Android Signing Certificates

Androguard has the ability to get information about the signing certificate found in APKs. Over the last versions of Androguard, different parsers has been used to get certificate information. The first parser was Chilkat, then a mixture of pyasn1 and cryptography was used, while the latest parser uses the asn1crypto library. Not all x509 parsers work with all certificates as there are plenty of examples where the certificate creator does not follow the RFCs for creating certificates. Some parsers do not accept such broken certificates and will fail to parse them.

The purpose of Androids signing process is not to provide verified information about the author, like with JAR signing, but only provide a way to check the integrity of the APK as well as check if an APK can be upgraded by comparing the certificate fingerprints. In some sense, the certificate information can be used to find other APKs from the same author - as long as the signing key was kept secret! There are also public available signing keys, like the ones from AOSP, thus the same fingerprint of two APKs does not always tell you it was signed by the same person.

If you like to know more about the APK signing process, please read the official documentation about Signing. There is also an official tool to verify and sign APKs called apksigner.

Working with certificates

Inside the APK, there are two places for certificates:

  • v1 aka JAR signing: PKCS#7 files in the META-INF folder

  • v2 aka APK signing: a special section in the ZIP containing DER coded certifcates

The easiest way to get to the certificate information is androguard sign - Print Certificate Fingerprints. It gives similar output to apksigner, but uses only androguard. It can not verify the integrity of the file though.

$ --all --show examples/signing/apksig/golden-aligned-v1v2-out.apk
golden-aligned-v1v2-out.apk, package: 'android.appsecurity.cts.tinyapp'
Is signed v1: True
Is signed v2: True
Found 1 unique certificates
Issuer: CN=rsa-2048
Subject: CN=rsa-2048
Serial Number: 0x8e35306cdd0115f7L
Hash Algorithm: sha256
Signature Algorithm: rsassa_pkcs1v15
Valid not before: 2016-03-31 14:57:49+00:00
Valid not after: 2043-08-17 14:57:49+00:00
sha1 0aa07c0f297b4ae834dc85a17eea8c2cf9380ff7
sha256 fb5dbd3c669af9fc236c6991e6387b7f11ff0590997f22d0f5c74ff40e04fca8
sha512 4da6e6744a4dabef192b198be13b4492b0ce97469f3ce223dd9b7e8df2ee952328e06651e5e65dd3b60ac5e3946e16cf7059b20d4d4a649957c1e3055c2e1fb8
md5 e995a5ed7137307661f854e66901ee9e

As a comparison, here is the output of apksigner:

$ apksigner verify -verbose --print-certs examples/signing/apksig/golden-aligned-v1v2-out.apk
Verified using v1 scheme (JAR signing): true
Verified using v2 scheme (APK Signature Scheme v2): true
Number of signers: 1
Signer #1 certificate DN: CN=rsa-2048
Signer #1 certificate SHA-256 digest: fb5dbd3c669af9fc236c6991e6387b7f11ff0590997f22d0f5c74ff40e04fca8
Signer #1 certificate SHA-1 digest: 0aa07c0f297b4ae834dc85a17eea8c2cf9380ff7
Signer #1 certificate MD5 digest: e995a5ed7137307661f854e66901ee9e
Signer #1 key algorithm: RSA
Signer #1 key size (bits): 2048
Signer #1 public key SHA-256 digest: 8cabaedf32f1052f6bc5edbeb84d1c500f8c1aa15f8944bf22c46e44c5c4f7e8
Signer #1 public key SHA-1 digest: a708f9a777bac814e6634b02521224537ec3e019
Signer #1 public key MD5 digest: c0c8801fabf2ad970282be1c41584003

The most interesting part is probaby the fingerprint of the certificate (not of the public key!). You can use it to search for similar APKs. Sometimes there is a confusion about this fingerprint: The fingerprint is not the checksum of the whole PKCS#7 file, but only of a certain part of it! Calculating the hash of a PKCS#7 file from two different, but equally signed APKs will result in a different hash. The fingerprint will stay the same though.

Androguard offers methods in the androguard.core.bytecodes.apk.APK class to iterate over the certificates found there.

from androguard.core.bytecodes.apk import APK

a = APK('examples/signing/apksig/golden-aligned-v1v2-out.apk')

# first check if this APK is signed
print("APK is signed: {}".format(a.is_signed()))

if a.is_signed():
    # Test if signed v1 or v2 or both
    print("APK is signed with: {}".format("both" if a.is_signed_v1() and
    a.is_signed_v2() else "v1" if a.is_signed_v1() else "v2"))

# Iterate over all certificates
for cert in a.get_certificates():
    # Each cert is now a asn1crypt.x509.Certificate object
    # From the Certificate object, we can query stuff like:
    cert.sha1  # the sha1 fingerprint
    cert.sha256  # the sha256 fingerprint
    cert.issuer.human_friendly  # issuer
    cert.subject.human_friendly  # subject, usually the same
    cert.hash_algo  # hash algorithm
    cert.signature_algo  # Signature algorithm
    cert.serial_number  # Serial number
    cert.contents  # The DER coded bytes of the certificate itself
    # ...

Please referr to the asn1crypto documentation for more information on the features of the Certificate class!