Add support for exporting symmetric keys from connections

This is currently only supported for TLS 1.3 and is standard compliant, using the TLS-Exporter function defined by RFC 8446: https://www.rfc-editor.org/rfc/rfc8446#section-7.5
aws · Oct 4, 2023 · e6c57d8 · e6c57d8
1 parent ab7bfce
commit e6c57d8
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Showing 15 changed files with 275 additions and 12 deletions.
diff --git a/api/s2n.h b/api/s2n.h
@@ -2870,6 +2870,20 @@ S2N_API extern int s2n_connection_client_cert_used(struct s2n_connection *conn);
  */
 S2N_API extern const char *s2n_connection_get_cipher(struct s2n_connection *conn);
 
+/**
+ * Provides access to the TLS-Exporter functionality.
+ *
+ * See https://datatracker.ietf.org/doc/html/rfc5705 and https://www.rfc-editor.org/rfc/rfc8446.
+ *
+ * @note This is currently only available with TLS 1.3 connections which have finished a handshake.
+ *
+ * @param conn A pointer to the connection
+ * @returns A POSIX error signal. If an error was returned, the value contained in `output` should be considered invalid.
+ */
+S2N_API extern int s2n_connection_tls_exporter(struct s2n_connection *conn,
+        const uint8_t *label, uint32_t label_length, const uint8_t *context, uint32_t context_length,
+        uint8_t *output, uint32_t output_length);
+
 /**
  * Returns the IANA value for the connection's negotiated cipher suite.
  *

diff --git a/bindings/rust/s2n-tls/src/connection.rs b/bindings/rust/s2n-tls/src/connection.rs
@@ -809,6 +809,32 @@ impl Connection {
             hash_alg => Some(hash_alg.try_into()?),
         })
     }
+
+    /// Provides access to the TLS-Exporter functionality.
+    ///
+    /// See https://datatracker.ietf.org/doc/html/rfc5705 and https://www.rfc-editor.org/rfc/rfc8446.
+    ///
+    /// This is currently only available with TLS 1.3 connections which have finished a handshake.
+    pub fn tls_exporter(
+        &self,
+        label: &[u8],
+        context: &[u8],
+        output: &mut [u8],
+    ) -> Result<(), Error> {
+        unsafe {
+            s2n_connection_tls_exporter(
+                self.connection.as_ptr(),
+                label.as_ptr(),
+                label.len().try_into().map_err(|_| Error::INVALID_INPUT)?,
+                context.as_ptr(),
+                context.len().try_into().map_err(|_| Error::INVALID_INPUT)?,
+                output.as_mut_ptr(),
+                output.len().try_into().map_err(|_| Error::INVALID_INPUT)?,
+            )
+            .into_result()
+            .map(|_| ())
+        }
+    }
 }
 
 struct Context {

diff --git a/crypto/s2n_hkdf.c b/crypto/s2n_hkdf.c
@@ -257,10 +257,7 @@ int s2n_hkdf_expand_label(struct s2n_hmac_state *hmac, s2n_hmac_algorithm alg, c
     struct s2n_blob hkdf_label_blob = { 0 };
     struct s2n_stuffer hkdf_label = { 0 };
 
-    /* RFC8446 specifies that labels must be 12 characters or less, to avoid
-    ** incurring two hash rounds.
-    */
-    POSIX_ENSURE_LTE(label->size, 12);
+    POSIX_ENSURE_LTE(label->size, S2N_MAX_HKDF_EXPAND_LABEL_LENGTH);
 
     POSIX_GUARD(s2n_blob_init(&hkdf_label_blob, hkdf_label_buf, sizeof(hkdf_label_buf)));
     POSIX_GUARD(s2n_stuffer_init(&hkdf_label, &hkdf_label_blob));

diff --git a/crypto/s2n_hkdf.h b/crypto/s2n_hkdf.h
@@ -20,6 +20,16 @@
 #include "crypto/s2n_hmac.h"
 #include "utils/s2n_blob.h"
 
+/*
+ * Label structure is `opaque label<7..255> = "tls13 " + Label` per RFC8446.
+ * So, we have 255-sizeof("tls13 ") = 249, the maximum label length.
+ *
+ * Note that all labels defined by RFC 8446 are <12 characters, which
+ * avoids an extra hash iteration. However, the exporter functionality
+ * (s2n_connection_tls_exporter) allows for longer labels.
+ */
+#define S2N_MAX_HKDF_EXPAND_LABEL_LENGTH 249
+
 int s2n_hkdf(struct s2n_hmac_state *hmac, s2n_hmac_algorithm alg, const struct s2n_blob *salt,
         const struct s2n_blob *key, const struct s2n_blob *info, struct s2n_blob *output);
 

