public static int GenerateToken(string base32Key)
{
var keySize = Encoding.UTF8.GetByteCount(base32Key);
int bufferSize = (keySize + 7) / 8 * 5;
byte[] decoded = new byte[bufferSize];
int hmacResSize = HMAC_RES_SIZE;
Span <byte> hmacRes = stackalloc byte[HMAC_RES_SIZE];
long timestamp = Time.GetTimestamp(Time.Now) / 30;
Span <byte> challenge = stackalloc byte[8];
for (int i = 7; i >= 0; i--, timestamp >>= 8)
{
challenge[i] = (byte)timestamp;
}
Base32Decode(base32Key, decoded);
var hmac = new HMACSHA1(decoded);
hmac.TryComputeHash(challenge, hmacRes, out hmacResSize);
hmac.Dispose();
int offset = hmacRes[19] & 0xF;
int truncHash = (hmacRes[offset] << 24) | (hmacRes[offset + 1] << 16) | (hmacRes[offset + 2] << 8) | (hmacRes[offset + 3]);
truncHash &= 0x7FFFFFFF;
return(truncHash % 1000000);
}
