ft_otp

全体像のざっくりの理解

TOTP: Time based One Time Password

TOTP:: Master Key -> Time -> OTP

HOTP: HMAC One Time Password

code: hotp.bnf

TOTP = HOTP(K, T)

T = (Current Unix time - T0) / X, where the default floor function is used in the computation.

K = shared secret between client and server; each HOTP generator has a different and unique secret K.

T0 = 0

X = 30

HOTP(K,C) = Truncate(HMAC-SHA-1(K,C))

C = 8-byte counter value, the moving factor. This counter MUST be synchronized between the HOTP generator (client) and the HOTP validator (server).

Digit = 6

Step 1: Generate an HMAC-SHA-1 value

Let HS = HMAC-SHA-1(K,C) // HS is a 20-byte string

Step 2: Generate a 4-byte string (Dynamic Truncation)

Let Sbits = DT(HS) // DT, defined below, returns a 31-bit string

DT(String) // String = String[0]...String[19]

Let OffsetBits be the low-order 4 bits of String[19]

Offset = StToNum(OffsetBits) // 0 <= OffSet <= 15

Let P = String[OffSet]...String[OffSet+3]

Return the Last 31 bits of P

Step 3: Compute an HOTP value

Let Snum = StToNum(Sbits) // Convert S to a number in 0...2^{31}-1

Return D = Snum mod 10^Digit // D is a number in the range 0...10^{Digit}-1

低レイヤーの理解

上記Truncateを理解する

✅高レイヤーの描き出し

✅データ構造の策定

✅環境構築

⏳debug環境

✅テストの作成

設計

構築

✅コーディング

✅RED

✅GREEN

✅CLEAN