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