Add support for SCRAM-*-PLUS SASL mechanisms

This fixes #133

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

src/auth.c

@@ -83,7 +83,8 @@ static int _handle_digestmd5_rspauth(xmpp_conn_t *conn,
 static int _handle_scram_challenge(xmpp_conn_t *conn,
                                    xmpp_stanza_t *stanza,
                                    void *userdata);
-static char *_make_scram_init_msg(xmpp_conn_t *conn);
+struct scram_user_data;
+static int _make_scram_init_msg(struct scram_user_data *scram);
 
 static int _handle_missing_features_sasl(xmpp_conn_t *conn, void *userdata);
 static int _handle_missing_bind(xmpp_conn_t *conn, void *userdata);
@@ -243,21 +244,24 @@ _handle_features(xmpp_conn_t *conn, xmpp_stanza_t *stanza, void *userdata)
                 if (text == NULL)
                     continue;
 
-                if (strcasecmp(text, "PLAIN") == 0)
+                if (strcasecmp(text, "PLAIN") == 0) {
                     conn->sasl_support |= SASL_MASK_PLAIN;
-                else if (strcasecmp(text, "EXTERNAL") == 0 &&
+                } else if (strcasecmp(text, "EXTERNAL") == 0 &&
-                         (conn->tls_client_cert || conn->tls_client_key))
+                           (conn->tls_client_cert || conn->tls_client_key)) {
                     conn->sasl_support |= SASL_MASK_EXTERNAL;
-                else if (strcasecmp(text, "DIGEST-MD5") == 0)
+                } else if (strcasecmp(text, "DIGEST-MD5") == 0) {
                     conn->sasl_support |= SASL_MASK_DIGESTMD5;
-                else if (strcasecmp(text, "SCRAM-SHA-1") == 0)
+                } else if (strcasecmp(text, "ANONYMOUS") == 0) {
-                    conn->sasl_support |= SASL_MASK_SCRAMSHA1;
-                else if (strcasecmp(text, "SCRAM-SHA-256") == 0)
-                    conn->sasl_support |= SASL_MASK_SCRAMSHA256;
-                else if (strcasecmp(text, "SCRAM-SHA-512") == 0)
-                    conn->sasl_support |= SASL_MASK_SCRAMSHA512;
-                else if (strcasecmp(text, "ANONYMOUS") == 0)
                     conn->sasl_support |= SASL_MASK_ANONYMOUS;
+                } else {
+                    size_t n;
+                    for (n = 0; n < scram_algs_num; ++n) {
+                        if (strcasecmp(text, scram_algs[n]->scram_name) == 0) {
+                            conn->sasl_support |= scram_algs[n]->mask;
+                            break;
+                        }
+                    }
+                }
 
                 strophe_free(conn->ctx, text);
             }
@@ -439,7 +443,11 @@ static int _handle_digestmd5_rspauth(xmpp_conn_t *conn,
 }
 
 struct scram_user_data {
+    xmpp_conn_t *conn;
+    int sasl_plus;
     char *scram_init;
+    char *channel_binding;
+    const char *first_bare;
     const struct hash_alg *alg;
 };
 
@@ -471,8 +479,9 @@ static int _handle_scram_challenge(xmpp_conn_t *conn,
         if (!challenge)
             goto err;
 
-        response = sasl_scram(conn->ctx, scram_ctx->alg, challenge,
+        response =
-                              scram_ctx->scram_init, conn->jid, conn->pass);
+            sasl_scram(conn->ctx, scram_ctx->alg, scram_ctx->channel_binding,
+                       challenge, scram_ctx->first_bare, conn->jid, conn->pass);
         strophe_free(conn->ctx, challenge);
         if (!response)
             goto err;
@@ -506,7 +515,8 @@ static int _handle_scram_challenge(xmpp_conn_t *conn,
          */
         rc = _handle_sasl_result(conn, stanza,
                                  (void *)scram_ctx->alg->scram_name);
-        strophe_free(conn->ctx, scram_ctx->scram_init);
+        strophe_free_and_null(conn->ctx, scram_ctx->channel_binding);
+        strophe_free_and_null(conn->ctx, scram_ctx->scram_init);
         strophe_free(conn->ctx, scram_ctx);
     }
 
@@ -517,33 +527,103 @@ err_release_auth:
 err_free_response:
     strophe_free(conn->ctx, response);
 err:
-    strophe_free(conn->ctx, scram_ctx->scram_init);
+    strophe_free_and_null(conn->ctx, scram_ctx->channel_binding);
+    strophe_free_and_null(conn->ctx, scram_ctx->scram_init);
     strophe_free(conn->ctx, scram_ctx);
     disconnect_mem_error(conn);
     return 0;
 }
 
