test(applet): pin deriveStepUpSessionKeys to spec ASCII info strings

Adds a second regression test for AliroCrypto.deriveStepUpSessionKeys
that hard-codes the exact RFC 5869 reference computation (empty salt,
"SKDevice" / "SKReader" ASCII info) with a different IKM byte pattern
and independent expand-once helper. Complements the existing
deriveStepUpSessionKeysMatchesManualHkdf so a future spec misread in
shared helper code can't slip through.

Implementation itself was landed earlier in f94e416 ("fix: AUTH1 crypto
interop with stock X-CUBE-ALIRO + EXCHANGE compat stub"); this is the
M1A.1 task's tightened pin from the Step-Up v2 plan.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

applet/src/test/java/com/dangerousthings/aliro/AliroCryptoTest.java

@@ -167,6 +167,50 @@ class AliroCryptoTest {
         }
     }
 
+    /**
+     * Independent pin against an inline RFC 5869 reference (one HKDF-Expand
+     * block: T(1) = HMAC(PRK, info || 0x01)). Complements
+     * {@link #deriveStepUpSessionKeysMatchesManualHkdf} by anchoring on the
+     * exact ASCII info strings the spec specifies ("SKDevice" / "SKReader")
+     * with a different IKM byte pattern, so a future spec misread can't be
+     * masked by sharing helper code with the equivalent test above.
+     */
+    @Test
+    void deriveStepUpSessionKeysMatchesRfc5869WithSpecInfoStrings() throws Exception {
+        AliroCrypto crypto = new AliroCrypto();
+        byte[] stepUpSK = new byte[32];
+        for (byte i = 0; i < 32; i++) stepUpSK[i] = (byte) (0x10 + i);
+
+        byte[] skDevice = new byte[32];
+        byte[] skReader = new byte[32];
+        crypto.deriveStepUpSessionKeys(
+            stepUpSK, (short) 0,
+            skDevice, (short) 0,
+            skReader, (short) 0);
+
+        // Reference HKDF-SHA-256 with empty salt
+        javax.crypto.Mac extractMac = javax.crypto.Mac.getInstance("HmacSHA256");
+        extractMac.init(new javax.crypto.spec.SecretKeySpec(new byte[32], "HmacSHA256"));
+        byte[] prk = extractMac.doFinal(stepUpSK);
+
+        byte[] expectedDevice = expandOnce(prk, "SKDevice".getBytes("US-ASCII"));
+        byte[] expectedReader = expandOnce(prk, "SKReader".getBytes("US-ASCII"));
+
+        assertArrayEquals(expectedDevice, skDevice,
+            "StepUpSKDevice = HKDF-Expand(PRK, info=ASCII(\"SKDevice\"), L=32)");
+        assertArrayEquals(expectedReader, skReader,
+            "StepUpSKReader = HKDF-Expand(PRK, info=ASCII(\"SKReader\"), L=32)");
+    }
+
+    private static byte[] expandOnce(byte[] prk, byte[] info) throws Exception {
+        // T(1) = HMAC-SHA-256(PRK, info || 0x01); first 32 bytes is the output
+        javax.crypto.Mac mac = javax.crypto.Mac.getInstance("HmacSHA256");
+        mac.init(new javax.crypto.spec.SecretKeySpec(prk, "HmacSHA256"));
+        mac.update(info);
+        mac.update((byte) 0x01);
+        return mac.doFinal();
+    }
+
     @Test
     void hkdfExpandRfc5869TestCase1() {
         AliroCrypto crypto = new AliroCrypto();