1 <html devsite> 2 <head> 3 <title>Application Signing</title> 4 <meta name="project_path" value="/_project.yaml" /> 5 <meta name="book_path" value="/_book.yaml" /> 6 </head> 7 <body> 8 <!-- 9 Copyright 2017 The Android Open Source Project 10 11 Licensed under the Apache License, Version 2.0 (the "License"); 12 you may not use this file except in compliance with the License. 13 You may obtain a copy of the License at 14 15 http://www.apache.org/licenses/LICENSE-2.0 16 17 Unless required by applicable law or agreed to in writing, software 18 distributed under the License is distributed on an "AS IS" BASIS, 19 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 20 See the License for the specific language governing permissions and 21 limitations under the License. 22 --> 23 24 25 26 <p> 27 Application signing allows developers to identify the author of the application 28 and to update their application without creating complicated interfaces and 29 permissions. Every application that is run on the Android platform must be <a 30 href="https://developer.android.com/studio/publish/app-signing.html">signed by 31 the developer</a>. Applications that attempt to install without being signed 32 will be rejected by either Google Play or the package installer on the Android 33 device. 34 </p> 35 <p> 36 On Google Play, application signing bridges the trust Google has with the 37 developer and the trust the developer has with their application. Developers 38 know their application is provided, unmodified, to the Android device; and 39 developers can be held accountable for behavior of their application. 40 </p> 41 <p> 42 On Android, application signing is the first step to placing an application in 43 its Application Sandbox. The signed application certificate defines which user 44 ID is associated with which application; different applications run under 45 different user IDs. Application signing ensures that one application cannot 46 access any other application except through well-defined IPC. 47 </p> 48 <p> 49 When an application (APK file) is installed onto an Android device, the Package 50 Manager verifies that the APK has been properly signed with the certificate 51 included in that APK. If the certificate (or, more accurately, the public key in 52 the certificate) matches the key used to sign any other APK on the device, the 53 new APK has the option to specify in the manifest that it will share a UID with 54 the other similarly-signed APKs. 55 </p> 56 <p> 57 Applications can be signed by a third-party (OEM, operator, alternative market) 58 or self-signed. Android provides code signing using self-signed certificates 59 that developers can generate without external assistance or permission. 60 Applications do not have to be signed by a central authority. Android currently 61 does not perform CA verification for application certificates. 62 </p> 63 <p> 64 Applications are also able to declare security permissions at the Signature 65 protection level, restricting access only to applications signed with the same 66 key while maintaining distinct UIDs and Application Sandboxes. A closer 67 relationship with a shared Application Sandbox is allowed via the <a 68 href="https://developer.android.com/guide/topics/manifest/manifest-element.html#uid">shared 69 UID feature</a> where two or more applications signed with same developer key 70 can declare a shared UID in their manifest. 71 </p> 72 <h2>APK signing schemes</h2> 73 <p> 74 Android supports two application signing schemes, one based on JAR signing (v1 75 scheme) and <a href="v2.html">APK Signature Scheme v2 (v2 scheme)</a>, which 76 was introduced in Android Nougat (Android 7.0). 77 </p> 78 <p> 79 For maximum compatibility, applications should be signed both with v1 and v2 80 schemes. Android Nougat and newer devices install apps signed with v2 scheme 81 more quickly than those signed only with v1 scheme. Older Android platforms 82 ignore v2 signatures and thus need apps to contain v1 signatures. 83 </p> 84 <h3 id="v1">JAR signing (v1 scheme)</h3> 85 <p> 86 APK signing has been a part of Android from the beginning. It is based on <a 87 href="https://docs.oracle.com/javase/8/docs/technotes/guides/jar/jar.html#Signed_JAR_File"> 88 signed JAR</a>. For details on using this scheme, see the Android Studio documentation on 89 <a href="https://developer.android.com/studio/publish/app-signing.html">Signing 90 your app</a>. 91 </p> 92 <p> 93 v1 signatures do not protect some parts of the APK, such as ZIP metadata. The 94 APK verifier needs to process lots of untrusted (not yet verified) data 95 structures and then discard data not covered by the signatures. This offers a 96 sizeable attack surface. Moreover, the APK verifier must uncompress all 97 compressed entries, consuming more time and memory. To address these issues, 98 Android 7.0 introduced APK Signature Scheme v2. 99 </p> 100 <h3 id="v2">APK Signature Scheme v2 (v2 scheme)</h3> 101 <p> 102 Android 7.0 introduces APK signature scheme v2 (v2 scheme). The contents of the 103 APK are hashed and signed, then the resulting APK Signing Block is inserted 104 into the APK. For details on applying the v2 scheme to an application, refer to 105 <a href="https://developer.android.com/about/versions/nougat/android-7.0.html#apk_signature_v2">APK 106 Signature Scheme v2</a> in the Android N Developer Preview. 107 </p> 108 <p> 109 During validation, v2 scheme treats the APK file as a blob and performs signature 110 checking across the entire file. Any modification to the APK, including ZIP metadata 111 modifications, invalidates the APK signature. This form of APK verification is 112 substantially faster and enables detection of more classes of unauthorized 113 modifications. 114 </p> 115 <p> 116 The new format is backwards compatible, so APKs signed with the new signature 117 format can be installed on older Android devices (which simply ignore the extra 118 data added to the APK), as long as these APKs are also v1-signed. 119 </p> 120 <p> 121 <img src="../images/apk-validation-process.png" alt="APK signature verification process" id="figure1" /> 122 </p> 123 <p class="img-caption"><strong>Figure 1.</strong> APK signature verification 124 process (new steps in red)</p> 125 126 <p> 127 Whole-file hash of the APK is verified against the v2 signature stored in the 128 APK Signing Block. The hash covers everything except the APK Signing Block, 129 which contains the v2 signature. Any modification to the APK outside of the APK 130 Signing Block invalidates the APK's v2 signature. APKs with stripped v2 131 signature are rejected as well, because their v1 signature specifies that the 132 APK was v2-signed, which makes Android Nougat and newer refuse to verify APKs 133 using their v1 signatures. 134 </p> 135 136 <p>For details on the APK signature verification process, see the <a href="v2.html#verification"> 137 Verification section</a> of APK Signature Scheme v2.</p> 138 139 </body> 140 </html> 141
