First you need to right-click at the gradlew at android folder, and choose open in terminal.
In the directory type ./gradlew signingReport
if success, it will show you the SHA-1.
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run this command inside the android folder in your flutter app
.\gradlew signingReport
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Windows:
keytool -list -v -keystore "C:\Users\yourusernamehere\.android\debug.keystore" -alias androiddebugkey -storepass android -keypass android
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keytool -list -v \ -alias androiddebugkey -keystore %USERPROFILE%\.android\debug.keystore