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Question

I tried using the secure key import feature of AndroidKeyStore, following the example at:

https://android.googlesource.com/platform/cts/+/master/tests/tests/keystore/src/android/keystore/cts/ImportWrappedKeyTest.java

When I want to use the imported private key afterwards, calling androidKeyStore.getKey(importedKeyAlias, null), I get the following error:

java.security.UnrecoverableKeyException: Failed to obtain X.509 form of public key. Keystore has no public certificate stored.

I tried manually adding a certificate using androidKeyStore.setCertificateEntry(importedKeyAlias, generateCertificate(...)), but this call results in the error message

java.security.KeyStoreException: Entry exists and is not a trusted certificate.

I am executing the code on a Samsung Galaxy A34 5G, hence missing the StrongBox hardware, but according to my understanding this should still work with isStrongBox = false.

Question: How can I use the imported key?

Maybe there is an error in my code, so here is a MWE of the first approach, without trying to manually add a certificate.

fun minimumWorkingExample(isStrongBox: Boolean) {
    // primitives
    val wrappingKeyAlias = "wrappingKey"
    val importedKeyAlias = "importedKey"

    val importedKeySizeInBits = 2048
    val symmetricWrappingKeyTransformation = "RSA/ECB/OAEPPadding"

    // provisioning
    val androidKeyStoreProviderName = "AndroidKeyStore"
    val androidKeyStore =
        KeyStore.getInstance(androidKeyStoreProviderName).apply { load(null, null) }

    val challenge = Random.nextBytes(256 / 8)

    val wrappingKeyPairSpec = wrappingKeyPairSpecification(
        alias = wrappingKeyAlias,
        isStrongBoxBacked = isStrongBox,
        challenge = challenge,
    )

    KeyPairGenerator.getInstance(KeyProperties.KEY_ALGORITHM_RSA).apply {
        initialize(wrappingKeyPairSpec)
    }.generateKeyPair()

    val keyPairToBeImported =
        KeyPairGenerator.getInstance(KeyProperties.KEY_ALGORITHM_RSA).apply {
            initialize(importedKeySizeInBits, SecureRandom())
        }.generateKeyPair()

    // looking at data https://8gwifi.org/PemParserFunctions.jsp
    System.out.println("private key format: ${keyPairToBeImported.private.format}")
    System.out.println(
        "private key data: ${
            android.util.Base64.encodeToString(
                keyPairToBeImported.private.encoded, android.util.Base64.DEFAULT
            )
        }"
    )

    val wrappedPrivateKey = wrapKey(
        // as in the example source code
        androidKeyStore.getCertificateChain(wrappingKeyAlias).first().publicKey,
        keyPairToBeImported.private.encoded,
        keyFormat = 1, // represents "PKCS8" according to https://android.googlesource.com/platform/hardware/interfaces/+/master/keymaster/4.0/types.hal
        transformation = symmetricWrappingKeyTransformation,
        wrappedKeyAuthorizationList(importedKeySizeInBits), // similar to the the example source code
    )

    val wrappedKeyEntry: KeyStore.Entry = WrappedKeyEntry(
        wrappedPrivateKey,
        wrappingKeyAlias,
        symmetricWrappingKeyTransformation,
        wrappingKeyPairSpec,
    )

    androidKeyStore.setEntry(importedKeyAlias, wrappedKeyEntry, null);
    // androidKeyStore.setCertificateEntry(keyAlias, generateCertificate(wrappedKeyPair.publicKey, somePrivateKeyThatIsNotImportant))
    System.out.println("Key available: ${androidKeyStore.containsAlias(importedKeyAlias)}")

    // load
    val importedKey = androidKeyStore.getEntry(importedKeyAlias, null)
    System.out.println("isPrivateKey: ${importedKey is PrivateKey}")
}

fun wrappingKeyPairSpecification(
    alias: String,
    isStrongBoxBacked: Boolean,
    challenge: ByteArray,
) = KeyGenParameterSpec.Builder(alias, KeyProperties.PURPOSE_WRAP_KEY)
    .setDigests(KeyProperties.DIGEST_SHA256)
    .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_RSA_OAEP)
    .setBlockModes(KeyProperties.BLOCK_MODE_ECB).setIsStrongBoxBacked(isStrongBoxBacked)
    .setAttestationChallenge(challenge)
    .build()

@Throws(Exception::class)
fun wrapKey(
    publicKey: PublicKey?,
    keyMaterial: ByteArray?,
    keyFormat: Long,
    transformation: String,
    authorizationList: DERSequence
): ByteArray {
    return wrapKey(
        publicKey,
        keyMaterial,
        keyFormat,
        authorizationList,
        transformation,
        true,
    )
}

@Throws(Exception::class)
fun wrapKey(
    publicKey: PublicKey?,
    keyMaterial: ByteArray?,
    keyFormat: Long,
    authorizationList: DERSequence,
    transformation: String,
    correctWrappingRequired: Boolean, // this should be true, but let's keep this for testing purposes
): ByteArray {
    // Build description
    val descriptionItems: ASN1EncodableVector = ASN1EncodableVector()
    descriptionItems.add(ASN1Integer(keyFormat))
    descriptionItems.add(authorizationList)
    val wrappedKeyDescription: DERSequence = DERSequence(descriptionItems)

