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/*
* Copyright (C) 2022 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.annotation.NonNull;
import android.security.KeyStoreSecurityLevel;
import android.system.keystore2.KeyDescriptor;
import android.system.keystore2.KeyMetadata;
import java.math.BigInteger;
import java.security.interfaces.EdECPublicKey;
import java.security.spec.EdECPoint;
import java.security.spec.NamedParameterSpec;
import java.util.Arrays;
import java.util.Objects;
/**
* {@link EdECPublicKey} backed by keystore.
*
* @hide
*/
public class AndroidKeyStoreEdECPublicKey extends AndroidKeyStorePublicKey
implements EdECPublicKey {
/**
* DER sequence, as defined in https://datatracker.ietf.org/doc/html/rfc8410#section-4 and
* https://datatracker.ietf.org/doc/html/rfc5280#section-4.1.
* SEQUENCE (2 elem)
* SEQUENCE (1 elem)
* OBJECT IDENTIFIER 1.3.101.112 curveEd25519 (EdDSA 25519 signature algorithm)
* as defined in https://datatracker.ietf.org/doc/html/rfc8410#section-3
* BIT STRING (256 bit) as defined in
* https://datatracker.ietf.org/doc/html/rfc8032#section-5.1.2
*/
private static final byte[] DER_KEY_PREFIX = new byte[] {
0x30,
0x2a,
0x30,
0x05,
0x06,
0x03,
0x2b,
0x65,
0x70,
0x03,
0x21,
0x00,
};
private static final int ED25519_KEY_SIZE_BYTES = 32;
private byte[] mEncodedKey;
private EdECPoint mPoint;
public AndroidKeyStoreEdECPublicKey(
@NonNull KeyDescriptor descriptor,
@NonNull KeyMetadata metadata,
@NonNull String algorithm,
@NonNull KeyStoreSecurityLevel iSecurityLevel,
@NonNull byte[] encodedKey) {
super(descriptor, metadata, encodedKey, algorithm, iSecurityLevel);
mEncodedKey = encodedKey;
int preambleLength = matchesPreamble(DER_KEY_PREFIX, encodedKey);
if (preambleLength == 0) {
throw new IllegalArgumentException("Key size is not correct size");
}
mPoint = pointFromKeyByteArray(
Arrays.copyOfRange(encodedKey, preambleLength, encodedKey.length));
}
@Override
AndroidKeyStorePrivateKey getPrivateKey() {
return new AndroidKeyStoreEdECPrivateKey(
getUserKeyDescriptor(),
getKeyIdDescriptor().nspace,
getAuthorizations(),
"EdDSA",
getSecurityLevel());
}
@Override
public NamedParameterSpec getParams() {
return NamedParameterSpec.ED25519;
}
@Override
public EdECPoint getPoint() {
return mPoint;
}
private static int matchesPreamble(byte[] preamble, byte[] encoded) {
if (encoded.length != (preamble.length + ED25519_KEY_SIZE_BYTES)) {
return 0;
}
if (Arrays.compare(preamble, Arrays.copyOf(encoded, preamble.length)) != 0) {
return 0;
}
return preamble.length;
}
private static EdECPoint pointFromKeyByteArray(byte[] coordinates) {
Objects.requireNonNull(coordinates);
// Oddity of the key is the most-significant bit of the last byte.
boolean isOdd = (0x80 & coordinates[coordinates.length - 1]) != 0;
// Zero out the oddity bit.
coordinates[coordinates.length - 1] &= (byte) 0x7f;
// Representation of Y is in little-endian, according to rfc8032 section-3.1.
reverse(coordinates);
// The integer representing Y starts from the first bit in the coordinates array.
BigInteger y = new BigInteger(1, coordinates);
return new EdECPoint(isOdd, y);
}
private static void reverse(byte[] coordinateArray) {
int start = 0;
int end = coordinateArray.length - 1;
while (start < end) {
byte tmp = coordinateArray[start];
coordinateArray[start] = coordinateArray[end];
coordinateArray[end] = tmp;
start++;
end--;
}
}
@Override
public byte[] getEncoded() {
return mEncodedKey.clone();
}
}
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