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Change a secret ninja guru one-line expression into a function and document what it is supposed to do and how it does it.

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This commit is contained in:
Romain Tartiere 2010-02-23 12:05:14 +00:00
parent 5f7f8ffe2a
commit ce006c3cc9
1 changed files with 46 additions and 3 deletions
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49
libfreefare/mifare_classic.c
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@ -127,7 +127,8 @@ uint16_t mifare_trailer_access_permissions[] = {
* Private functions
*/
int get_block_access_bits (MifareTag tag, const MifareClassicBlockNumber block, MifareClassicAccessBits *block_access_bits);
int get_block_access_bits_shift (MifareClassicBlockNumber block, MifareClassicBlockNumber trailer);
int get_block_access_bits (MifareTag tag, const MifareClassicBlockNumber block, MifareClassicAccessBits *block_access_bits);
/*
@ -451,9 +452,38 @@ mifare_classic_transfer (MifareTag tag, const MifareClassicBlockNumber block)
* The following functions provide a convenient API for reading MIFARE card
* access bits. A cache system makes these functions query a single time the
* MIFARE card regardless of the number of information requested between two
* authentifications (i.e. for the current sector).
* authentications (i.e. for the current sector).
*/
/*
* Given a block, determine the rank of applicable access bits in the trailer
* block.
*
* For 4 blocks sectors, each access bit applies to a single block; but for 16
* blocks sectors (second part of MIFARE Classic 4k), the first 3 access bits
* apply to 5 blocks, and the last access bits apply to the trailer block:
*
* Sector | Access bits | Shift
* -------------------------------------+--------------+-------
* 4, 128, 129, 130, 131, 132, 144 | ---x---x---x | 0
* 1, 5, 133, 134, 135, 136, 137 | --x---x---x- | 1
* 2, 6, 138, 138, 140, 141, 142 | -x---x---x-- | 2
* 3, 7, 143 | x---x---x--- | 3
*
*/
int
get_block_access_bits_shift (MifareClassicBlockNumber block, MifareClassicBlockNumber trailer)
{
if (block == trailer) {
return 3;
} else {
if (block < 128)
return block % 4;
else
return ((block - 128) % 16) / 5;
}
}
/*
* Fetch access bits for a given block from the block's sector's trailing
* block.
@ -474,6 +504,15 @@ get_block_access_bits (MifareTag tag, const MifareClassicBlockNumber block, Mifa
MifareClassicBlockNumber trailer = mifare_classic_last_sector_block (block);
/*
* The trailer block contains access bits for the whole sector in a 3 bytes
* structure that holds 2 times the permissions (once inverted, once
* not-inverted).
*
* First we get these bytes, and check the inverted and non-inverted
* permissions match. A cache mechanism prevents many read access to the
* NFC target if the function is called multiple times on the same block.
*/
if (MIFARE_CLASSIC(tag)->cached_access_bits.sector_trailer_block_number == trailer) {
/* cache hit! */
sector_access_bits = MIFARE_CLASSIC(tag)->cached_access_bits.sector_access_bits;
@ -497,6 +536,10 @@ get_block_access_bits (MifareTag tag, const MifareClassicBlockNumber block, Mifa
MIFARE_CLASSIC(tag)->cached_access_bits.sector_access_bits = sector_access_bits;
}
/*
* To ease permissions lookup, related permission bits which are not
* contiguous are assembled in a quartet.
*/
if (MIFARE_CLASSIC(tag)->cached_access_bits.block_number == block) {
/* cache hit! */
*block_access_bits = MIFARE_CLASSIC(tag)->cached_access_bits.block_access_bits;
@ -506,7 +549,7 @@ get_block_access_bits (MifareTag tag, const MifareClassicBlockNumber block, Mifa
* |,------C2
* ||,---- C1
* ||| */
uint16_t block_access_bits_mask = 0x0111 << ((block == trailer) ? 3 : ((block < 128) ? block : ((block - 128) % 16) / 5) % 4);
uint16_t block_access_bits_mask = 0x0111 << get_block_access_bits_shift (block, trailer);
/* |||
* ||`---------------.
* |`---------------.|