-static char *_make_scram_init_msg(xmpp_conn_t *conn)
+static int _make_scram_init_msg(struct scram_user_data *scram)
 {
+    xmpp_conn_t *conn = scram->conn;
     xmpp_ctx_t *ctx = conn->ctx;
-    size_t message_len;
+    const void *binding_data;
-    char *node;
+    const char *binding_type;
-    char *message;
+    char *node, *message;
-    char nonce[32];
+    size_t message_len, binding_type_len = 0, binding_data_len;
+    int l, is_secured = xmpp_conn_is_secured(conn);
+    /* This buffer must be able to hold:
+     * "p=<10 bytes binding type>,,<36 bytes binding data>"
+     * + alignment */
+    char buf[56];
+
+    if (scram->sasl_plus) {
+        if (!is_secured) {
+            strophe_error(
+                ctx, "xmpp",
+                "SASL: Server requested a -PLUS variant to authenticate, "
+                "but the connection is not secured. This is an error on "
+                "the server side we can't do anything about.");
+            return -1;
+        }
+        if (tls_init_channel_binding(conn->tls, &binding_type,
+                                     &binding_type_len)) {
+            return -1;
+        }
+        /* directly account for the '=' char in 'p=<binding-type>' */
+        binding_type_len += 1;
+    }
 
     node = xmpp_jid_node(ctx, conn->jid);
     if (!node) {
-        return NULL;
+        return -1;
     }
-    xmpp_rand_nonce(ctx->rand, nonce, sizeof(nonce));
+    /* 32 bytes nonce is enough */
-    message_len = strlen(node) + strlen(nonce) + 8 + 1;
+    xmpp_rand_nonce(ctx->rand, buf, 33);
+    message_len = strlen(node) + strlen(buf) + 8 + binding_type_len + 1;
     message = strophe_alloc(ctx, message_len);
-    if (message) {
+    if (!message) {
-        strophe_snprintf(message, message_len, "n,,n=%s,r=%s", node, nonce);
+        goto err_node;
     }
+    /* increase length to account for 'y,,', 'n,,' or 'p,,'.
+     * In the 'p' case the '=' sign has already been accounted for above.
+     */
+    binding_type_len += 3;
+    if (scram->sasl_plus) {
+        l = strophe_snprintf(message, message_len, "p=%s,,n=%s,r=%s",
+                             binding_type, node, buf);
+    } else {
+        l = strophe_snprintf(message, message_len, "%c,,n=%s,r=%s",
+                             is_secured ? 'y' : 'n', node, buf);
+    }
+    if (l < 0 || (size_t)l >= message_len) {
+        goto err_msg;
+    }
+    if (binding_type_len > sizeof(buf)) {
+        goto err_msg;
+    }
+    /* Make `first_bare` point to the 'n' of 'n=<node>' of the
+     * client-first-message */
+    scram->first_bare = message + binding_type_len;
+    memcpy(buf, message, binding_type_len);
+    if (scram->sasl_plus) {
+        binding_data =
+            tls_get_channel_binding_data(conn->tls, &binding_data_len);
+        if (!binding_data) {
+            goto err_msg;
+        }
+        if (binding_data_len > sizeof(buf) - binding_type_len) {
+            strophe_error(ctx, "xmpp", "Channel binding data is too long (%zu)",
+                          binding_data_len);
+            goto err_msg;
+        }
+        memcpy(&buf[binding_type_len], binding_data, binding_data_len);
+        binding_type_len += binding_data_len;
+    }
+    scram->channel_binding =
+        xmpp_base64_encode(ctx, (void *)buf, binding_type_len);
+    memset(buf, 0, binding_type_len);
     strophe_free(ctx, node);
+    scram->scram_init = message;
 
-    return message;
+    return 0;
+
+err_msg:
+    strophe_free(ctx, message);
+err_node:
+    strophe_free(ctx, node);
+    return -1;
 }
 
 static xmpp_stanza_t *_make_starttls(xmpp_conn_t *conn)
@@ -636,7 +716,7 @@ static void _auth(xmpp_conn_t *conn)
         return;
     }
 
