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fix: add const correctness to S2N_BLOB_LABEL macro #5566

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fix: add const correctness to S2N_BLOB_LABEL macro #5566

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131f3f2
fix: add const correctness to S2N_BLOB_LABEL macro
akshatsinha0 Oct 16, 2025
92e05eb
fix: use union to cast away const
akshatsinha0 Oct 24, 2025
0d048d6
tls13: added read-only blob API and migrated TLS 1.3 labels
akshatsinha0 Nov 9, 2025
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27 changes: 27 additions & 0 deletions crypto/s2n_hkdf.c
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Original file line number Diff line number Diff line change
Expand Up @@ -376,6 +376,33 @@ int s2n_hkdf_expand_label(struct s2n_hmac_state *hmac, s2n_hmac_algorithm alg, c
return S2N_SUCCESS;
}

int s2n_hkdf_expand_label_ro(struct s2n_hmac_state *hmac, s2n_hmac_algorithm alg, const struct s2n_blob *secret, const struct s2n_ro_blob *label,
const struct s2n_blob *context, struct s2n_blob *output)
{
POSIX_ENSURE_REF(label);
POSIX_ENSURE_REF(context);
POSIX_ENSURE_REF(output);

uint8_t hkdf_label_buf[2 + 256 + 256];
struct s2n_blob hkdf_label_blob = { 0 };
struct s2n_stuffer hkdf_label = { 0 };

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));
POSIX_GUARD(s2n_stuffer_write_uint16(&hkdf_label, output->size));
POSIX_GUARD(s2n_stuffer_write_uint8(&hkdf_label, label->size+sizeof("tls13 ") - 1));
POSIX_GUARD(s2n_stuffer_write_str(&hkdf_label, "tls13 "));
POSIX_GUARD(s2n_stuffer_write_bytes(&hkdf_label, label->data,label->size));
POSIX_GUARD(s2n_stuffer_write_uint8(&hkdf_label, context->size));
POSIX_GUARD(s2n_stuffer_write(&hkdf_label, context));
hkdf_label_blob.size = s2n_stuffer_data_available(&hkdf_label);
POSIX_GUARD(s2n_hkdf_expand(hmac, alg, secret, &hkdf_label_blob, output));

return S2N_SUCCESS;
}

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)
{
Expand Down
4 changes: 4 additions & 0 deletions crypto/s2n_hkdf.h
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Original file line number Diff line number Diff line change
Expand Up @@ -39,4 +39,8 @@ int s2n_hkdf_extract(struct s2n_hmac_state *hmac, s2n_hmac_algorithm alg, const
int s2n_hkdf_expand_label(struct s2n_hmac_state *hmac, s2n_hmac_algorithm alg, const struct s2n_blob *secret, const struct s2n_blob *label,
const struct s2n_blob *context, struct s2n_blob *output);

/* Read-only label variant to avoid non-const data pointers */
int s2n_hkdf_expand_label_ro(struct s2n_hmac_state *hmac, s2n_hmac_algorithm alg, const struct s2n_blob *secret, const struct s2n_ro_blob *label,
const struct s2n_blob *context, struct s2n_blob *output);

bool s2n_libcrypto_supports_hkdf();
44 changes: 22 additions & 22 deletions crypto/s2n_tls13_keys.c
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Original file line number Diff line number Diff line change
Expand Up @@ -55,44 +55,44 @@
* Define TLS 1.3 HKDF labels as specified in
* https://tools.ietf.org/html/rfc8446#section-7.1
*/
S2N_BLOB_LABEL(s2n_tls13_label_derived_secret, "derived")
S2N_RO_BLOB_LABEL(s2n_tls13_label_derived_secret, "derived")

S2N_BLOB_LABEL(s2n_tls13_label_external_psk_binder_key, "ext binder")
S2N_BLOB_LABEL(s2n_tls13_label_resumption_psk_binder_key, "res binder")
S2N_RO_BLOB_LABEL(s2n_tls13_label_external_psk_binder_key, "ext binder")
S2N_RO_BLOB_LABEL(s2n_tls13_label_resumption_psk_binder_key, "res binder")

