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79 lines
3.1 KiB
C
79 lines
3.1 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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#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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/* first check actual parsing */
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assert(scan_asn1derlengthvalue("\x05",1,&i)==1 && i==5);
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assert(scan_asn1derlengthvalue("\x81\xc2",2,&i)==2 && i==0xc2);
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assert(scan_asn1derlengthvalue("\x82\x01\x23",3,&i)==3 && i==0x123);
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assert(scan_asn1derlengthvalue("\x83\x01\x23\x45",4,&i)==4 && i==0x12345);
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assert(scan_asn1derlengthvalue("\x83\x01\x23\x45",5,&i)==4 && i==0x12345);
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assert(scan_asn1derlengthvalue("\x80",1,&i)==0); // reject indefinite length encoding
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assert(scan_asn1derlengthvalue("\x81\x05",2,&i)==0); // reject non-minimal encoding
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assert(scan_asn1derlengthvalue("\x81\xc2",1,&i)==0); // reject truncated message
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assert(scan_asn1derlengthvalue("\x82\xc2",2,&i)==0); // reject truncated message
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assert(scan_asn1derlengthvalue("\x82\x00\xc2",3,&i)==0); // reject non-minimal encoding
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assert(scan_asn1derlengthvalue("\x89\x01\x02\x03\x04\x05\x06\x07\x08\x09",10,&i)==0); // value does not fit in target integer
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/* now check buffer length checking in scan_asn1derlength */
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assert(scan_asn1derlength("\x01",2,&i)==1 && i==1); // ok
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assert(scan_asn1derlength("\x02",2,&i)==0); // buffer too small
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assert(scan_asn1derlength("\x88\xff\xff\xff\xff\xff\xff\xff\xff",9,&i)==0); // buffer too small, and integer overflow in naive check
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return 0;
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}
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#endif
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