diff --git a/crypto/s2n_tls13_keys.c b/crypto/s2n_tls13_keys.c
@@ -37,7 +37,7 @@
  * [x] server_handshake_traffic_secret
  * [x] client_application_traffic_secret_0
  * [x] server_application_traffic_secret_0
- * [ ] exporter_master_secret
+ * [x] exporter_master_secret
  * [x] resumption_master_secret
  *
  * The TLS 1.3 key generation can be divided into 3 phases
@@ -79,6 +79,11 @@ S2N_BLOB_LABEL(s2n_tls13_label_session_ticket_secret, "resumption")
 S2N_BLOB_LABEL(s2n_tls13_label_traffic_secret_key, "key")
 S2N_BLOB_LABEL(s2n_tls13_label_traffic_secret_iv, "iv")
 
+/*
+ * TLS 1.3 Exporter label
+ */
+S2N_BLOB_LABEL(s2n_tls13_label_exporter, "exporter")
+
 /*
  * TLS 1.3 Finished label
  */

diff --git a/crypto/s2n_tls13_keys.h b/crypto/s2n_tls13_keys.h
@@ -70,6 +70,8 @@ extern const struct s2n_blob s2n_tls13_label_resumption_master_secret;
 
 extern const struct s2n_blob s2n_tls13_label_finished;
 
+extern const struct s2n_blob s2n_tls13_label_exporter;
+
 /* Traffic secret labels */
 
 extern const struct s2n_blob s2n_tls13_label_traffic_secret_key;

diff --git a/tests/unit/s2n_self_talk_key_log_test.c b/tests/unit/s2n_self_talk_key_log_test.c
@@ -56,6 +56,7 @@ S2N_RESULT s2n_test_check_tls13(struct s2n_stuffer *stuffer)
     RESULT_ENSURE_REF(strstr(out, "SERVER_HANDSHAKE_TRAFFIC_SECRET "));
     RESULT_ENSURE_REF(strstr(out, "CLIENT_TRAFFIC_SECRET_0 "));
     RESULT_ENSURE_REF(strstr(out, "SERVER_TRAFFIC_SECRET_0 "));
+    RESULT_ENSURE_REF(strstr(out, "EXPORTER_SECRET "));
     return S2N_RESULT_OK;
 }
 

diff --git a/tests/unit/s2n_self_talk_quic_support_test.c b/tests/unit/s2n_self_talk_quic_support_test.c
@@ -18,7 +18,7 @@
 #include "tls/s2n_quic_support.h"
 
 #define S2N_MODE_COUNT        2
-#define S2N_SECRET_TYPE_COUNT 5
+#define S2N_SECRET_TYPE_COUNT 6
 
 static const uint8_t CLIENT_TRANSPORT_PARAMS[] = "client transport params";
 static const uint8_t SERVER_TRANSPORT_PARAMS[] = "server transport params";

diff --git a/tests/unit/s2n_tls13_key_schedule_test.c b/tests/unit/s2n_tls13_key_schedule_test.c
@@ -24,7 +24,7 @@
 #include "tls/s2n_tls13_secrets.c"
 
 #define NO_TRIGGER_MSG       APPLICATION_DATA
-#define TRAFFIC_SECRET_COUNT 5
+#define TRAFFIC_SECRET_COUNT 6
 
 static uint8_t empty_secret[S2N_TLS13_SECRET_MAX_LEN] = { 0 };
 

