From 2adda9c12cae778a7ed37f9e04ce998f6fe5c912 Mon Sep 17 00:00:00 2001 From: Miroslav Lichvar Date: Mon, 16 Sep 2024 14:15:38 +0200 Subject: [PATCH] nts: construct key exporter context When the NTS client and server negotiated use of AES-128-GCM-SIV keys, the keys exported from the TLS session and used for authentication and encryption of NTP messages do not comply to RFC8915. The exporter context value specified in the section 5.1 of RFC8915 function is incorrect. It is a hardcoded string which contains 15 (AES-SIV-CMAC-256) instead of 30 (AES-128-GCM-SIV). This causes chrony to not interoperate with NTS implementations that follow RFC8915 correctly. (At this time, there doesn't seem to be another implementation with AES-128-GCM-SIV support yet.) Replace the string with a proper construction of the exporter context from a specified AEAD ID and next protocol. Keep using the incorrect AEAD ID for AES-128-GCM-SIV to not break compatibility with existing chrony servers and clients. A new NTS-KE record will be added to negotiate the compliant exporter context. Reported-by: Martin Mayer --- nts_ke.h | 2 -- nts_ke_client.c | 12 ++++++++++-- nts_ke_server.c | 10 +++++++++- nts_ke_session.c | 29 +++++++++++++++++++++++------ nts_ke_session.h | 7 +++++-- test/unit/nts_ke_server.c | 10 ++++++---- test/unit/nts_ke_session.c | 16 +++++++++++++--- 7 files changed, 66 insertions(+), 20 deletions(-) diff --git a/nts_ke.h b/nts_ke.h index e7497af..3ecadd9 100644 --- a/nts_ke.h +++ b/nts_ke.h @@ -49,8 +49,6 @@ #define NKE_ALPN_NAME "ntske/1" #define NKE_EXPORTER_LABEL "EXPORTER-network-time-security" -#define NKE_EXPORTER_CONTEXT_C2S "\x0\x0\x0\xf\x0" -#define NKE_EXPORTER_CONTEXT_S2C "\x0\x0\x0\xf\x1" #define NKE_MAX_MESSAGE_LENGTH 16384 #define NKE_MAX_RECORD_BODY_LENGTH 256 diff --git a/nts_ke_client.c b/nts_ke_client.c index 3891f71..f10aed5 100644 --- a/nts_ke_client.c +++ b/nts_ke_client.c @@ -255,6 +255,7 @@ process_response(NKC_Instance inst) static int handle_message(void *arg) { + SIV_Algorithm exporter_algorithm; NKC_Instance inst = arg; if (!process_response(inst)) { @@ -262,8 +263,15 @@ handle_message(void *arg) return 0; } - if (!NKSN_GetKeys(inst->session, inst->context.algorithm, - &inst->context.c2s, &inst->context.s2c)) + exporter_algorithm = inst->context.algorithm; + + /* With AES-128-GCM-SIV, set the algorithm ID in the RFC5705 key exporter + context incorrectly for compatibility with older chrony servers */ + if (exporter_algorithm == AEAD_AES_128_GCM_SIV) + exporter_algorithm = AEAD_AES_SIV_CMAC_256; + + if (!NKSN_GetKeys(inst->session, inst->context.algorithm, exporter_algorithm, + NKE_NEXT_PROTOCOL_NTPV4, &inst->context.c2s, &inst->context.s2c)) return 0; if (inst->server_name[0] != '\0') { diff --git a/nts_ke_server.c b/nts_ke_server.c index 6d617b4..d5b240b 100644 --- a/nts_ke_server.c +++ b/nts_ke_server.c @@ -339,6 +339,7 @@ helper_signal(int x) static int prepare_response(NKSN_Instance session, int error, int next_protocol, int aead_algorithm) { + SIV_Algorithm exporter_algorithm; NKE_Context context; NKE_Cookie cookie; char *ntp_server; @@ -385,8 +386,15 @@ prepare_response(NKSN_Instance session, int error, int next_protocol, int aead_a } context.algorithm = aead_algorithm; + exporter_algorithm = aead_algorithm; - if (!NKSN_GetKeys(session, aead_algorithm, &context.c2s, &context.s2c)) + /* With AES-128-GCM-SIV, set the algorithm ID in the RFC5705 key exporter + context incorrectly for compatibility with older chrony clients */ + if (exporter_algorithm == AEAD_AES_128_GCM_SIV) + exporter_algorithm = AEAD_AES_SIV_CMAC_256; + + if (!NKSN_GetKeys(session, aead_algorithm, exporter_algorithm, + NKE_NEXT_PROTOCOL_NTPV4, &context.c2s, &context.s2c)) return 0; for (i = 0; i < NKE_MAX_COOKIES; i++) { diff --git a/nts_ke_session.c b/nts_ke_session.c index 2ae1e91..6ad662a 100644 --- a/nts_ke_session.c +++ b/nts_ke_session.c @@ -877,22 +877,39 @@ NKSN_GetRecord(NKSN_Instance inst, int *critical, int *type, int *body_length, /* ================================================== */ int -NKSN_GetKeys(NKSN_Instance inst, SIV_Algorithm siv, NKE_Key *c2s, NKE_Key *s2c) +NKSN_GetKeys(NKSN_Instance inst, SIV_Algorithm algorithm, SIV_Algorithm exporter_algorithm, + int next_protocol, NKE_Key *c2s, NKE_Key *s2c) { - int length = SIV_GetKeyLength(siv); + int length = SIV_GetKeyLength(algorithm); + struct { + uint16_t next_protocol; + uint16_t algorithm; + uint8_t is_s2c; + uint8_t _pad; + } context; if (length <= 0 || length > sizeof (c2s->key) || length > sizeof (s2c->key)) { DEBUG_LOG("Invalid algorithm"); return 0; } + assert(sizeof (context) == 6); + context.next_protocol = htons(next_protocol); + context.algorithm = htons(exporter_algorithm); + + context.is_s2c = 0; if (gnutls_prf_rfc5705(inst->tls_session, sizeof (NKE_EXPORTER_LABEL) - 1, NKE_EXPORTER_LABEL, - sizeof (NKE_EXPORTER_CONTEXT_C2S) - 1, NKE_EXPORTER_CONTEXT_C2S, - length, (char *)c2s->key) < 0 || - gnutls_prf_rfc5705(inst->tls_session, + sizeof (context) - 1, (char *)&context, + length, (char *)c2s->key) < 0) { + DEBUG_LOG("Could not export key"); + return 0; + } + + context.is_s2c = 1; + if (gnutls_prf_rfc5705(inst->tls_session, sizeof (NKE_EXPORTER_LABEL) - 1, NKE_EXPORTER_LABEL, - sizeof (NKE_EXPORTER_CONTEXT_S2C) - 1, NKE_EXPORTER_CONTEXT_S2C, + sizeof (context) - 1, (char *)&context, length, (char *)s2c->key) < 0) { DEBUG_LOG("Could not export key"); return 0; diff --git a/nts_ke_session.h b/nts_ke_session.h index 2735e04..b046144 100644 --- a/nts_ke_session.h +++ b/nts_ke_session.h @@ -77,8 +77,11 @@ extern int NKSN_EndMessage(NKSN_Instance inst); extern int NKSN_GetRecord(NKSN_Instance inst, int *critical, int *type, int *body_length, void *body, int buffer_length); -/* Export NTS keys for a specified algorithm */ -extern int NKSN_GetKeys(NKSN_Instance inst, SIV_Algorithm siv, NKE_Key *c2s, NKE_Key *s2c); +/* Export NTS keys for a specified algorithm (for compatibility reasons the + RFC5705 exporter context is allowed to have a different algorithm) */ +extern int NKSN_GetKeys(NKSN_Instance inst, SIV_Algorithm algorithm, + SIV_Algorithm exporter_algorithm, + int next_protocol, NKE_Key *c2s, NKE_Key *s2c); /* Check if the session has stopped */ extern int NKSN_IsStopped(NKSN_Instance inst); diff --git a/test/unit/nts_ke_server.c b/test/unit/nts_ke_server.c index 3d2f295..be3f6d8 100644 --- a/test/unit/nts_ke_server.c +++ b/test/unit/nts_ke_server.c @@ -30,11 +30,12 @@ #define NKSN_GetKeys get_keys static int -get_keys(NKSN_Instance session, SIV_Algorithm siv, NKE_Key *c2s, NKE_Key *s2c) +get_keys(NKSN_Instance session, SIV_Algorithm algorithm, SIV_Algorithm exporter_algorithm, + int next_protocol, NKE_Key *c2s, NKE_Key *s2c) { - c2s->length = SIV_GetKeyLength(siv); + c2s->length = SIV_GetKeyLength(algorithm); UTI_GetRandomBytes(c2s->key, c2s->length); - s2c->length = SIV_GetKeyLength(siv); + s2c->length = SIV_GetKeyLength(algorithm); UTI_GetRandomBytes(s2c->key, s2c->length); return 1; } @@ -174,7 +175,8 @@ test_unit(void) for (i = 0; i < 10000; i++) { context.algorithm = AEAD_AES_SIV_CMAC_256; - get_keys(session, context.algorithm, &context.c2s, &context.s2c); + get_keys(session, context.algorithm, random() % 100, NKE_NEXT_PROTOCOL_NTPV4, + &context.c2s, &context.s2c); memset(&cookie, 0, sizeof (cookie)); TEST_CHECK(NKS_GenerateCookie(&context, &cookie)); TEST_CHECK(NKS_DecodeCookie(&cookie, &context2)); diff --git a/test/unit/nts_ke_session.c b/test/unit/nts_ke_session.c index d0e72c7..c2db4a2 100644 --- a/test/unit/nts_ke_session.c +++ b/test/unit/nts_ke_session.c @@ -116,9 +116,19 @@ verify_message(NKSN_Instance inst) TEST_CHECK(!NKSN_GetRecord(inst, &critical, &t, &length, buffer, sizeof (buffer))); - TEST_CHECK(NKSN_GetKeys(inst, AEAD_AES_SIV_CMAC_256, &c2s, &s2c)); - TEST_CHECK(c2s.length == SIV_GetKeyLength(AEAD_AES_SIV_CMAC_256)); - TEST_CHECK(s2c.length == SIV_GetKeyLength(AEAD_AES_SIV_CMAC_256)); + for (i = 0; i < 10; i++) { + TEST_CHECK(NKSN_GetKeys(inst, AEAD_AES_SIV_CMAC_256, random(), random(), &c2s, &s2c)); + TEST_CHECK(c2s.length == SIV_GetKeyLength(AEAD_AES_SIV_CMAC_256)); + TEST_CHECK(s2c.length == SIV_GetKeyLength(AEAD_AES_SIV_CMAC_256)); + + if (SIV_GetKeyLength(AEAD_AES_128_GCM_SIV) > 0) { + TEST_CHECK(NKSN_GetKeys(inst, AEAD_AES_128_GCM_SIV, random(), random(), &c2s, &s2c)); + TEST_CHECK(c2s.length == SIV_GetKeyLength(AEAD_AES_128_GCM_SIV)); + TEST_CHECK(s2c.length == SIV_GetKeyLength(AEAD_AES_128_GCM_SIV)); + } else { + TEST_CHECK(!NKSN_GetKeys(inst, AEAD_AES_128_GCM_SIV, random(), random(), &c2s, &s2c)); + } + } } static int