How to regenerate private key using EdDSA/Ed448 signature algorithm?

Question

Here is how I generated the key pair:

var keyPairGenerator = KeyPairGenerator.getInstance("Ed448");
keyPairGenerator.initialize(448, SecureRandom.getInstanceStrong());
var keyPair = keyPairGenerator.generateKeyPair();
var privateKey = keyPair.getPrivate();
var publicKey = keyPair.getPublic();
byte[] privateKeyBytes = privateKey.getEncoded();
byte[] publicKeyBytes = publicKey.getEncoded();

However, when I tried to regenerate the keys again, I got an exception:

var keyFactory = KeyFactory.getInstance("EdDSA");
var edECPoint = byteArrayToEdPoint(publicKeyBytes);
var edECPublicKeySpec = new EdECPublicKeySpec(new NamedParameterSpec("Ed448"), edECPoint);
var publicKey = keyFactory.generatePublic(edECPublicKeySpec);
var edECPrivateKeySpec = new EdECPrivateKeySpec(new NamedParameterSpec("Ed448"), privateKeyBytes);
var privateKey = keyFactory.generatePrivate(edECPrivateKeySpec); // generatePrivate() throws exception

The exception stacktrace:

Exception in thread "main" java.security.spec.InvalidKeySpecException: java.security.InvalidKeyException: key length is 73, key length must be 57
    at jdk.crypto.ec/sun.security.ec.ed.EdDSAKeyFactory.engineGeneratePrivate(EdDSAKeyFactory.java:129)
    at java.base/java.security.KeyFactory.generatePrivate(KeyFactory.java:390)
    at io.fouad.Main.main(Main.java:48)
Caused by: java.security.InvalidKeyException: key length is 73, key length must be 57
    at jdk.crypto.ec/sun.security.ec.ed.EdDSAPrivateKeyImpl.checkLength(EdDSAPrivateKeyImpl.java:84)
    at jdk.crypto.ec/sun.security.ec.ed.EdDSAPrivateKeyImpl.<init>(EdDSAPrivateKeyImpl.java:61)
    at jdk.crypto.ec/sun.security.ec.ed.EdDSAKeyFactory.generatePrivateImpl(EdDSAKeyFactory.java:171)
    at jdk.crypto.ec/sun.security.ec.ed.EdDSAKeyFactory.engineGeneratePrivate(EdDSAKeyFactory.java:127)
    ... 2 more

and here is some utilities I copied from OpenJDK repo:

private static EdECPoint byteArrayToEdPoint(byte[] arr)
{
    byte msb = arr[arr.length - 1];
    boolean xOdd = (msb & 0x80) != 0;
    arr[arr.length - 1] &= (byte) 0x7F;
    reverse(arr);
    BigInteger y = new BigInteger(1, arr);
    return new EdECPoint(xOdd, y);
}

private static void reverse(byte [] arr)
{
    int i = 0;
    int j = arr.length - 1;
    
    while(i < j)
    {
        swap(arr, i, j);
        i++;
        j--;
    }
}

private static void swap(byte[] arr, int i, int j)
{
    byte tmp = arr[i];
    arr[i] = arr[j];
    arr[j] = tmp;
}

Did you try the example of the [JEP-339](https://openjdk.java.net/jeps/339) ticket? — flaxel, Sep 21 '20 at 10:49
@flaxel The example is about regenerating public key only, and with `Ed25519` not `Ed448`. — Eng.Fouad, Sep 21 '20 at 10:51
Use `PKCS8EncodedKeySpec pkcs8EncodedKeySpec = new PKCS8EncodedKeySpec(privateKeyBytes);` and pass it to `keyFactory.generatePrivate` — Michał Krzywański, Sep 21 '20 at 11:05
@michalk `PKCS8EncodedKeySpec` is used to generate RSA private keys. I am not sure if this is the proper way to do it. — Eng.Fouad, Sep 21 '20 at 11:32
PKCS8 is standard that describes the format of private keys. It can represent not only RSA keys... — Michał Krzywański, Sep 21 '20 at 11:39

score 4 · Accepted Answer · edited Oct 07 '21 at 08:58

4

Since privateKey.getEncoded(the instance will be of EdDSAPrivateKeyImpl type) will return a key in PKCS8 format, you should use PKCS8EncodedKeySpec :

var keyFactory = KeyFactory.getInstance("EdDSA");
var pkcs8EncodedKeySpec = new PKCS8EncodedKeySpec(privateKeyBytes);
var privateKey = keyFactory.generatePrivate(pkcs8EncodedKeySpec);

