public static KeyPair newKeyPairFromSeed(byte[] secretSeed) throws GeneralSecurityException {
      if (secretSeed.length != Field25519.FIELD_LEN) {
        throw new IllegalArgumentException(
            String.format("Given secret seed length is not %s", Field25519.FIELD_LEN));
      }
      byte[] privateKey = secretSeed;
      byte[] publicKey = Ed25519.scalarMultWithBaseToBytes(Ed25519.getHashedScalar(privateKey));
      return new KeyPair(publicKey, privateKey);
    }
Returns a new <publicKey, privateKey> KeyPair generated from a seed.
java_src/src/main/java/com/google/crypto/tink/subtle/Ed25519Sign.java