Updated on 2026-08-14

app/src/main/java/org/stellar/sdk/SLIP10.java

@@ -0,0 +1,65 @@
+package org.stellar.sdk;
+
+import javax.crypto.Mac;
+import javax.crypto.ShortBufferException;
+import javax.crypto.spec.SecretKeySpec;
+import java.nio.charset.Charset;
+import java.security.InvalidKeyException;
+import java.security.NoSuchAlgorithmException;
+
+final class SLIP10 {
+
+  private SLIP10() {
+  }
+
+  private static final String hmacSHA512algorithm = "HmacSHA512";
+
+  /**
+   * Derives only the private key for ED25519 in the manor defined in
+   * <a href="https://github.com/satoshilabs/slips/blob/master/slip-0010.md">SLIP-0010</a>.
+   *
+   * @param seed    Seed, the BIP0039 output.
+   * @param indexes an array of indexes that define the path. E.g. for m/1'/2'/3', pass 1, 2, 3.
+   *                As with Ed25519 non-hardened child indexes are not supported, this function treats all indexes
+   *                as hardened.
+   * @return Private key.
+   * @throws NoSuchAlgorithmException If it cannot find the HmacSHA512 algorithm by name.
+   * @throws ShortBufferException     Occurrence not expected.
+   * @throws InvalidKeyException      Occurrence not expected.
+   */
+  static byte[] deriveEd25519PrivateKey(final byte[] seed, final int... indexes)
+          throws NoSuchAlgorithmException, ShortBufferException, InvalidKeyException {
+
+    final byte[] I = new byte[64];
+    final Mac mac = Mac.getInstance(hmacSHA512algorithm);
+
+    // I = HMAC-SHA512(Key = bytes("ed25519 seed"), Data = seed)
+    mac.init(new SecretKeySpec("ed25519 seed".getBytes(Charset.forName("UTF-8")), hmacSHA512algorithm));
+    mac.update(seed);
+    mac.doFinal(I, 0);
+
+    for (int i : indexes) {
+      // I = HMAC-SHA512(Key = c_par, Data = 0x00 || ser256(k_par) || ser32(i'))
+      // which is simply:
+      // I = HMAC-SHA512(Key = Ir, Data = 0x00 || Il || ser32(i'))
+      // Key = Ir
+      mac.init(new SecretKeySpec(I, 32, 32, hmacSHA512algorithm));
+      // Data = 0x00
+      mac.update((byte) 0x00);
+      // Data += Il
+      mac.update(I, 0, 32);
+      // Data += ser32(i')
+      mac.update((byte) (i >> 24 | 0x80));
+      mac.update((byte) (i >> 16));
+      mac.update((byte) (i >> 8));
+      mac.update((byte) i);
+      // Write to I
+      mac.doFinal(I, 0);
+    }
+
+    final byte[] Il = new byte[32];
+    // copy head 32 bytes of I into Il
+    System.arraycopy(I, 0, Il, 0, 32);
+    return Il;
+  }
+}