diff --git a/app/src/main/java/com/stevesoltys/seedvault/repo/Checker.kt b/app/src/main/java/com/stevesoltys/seedvault/repo/Checker.kt
index 9dbb4c1a..4c077f3a 100644
--- a/app/src/main/java/com/stevesoltys/seedvault/repo/Checker.kt
+++ b/app/src/main/java/com/stevesoltys/seedvault/repo/Checker.kt
@@ -8,16 +8,31 @@ package com.stevesoltys.seedvault.repo
 import androidx.annotation.WorkerThread
 import com.stevesoltys.seedvault.backend.BackendManager
 import com.stevesoltys.seedvault.crypto.Crypto
-import kotlinx.coroutines.delay
+import com.stevesoltys.seedvault.proto.Snapshot
+import com.stevesoltys.seedvault.proto.Snapshot.Blob
+import io.github.oshai.kotlinlogging.KotlinLogging
 import org.calyxos.seedvault.core.backends.AppBackupFileType
 import org.calyxos.seedvault.core.backends.TopLevelFolder
+import org.calyxos.seedvault.core.toHexString
+import org.koin.core.time.measureTimedValue
+import java.security.DigestInputStream
+import java.security.GeneralSecurityException
+import java.security.MessageDigest
+import kotlin.math.min
+import kotlin.math.roundToInt
+import kotlin.math.roundToLong
+import kotlin.time.Duration.Companion.milliseconds
 
 @WorkerThread
 internal class Checker(
     private val crypto: Crypto,
     private val backendManager: BackendManager,
     private val snapshotManager: SnapshotManager,
+    private val loader: Loader,
 ) {
+    private val log = KotlinLogging.logger { }
+
+    private var snapshots: List<Snapshot>? = null
 
     suspend fun getBackupSize(): Long {
         // get all snapshots
@@ -27,6 +42,7 @@ internal class Checker(
             handles.add(fileInfo.fileHandle as AppBackupFileType.Snapshot)
         }
         val snapshots = snapshotManager.onSnapshotsLoaded(handles)
+        this.snapshots = snapshots // remember loaded snapshots
 
         // get total disk space used by snapshots
         val sizeMap = mutableMapOf<String, Int>()
@@ -39,7 +55,82 @@ internal class Checker(
 
     suspend fun check(percent: Int) {
         check(percent in 0..100) { "Percent $percent out of bounds." }
-        delay(20_000)
+
+        if (snapshots == null) getBackupSize() // just get size again to be sure we get snapshots
+        val snapshots = snapshots ?: error("Snapshots still null")
+
+        val messageDigest = MessageDigest.getInstance("SHA-256")
+        val (checkedSize, time) = measureTimedValue {
+            checkBlobs(snapshots, percent) { chunkId, blob ->
+                val storageId = blob.id.hexFromProto()
+                log.info { "Checking blob $storageId..." }
+                val handle = AppBackupFileType.Blob(crypto.repoId, storageId)
+                loader.loadFile(handle, null).close()
+                val readChunkId = loader.loadFile(handle, null).use { inputStream ->
+                    DigestInputStream(inputStream, messageDigest).use { digestStream ->
+                        digestStream.readAllBytes()
+                        digestStream.messageDigest.digest().toHexString()
+                    }
+                }
+                if (readChunkId != chunkId) throw GeneralSecurityException("ChunkId doesn't match")
+            }
+        }
+        val bandwidth = (checkedSize / 1024 / (time / 1000)).roundToInt()
+        log.info { "Took ${time.milliseconds} for $checkedSize bytes, $bandwidth KB/s" }
     }
 
+    private suspend fun checkBlobs(
+        snapshots: List<Snapshot>,
+        percent: Int,
+        blobChecker: suspend (String, Blob) -> Unit,
+    ): Long {
+        // split up blobs for app data and for APKs
+        val appBlobs = mutableMapOf<String, Blob>()
+        val apkBlobs = mutableMapOf<String, Blob>()
+        snapshots.forEach { snapshot ->
+            val appChunkIds = snapshot.appsMap.flatMap { it.value.chunkIdsList.hexFromProto() }
+            val apkChunkIds = snapshot.appsMap.flatMap {
+                it.value.apk.splitsList.flatMap { split -> split.chunkIdsList.hexFromProto() }
+            }
+            appChunkIds.forEach { chunkId ->
+                appBlobs[chunkId] = snapshot.blobsMap[chunkId] ?: error("No Blob for chunkId")
+            }
+            apkChunkIds.forEach { chunkId ->
+                apkBlobs[chunkId] = snapshot.blobsMap[chunkId] ?: error("No Blob for chunkId")
+            }
+        }
+        // calculate sizes
+        val appSize = appBlobs.values.sumOf { it.length.toLong() }
+        val apkSize = apkBlobs.values.sumOf { it.length.toLong() }
+        // let's assume it is unlikely that app data and APKs have blobs in common
+        val totalSize = appSize + apkSize
+        log.info { "Got ${appBlobs.size + apkBlobs.size} blobs worth $totalSize bytes to check." }
+
+        // calculate target sizes (how much do we want to check)
+        val targetSize = (totalSize * (percent.toDouble() / 100)).roundToLong()
+        val appTargetSize = min((targetSize * 0.75).roundToLong(), appSize) // 75% of targetSize
+        log.info { "Sampling $targetSize bytes of which $appTargetSize bytes for apps." }
+
+        var currentSize = 0L
+        // check apps first until we reach their target size
+        val appIterator = appBlobs.keys.shuffled().iterator() // random app blob iterator
+        while (currentSize < appTargetSize && appIterator.hasNext()) {
+            val randomChunkId = appIterator.next()
+            val blob = appBlobs[randomChunkId] ?: error("No blob")
+            blobChecker(randomChunkId, blob)
+            currentSize += blob.length
+            // TODO do progress reporting via system notification instead
+            log.info { "  ${((currentSize.toDouble() / targetSize) * 100).roundToInt()}%" }
+        }
+        // now check APKs until we reach total targetSize
+        val apkIterator = apkBlobs.keys.shuffled().iterator() // random APK blob iterator
+        while (currentSize < targetSize && apkIterator.hasNext()) {
+            val randomChunkId = apkIterator.next()
+            val blob = apkBlobs[randomChunkId] ?: error("No blob")
+            blobChecker(randomChunkId, blob)
+            currentSize += blob.length
+            log.info { "  ${((currentSize.toDouble() / targetSize) * 100).roundToInt()}%" }
+        }
+        return currentSize
+    }
 }
diff --git a/app/src/main/java/com/stevesoltys/seedvault/repo/RepoModule.kt b/app/src/main/java/com/stevesoltys/seedvault/repo/RepoModule.kt
index bf1dca9f..e265433f 100644
--- a/app/src/main/java/com/stevesoltys/seedvault/repo/RepoModule.kt
+++ b/app/src/main/java/com/stevesoltys/seedvault/repo/RepoModule.kt
@@ -21,5 +21,5 @@ val repoModule = module {
     }
     factory { SnapshotCreatorFactory(androidContext(), get(), get(), get()) }
     factory { Pruner(get(), get(), get()) }
-    single { Checker(get(), get(), get()) }
+    single { Checker(get(), get(), get(), get()) }
 }