ria-ran--sim-drop/server/profile_generator.py

import json
import secrets
import logging

from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
from cryptography.hazmat.backends import default_backend

logger = logging.getLogger(__name__)


def generate_imsi(mcc: str, mnc: str) -> str:
    """Generate a random 15-digit IMSI: MCC + MNC + MSIN.

    IMSI is always 15 digits. MCC is 3 digits; MNC is 2 or 3 digits, so the
    MSIN fills the remaining 9 or 10 digits.
    """
    msin_len = 15 - len(mcc) - len(mnc)
    if msin_len <= 0:
        raise ValueError(f"MCC/MNC too long for a 15-digit IMSI: {mcc!r}/{mnc!r}")
    msin = str(secrets.randbelow(10**msin_len)).zfill(msin_len)
    imsi = mcc + mnc + msin
    assert len(imsi) == 15, f"IMSI length error: {imsi!r}"
    return imsi


def generate_ki() -> bytes:
    """Generate a 16-byte cryptographically random authentication key."""
    return secrets.token_bytes(16)


def luhn_checksum(number: str) -> int:
    """Compute the Luhn check digit for a string of digits."""
    digits = [int(d) for d in number]
    odd_digits = digits[-1::-2]
    even_digits = digits[-2::-2]
    total = sum(odd_digits)
    for d in even_digits:
        total += sum(divmod(d * 2, 10))
    return (10 - (total % 10)) % 10


def generate_iccid() -> str:
    """Generate a 19-digit ICCID with a valid Luhn check digit.

    Format: 8910 (telecom prefix) + 10 random digits + 5 more + Luhn digit = 19 total.
    """
    prefix = "8910"
    body = prefix + str(secrets.randbelow(10**14)).zfill(14)  # 18 digits
    check = luhn_checksum(body)
    return body + str(check)


def derive_opc(ki: bytes, op: bytes) -> bytes:
    """Derive OPc from Ki and OP using the Milenage algorithm.

    OPc = AES_Ki(OP) XOR OP
    """
    cipher = Cipher(algorithms.AES(ki), modes.ECB(), backend=default_backend())
    enc = cipher.encryptor()
    encrypted_op = enc.update(op) + enc.finalize()
    return bytes(a ^ b for a, b in zip(encrypted_op, op))


def swap_nibbles(s: str) -> str:
    """Swap the nibbles of every byte in a hex string.

    ``"1234"`` -> ``"2143"``.  Used for the BCD-with-swapped-nibbles layout
    that 3GPP uses for ICCID and IMSI elementary files.
    """
    if len(s) % 2:
        raise ValueError(f"hex string must have even length: {s!r}")
    return "".join(s[i + 1] + s[i] for i in range(0, len(s), 2))


def enc_iccid(iccid: str) -> bytes:
    """Encode an ICCID as EF_ICCID bytes (3GPP TS 31.102 §4.2.1).

    Digits are BCD packed with swapped nibbles and right-padded to 10 bytes
    (20 nibbles) with 0xF.
    """
    if not iccid.isdigit():
        raise ValueError(f"ICCID must be digits only: {iccid!r}")
    digits = iccid.ljust(20, "f")
    return bytes.fromhex(swap_nibbles(digits))


def enc_imsi(imsi: str) -> bytes:
    """Encode an IMSI as EF_IMSI bytes (3GPP TS 31.102 §4.2.2).

    Layout: length octet, then a parity nibble ((odd<<3)|1) prepended to the
    IMSI digits, the whole run nibble-swapped.
    """
    if not imsi.isdigit():
        raise ValueError(f"IMSI must be digits only: {imsi!r}")
    odd = len(imsi) & 1
    value = swap_nibbles(f"{(odd << 3) | 1:01x}{imsi}")
    length = len(value) // 2
    return bytes([length]) + bytes.fromhex(value)


def build_lpa_activation_code(iccid: str, smdp_address: str) -> str:
    """Build an SGP.22 LPA activation code string.

    Format: ``LPA:1$<smdp-address>$<matching-id>``.  The ICCID is used as the
    matching id so the profile is self-referencing for this ad-hoc deployment.
    """
    return f"LPA:1${smdp_address}${iccid}"


def create_esim_profile(
    imsi: str,
    ki: bytes,
    opc: bytes,
    iccid: str,
    profile_name: str,
    carrier_name: str,
    smdp_address: str = "esim.local",
    apn: str = "internet",
) -> bytes:
    """Create an eSIM profile package (``profile-package-v2``).

    Emits a structured package embedding the raw credentials, the real 3GPP
    EF_ICCID / EF_IMSI byte encodings, and an SGP.22 LPA activation code.

    Full SGP.22 SIMalliance Interoperable Profile (ASN.1 UPP) generation via
    osmocom pySim remains the follow-up step; see CHANGELOG.
    """
    ki_hex = ki.hex() if isinstance(ki, bytes) else ki
    opc_hex = opc.hex() if isinstance(opc, bytes) else opc

    profile = {
        "version": "2.0",
        "type": "profile-package-v2",
        "iccid": iccid,
        "imsi": imsi,
        "ki": ki_hex,
        "opc": opc_hex,
        "profile_name": profile_name,
        "carrier_name": carrier_name,
        "apn": apn,
        "activation_code": build_lpa_activation_code(iccid, smdp_address),
        "ef": {
            "iccid": enc_iccid(iccid).hex(),
            "imsi": enc_imsi(imsi).hex(),
        },
    }
    logger.debug(f"Created profile-package-v2 for IMSI {imsi}")
    return json.dumps(profile, indent=2).encode("utf-8")