S2N_BLOB_LABEL(s2n_tls13_label_client_early_traffic_secret, "c e traffic")
S2N_BLOB_LABEL(s2n_tls13_label_early_exporter_master_secret, "e exp master")
S2N_RO_BLOB_LABEL(s2n_tls13_label_client_early_traffic_secret, "c e traffic")
S2N_RO_BLOB_LABEL(s2n_tls13_label_early_exporter_master_secret, "e exp master")

S2N_BLOB_LABEL(s2n_tls13_label_client_handshake_traffic_secret, "c hs traffic")
S2N_BLOB_LABEL(s2n_tls13_label_server_handshake_traffic_secret, "s hs traffic")
S2N_RO_BLOB_LABEL(s2n_tls13_label_client_handshake_traffic_secret, "c hs traffic")
S2N_RO_BLOB_LABEL(s2n_tls13_label_server_handshake_traffic_secret, "s hs traffic")

S2N_BLOB_LABEL(s2n_tls13_label_client_application_traffic_secret, "c ap traffic")
S2N_BLOB_LABEL(s2n_tls13_label_server_application_traffic_secret, "s ap traffic")
S2N_RO_BLOB_LABEL(s2n_tls13_label_client_application_traffic_secret, "c ap traffic")
S2N_RO_BLOB_LABEL(s2n_tls13_label_server_application_traffic_secret, "s ap traffic")

S2N_BLOB_LABEL(s2n_tls13_label_exporter_master_secret, "exp master")
S2N_BLOB_LABEL(s2n_tls13_label_resumption_master_secret, "res master")
S2N_BLOB_LABEL(s2n_tls13_label_session_ticket_secret, "resumption")
S2N_RO_BLOB_LABEL(s2n_tls13_label_exporter_master_secret, "exp master")
S2N_RO_BLOB_LABEL(s2n_tls13_label_resumption_master_secret, "res master")
S2N_RO_BLOB_LABEL(s2n_tls13_label_session_ticket_secret, "resumption")

/*
* Traffic secret labels
*/
S2N_BLOB_LABEL(s2n_tls13_label_traffic_secret_key, "key")
S2N_BLOB_LABEL(s2n_tls13_label_traffic_secret_iv, "iv")
S2N_RO_BLOB_LABEL(s2n_tls13_label_traffic_secret_key, "key")
S2N_RO_BLOB_LABEL(s2n_tls13_label_traffic_secret_iv, "iv")

/*
* TLS 1.3 Exporter label
*/
S2N_BLOB_LABEL(s2n_tls13_label_exporter, "exporter")
S2N_RO_BLOB_LABEL(s2n_tls13_label_exporter, "exporter")

/*
* TLS 1.3 Finished label
*/
S2N_BLOB_LABEL(s2n_tls13_label_finished, "finished")
S2N_RO_BLOB_LABEL(s2n_tls13_label_finished, "finished")

/*
* TLS 1.3 KeyUpdate label
*/
S2N_BLOB_LABEL(s2n_tls13_label_application_traffic_secret_update, "traffic upd")
S2N_RO_BLOB_LABEL(s2n_tls13_label_application_traffic_secret_update, "traffic upd")

static const struct s2n_blob zero_length_blob = { .data = NULL, .size = 0 };

Expand Down Expand Up @@ -135,9 +135,9 @@ int s2n_tls13_derive_traffic_keys(struct s2n_tls13_keys *keys, struct s2n_blob *
POSIX_ENSURE_REF(key);
POSIX_ENSURE_REF(iv);

POSIX_GUARD(s2n_hkdf_expand_label(&keys->hmac, keys->hmac_algorithm, secret,
POSIX_GUARD(s2n_hkdf_expand_label_ro(&keys->hmac, keys->hmac_algorithm, secret,
&s2n_tls13_label_traffic_secret_key, &zero_length_blob, key));
POSIX_GUARD(s2n_hkdf_expand_label(&keys->hmac, keys->hmac_algorithm, secret,
POSIX_GUARD(s2n_hkdf_expand_label_ro(&keys->hmac, keys->hmac_algorithm, secret,
&s2n_tls13_label_traffic_secret_iv, &zero_length_blob, iv));
return 0;
}
Expand All @@ -148,7 +148,7 @@ int s2n_tls13_derive_traffic_keys(struct s2n_tls13_keys *keys, struct s2n_blob *
*/
int s2n_tls13_derive_finished_key(struct s2n_tls13_keys *keys, struct s2n_blob *secret_key, struct s2n_blob *output_finish_key)
{
POSIX_GUARD(s2n_hkdf_expand_label(&keys->hmac, keys->hmac_algorithm, secret_key, &s2n_tls13_label_finished, &zero_length_blob, output_finish_key));
POSIX_GUARD(s2n_hkdf_expand_label_ro(&keys->hmac, keys->hmac_algorithm, secret_key, &s2n_tls13_label_finished, &zero_length_blob, output_finish_key));

