Implement RFC-5869 HMAC-based Extract-and-Expand Key Derivation Function (HKDF)

The HKDF is initialized ("extracted") from a (non-secret) salt and a secret key.
From this, any number of secret keys can be derived ("expanded") deterministically.

When multiple keys are to be derived from the same initial keying/salting material,
each separate key should use a distinct "context". This ensures that there exists
a domain separation between the keys. Using the same context as another key on a
HKDF initialized with the same salt+key results in the exact same derived key
material as that key.

This implementation only offers HMAC-SHA256-based key derivation.

Tested with all HMAC-SHA256 test vectors in RFC-5869, with added edge case tests.

Analogous to BouncyCastle's `HKDFBytesGenerator`, but with a simpler API that
tries to be very explicit in its operation, as well as fully thread safe due to
not storing intermediate calculations in member fields.

1 files changed, 214 insertions, 0 deletions

diff --git a/security-utils/src/test/java/com/yahoo/security/HKDFTest.java b/security-utils/src/test/java/com/yahoo/security/HKDFTest.java
new file mode 100644
index 00000000000..c2d8b99e9a7
--- /dev/null
+++ b/security-utils/src/test/java/com/yahoo/security/HKDFTest.java
@@ -0,0 +1,214 @@
+// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.security;
+
+import org.bouncycastle.util.encoders.Hex;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+/**
+ * HKDF tests that ensure that the output of our own implementation matches the test
+ * vectors given in <a href="https://tools.ietf.org/html/rfc5869">RFC-5869</a>.
+ *
+ * We don't expose the internal PRK (pseudo-random key) value of the HKDF itself,
+ * so we don't test it explicitly. The actual OKM (output keying material) inherently
+ * depends on it, so its correctness is verified transitively.
+ *
+ * @author vekterli
+ */
+public class HKDFTest {
+
+    private static byte[] fromHex(String hex) {
+        return Hex.decode(hex);
+    }
+
+    private static String toHex(byte[] bytes) {
+        return Hex.toHexString(bytes);
+    }
+
+    /*
+       A.1.  Test Case 1
+
+       Basic test case with SHA-256
+
+       Hash = SHA-256
+       IKM  = 0x0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b (22 octets)
+       salt = 0x000102030405060708090a0b0c (13 octets)
+       info = 0xf0f1f2f3f4f5f6f7f8f9 (10 octets)
+       L    = 42
+
+       PRK  = 0x077709362c2e32df0ddc3f0dc47bba63
+              90b6c73bb50f9c3122ec844ad7c2b3e5 (32 octets)
+       OKM  = 0x3cb25f25faacd57a90434f64d0362f2a
+              2d2d0a90cf1a5a4c5db02d56ecc4c5bf
+              34007208d5b887185865 (42 octets)
+     */
+    @Test
+    void rfc_5869_test_vector_case_1() {
+        var ikm  = fromHex("0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b");
+        var salt = fromHex("000102030405060708090a0b0c");
+        var info = fromHex("f0f1f2f3f4f5f6f7f8f9");
+
+        var hkdf = HKDF.extractedFrom(salt, ikm);
+        var okm = hkdf.expand(42, info);
+        assertEquals(toHex(okm),
+                     "3cb25f25faacd57a90434f64d0362f2a" +
+                     "2d2d0a90cf1a5a4c5db02d56ecc4c5bf" +
+                     "34007208d5b887185865");
+    }
+
+    @Test
+    void rfc_5869_test_vector_case_1_block_boundary_edge_cases() {
+        var ikm  = fromHex("0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b");
+        var salt = fromHex("000102030405060708090a0b0c");
+        var info = fromHex("f0f1f2f3f4f5f6f7f8f9");
+
+        var hkdf = HKDF.extractedFrom(salt, ikm);
+        var okm = hkdf.expand(31, info); // One less than block size
+        assertEquals(toHex(okm),
+                     "3cb25f25faacd57a90434f64d0362f2a" +
+                     "2d2d0a90cf1a5a4c5db02d56ecc4c5");
+
+        okm = hkdf.expand(32, info); // Exactly equal to block size
+        assertEquals(toHex(okm),
+                     "3cb25f25faacd57a90434f64d0362f2a" +
+                     "2d2d0a90cf1a5a4c5db02d56ecc4c5bf");
+
+        okm = hkdf.expand(33, info); // One more than block size
+        assertEquals(toHex(okm),
+                     "3cb25f25faacd57a90434f64d0362f2a" +
+                     "2d2d0a90cf1a5a4c5db02d56ecc4c5bf" +
+                     "34");
+    }
+
+    /*
+       A.2.  Test Case 2
+
+       Test with SHA-256 and longer inputs/outputs
+
+       Hash = SHA-256
+       IKM  = 0x000102030405060708090a0b0c0d0e0f
+              101112131415161718191a1b1c1d1e1f
+              202122232425262728292a2b2c2d2e2f
+              303132333435363738393a3b3c3d3e3f
+              404142434445464748494a4b4c4d4e4f (80 octets)
+       salt = 0x606162636465666768696a6b6c6d6e6f
+              707172737475767778797a7b7c7d7e7f
+              808182838485868788898a8b8c8d8e8f
+              909192939495969798999a9b9c9d9e9f
+              a0a1a2a3a4a5a6a7a8a9aaabacadaeaf (80 octets)
+       info = 0xb0b1b2b3b4b5b6b7b8b9babbbcbdbebf
+              c0c1c2c3c4c5c6c7c8c9cacbcccdcecf
+              d0d1d2d3d4d5d6d7d8d9dadbdcdddedf
