Step-by-Step TOTP

Inputs

K (shared secret)

Digit (number of digits)

WARNING: At least 6 digits are required, according to RFC 4226.

C (counter value)

HOTP

C (counter value)

TOTP

000000

K (Base32-decoded)

C (converted to bytes)

HS = HMAC-SHA-1(K,C)

Details

key	(use H(K) because the input K is longer than 64 bytes)
K XOR ipad
K XOR opad
K XOR ipad, text
H(K XOR ipad, text)
K XOR opad, H(K XOR ipad, text)
H(K XOR opad, H(K XOR ipad, text))

Sbits = DT(HS)

Details

OffsetBits = low-order 4 bits of HS[19]
Offset = StToNum(OffsetBits)
P = HS[OffSet]...HS[OffSet+3]
Last 31 bits of P

Snum = StToNum(Sbits)

D = Snum mod 10^Digit