support metadata signature verification using otacerts.zip #33
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We sign using RSASSA-PSS as recommended for new applications in PKCS # 1 v2.2 (RFC 8017) [1]. SHA256withRSA/PSS is supported on Android [2]. Test: With a self-hosted server, a GeneralSecurityException is thrown in Service when the metadata doesn't match with the original contents, and no GeneralSecurityException is thrown when it matches. Test: The following steps were done successfully with an RSA-2048 release key and then repeated with an RSA-4096 release key: 1. Flash latest factory zip and boot the OS and have the latest version also on the server 2. Check for updates on the server (sargo-stable) with a correct signature to get the "targetBuildDate: ... not higher than sourceBuildDate" message 3. Edit sargo-stable on the server to something different, and check again for updates in the app. The logcat shows "java.security.GeneralSecurityException: verifier.verify returned false" 4. Repeat steps 2-3 for the beta and testing channels 5. Upload a newer release with the metadata altered from the correct value; the same exception from step 3 shows up 6. Change metadata to correct value; the update downloads and installs correctly. Test: In Direct Boot mode, the Updater app was able to download metadata, verify the metadata, download OTA packages, and install updates. [1] https://tools.ietf.org/html/rfc8017#section-8 [2] https://developer.android.com/reference/java/security/Signature
The actual GrapheneOS `releasekeys` are RSA keys right now, but for completeness, this change will accommodate usages of an ECDSA `releasekey` as generated by `development/tools/make_key`. Test: Ran through a full update check then download and install cycle with an EC key, and also tested that editing the metadata without resigning causes a GeneralSecurityException.
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Test: A previous version of the Updater app was able to downloadn and verify the new metadata format without issues.
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Closes #7
Needs to be looked at with GrapheneOS/script#8
This PR adds metadata signature verification. It also verifies the new added device model
field in the metadata as noted in #7.
For RSA, we sign using RSASSA-PSS as recommended for new applications in PKCS #`1
v2.2 (RFC 8017) [1]. SHA256withRSA/PSS is supported on Android [2].
Test: With a self-hosted server, a GeneralSecurityException is thrown in
Service when the metadata doesn't match with the original contents, and
no GeneralSecurityException when it matches.
Test: The following steps were done successfully with an RSA-2048
release key and then repeated with an RSA-4096 release key and an
ECDSA release key (generated from
development/tools/make_key):latest version also on the server
signature to get the "targetBuildDate: ... not higher than
sourceBuildDate" message
again for updates in the app. The logcat shows
java.security.GeneralSecurityException: verifier.verify returned false
value; the same exception from step 3 shows up
correctly.
Test: A previous version of the Updater app was able to download and
verify the new metadata format without issues.
[1] https://tools.ietf.org/html/rfc8017#section-8
[2] https://developer.android.com/reference/java/security/Signature