return 0;
}
Expand All @@ -175,7 +175,7 @@ int s2n_tls13_update_application_traffic_secret(struct s2n_tls13_keys *keys, str
POSIX_ENSURE_REF(old_secret);
POSIX_ENSURE_REF(new_secret);

POSIX_GUARD(s2n_hkdf_expand_label(&keys->hmac, keys->hmac_algorithm, old_secret,
POSIX_GUARD(s2n_hkdf_expand_label_ro(&keys->hmac, keys->hmac_algorithm, old_secret,
&s2n_tls13_label_application_traffic_secret_update, &zero_length_blob, new_secret));

return 0;
Expand All @@ -190,7 +190,7 @@ S2N_RESULT s2n_tls13_derive_session_ticket_secret(struct s2n_tls13_keys *keys, s
RESULT_ENSURE_REF(secret_blob);

/* Derive session ticket secret from master session resumption secret */
RESULT_GUARD_POSIX(s2n_hkdf_expand_label(&keys->hmac, keys->hmac_algorithm, resumption_secret,
RESULT_GUARD_POSIX(s2n_hkdf_expand_label_ro(&keys->hmac, keys->hmac_algorithm, resumption_secret,
&s2n_tls13_label_session_ticket_secret, ticket_nonce, secret_blob));

return S2N_RESULT_OK;
Expand Down
32 changes: 16 additions & 16 deletions crypto/s2n_tls13_keys.h
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Original file line number Diff line number Diff line change
Expand Up @@ -51,31 +51,31 @@ struct s2n_tls13_keys {
struct s2n_hmac_state hmac;
};

/* Defines TLS 1.3 HKDF Labels */
extern const struct s2n_blob s2n_tls13_label_derived_secret;
extern const struct s2n_blob s2n_tls13_label_external_psk_binder_key;
extern const struct s2n_blob s2n_tls13_label_resumption_psk_binder_key;
/* Defines TLS 1.3 HKDF Labels (read-only) */
extern const struct s2n_ro_blob s2n_tls13_label_derived_secret;
extern const struct s2n_ro_blob s2n_tls13_label_external_psk_binder_key;
extern const struct s2n_ro_blob s2n_tls13_label_resumption_psk_binder_key;

extern const struct s2n_blob s2n_tls13_label_client_early_traffic_secret;
extern const struct s2n_blob s2n_tls13_label_early_exporter_master_secret;
extern const struct s2n_ro_blob s2n_tls13_label_client_early_traffic_secret;
extern const struct s2n_ro_blob s2n_tls13_label_early_exporter_master_secret;

extern const struct s2n_blob s2n_tls13_label_client_handshake_traffic_secret;
extern const struct s2n_blob s2n_tls13_label_server_handshake_traffic_secret;
extern const struct s2n_ro_blob s2n_tls13_label_client_handshake_traffic_secret;
extern const struct s2n_ro_blob s2n_tls13_label_server_handshake_traffic_secret;

extern const struct s2n_blob s2n_tls13_label_client_application_traffic_secret;
extern const struct s2n_blob s2n_tls13_label_server_application_traffic_secret;
extern const struct s2n_ro_blob s2n_tls13_label_client_application_traffic_secret;
extern const struct s2n_ro_blob s2n_tls13_label_server_application_traffic_secret;

extern const struct s2n_blob s2n_tls13_label_exporter_master_secret;
extern const struct s2n_blob s2n_tls13_label_resumption_master_secret;
extern const struct s2n_ro_blob s2n_tls13_label_exporter_master_secret;
extern const struct s2n_ro_blob s2n_tls13_label_resumption_master_secret;

extern const struct s2n_blob s2n_tls13_label_finished;
extern const struct s2n_ro_blob s2n_tls13_label_finished;

extern const struct s2n_blob s2n_tls13_label_exporter;
extern const struct s2n_ro_blob s2n_tls13_label_exporter;

