android/security/keystore2/KeymasterUtils.java

/*
 * Copyright (C) 2021 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.security.keystore2;

import android.security.keymaster.KeymasterArguments;
import android.security.keymaster.KeymasterDefs;
import android.security.keystore.KeyProperties;

import java.security.ProviderException;

/**
 * @hide
 */
public abstract class KeymasterUtils {

    private KeymasterUtils() {}

    /** @hide */
    static int getDigestOutputSizeBits(int keymasterDigest) {
        switch (keymasterDigest) {
            case KeymasterDefs.KM_DIGEST_NONE:
                return -1;
            case KeymasterDefs.KM_DIGEST_MD5:
                return 128;
            case KeymasterDefs.KM_DIGEST_SHA1:
                return 160;
            case KeymasterDefs.KM_DIGEST_SHA_2_224:
                return 224;
            case KeymasterDefs.KM_DIGEST_SHA_2_256:
                return 256;
            case KeymasterDefs.KM_DIGEST_SHA_2_384:
                return 384;
            case KeymasterDefs.KM_DIGEST_SHA_2_512:
                return 512;
            default:
                throw new IllegalArgumentException("Unknown digest: " + keymasterDigest);
        }
    }

    /** @hide */
    static boolean isKeymasterBlockModeIndCpaCompatibleWithSymmetricCrypto(
            int keymasterBlockMode) {
        switch (keymasterBlockMode) {
            case KeymasterDefs.KM_MODE_ECB:
                return false;
            case KeymasterDefs.KM_MODE_CBC:
            case KeymasterDefs.KM_MODE_CTR:
            case KeymasterDefs.KM_MODE_GCM:
                return true;
            default:
                throw new IllegalArgumentException("Unsupported block mode: " + keymasterBlockMode);
        }
    }

    /** @hide */
    static boolean isKeymasterPaddingSchemeIndCpaCompatibleWithAsymmetricCrypto(
            int keymasterPadding) {
        switch (keymasterPadding) {
            case KeymasterDefs.KM_PAD_NONE:
                return false;
            case KeymasterDefs.KM_PAD_RSA_OAEP:
            case KeymasterDefs.KM_PAD_RSA_PKCS1_1_5_ENCRYPT:
                return true;
            default:
                throw new IllegalArgumentException(
                        "Unsupported asymmetric encryption padding scheme: " + keymasterPadding);
        }
    }

    /**
     * Adds {@code KM_TAG_MIN_MAC_LENGTH} tag, if necessary, to the keymaster arguments for
     * generating or importing a key. This tag may only be needed for symmetric keys (e.g., HMAC,
     * AES-GCM).
     */
    public static void addMinMacLengthAuthorizationIfNecessary(KeymasterArguments args,
            int keymasterAlgorithm,
            int[] keymasterBlockModes,
            int[] keymasterDigests) {
        switch (keymasterAlgorithm) {
            case KeymasterDefs.KM_ALGORITHM_AES:
                if (com.android.internal.util.ArrayUtils.contains(
                        keymasterBlockModes, KeymasterDefs.KM_MODE_GCM)) {
                    // AES GCM key needs the minimum length of AEAD tag specified.
                    args.addUnsignedInt(KeymasterDefs.KM_TAG_MIN_MAC_LENGTH,
                            AndroidKeyStoreAuthenticatedAESCipherSpi.GCM
                                    .MIN_SUPPORTED_TAG_LENGTH_BITS);
                }
                break;
            case KeymasterDefs.KM_ALGORITHM_HMAC:
                // HMAC key needs the minimum length of MAC set to the output size of the associated
                // digest. This is because we do not offer a way to generate shorter MACs and
                // don't offer a way to verify MACs (other than by generating them).
                if (keymasterDigests.length != 1) {
                    throw new ProviderException(
                            "Unsupported number of authorized digests for HMAC key: "
                                    + keymasterDigests.length
                                    + ". Exactly one digest must be authorized");
                }
                int keymasterDigest = keymasterDigests[0];
                int digestOutputSizeBits = getDigestOutputSizeBits(keymasterDigest);
                if (digestOutputSizeBits == -1) {
                    throw new ProviderException(
                            "HMAC key authorized for unsupported digest: "
                                    + KeyProperties.Digest.fromKeymaster(keymasterDigest));
                }
                args.addUnsignedInt(KeymasterDefs.KM_TAG_MIN_MAC_LENGTH, digestOutputSizeBits);
                break;
        }
    }
}