Otherwise you would have to unwrap the the sequence yourself and get PrivateKey (this is OctetString) as described in RFC5958-section2. Then you could use EdECPrivateKeySpec.

edited Oct 07 '21 at 08:58

Community

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answered Sep 21 '20 at 11:35

Michał Krzywański

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+1 Thanks for you help. Actually, I got it working by calling `((EdECPrivateKey) keyPair.getPrivate()).getBytes().orElseThrow()` instead. – Eng.Fouad Sep 21 '20 at 11:56
This will work if you have the instance of the `EdECPrivateKey` already, but if you wanted to reconstruct it for example from a text file which contains PKCS8 format key - you would not be able to do this. – Michał Krzywański Sep 21 '20 at 11:58
`((EdECPrivateKey) keyPair.getPrivate()).getBytes().orElseThrow()` will return a byte array which I then can use it like this: `new EdECPrivateKeySpec(NamedParameterSpec.ED448, byteArray);`. – Eng.Fouad Sep 21 '20 at 12:00
However, `byteArrayToEdPoint()` seems to be not working well, I got `y value is too large` when to do verification :( I will try to look into it. – Eng.Fouad Sep 21 '20 at 12:01
I ended up using `keyFactory.generatePublic(new X509EncodedKeySpec(publicKey.getEncoded()))` and `keyFactory.generatePrivate(new PKCS8EncodedKeySpec(privateKey.getEncoded()))`. Thanks again. – Eng.Fouad Sep 21 '20 at 12:23

score 1 · Answer 2 · answered Sep 06 '22 at 14:10

Method #1:

var keyPairGenerator = KeyPairGenerator.getInstance("Ed25519");
var keyPair = keyPairGenerator.generateKeyPair();
var privateKey = (EdECPrivateKey) keyPair.getPrivate();
var publicKey = (EdECPublicKey) keyPair.getPublic();
byte[] privateKeyBytes = privateKey.getBytes().orElseThrow();
var edecPoint = publicKey.getPoint();
BigInteger y = edecPoint.getY();
boolean xOdd = edecPoint.isXOdd();

// reconstruct the keys using (privateKeyBytes), (y) and (xOdd)

var keyFactory = KeyFactory.getInstance("Ed25519");
var edECPublicKeySpec = new EdECPublicKeySpec(NamedParameterSpec.ED25519, new EdECPoint(xOdd, y));
var publicKey2 = keyFactory.generatePublic(edECPublicKeySpec);
var edECPrivateKeySpec = new EdECPrivateKeySpec(NamedParameterSpec.ED25519, privateKeyBytes);
var privateKey2 = keyFactory.generatePrivate(edECPrivateKeySpec);

Method #2:

var keyPairGenerator = KeyPairGenerator.getInstance("Ed25519");
var keyPair = keyPairGenerator.generateKeyPair();
var privateKey = keyPair.getPrivate();
var publicKey = keyPair.getPublic();
byte[] privateKeyBytes = privateKey.getEncoded();
byte[] publicKeyBytes = publicKey.getEncoded();

// reconstruct the keys using (privateKeyBytes) and (publicKeyBytes)

var keyFactory = KeyFactory.getInstance("Ed25519");
var publicKey2 = keyFactory.generatePublic(new X509EncodedKeySpec(publicKeyBytes));
var privateKey2 = keyFactory.generatePrivate(new PKCS8EncodedKeySpec(privateKeyBytes));

How to regenerate private key using EdDSA/Ed448 signature algorithm?

2 Answers2