Example

The code below is a basic example of how the wire format of a byte string can be unmarshaled into a Python object. The byte string used is the modstatesig field of the first example provided in the Key attestation format example.

class M_CipherText:
    def __init__(self, mech, data, iv):
        self.mech = mech
        self.data = data
        self.iv = iv

        known_mechs = {170: "DSAhSHA256", 187: "ECDSAhSHA512"}
        self.mech_name = known_mechs.get(mech, "Undefined")

    def __str__(self):
        description = ""
        description += f"CipherText.mech= {self.mech_name}\n"
        description += f"          .data.r= {self.data.r}\n"
        description += f"               .s= {self.data.s}"
        if self.iv:
            description += f"\n          .iv= {self.iv}"
        return description

class M_Mech_DSA_Cipher:
    def __init__(self, r, s):
        self.r = r
        self.s = s

class M_Mech_ECDSA_Cipher:
    def __init__(self, r, s):
        self.r = r
        self.s = s

class M_Bignum(int):
    def __new__(cls, content):
        return super().__new__(cls, int.from_bytes(content, byteorder='little'))

    def __str__(self):
        return hex(self)

def unmarshal(bytes_, t):
    if t == M_Bignum:
        length = int.from_bytes(bytes_[:4], byteorder='little')
        return t(bytes_[4:4+length]), bytes_[4+length:]
    elif t == M_Mech_DSA_Cipher:
        r, bytes_ = unmarshal(bytes_, M_Bignum)
        s, bytes_ = unmarshal(bytes_, M_Bignum)
        return t(r, s), bytes_
    elif t == M_Mech_ECDSA_Cipher:
        r, bytes_ = unmarshal(bytes_, M_Bignum)
        s, bytes_ = unmarshal(bytes_, M_Bignum)
        return t(r, s), bytes_
    elif t == M_CipherText:
        mech = int.from_bytes(bytes_[:4], byteorder='little')
        bytes_ = bytes_[4:]
        iv = None # none of the defined variants populate iv
        if mech == 170:
            data, bytes_ = unmarshal(bytes_, M_Mech_DSA_Cipher)
        elif mech == 187:
            data, bytes_ = unmarshal(bytes_, M_Mech_ECDSA_Cipher)
        else:
            print(f"Mech {mech} not yet implemented")
            data = None
        return t(mech, data, iv), bytes_
    else:
        print(f"Type {t.__name__} not yet implemented")
        return None, bytes_

sig = base64.urlsafe_b64decode("uwAAAEQAAABIF582_Xr16pI3UgQnbJ6BVC_qkiHGmIqnidVnXo2eFxSl5iKKTRCaIbHzB9LADNKwNygbldB7Idbn6BUP4bpayQEAAEQAAACv8Z4kRqTNrxXdiZTyyBDWRFVY9Tm0Qq2Pm6EHbSgZyCxjH_XpKVZHxlTVnNW75DE7PT_vMKgDgU5joCLtuzrUigAAAA==")
print(unmarshal(sig, M_CipherText)[0])
print("---")
print(nfkm.unmarshal(sig, nfkm.CipherText)[0])

CipherText.mech= ECDSAhSHA512
          .data.r= 0x1c95abae10f15e8e7d6217bd0951b2837b0d20cc0d207f3b1219a104d8a22e6a514179e8d5e67d589a78a98c62192ea2f54819e6c2704523792eaf57afd369f1748
               .s= 0x8ad43abbed22a0634e8103a830ef3f3d3b31e4bbd59cd554c6475629e9f51f632cc819286d07a19b8fad42b439f5585544d610c8f29489dd15afcda446249ef1af
---
CipherText.mech= ECDSAhSHA512
          .data.r= 0x1c95abae10f15e8e7d6217bd0951b2837b0d20cc0d207f3b1219a104d8a22e6a514179e8d5e67d589a78a98c62192ea2f54819e6c2704523792eaf57afd369f1748
               .s= 0x8ad43abbed22a0634e8103a830ef3f3d3b31e4bbd59cd554c6475629e9f51f632cc819286d07a19b8fad42b439f5585544d610c8f29489dd15afcda446249ef1af