1 files changed, 43 insertions, 0 deletions
diff --git a/keystore/java/android/security/keystore/KeymasterUtils.java b/keystore/java/android/security/keystore/KeymasterUtils.java
index 92d636c..feafbfa 100644
--- a/keystore/java/android/security/keystore/KeymasterUtils.java
+++ b/keystore/java/android/security/keystore/KeymasterUtils.java
@@ -22,6 +22,8 @@ import android.security.KeyStore;
 import android.security.keymaster.KeymasterArguments;
 import android.security.keymaster.KeymasterDefs;
 
+import java.security.ProviderException;
+
 /**
  * @hide
  */
@@ -133,4 +135,45 @@ public abstract class KeymasterUtils {
                     userAuthenticationValidityDurationSeconds);
         }
     }
+
+    /**
+     * 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;
+        }
+    }
 }