Add support for token resealing

Adds underlying support--and tooling--for resealing a token for
another recipient. This allows for delegating decryption to another
party without having to reveal the private key of the original
recipient (or having to send the raw underlying secret key over a
potentially insecure channel). Key ID can/should change as part of
this operation.

1 files changed, 12 insertions, 4 deletions

diff --git a/security-utils/src/main/java/com/yahoo/security/SharedKeyGenerator.java b/security-utils/src/main/java/com/yahoo/security/SharedKeyGenerator.java
index 8a1a7dd3688..66a87a94707 100644
--- a/security-utils/src/main/java/com/yahoo/security/SharedKeyGenerator.java
+++ b/security-utils/src/main/java/com/yahoo/security/SharedKeyGenerator.java
@@ -62,10 +62,7 @@ public class SharedKeyGenerator {
 
     public static SecretSharedKey generateForReceiverPublicKey(PublicKey receiverPublicKey, KeyId keyId) {
         var secretKey = generateRandomSecretAesKey();
-        // We protect the integrity of the key ID by passing it as AAD.
-        var sealed = HPKE.sealBase((XECPublicKey) receiverPublicKey, EMPTY_BYTES, keyId.asBytes(), secretKey.getEncoded());
-        var sealedSharedKey = new SealedSharedKey(keyId, sealed.enc(), sealed.ciphertext());
-        return new SecretSharedKey(secretKey, sealedSharedKey);
+        return internalSealSecretKeyForReceiver(secretKey, receiverPublicKey, keyId);
     }
 
     public static SecretSharedKey fromSealedKey(SealedSharedKey sealedKey, PrivateKey receiverPrivateKey) {
@@ -74,6 +71,17 @@ public class SharedKeyGenerator {
         return new SecretSharedKey(new SecretKeySpec(secretKeyBytes, "AES"), sealedKey);
     }
 
+    public static SecretSharedKey reseal(SecretSharedKey secret, PublicKey receiverPublicKey, KeyId keyId) {
+        return internalSealSecretKeyForReceiver(secret.secretKey(), receiverPublicKey, keyId);
+    }
+
+    private static SecretSharedKey internalSealSecretKeyForReceiver(SecretKey secretKey, PublicKey receiverPublicKey, KeyId keyId) {
+        // We protect the integrity of the key ID by passing it as AAD.
+        var sealed = HPKE.sealBase((XECPublicKey) receiverPublicKey, EMPTY_BYTES, keyId.asBytes(), secretKey.getEncoded());
+        var sealedSharedKey = new SealedSharedKey(keyId, sealed.enc(), sealed.ciphertext());
+        return new SecretSharedKey(secretKey, sealedSharedKey);
+    }
+
     // A given key+IV pair can only be used for one single encryption session, ever.
     // Since our keys are intended to be inherently single-use, we can satisfy that
     // requirement even with a fixed IV. This avoids the need for explicitly including