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