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otp.c
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otp.c
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#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <time.h>
#include "sha1.h"
#include "otp.h"
static int base32_decode(const char *src, const int srclen, uint8_t *dst, const int dstlen)
{
const int len = srclen == -1 ? strlen((const char *) src) : srclen;
int retval = 0;
uint32_t accum = 0;
int shifter = 0;
int i;
for (i = 0; i < len; i++) {
const uint8_t ch = (uint8_t) src[i];
uint8_t val;
if ((ch >= 'A') && (ch <= 'Z')) {
val = ch - 'A';
} else if ((ch >= '2') && (ch <= '7')) {
val = (ch - '2') + 26;
} else if (ch == '=') {
val = 0;
/* these are illegal in base32, but GAuth keys might have them. */
} else if (ch == ' ') {
continue; /* skip these. */
} else if ((ch >= 'a') && (ch <= 'z')) {
val = ch - 'a'; /* treat like uppercase. */
} else {
return -1; /* invalid string. */
}
accum = (accum << 5) | ((uint32_t) val);
shifter += 5;
if (shifter >= 8) {
if (retval > dstlen) {
return -1; /* dst too small */
}
dst[retval] = (uint8_t) ((accum >> (shifter - 8)) & 0xFF);
retval++;
shifter -= 8;
}
}
if (shifter > 0) {
if (retval > dstlen) {
return -1; /* dst too small */
}
dst[retval] = (uint8_t) (accum & 0xFF);
retval++;
}
return retval;
}
int totp(const char *base32_secret, char *dst, int dstlen)
{
uint8_t decoded[64];
int decodedlen;
uint64_t secs;
uint8_t timebytes[8];
unsigned char digest[SHA1_DIGEST_LENGTH];
uint8_t *bytes;
uint32_t val;
decodedlen = base32_decode(base32_secret, -1, decoded, sizeof (decoded));
if (decodedlen == -1) {
return -1;
}
secs = ((uint64_t) time(NULL)) / 30;
for (int i = 0; i < 8; i++) {
timebytes[i] = (uint8_t) (secs >> ((7-i) * 8));
}
SHA1Hmac(decoded, decodedlen, timebytes, 8, digest);
bytes = (uint8_t *) (digest + (digest[SHA1_DIGEST_LENGTH-1] & 0xF));
val = (((uint32_t) bytes[0]) << 24) | (((uint32_t) bytes[1]) << 16) |
(((uint32_t) bytes[2]) << 8) | (((uint32_t) bytes[3]));
val &= 0x7FFFFFFF; /* drop most significant bit. */
val %= 1000000; /* make it six digits long. */
snprintf(dst, dstlen, "%06u", (unsigned int) val);
return 0;
}
/* end of otp.c ... */