sch-UID2-5339 salt rotation populate previous salt

src/main/java/com/uid2/admin/secret/SaltRotation.java

@@ -19,6 +19,7 @@
 public class SaltRotation {
     private final IKeyGenerator keyGenerator;
     private final long THIRTY_DAYS_IN_MS = Duration.ofDays(30).toMillis();
+    private final long DAY_IN_MS = Duration.ofDays(1).toMillis();
 
     public SaltRotation(IKeyGenerator keyGenerator) {
         this.keyGenerator = keyGenerator;
@@ -63,14 +64,15 @@ private SaltEntry updateSalt(SaltEntry oldSalt, boolean shouldRotate, long nextE
         var currentSalt = shouldRotate ? this.keyGenerator.generateRandomKeyString(32) : oldSalt.currentSalt();
         var lastUpdated = shouldRotate ? nextEffective : oldSalt.lastUpdated();
         var refreshFrom = calculateRefreshFrom(oldSalt.lastUpdated(), nextEffective);
+        var previousSalt = calculatePreviousSalt(oldSalt, shouldRotate, nextEffective);
 
         return new SaltEntry(
                 oldSalt.id(),
                 oldSalt.hashedId(),
                 lastUpdated,
                 currentSalt,
                 refreshFrom,
-                null,
+                previousSalt,
                 null,
                 null
         );
@@ -82,6 +84,17 @@ private long calculateRefreshFrom(long lastUpdated, long nextEffective) {
         return lastUpdated + (multiplier * THIRTY_DAYS_IN_MS);
     }
 
+    private String calculatePreviousSalt(SaltEntry salt, boolean shouldRotate, long nextEffective) throws Exception {
+        if (shouldRotate) {
+            return salt.currentSalt();
+        }
+        long age = nextEffective - salt.lastUpdated();
+        if ( age / DAY_IN_MS < 90) {
+            return salt.previousSalt();
+        }
+        return null;
+    }
+
     private List<Integer> pickSaltIndexesToRotate(
             SaltSnapshot lastSnapshot,
             Instant nextEffective,

src/test/java/com/uid2/admin/secret/SaltRotationTest.java

@@ -239,4 +239,83 @@ void testRefreshFromCalculation(int lastRotationDaysAgo, int refreshFromDaysFrom
 
         assertThat(actual.refreshFrom()).isEqualTo(expected);
     }
+
+    @Test
+    void rotateSaltsPopulatePreviousSaltsOnRotation() throws Exception {
+        final Duration[] minAges = {
+                Duration.ofDays(90),
+                Duration.ofDays(60),
+                Duration.ofDays(30)
+        };
+
+        var lessThan90Days  = daysEarlier(60).toEpochMilli();
+        var exactly90Days = daysEarlier(90).toEpochMilli();
+        var over90Days = daysEarlier(120).toEpochMilli();
+        var lastSnapshot = SnapshotBuilder.start()
+                .withEntries(
+                    new SaltEntry(1, "1", lessThan90Days, "salt1", null, null, null, null),
+                    new SaltEntry(3, "2", exactly90Days, "salt2", null, null, null, null),
+                    new SaltEntry(5, "3", over90Days, "salt3", null, null, null, null)
+                )
+                .build(daysEarlier(1), daysLater(6));
+
+        var result = saltRotation.rotateSalts(lastSnapshot, minAges, 1, targetDate);
+        assertTrue(result.hasSnapshot());
+
+        var salts = result.getSnapshot().getAllRotatingSalts();
+        assertEquals("salt1", salts[0].previousSalt());
+        assertEquals("salt2", salts[1].previousSalt());
+        assertEquals("salt3", salts[2].previousSalt());
+    }
+
+    @Test
+    void rotateSaltsPreservePreviousSaltsLessThan90DaysOld() throws Exception {
+        final Duration[] minAges = {
+                Duration.ofDays(60),
+        };
+
+        var notValidForRotation1 = daysEarlier(40).toEpochMilli();
+        var notValidForRotation2 = daysEarlier(50).toEpochMilli();
+        var validForRotation = daysEarlier(70);
+        var lastSnapshot = SnapshotBuilder.start()
+                .withEntries(
+                        new SaltEntry(1, "1", notValidForRotation1, "salt1", null, "previousSalt1", null, null),
+                        new SaltEntry(2, "2", notValidForRotation2, "salt2", null, null, null, null)
+                )
+                .withEntries(1, validForRotation)
+                .build(daysEarlier(1), daysLater(6));
+
+        var result = saltRotation.rotateSalts(lastSnapshot, minAges, 1, targetDate);
+        assertTrue(result.hasSnapshot());
+
+        var salts = result.getSnapshot().getAllRotatingSalts();
+        assertEquals("previousSalt1", salts[0].previousSalt());
+        assertNull(salts[1].previousSalt());
+    }
+
+    @Test
+    void rotateSaltsRemovePreviousSaltsOver90DaysOld() throws Exception {
+        final Duration[] minAges = {
+                Duration.ofDays(100),
+        };
+
+        var exactly90Days = daysEarlier(90).toEpochMilli();
+        var over90Days = daysEarlier(100).toEpochMilli();
+        var validForRotation = daysEarlier(120);
+        var lastSnapshot = SnapshotBuilder.start()
+                .withEntries(
+                    new SaltEntry(1, "1", exactly90Days, "salt1", null, "90DaysOld", null, null),
+                    new SaltEntry(2, "2", over90Days, "salt2", null, "over90DaysOld", null, null)
+                )
+                .withEntries(1, validForRotation)
+                .build(daysEarlier(1), daysLater(6));
+
+        var result = saltRotation.rotateSalts(lastSnapshot, minAges, 0.5, targetDate);
+        assertTrue(result.hasSnapshot());
+
+        var salts = result.getSnapshot().getAllRotatingSalts();
+        assertNull(salts[0].previousSalt());
+        assertNull(salts[1].previousSalt());
+    }
+
 }