podlite/tools/signapk.py

"""v1 (JAR) APK signer in the format Android's own SignApk used in the 2.x era, plus zipalign.

Why not jarsigner: modern JDKs write "SHA-1-Digest" entries (Android <= 4.2 only looks for
"SHA1-Digest"/"SHA-Digest") and put sha1WithRSAEncryption in the PKCS#7 SignerInfo, which old
JarUtils can't map. Either makes Android 2.x treat the APK as unsigned and refuse to install it.

Usage: signapk.py <keystore.p12> <password> <in.apk> <out.apk>
"""
import base64
import hashlib
import struct
import sys
import zipfile

from cryptography.hazmat.primitives import hashes, serialization
from cryptography.hazmat.primitives.asymmetric import padding
from cryptography.hazmat.primitives.serialization import pkcs12

ALIGN = 4
FIXED_TIME = (2008, 1, 1, 0, 0, 0)


# ---------------------------------------------------------------- minimal DER

def der_len(n):
    if n < 0x80:
        return bytes([n])
    b = n.to_bytes((n.bit_length() + 7) // 8, "big")
    return bytes([0x80 | len(b)]) + b


def tlv(tag, body):
    return bytes([tag]) + der_len(len(body)) + body


def seq(*parts):
    return tlv(0x30, b"".join(parts))


def set_of(*parts):
    return tlv(0x31, b"".join(sorted(parts)))


def oid(dotted):
    nums = [int(x) for x in dotted.split(".")]
    body = bytes([nums[0] * 40 + nums[1]])
    for n in nums[2:]:
        chunk = [n & 0x7F]
        n >>= 7
        while n:
            chunk.append(0x80 | (n & 0x7F))
            n >>= 7
        body += bytes(reversed(chunk))
    return tlv(0x06, body)


def integer(n):
    b = n.to_bytes((n.bit_length() + 8) // 8 or 1, "big", signed=True)
    return tlv(0x02, b)


NULL = b"\x05\x00"
OID_SHA1 = "1.3.14.3.2.26"
OID_RSA = "1.2.840.113549.1.1.1"
OID_DATA = "1.2.840.113549.1.7.1"
OID_SIGNED_DATA = "1.2.840.113549.1.7.2"


def pkcs7_detached(sf_bytes, key, cert):
    signature = key.sign(sf_bytes, padding.PKCS1v15(), hashes.SHA1())
    cert_der = cert.public_bytes(serialization.Encoding.DER)
    sha1_alg = seq(oid(OID_SHA1), NULL)
    signer_info = seq(
        integer(1),
        seq(cert.issuer.public_bytes(), integer(cert.serial_number)),
        sha1_alg,
        seq(oid(OID_RSA), NULL),          # rsaEncryption, as old JarUtils expects
        tlv(0x04, signature),
    )
    signed_data = seq(
        integer(1),
        set_of(sha1_alg),
        seq(oid(OID_DATA)),               # detached: content is META-INF/CERT.SF
        tlv(0xA0, cert_der),              # [0] IMPLICIT certificates
        set_of(signer_info),
    )
    return seq(oid(OID_SIGNED_DATA), tlv(0xA0, signed_data))


# ---------------------------------------------------------------- manifest text

def b64sha1(data):
    return base64.b64encode(hashlib.sha1(data).digest()).decode()


def wrap(line):
    """JAR manifest lines are at most 72 bytes; continuation lines start with a space."""
    raw = line.encode("utf-8")
    out = [raw[:70]]
    raw = raw[70:]
    while raw:
        out.append(b" " + raw[:69])
        raw = raw[69:]
    return b"\r\n".join(out) + b"\r\n"


def sign(p12_path, password, src, dst):
    key, cert, _ = pkcs12.load_key_and_certificates(open(p12_path, "rb").read(), password.encode())
    zin = zipfile.ZipFile(src)
    entries = [i for i in zin.infolist()
               if not i.filename.startswith("META-INF/") and not i.is_dir()]
    data = {i.filename: zin.read(i.filename) for i in entries}

    manifest = b"Manifest-Version: 1.0\r\nCreated-By: 1.0 (Android SignApk)\r\n\r\n"
    sf_entries = b""
    for i in entries:
        section = wrap("Name: " + i.filename) + wrap("SHA1-Digest: " + b64sha1(data[i.filename])) + b"\r\n"
        manifest += section
        sf_entries += wrap("Name: " + i.filename) + wrap("SHA1-Digest: " + b64sha1(section)) + b"\r\n"
    sf = (b"Signature-Version: 1.0\r\nCreated-By: 1.0 (Android SignApk)\r\n"
          + wrap("SHA1-Digest-Manifest: " + b64sha1(manifest)) + b"\r\n" + sf_entries)
    rsa = pkcs7_detached(sf, key, cert)

    with zipfile.ZipFile(dst, "w") as zout:
        def put(name, payload, compress):
            zi = zipfile.ZipInfo(name, date_time=FIXED_TIME)
            zi.compress_type = compress
            zi.external_attr = 0o644 << 16
            zi.create_system = 0
            extra = b""
            if compress == zipfile.ZIP_STORED:
                base = zout.fp.tell() + 30 + len(name.encode("utf-8")) + 6
                pad = (ALIGN - base % ALIGN) % ALIGN
                extra = struct.pack("<HHH", 0xD935, 2 + pad, ALIGN) + b"\0" * pad
            zi.extra = extra
            zout.writestr(zi, payload)

        put("META-INF/MANIFEST.MF", manifest, zipfile.ZIP_DEFLATED)
        put("META-INF/CERT.SF", sf, zipfile.ZIP_DEFLATED)
        put("META-INF/CERT.RSA", rsa, zipfile.ZIP_DEFLATED)
        for i in entries:
            put(i.filename, data[i.filename], i.compress_type)


if __name__ == "__main__":
    sign(*sys.argv[1:5])