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libowfat/scan/scan_asn1derlength.c

52 lines
1.8 KiB

#include "scan.h"
/* ASN.1 DER encoded length:
* if (value<=0x80):
* emit value;
* otherwise:
* emit 0x80+bytes_that_follow
* emit all the bytes in the number, number saved big endian
* examples:
* 5 -> 0x05
* 0xc2 -> 0x81 0xc2
* 0x123 -> 0x82 0x01 0x23
* 0x12345 -> 0x83 0x01 0x23 0x45
*/
/* We provide two functions.
* One that only parses the length value (scan_asn1derlengthvalue), and
* one that also makes sure that as many bytes as specified by the
* length are in the input buffer (scan_asn1derlength).
* If you are trying to parse ASN.1, use scan_asn1derlength.
* If you just want to use the integer encoding format used by ASN.1 DER
* for lengths, use scan_asn1derlengthvalue. */
size_t scan_asn1derlengthvalue(const char* src,size_t len,unsigned long long* value) {
if (len==0 || len>=-(uintptr_t)src) return 0;
unsigned int i,c=*src;
unsigned long long l;
if ((c&0x80)==0) {
*value=c;
return 1;
}
/* Highest bit set: lower 7 bits is the length of the length value in bytes. */
c&=0x7f;
if (!c) return 0; /* length 0x80 means indefinite length encoding, not supported here */
l=(unsigned char)src[1];
if (l==0) return 0; /* not minimally encoded: 0x82 0x00 0xc2 instead of 0x81 0xc2 */
if (c>sizeof(l)) return 0; /* too many bytes, does not fit into target integer type */
if (c+1>len) return 0; /* not enough data in input buffer */
for (i=2; i<=c; ++i)
l=l*256+(unsigned char)src[i];
if (l<0x7f) return 0; /* not minimally encoded: 0x81 0x70 instead of 0x70 */
*value=l;
return i;
}
size_t scan_asn1derlength(const char* src,size_t len,unsigned long long* value) {
unsigned long long l;
size_t i=scan_asn1derlengthvalue(src,len,&l);
if (l > len-i) return 0; /* make sure data would fit into buffer */
*value=l;
return i;
}