* TOTP (Time-based one-time password) on TPM 2.0

Previously:
https://yurichev.com/blog/TOTP1/
https://yurichev.com/blog/TOTP2/
https://yurichev.com/blog/TPM1/
https://yurichev.com/raw-notes/cryptography/TPM/HMAC-SHA1-on-TPM-2.0.org

** Create hmac.ctx from TOTP key

( The code is the same as in lichess example: https://yurichev.com/blog/TOTP1/ )

#+BEGIN_CODE
#!/usr/bin/env python3
import base64

s=base64.b32decode("XYYUFRCR3YP7DMHSZOLAQWBRTEODNAZL")
with open("hmac.key", "wb") as f:
    f.write(s)
#+END_CODE

Run:

#+BEGIN_CODE
tpm2 loadexternal -C n -G hmac:sha1 -a "sign|userwithauth" -r hmac.key -c hmac.ctx
rm hmac.key
#+END_CODE

** Generate TOTP codes

#+BEGIN_CODE
#!/usr/bin/env python3
import time, os, tempfile

def hotp(ctr: int, length: int) -> str:
    # https://stackoverflow.com/questions/3924117/how-to-use-tempfile-namedtemporaryfile-in-python
    tmp_in=tempfile.NamedTemporaryFile(delete=False)
    tmp_in.write(ctr.to_bytes(8, 'big'))
    tmp_in.close()
    tmp_out=tempfile.NamedTemporaryFile(delete=False)
    tmp_out.close()
    os.system (f"tpm2 hmac -g sha1 -c hmac.ctx -o {tmp_out.name} {tmp_in.name}")
    os.unlink(tmp_in.name)
    with open(tmp_out.name, "rb") as f:
        mac=f.read()
        os.unlink(tmp_out.name)
        offset = mac[-1] & 0xf
        truncated = bytearray(mac[offset:offset+4])
        truncated[0] &= 0x7f
        value = int.from_bytes(truncated, 'big') % (10**length)

        return str(value).rjust(length, '0')

ctr=int(time.time()//30)
print (hotp(ctr,6))
valid=30-(int(time.time()) % 30)
print (f"The code is valid for {valid} seconds")
#+END_CODE

Run it:

#+BEGIN_CODE
165457
The code is valid for 28 seconds
#+END_CODE

** Weakness

But this solution has the same weakness as with Yubikey ( https://yurichev.com/blog/TOTP2/ ).
An attacker, having access to your TPM, can generate some codes for future use.
This is to be kept in mind.
But TPM context can be protected with password.