+              e0e1e2e3e4e5e6e7e8e9eaebecedeeef
+              f0f1f2f3f4f5f6f7f8f9fafbfcfdfeff (80 octets)
+       L    = 82
+
+       PRK  = 0x06a6b88c5853361a06104c9ceb35b45c
+              ef760014904671014a193f40c15fc244 (32 octets)
+       OKM  = 0xb11e398dc80327a1c8e7f78c596a4934
+              4f012eda2d4efad8a050cc4c19afa97c
+              59045a99cac7827271cb41c65e590e09
+              da3275600c2f09b8367793a9aca3db71
+              cc30c58179ec3e87c14c01d5c1f3434f
+              1d87 (82 octets)
+     */
+    @Test
+    void rfc_5869_test_vector_case_2() {
+        var ikm  = fromHex("000102030405060708090a0b0c0d0e0f" +
+                           "101112131415161718191a1b1c1d1e1f" +
+                           "202122232425262728292a2b2c2d2e2f" +
+                           "303132333435363738393a3b3c3d3e3f" +
+                           "404142434445464748494a4b4c4d4e4f");
+        var salt = fromHex("606162636465666768696a6b6c6d6e6f" +
+                           "707172737475767778797a7b7c7d7e7f" +
+                           "808182838485868788898a8b8c8d8e8f" +
+                           "909192939495969798999a9b9c9d9e9f" +
+                           "a0a1a2a3a4a5a6a7a8a9aaabacadaeaf");
+        var info = fromHex("b0b1b2b3b4b5b6b7b8b9babbbcbdbebf" +
+                           "c0c1c2c3c4c5c6c7c8c9cacbcccdcecf" +
+                           "d0d1d2d3d4d5d6d7d8d9dadbdcdddedf" +
+                           "e0e1e2e3e4e5e6e7e8e9eaebecedeeef" +
+                           "f0f1f2f3f4f5f6f7f8f9fafbfcfdfeff");
+
+        var hkdf = HKDF.extractedFrom(salt, ikm);
+        var okm = hkdf.expand(82, info);
+        assertEquals(toHex(okm),
+                     "b11e398dc80327a1c8e7f78c596a4934" +
+                     "4f012eda2d4efad8a050cc4c19afa97c" +
+                     "59045a99cac7827271cb41c65e590e09" +
+                     "da3275600c2f09b8367793a9aca3db71" +
+                     "cc30c58179ec3e87c14c01d5c1f3434f" +
+                     "1d87");
+    }
+
+    /*
+       A.3.  Test Case 3
+
+       Test with SHA-256 and zero-length salt/info
+
+       Hash = SHA-256
+       IKM  = 0x0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b (22 octets)
+       salt = (0 octets)
+       info = (0 octets)
+       L    = 42
+
+       PRK  = 0x19ef24a32c717b167f33a91d6f648bdf
+              96596776afdb6377ac434c1c293ccb04 (32 octets)
+       OKM  = 0x8da4e775a563c18f715f802a063c5a31
+              b8a11f5c5ee1879ec3454e5f3c738d2d
+              9d201395faa4b61a96c8 (42 octets)
+     */
+    @Test
+    void rfc_5869_test_vector_case_3() {
+        var ikm  = fromHex("0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b");
+        var info = new byte[0];
+
+        // We don't allow empty salt to the salted factory function, so this is equivalent.
+        var hkdf = HKDF.unsaltedExtractedFrom(ikm);
+        var okm = hkdf.expand(42, info);
+        var expectedKey = "8da4e775a563c18f715f802a063c5a31" +
+                          "b8a11f5c5ee1879ec3454e5f3c738d2d" +
+                          "9d201395faa4b61a96c8";
+        assertEquals(toHex(okm), expectedKey);
+
+        // expand() without explicit context should return as if an empty context array was passed
+        okm = hkdf.expand(42);
+        assertEquals(toHex(okm), expectedKey);
+    }
+
+    @Test
+    void requested_key_size_is_bounded_and_checked() {
+        var ikm  = fromHex("0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b");
+        var salt = fromHex("000102030405060708090a0b0c");
+        var hkdf = HKDF.extractedFrom(salt, ikm);
+
+        assertThrows(IllegalArgumentException.class, () -> hkdf.expand(-1)); // Can't request negative output size
+
+        assertThrows(IllegalArgumentException.class, () -> hkdf.expand(0)); // Need at least 1 key byte
+
+        assertThrows(IllegalArgumentException.class, () -> hkdf.expand(HKDF.MAX_OUTPUT_SIZE + 1)); // 1 too large
+
+        // However, exactly max should work (though a strange size to request in practice)
+        var okm = hkdf.expand(HKDF.MAX_OUTPUT_SIZE);
+        assertEquals(okm.length, HKDF.MAX_OUTPUT_SIZE);
+    }
+
+    @Test
+    void missing_salt_to_salted_factory_function_throws_exception() {
+        var ikm  = fromHex("0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b");
+        assertThrows(NullPointerException.class, () -> HKDF.extractedFrom(null, ikm));
+        assertThrows(IllegalArgumentException.class, () -> HKDF.extractedFrom(new byte[0], ikm));
+    }
+
+    @Test
+    void ikm_can_not_be_null_or_empty() {
+        var salt = fromHex("000102030405060708090a0b0c");
+        assertThrows(NullPointerException.class, () -> HKDF.extractedFrom(salt, null));
+        assertThrows(IllegalArgumentException.class, () -> HKDF.extractedFrom(salt, new byte[0]));
+        assertThrows(NullPointerException.class, () -> HKDF.unsaltedExtractedFrom(null));
+        assertThrows(IllegalArgumentException.class, () -> HKDF.unsaltedExtractedFrom(new byte[0]));
+    }
+
+}