from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives.kdf.pbkdf2 import PBKDF2HMAC
from cryptography.hazmat.primitives.ciphers.aead import AESGCM
from cryptography.hazmat.backends import default_backend
import sys

CHUNK = 64 * 1024  # plaintext bytes per chunk (must match src/crypto.ts)
TAG = 16
ENC_CHUNK = CHUNK + TAG  # ciphertext bytes for a full chunk
HEADER = 16 + 7  # salt[16] | noncePrefix[7]


def decrypt(data: bytes, password: str) -> bytes:
    """
    Decrypts content produced by encrypt.py / the server / browser.

    Uses chunked AES-256-GCM with a PBKDF2-HMAC-SHA512 derived key, matching the
    implementation in src/crypto.ts.

    Wire format: salt[16] | noncePrefix[7] | encChunk_0 | encChunk_1 | ...
    where encChunk_i = AES-GCM(key, nonce_i, plaintext_i) (16-byte tag appended)
    and nonce_i = noncePrefix[7] || uint32_be(i) || flag, flag=1 on the final
    chunk else 0. Each non-final plaintext chunk is exactly CHUNK bytes, so chunk
    boundaries (and which chunk is last) follow from the total length.

    Args:
    data (bytes): The encrypted wire format.
    password (str): The password used for encryption.

    Returns:
    bytes: the decrypted content

    Raises:
    Exception: if the password is wrong or the data is corrupt/truncated (the
    AES-GCM tag, chunk counter, and final-chunk flag are all authenticated).
    """

    if len(data) < HEADER + TAG:
        raise ValueError("ciphertext too short")

    salt = data[:16]
    prefix = data[16:HEADER]
    body = data[HEADER:]

    kdf = PBKDF2HMAC(
        algorithm=hashes.SHA512(),
        length=32,
        salt=salt,
        iterations=210000,
        backend=default_backend()
    )
    aes = AESGCM(kdf.derive(password.encode()))

    out = bytearray()
    offset = 0
    i = 0
    while offset < len(body):
        enc_len = min(ENC_CHUNK, len(body) - offset)
        last = 1 if offset + enc_len >= len(body) else 0
        nonce = prefix + i.to_bytes(4, "big") + bytes([last])
        out += aes.decrypt(nonce, body[offset:offset + enc_len], None)
        offset += enc_len
        i += 1
    return bytes(out)


if __name__ == "__main__":
    if len(sys.argv) != 4:
        print("Usage: decrypt.py <file> <password> <outfile>")
        sys.exit(1)
    data = decrypt(open(sys.argv[1], "rb").read(), sys.argv[2])
    open(sys.argv[3], "wb").write(data)