diff --git a/tests/unit/s2n_tls13_secrets_rfc8448_test.c b/tests/unit/s2n_tls13_secrets_rfc8448_test.c
@@ -498,6 +498,50 @@ int main(int argc, char **argv)
                         secret.data, secret.size);
             }
         };
+
+        /* Derive EXPORTER_MASTER_SECRET */
+        {
+            /**
+             *= https://www.rfc-editor.org/rfc/rfc8448.html#section-3
+             *= type=test
+             *#    {client}  derive secret "tls13 exp master" (same as server)
+             *
+             *= https://www.rfc-editor.org/rfc/rfc8448.html#section-3
+             *= type=test
+             *#    {server}  derive secret "tls13 exp master":
+             *#
+             *#       PRK (32 octets):  18 df 06 84 3d 13 a0 8b f2 a4 49 84 4c 5f 8a 47
+             *#          80 01 bc 4d 4c 62 79 84 d5 a4 1d a8 d0 40 29 19
+             *#
+             *#       hash (32 octets):  96 08 10 2a 0f 1c cc 6d b6 25 0b 7b 7e 41 7b 1a
+             *#          00 0e aa da 3d aa e4 77 7a 76 86 c9 ff 83 df 13
+             *#
+             **
+             *= https://www.rfc-editor.org/rfc/rfc8448.html#section-3
+             *= type=test
+             *#       info (52 octets):  00 20 10 74 6c 73 31 33 20 65 78 70 20 6d 61 73
+             *#          74 65 72 20 96 08 10 2a 0f 1c cc 6d b6 25 0b 7b 7e 41 7b 1a 00
+             *#          0e aa da 3d aa e4 77 7a 76 86 c9 ff 83 df 13
+             *#
+             *#       expanded (32 octets):  fe 22 f8 81 17 6e da 18 eb 8f 44 52 9e 67
+             *#          92 c5 0c 9a 3f 89 45 2f 68 d8 ae 31 1b 43 09 d3 cf 50
+             */
+            S2N_BLOB_FROM_HEX(hash, "96 08 10 2a 0f 1c cc 6d b6 25 0b 7b 7e 41 7b 1a \
+                       00 0e aa da 3d aa e4 77 7a 76 86 c9 ff 83 df 13");
+            S2N_BLOB_FROM_HEX(secret, "fe 22 f8 81 17 6e da 18 eb 8f 44 52 9e 67 \
+                       92 c5 0c 9a 3f 89 45 2f 68 d8 ae 31 1b 43 09 d3 cf 50");
+
+            for (size_t i = 0; i < s2n_array_len(modes); i++) {
+                DEFER_CLEANUP(struct s2n_connection *conn = s2n_connection_new(modes[i]), s2n_connection_ptr_free);
+                conn->secure->cipher_suite = cipher_suite;
+                EXPECT_OK(s2n_connection_set_test_master_secret(conn, &master_secret));
+                EXPECT_OK(s2n_connection_set_test_transcript_hash(conn, SERVER_FINISHED, &hash));
+
+                EXPECT_OK(s2n_derive_exporter_master_secret(conn, &derived_secret));
+                EXPECT_EQUAL(derived_secret.size, secret.size);
+                EXPECT_BYTEARRAY_EQUAL(derived_secret.data, secret.data, secret.size);
+            }
+        };
     };
 
     /* Resumed 0-RTT Handshake */

diff --git a/tests/unit/s2n_tls13_secrets_test.c b/tests/unit/s2n_tls13_secrets_test.c
@@ -281,6 +281,7 @@ int main(int argc, char **argv)
             EXPECT_MEMCPY_SUCCESS(conn->secrets.version.tls13.server_handshake_secret, test_secret.data, test_secret.size);
             EXPECT_MEMCPY_SUCCESS(conn->secrets.version.tls13.client_app_secret, test_secret.data, test_secret.size);
             EXPECT_MEMCPY_SUCCESS(conn->secrets.version.tls13.server_app_secret, test_secret.data, test_secret.size);
+            EXPECT_MEMCPY_SUCCESS(conn->secrets.version.tls13.exporter_master_secret, test_secret.data, test_secret.size);
             EXPECT_MEMCPY_SUCCESS(conn->secrets.version.tls13.resumption_master_secret, test_secret.data, test_secret.size);
 
             EXPECT_BYTEARRAY_NOT_EQUAL(conn->secrets.version.tls13.extract_secret, empty_secret, sizeof(empty_secret));
@@ -289,6 +290,7 @@ int main(int argc, char **argv)
             EXPECT_BYTEARRAY_NOT_EQUAL(conn->secrets.version.tls13.server_handshake_secret, empty_secret, sizeof(empty_secret));
             EXPECT_BYTEARRAY_NOT_EQUAL(conn->secrets.version.tls13.client_app_secret, empty_secret, sizeof(empty_secret));
             EXPECT_BYTEARRAY_NOT_EQUAL(conn->secrets.version.tls13.server_app_secret, empty_secret, sizeof(empty_secret));
+            EXPECT_BYTEARRAY_NOT_EQUAL(conn->secrets.version.tls13.exporter_master_secret, empty_secret, sizeof(empty_secret));
             EXPECT_BYTEARRAY_NOT_EQUAL(conn->secrets.version.tls13.resumption_master_secret, empty_secret, sizeof(empty_secret));
 
             EXPECT_OK(s2n_tls13_secrets_clean(conn));
@@ -299,6 +301,7 @@ int main(int argc, char **argv)
             EXPECT_BYTEARRAY_EQUAL(conn->secrets.version.tls13.server_handshake_secret, empty_secret, sizeof(empty_secret));
             EXPECT_BYTEARRAY_NOT_EQUAL(conn->secrets.version.tls13.client_app_secret, empty_secret, sizeof(empty_secret));
             EXPECT_BYTEARRAY_NOT_EQUAL(conn->secrets.version.tls13.server_app_secret, empty_secret, sizeof(empty_secret));
+            EXPECT_BYTEARRAY_NOT_EQUAL(conn->secrets.version.tls13.exporter_master_secret, empty_secret, sizeof(empty_secret));
             EXPECT_BYTEARRAY_NOT_EQUAL(conn->secrets.version.tls13.resumption_master_secret, empty_secret, sizeof(empty_secret));
         };
     };
@@ -488,5 +491,58 @@ int main(int argc, char **argv)
         };
     };
 
