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#include "scan.h"
/* ASN.1 DER tag encoding parser.
* Write value as big endian series of bytes, each containing seven
* bits. In each byte except the last, set the highest bit.
* Examples:
* 0 -> 0x00
* 5 -> 0x05
* 0xc2 -> 0x81 0x42 */
size_t scan_asn1dertag(const char* src,size_t len,unsigned long long* length) {
size_t n;
unsigned long long l=0;
if (len==0 || (unsigned char)src[0]==0x80) return 0; // catch non-minimal encoding
for (n=0; n<len; ++n) {
/* make sure we can represent the stored number in l */
if (l>>(sizeof(l)*8-7)) return 0; // catch integer overflow
l=(l<<7) | (src[n]&0x7f);
/* if high bit not set, this is the last byte in the sequence */
if (!(src[n]&0x80)) {
*length=l;
return n+1;
}
}
return 0;
}
#ifdef UNITTEST
#include <assert.h>
int main() {
unsigned long long i;
assert(scan_asn1dertag("\x00",1,&i)==1 && i==0);
assert(scan_asn1dertag("\x05",1,&i)==1 && i==5);
assert(scan_asn1dertag("\x81\x42",2,&i)==2 && i==0xc2);
assert(scan_asn1dertag("\x05",0,&i)==0); // truncated
assert(scan_asn1dertag("\x81\x42",1,&i)==0); // truncated
assert(scan_asn1dertag("\x80\x05",2,&i)==0); // non-minimal encoding
assert(scan_asn1dertag("\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff",10,&i)==0); // value does not fit
return 0;
}
#endif