en/security/apksigning/index.html

page.title=Application Signing
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    <h2>In this document</h2>
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<p>
Application signing allows developers to identify the author of the application
and to update their application without creating complicated interfaces and
permissions. Every application that is run on the Android platform must be <a
href="https://developer.android.com/studio/publish/app-signing.html">signed by
the developer</a>. Applications that attempt to install without being signed
will be rejected by either Google Play or the package installer on the Android
device.
</p>
<p>
On Google Play, application signing bridges the trust Google has with the
developer and the trust the developer has with their application. Developers
know their application is provided, unmodified, to the Android device; and
developers can be held accountable for behavior of their application.
</p>
<p>
On Android, application signing is the first step to placing an application in
its Application Sandbox. The signed application certificate defines which user
ID is associated with which application; different applications run under
different user IDs. Application signing ensures that one application cannot
access any other application except through well-defined IPC.
</p>
<p>
When an application (APK file) is installed onto an Android device, the Package
Manager verifies that the APK has been properly signed with the certificate
included in that APK. If the certificate (or, more accurately, the public key in
the certificate) matches the key used to sign any other APK on the device, the
new APK has the option to specify in the manifest that it will share a UID with
the other similarly-signed APKs.
</p>
<p>
Applications can be signed by a third-party (OEM, operator, alternative market)
or self-signed. Android provides code signing using self-signed certificates
that developers can generate without external assistance or permission.
Applications do not have to be signed by a central authority. Android currently
does not perform CA verification for application certificates.
</p>
<p>
Applications are also able to declare security permissions at the Signature
protection level, restricting access only to applications signed with the same
key while maintaining distinct UIDs and Application Sandboxes. A closer
relationship with a shared Application Sandbox is allowed via the <a
href="https://developer.android.com/guide/topics/manifest/manifest-element.html#uid">shared
UID feature</a> where two or more applications signed with same developer key
can declare a shared UID in their manifest.
</p>
<h2>APK signing schemes</h2>
<p>
Android supports two application signing schemes, one based on JAR signing (v1
scheme) and <a href="v2.html">APK Signature Scheme v2 (v2 scheme)</a>, which
was introduced in Android Nougat (Android 7.0).
</p>
<p>
For maximum compatibility, applications should be signed both with v1 and v2
schemes. Android Nougat and newer devices install apps signed with v2 scheme
more quickly than those signed only with v1 scheme. Older Android platforms
ignore v2 signatures and thus need apps to contain v1 signatures.
</p>
<h3 id="v1">JAR signing (v1 scheme)</h3>
<p>
APK signing has been a part of Android from the beginning. It is based on <a
href="https://docs.oracle.com/javase/8/docs/technotes/guides/jar/jar.html#Signed_JAR_File">
signed JAR</a>. For details on using this scheme, see the Android Studio documentation on
<a href="https://developer.android.com/studio/publish/app-signing.html">Signing
your app</a>.
</p>
<p>
v1 signatures do not protect some parts of the APK, such as ZIP metadata. The
APK verifier needs to process lots of untrusted (not yet verified) data
structures and then discard data not covered by the signatures. This offers a
sizeable attack surface. Moreover, the APK verifier must uncompress all
compressed entries, consuming more time and memory. To address these issues,
Android 7.0 introduced APK Signature Scheme v2.
</p>
<h3 id="v2">APK Signature Scheme v2 (v2 scheme)</h3>
<p>
Android 7.0 introduces APK signature scheme v2 (v2 scheme). The contents of the
APK are hashed and signed, then the resulting APK Signing Block is inserted
into the APK. For details on applying the v2 scheme to an application, refer to
<a href="https://developer.android.com/preview/api-overview.html#apk_signature_v2">APK
Signature Scheme v2</a> in the Android N Developer Preview.
</p>
<p>
During validation, v2 scheme treats the APK file as a blob and performs signature
checking across the entire file. Any modification to the APK, including ZIP metadata
modifications, invalidates the APK signature. This form of APK verification is
substantially faster and enables detection of more classes of unauthorized
modifications.
</p>
<p>
The new format is backwards compatible, so APKs signed with the new signature
format can be installed on older Android devices (which simply ignore the extra
data added to the APK), as long as these APKs are also v1-signed.
</p>
<p>
  <img src="../images/apk-validation-process.png" alt="APK signature verification process" id="figure1" />
</p>
<p class="img-caption"><strong>Figure 1.</strong> APK signature verification
process (new steps in red)</p>

<p>
Whole-file hash of the APK is verified against the v2 signature stored in the
APK Signing Block. The hash covers everything except the APK Signing Block,
which contains the v2 signature. Any modification to the APK outside of the APK
Signing Block invalidates the APK's v2 signature. APKs with stripped v2
signature are rejected as well, because their v1 signature specifies that the
APK was v2-signed, which makes Android Nougat and newer refuse to verify APKs
using their v1 signatures.
</p>

<p>For details on the APK signature verification process, see the <a href="v2.html#verification">
Verification section</a> of APK Signature Scheme v2.</p>