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Fix an overflow for 10-digits OTP

This commit is contained in:
Lucas 2021-02-11 21:20:06 +00:00
parent 0b93ea8653
commit 4d2024128e
1 changed files with 14 additions and 5 deletions
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19
otp.c
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@ -26,12 +26,20 @@ otp(const EVP_MD *evp_md, const void *key, int key_len, uint64_t counter,
unsigned char md_value[EVP_MAX_MD_SIZE];
uint8_t buf[8];
uint32_t res;
unsigned int md_len, modulo, offset;
int32_t modulo;
unsigned int md_len, offset;
if (digits < 6 || digits > 10)
return -1;
for (modulo = 1000000; digits > 6; digits--, modulo *= 10)
;
/*
* 10 digits would be 10_000_000_000, which overflows 32 bits.
* Nevertheless, given the number construction, 10 digits OTP are
* possible. We won't apply reminder operator in that case.
*/
if (digits < 10)
for (modulo = 1000000; digits > 6; digits--, modulo *= 10)
continue;
/* convert counter to big endian */
buf[0] = counter >> 56;
@ -46,14 +54,15 @@ otp(const EVP_MD *evp_md, const void *key, int key_len, uint64_t counter,
if (HMAC(evp_md, key, key_len, buf, sizeof(buf), md_value, &md_len) ==
NULL)
return -1;
offset = md_value[md_len - 1] & 0xf;
res = (md_value[offset] << 24) |
(md_value[offset + 1] << 16) |
(md_value[offset + 2] << 8) |
md_value[offset + 3];
res &= ~0x80000000;
res &= 0x7fffffff;
return (int32_t)res % modulo;
return digits == 10 ? (int32_t)res : (int32_t)res % modulo;
}
int32_t