $be0b1010, $c90c2086, $5768b525, $206f85b3, $b966d409, $ce61e49f, $5edef90e, $29d9c998, $b0d09822, $c7d7a8b4, $59b33d17, $2eb40d81, $b7bd5c3b, $c0ba6cad, $edb88320, $9abfb3b6, $03b6e20c, $74b1d29a, $ead54739, $9dd277af, $04db2615, $73dc1683, $e3630b12, $94643b84, $0d6d6a3e, $7a6a5aa8, $e40ecf0b, $9309ff9d, $0a00ae27, $7d079eb1, $f00f9344, $8708a3d2, $1e01f268, $6906c2fe, $f762575d, $806567cb, $196c3671, $6e6b06e7, $fed41b76, $89d32be0, $10da7a5a, $67dd4acc, $f9b9df6f, $8ebeeff9, $17b7be43, $60b08ed5, $d6d6a3e8, $a1d1937e, $38d8c2c4, $4fdff252, $d1bb67f1, $a6bc5767, $3fb506dd, $48b2364b, $d80d2bda, $af0a1b4c, $36034af6, $41047a60, $df60efc3, $a867df55, $316e8eef, $4669be79, $cb61b38c, $bc66831a, $256fd2a0, $5268e236, $cc0c7795, $bb0b4703, $220216b9, $5505262f, $c5ba3bbe, $b2bd0b28, $2bb45a92, $5cb36a04, $c2d7ffa7, $b5d0cf31, $2cd99e8b, $5bdeae1d, $9b64c2b0, $ec63f226, $756aa39c, $026d930a, $9c0906a9, $eb0e363f, $72076785, $05005713, $95bf4a82, $e2b87a14, $7bb12bae, $0cb61b38, $92d28e9b, $e5d5be0d, $7cdcefb7, $0bdbdf21, $86d3d2d4, $f1d4e242, $68ddb3f8, $1fda836e, $81be16cd, $f6b9265b, $6fb077e1, $18b74777, $88085ae6, $ff0f6a70, $66063bca, $11010b5c, $8f659eff, $f862ae69, $616bffd3, $166ccf45, $a00ae278, $d70dd2ee, $4e048354, $3903b3c2, $a7672661, $d06016f7, $4969474d, $3e6e77db, $aed16a4a, $d9d65adc, $40df0b66, $37d83bf0, $a9bcae53, $debb9ec5, $47b2cf7f, $30b5ffe9, $bdbdf21c, $cabac28a, $53b39330, $24b4a3a6, $bad03605, $cdd70693, $54de5729, $23d967bf, $b3667a2e, $c4614ab8, $5d681b02, $2a6f2b94, $b40bbe37, $c30c8ea1, $5a05df1b, $2d02ef8d); type tChunk = Array[1..60000] of Byte; var Blocks : Word; Remain : Word; Loop : Word; Work : Word; Chunk : ^tChunk; Crc : Longint; fCheck : File; Function FileCrc(Filename: String): Longint; Begin New(Chunk); Assign(fCheck, Filename); Reset(fCheck, 1); Blocks := FileSize(fCheck) div Sizeof(Chunk^); Remain := FileSize(fCheck) - (Blocks * SizeOf(Chunk^)); Crc := $ffffffff; For Loop := 1 to Blocks do Begin Blockread(fCheck, Chunk^, SizeOf(Chunk^)); If Loop = 1 then For Work := 8 to 16 do Chunk^[Work] := 0; For Work := 1 to SizeOf(Chunk^) do Crc := crc32tab[byte(crc xor longint(Chunk^[Work]))] xor ((crc shr 8) and $00ffffff); End; Blockread(fCheck, Chunk^, Remain); For Work := 1 to Remain do Crc := crc32tab[byte(crc xor longint(Chunk^[Work]))] xor ((crc shr 8) and $00ffffff); Crc := Crc xor $ffffffff; FileCRC := Crc; Close(fCheck); Dispose(Chunk); End;