/* Traffic secret labels */

extern const struct s2n_blob s2n_tls13_label_traffic_secret_key;
extern const struct s2n_blob s2n_tls13_label_traffic_secret_iv;
extern const struct s2n_ro_blob s2n_tls13_label_traffic_secret_key;
extern const struct s2n_ro_blob s2n_tls13_label_traffic_secret_iv;

#define s2n_tls13_key_blob(name, bytes) \
s2n_stack_blob(name, bytes, S2N_TLS13_SECRET_MAX_LEN)
Expand Down
4 changes: 2 additions & 2 deletions tests/unit/s2n_tls13_prf_test.c
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Expand Up @@ -127,12 +127,12 @@ int main(int argc, char **argv)
EXPECT_EQUAL(memcmp(secret_buf, expected_secret_bytes, sizeof(secret_buf)), 0);

/* Validate the derived secret */
S2N_BLOB_LABEL(label, "derived");
S2N_RO_BLOB_LABEL(label, "derived");

struct s2n_hmac_state hmac = { 0 };

EXPECT_SUCCESS(s2n_hmac_new(&hmac));
EXPECT_SUCCESS(s2n_hkdf_expand_label(&hmac, S2N_HMAC_SHA256, &secret, &label, &digest, &output));
EXPECT_SUCCESS(s2n_hkdf_expand_label_ro(&hmac, S2N_HMAC_SHA256, &secret, &label, &digest, &output));

EXPECT_EQUAL(memcmp(output_buf, expected_expanded_bytes, sizeof(output_buf)), 0);

Expand Down
2 changes: 1 addition & 1 deletion tests/unit/s2n_tls13_record_aead_test.c
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Expand Up @@ -292,7 +292,7 @@ int main(int argc, char **argv)
}

/* Test parsing of tls 1.3 aead record */
S2N_BLOB_LABEL(expect_plaintext, "Hello world");
S2N_RO_BLOB_LABEL(expect_plaintext, "Hello world");

static uint8_t hello_data[] = "Hello world";
struct s2n_blob plaintext = { 0 };
Expand Down
8 changes: 4 additions & 4 deletions tls/s2n_tls13_key_schedule.c
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Expand Up @@ -75,8 +75,8 @@ static S2N_RESULT s2n_tls13_key_schedule_get_keying_material(
*#
*# - A purpose value indicating the specific value being generated
**/
const struct s2n_blob *key_purpose = &s2n_tls13_label_traffic_secret_key;
const struct s2n_blob *iv_purpose = &s2n_tls13_label_traffic_secret_iv;
const struct s2n_ro_blob *key_purpose = &s2n_tls13_label_traffic_secret_key;
const struct s2n_ro_blob *iv_purpose = &s2n_tls13_label_traffic_secret_iv;

/**
*= https://www.rfc-editor.org/rfc/rfc8446#section-7.3
Expand Down Expand Up @@ -105,15 +105,15 @@ static S2N_RESULT s2n_tls13_key_schedule_get_keying_material(
**/
RESULT_ENSURE_LTE(key_size, key->size);
key->size = key_size;
RESULT_GUARD_POSIX(s2n_hkdf_expand_label(&hmac, hmac_alg,
RESULT_GUARD_POSIX(s2n_hkdf_expand_label_ro(&hmac, hmac_alg,
&secret, key_purpose, &s2n_zero_length_context, key));
/**
*= https://www.rfc-editor.org/rfc/rfc8446#section-7.3
*# [sender]_write_iv = HKDF-Expand-Label(Secret, "iv", "", iv_length)
**/
RESULT_ENSURE_LTE(iv_size, iv->size);
iv->size = iv_size;
RESULT_GUARD_POSIX(s2n_hkdf_expand_label(&hmac, hmac_alg,
RESULT_GUARD_POSIX(s2n_hkdf_expand_label_ro(&hmac, hmac_alg,
&secret, iv_purpose, &s2n_zero_length_context, iv));

return S2N_RESULT_OK;
Expand Down
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