    // Generate 12 byte initialization vector
    val iv = ByteArray(12)
    // TODO: use this for production code: SecureRandom.getInstanceStrong().nextBytes(iv)
    Random.nextBytes(iv)
    // Generate 256 bit AES key. This is the ephemeral key used to encrypt the secure key.
    val aesKeyBytes = ByteArray(32)
    // TODO: use this for production code: SecureRandom.getInstanceStrong().nextBytes(aesKeyBytes)
    Random.nextBytes(aesKeyBytes)

    // Encrypt ephemeral keys
    val spec = OAEPParameterSpec(
        "SHA-256", "MGF1", MGF1ParameterSpec.SHA1, PSource.PSpecified.DEFAULT
    )
    val pkCipher = Cipher.getInstance(transformation)
    if (correctWrappingRequired) {
        pkCipher.init(Cipher.ENCRYPT_MODE, publicKey, spec)
    } else {
        // Use incorrect OAEPParameters while initializing cipher. By default, main digest and
        // MGF1 digest are SHA-1 here.
        pkCipher.init(Cipher.ENCRYPT_MODE, publicKey)
    }
    val encryptedEphemeralKeys = pkCipher.doFinal(aesKeyBytes)

    // Encrypt secure key
    val cipher = Cipher.getInstance("AES/GCM/NoPadding")
    val secretKeySpec = SecretKeySpec(aesKeyBytes, "AES")
    val gcmParameterSpec = GCMParameterSpec(GCM_TAG_SIZE, iv)
    cipher.init(Cipher.ENCRYPT_MODE, secretKeySpec, gcmParameterSpec)
    val aad: ByteArray = wrappedKeyDescription.getEncoded()
    cipher.updateAAD(aad)
    var encryptedSecureKey = cipher.doFinal(keyMaterial)
    // Get GCM tag. Java puts the tag at the end of the ciphertext data :(
    val len = encryptedSecureKey.size
    val tagSize: Int = (GCM_TAG_SIZE / 8)
    val tag = Arrays.copyOfRange(encryptedSecureKey, len - tagSize, len)
    // Remove GCM tag from end of output
    encryptedSecureKey = Arrays.copyOfRange(encryptedSecureKey, 0, len - tagSize)

    // Build ASN.1 DER encoded sequence WrappedKeyWrapper
    val items: ASN1EncodableVector = ASN1EncodableVector()
    items.add(ASN1Integer(WRAPPED_FORMAT_VERSION))
    items.add(DEROctetString(encryptedEphemeralKeys))
    items.add(DEROctetString(iv))
    items.add(wrappedKeyDescription)
    items.add(DEROctetString(encryptedSecureKey))
    items.add(DEROctetString(tag))
    return DERSequence(items).getEncoded(ASN1Encoding.DER)
}


private fun removeTagType(tag: Int): Int {
    val kmTagTypeMask = 0x0FFFFFFF
    return tag and kmTagTypeMask
}

private fun wrappedKeyAuthorizationList(size: Int): DERSequence {
    // https://source.android.com/docs/security/features/keystore/tags?hl=de
    val algorithmRsaIdentifier = 1L
    val paddingNoneIdentifier = 1L

    val allPurposes = ASN1EncodableVector()
    allPurposes.add(ASN1Integer(KeyProperties.PURPOSE_SIGN.toLong()))
    val purposeSet: DERSet = DERSet(allPurposes)
    val purpose = DERTaggedObject(true, removeTagType(KM_TAG_PURPOSE), purposeSet)
    val algorithm = DERTaggedObject(
        true,
        removeTagType(KM_TAG_ALGORITHM),
        // see https://source.android.com/docs/security/features/keystore/tags?hl=de
        ASN1Integer(algorithmRsaIdentifier),
    )
    val keySize =
        DERTaggedObject(true, removeTagType(KM_TAG_KEY_SIZE), ASN1Integer(size.toLong()))
    val allDigests: ASN1EncodableVector = ASN1EncodableVector()
    val digestSet: DERSet = DERSet(allDigests)
    val digest = DERTaggedObject(true, removeTagType(KM_TAG_DIGEST), digestSet)
    val allPaddings: ASN1EncodableVector = ASN1EncodableVector()
    allPaddings.add(ASN1Integer(paddingNoneIdentifier)) // see https://source.android.com/docs/security/features/keystore/tags?hl=de
    val paddingSet: DERSet = DERSet(allPaddings)
    val padding = DERTaggedObject(true, removeTagType(KM_TAG_PADDING), paddingSet)
    val noAuthRequired = // TODO: add user authentication
        DERTaggedObject(true, removeTagType(KM_TAG_NO_AUTH_REQUIRED), DERNull.INSTANCE)
    // Build sequence
    val allItems: ASN1EncodableVector = ASN1EncodableVector()
    allItems.add(purpose)
    allItems.add(algorithm)
    allItems.add(keySize)
    allItems.add(digest)
    allItems.add(padding)
    allItems.add(noAuthRequired)
    return DERSequence(allItems)
}

Relevant dependencies

// https://mvnrepository.com/artifact/org.bouncycastle/bcprov-jdk18on
implementation("org.bouncycastle:bcprov-jdk18on:1.78.1")

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Danh mục

AndroidKeyStore – SecureKeyImport: How to use the imported key? (Receiving errors on calling `keyStore.getEntry`)