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35 lines
1.2 KiB
35 lines
1.2 KiB
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#include "trunc.h"
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int32_t get_otp_from_cnt(uint64_t cnt, uint8_t digits, uint8_t* secret, uint8_t sec_len){
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uint8_t tmp[8]={((uint8_t*)&cnt)[7],((uint8_t*)&cnt)[6],((uint8_t*)&cnt)[5],((uint8_t*)&cnt)[4],((uint8_t*)&cnt)[3],((uint8_t*)&cnt)[2],((uint8_t*)&cnt)[1],((uint8_t*)&cnt)[0]};
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/*Step 1 generate HMAC-SHA1*/
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uint8_t hmac_result[20];
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hmac_sha1(hmac_result, secret, sec_len*8, tmp, 64);
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return get_otp(hmac_result,digits);
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}
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int32_t get_otp(uint8_t* hmac, uint8_t digits){
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uint8_t n;
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uint32_t dt;
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/*Step 1 generate HMAC-SHA1*/
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/*Step 2 get DT(HMAC) */
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/* get the lower 4-bits and treat them as a number n*/
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/* DT is the 31bit integer at HMAC[n]+HMAC[n+1]+HMAC[n+2]+HMAC[n+3]*/
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/* the MSB is ignored */
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n=hmac[SHA1_SIZE-1] & 0x0F;
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dt= (uint32_t)(hmac[n ]&0x7F)<<(uint32_t)24 |
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(uint32_t)(hmac[n+1])<<(uint32_t)16 |
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(uint32_t)(hmac[n+2])<<(uint32_t)8 |
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(uint32_t)(hmac[n+3]);
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/*Step 3 return dt mod 10^digits*/
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switch(digits){
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case 6: return dt % 1000000;
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case 7: return dt % 10000000;
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default: case 8: return dt % 100000000;
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}
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}
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