+    /* s2n_connection_export_secret */
+    {
+        /* Derives exporter secret on SERVER_FINISHED */
+        {
+            DEFER_CLEANUP(struct s2n_connection *conn = s2n_connection_new(S2N_SERVER),
+                    s2n_connection_ptr_free);
+            conn->secure->cipher_suite = &s2n_tls13_aes_128_gcm_sha256;
+            conn->actual_protocol_version = S2N_TLS13;
+            EXPECT_OK(s2n_connection_set_test_master_secret(conn, &test_secret));
+            EXPECT_BYTEARRAY_EQUAL(conn->secrets.version.tls13.exporter_master_secret,
+                    empty_secret, sizeof(empty_secret));
+
+            while (s2n_conn_get_current_message_type(conn) != SERVER_FINISHED) {
+                conn->handshake.message_number++;
+            }
+            EXPECT_OK(s2n_tls13_secrets_update(conn));
+
+            EXPECT_BYTEARRAY_NOT_EQUAL(conn->secrets.version.tls13.exporter_master_secret,
+                    empty_secret, sizeof(empty_secret));
+
+            /*
+             * s2n_connection_tls_exporter requires us to finish the handshake.
+             * The above is needed since s2n_tls13_secrets_update will only
+             * initialize when it sees the SERVER_FINISHED frame.
+             */
+            EXPECT_OK(s2n_skip_handshake(conn));
+
+            uint8_t output[32] = { 0 };
+            int result = s2n_connection_tls_exporter(
+                    conn,
+                    (const uint8_t *) "label",
+                    sizeof("label") - 1,
+                    (const uint8_t *) "context",
+                    sizeof("context") - 1,
+                    output,
+                    sizeof(output));
+            EXPECT_SUCCESS(result);
+            /*
+             * If updating this value, it's a good idea to make sure the update
+             * matches OpenSSL's SSL_export_keying_material. The easiest known
+             * way of doing that is building a simple client/server pair and
+             * calling the s2n and OpenSSL APIs after a handshake on both
+             * sides; you should get identical results with identical
+             * label/context parameters. This particular value though is not
+             * checked as its dependent on the s2n-specific test master secret.
+             */
+            S2N_BLOB_FROM_HEX(expected, "3a 72 eb 08 10 a3 69 f3 06 f2 77 11 70 ad d5 76 bd 21 15 \
+                    46 d4 c8 fb 80 1a 93 04 1e ac 59 aa 47");
+            EXPECT_EQUAL(sizeof(output), expected.size);
+            EXPECT_BYTEARRAY_EQUAL(output, expected.data, expected.size);
+        };
+    };
+
     END_TEST();
 }
diff --git a/tls/s2n_key_log.c b/tls/s2n_key_log.c
@@ -84,6 +84,7 @@ S2N_RESULT s2n_key_log_tls13_secret(struct s2n_connection *conn, const struct s2
     const uint8_t server_handshake_label[] = "SERVER_HANDSHAKE_TRAFFIC_SECRET ";
     const uint8_t client_traffic_label[] = "CLIENT_TRAFFIC_SECRET_0 ";
     const uint8_t server_traffic_label[] = "SERVER_TRAFFIC_SECRET_0 ";
+    const uint8_t exporter_secret_label[] = "EXPORTER_SECRET ";
 
     const uint8_t *label = NULL;
     uint8_t label_size = 0;
@@ -109,6 +110,10 @@ S2N_RESULT s2n_key_log_tls13_secret(struct s2n_connection *conn, const struct s2
             label = server_traffic_label;
             label_size = sizeof(server_traffic_label) - 1;
             break;
+        case S2N_EXPORTER_SECRET:
+            label = exporter_secret_label;
+            label_size = sizeof(exporter_secret_label) - 1;
+            break;
         default:
             /* Ignore the secret types we don't understand */
             return S2N_RESULT_OK;

diff --git a/tls/s2n_quic_support.h b/tls/s2n_quic_support.h
@@ -56,6 +56,7 @@ typedef enum {
     S2N_SERVER_HANDSHAKE_TRAFFIC_SECRET,
     S2N_CLIENT_APPLICATION_TRAFFIC_SECRET,
     S2N_SERVER_APPLICATION_TRAFFIC_SECRET,
+    S2N_EXPORTER_SECRET,
 } s2n_secret_type_t;
 
 /*