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Tighten up the Westwood AUD detection. Probability of random detections

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used to be on the order of 2^8. It is now on the order of 2^45.

Originally committed as revision 12892 to svn://svn.ffmpeg.org/ffmpeg/trunk
This commit is contained in:
Mike Melanson 2008-04-18 17:29:58 +00:00
parent e5ab737950
commit 9e2ffc450b
1 changed files with 14 additions and 5 deletions
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19
libavformat/westwood.c
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@ -90,14 +90,14 @@ static int wsaud_probe(AVProbeData *p)
/* Probabilistic content detection strategy: There is no file signature /* Probabilistic content detection strategy: There is no file signature
* so perform sanity checks on various header parameters: * so perform sanity checks on various header parameters:
* 8000 <= sample rate (16 bits) <= 48000 ==> 40001 acceptable numbers * 8000 <= sample rate (16 bits) <= 48000 ==> 40001 acceptable numbers
* flags <= 0x03 (2 LSBs are used) ==> 4 acceptable numbers
* compression type (8 bits) = 1 or 99 ==> 2 acceptable numbers * compression type (8 bits) = 1 or 99 ==> 2 acceptable numbers
* There is a total of 24 bits. The number space contains 2^24 = * first audio chunk signature (32 bits) ==> 1 acceptable number
* 16777216 numbers. There are 40001 * 2 = 80002 acceptable combinations * The number space contains 2^64 numbers. There are 40001 * 4 * 2 * 1 =
* of numbers. There is a 80002/16777216 = 0.48% chance of a false * 320008 acceptable number combinations.
* positive.
*/ */
if (p->buf_size < AUD_HEADER_SIZE) if (p->buf_size < AUD_HEADER_SIZE + AUD_CHUNK_PREAMBLE_SIZE)
return 0; return 0;
/* check sample rate */ /* check sample rate */
@ -105,11 +105,20 @@ static int wsaud_probe(AVProbeData *p)
if ((field < 8000) || (field > 48000)) if ((field < 8000) || (field > 48000))
return 0; return 0;
/* enforce the rule that the top 6 bits of this flags field are reserved (0);
* this might not be true, but enforce it until deemed unnecessary */
if (p->buf[10] & 0xFC)
return 0;
/* note: only check for WS IMA (type 99) right now since there is no /* note: only check for WS IMA (type 99) right now since there is no
* support for type 1 */ * support for type 1 */
if (p->buf[11] != 99) if (p->buf[11] != 99)
return 0; return 0;
/* read ahead to the first audio chunk and validate the first header signature */
if (AV_RL32(&p->buf[16]) != AUD_CHUNK_SIGNATURE)
return 0;
/* return 1/2 certainty since this file check is a little sketchy */ /* return 1/2 certainty since this file check is a little sketchy */
return AVPROBE_SCORE_MAX / 2; return AVPROBE_SCORE_MAX / 2;
} }