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Modify OTP generation to run in constant time #148

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merged 1 commit into from Dec 14, 2022
Merged

Modify OTP generation to run in constant time #148

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9 changes: 4 additions & 5 deletions src/pyotp/otp.py
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Expand Up @@ -18,6 +18,8 @@ def __init__(
issuer: Optional[str] = None,
) -> None:
self.digits = digits
if digits > 10:
raise ValueError("digits must be no greater than 10")
Comment on lines +21 to +22
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This introduces a breaking limitation. It is needed because of the hardcoded 10_000_000_000 below. See there for a suggestion of how to deal with this.

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The limitation already existed. It comes from the fact that the HMAC is only 32 bits, so if you try to extract more than 10 digits you'll only get extra zeroes.

The proper limit is actually 9 digits, but the Steam subclass needs 10, so I allowed 10.

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The HMAC is as long as the hash digest (160 bits for SHA1) but @Changaco is correct that the output of the DT transform as specified in the HOTP RFC is shorter - it actually has 31 bits of information (the high order bit is masked out). That's still over 9 decimal digits worth of information, so 10 digits (not 9) is correct.

self.digest = digest
self.secret = s
self.name = name or "Secret"
Expand All @@ -39,11 +41,8 @@ def generate_otp(self, input: int) -> str:
| (hmac_hash[offset + 2] & 0xFF) << 8
| (hmac_hash[offset + 3] & 0xFF)
)
str_code = str(code % 10**self.digits)
while len(str_code) < self.digits:
str_code = "0" + str_code

return str_code
str_code = str(10_000_000_000 + (code % 10**self.digits))
return str_code[-self.digits:]
Comment on lines +44 to +45
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dec_power = 10**self.digits
return str(dec_power + code % dec_power)[1:]


def byte_secret(self) -> bytes:
secret = self.secret
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