-    if (anonjid && conn->sasl_support & SASL_MASK_ANONYMOUS) {
+    if (anonjid && (conn->sasl_support & SASL_MASK_ANONYMOUS)) {
         /* some crap here */
         auth = _make_sasl_auth(conn, "ANONYMOUS");
         if (!auth) {
@@ -702,22 +782,26 @@ static void _auth(xmpp_conn_t *conn)
             "Password hasn't been set, and SASL ANONYMOUS unsupported.");
         xmpp_disconnect(conn);
     } else if (conn->sasl_support & SASL_MASK_SCRAM) {
+        size_t n;
         scram_ctx = strophe_alloc(conn->ctx, sizeof(*scram_ctx));
-        if (conn->sasl_support & SASL_MASK_SCRAMSHA512)
+        memset(scram_ctx, 0, sizeof(*scram_ctx));
-            scram_ctx->alg = &scram_sha512;
+        for (n = 0; n < scram_algs_num; ++n) {
-        else if (conn->sasl_support & SASL_MASK_SCRAMSHA256)
+            if (conn->sasl_support & scram_algs[n]->mask) {
-            scram_ctx->alg = &scram_sha256;
+                scram_ctx->alg = scram_algs[n];
-        else if (conn->sasl_support & SASL_MASK_SCRAMSHA1)
+                break;
-            scram_ctx->alg = &scram_sha1;
+            }
+        }
+
         auth = _make_sasl_auth(conn, scram_ctx->alg->scram_name);
         if (!auth) {
             disconnect_mem_error(conn);
             return;
         }
 
-        /* don't free scram_init on success */
+        scram_ctx->conn = conn;
-        scram_ctx->scram_init = _make_scram_init_msg(conn);
+        scram_ctx->sasl_plus =
-        if (!scram_ctx->scram_init) {
+            scram_ctx->alg->mask & SASL_MASK_SCRAM_PLUS ? 1 : 0;
+        if (_make_scram_init_msg(scram_ctx)) {
             strophe_free(conn->ctx, scram_ctx);
             xmpp_stanza_release(auth);
             disconnect_mem_error(conn);
@@ -753,7 +837,7 @@ static void _auth(xmpp_conn_t *conn)
 
         send_stanza(conn, auth, XMPP_QUEUE_STROPHE);
 
-        /* SASL SCRAM-SHA-1 was tried, unset flag */
+        /* SASL algorithm was tried, unset flag */
         conn->sasl_support &= ~scram_ctx->alg->mask;
     } else if (conn->sasl_support & SASL_MASK_DIGESTMD5) {
         auth = _make_sasl_auth(conn, "DIGEST-MD5");

src/common.h

@@ -167,9 +167,14 @@ struct _xmpp_send_queue_t {
 #define SASL_MASK_SCRAMSHA256 (1 << 4)
 #define SASL_MASK_SCRAMSHA512 (1 << 5)
 #define SASL_MASK_EXTERNAL (1 << 6)
+#define SASL_MASK_SCRAMSHA1_PLUS (1 << 7)
+#define SASL_MASK_SCRAMSHA256_PLUS (1 << 8)
 
-#define SASL_MASK_SCRAM \
+#define SASL_MASK_SCRAM_PLUS \
+    (SASL_MASK_SCRAMSHA1_PLUS | SASL_MASK_SCRAMSHA256_PLUS)
+#define SASL_MASK_SCRAM_WEAK \
     (SASL_MASK_SCRAMSHA1 | SASL_MASK_SCRAMSHA256 | SASL_MASK_SCRAMSHA512)
+#define SASL_MASK_SCRAM (SASL_MASK_SCRAM_PLUS | SASL_MASK_SCRAM_WEAK)
 
 enum {
     XMPP_PORT_CLIENT = 5222,

src/sasl.c

@@ -375,6 +375,7 @@ char *sasl_digest_md5(xmpp_ctx_t *ctx,
 /** generate auth response string for the SASL SCRAM mechanism */
 char *sasl_scram(xmpp_ctx_t *ctx,
                  const struct hash_alg *alg,
+                 const char *channel_binding,
                  const char *challenge,
                  const char *first_bare,
                  const char *jid,
@@ -398,6 +399,7 @@ char *sasl_scram(xmpp_ctx_t *ctx,
     char *result = NULL;
     size_t response_len;
     size_t auth_len;
+    int l;
 
     UNUSED(jid);
 
@@ -428,37 +430,44 @@ char *sasl_scram(xmpp_ctx_t *ctx,
     }
     ival = strtol(i, &saveptr, 10);
 
-    auth_len = 10 + strlen(r) + strlen(first_bare) + strlen(challenge);
+    /* "c=<channel_binding>," + r + ",p=" + sign_b64 + '\0' */
-    auth = strophe_alloc(ctx, auth_len);
+    response_len = 3 + strlen(channel_binding) + strlen(r) + 3 +
-    if (!auth) {
+                   ((alg->digest_size + 2) / 3 * 4) + 1;
+    response = strophe_alloc(ctx, response_len);
+    if (!response) {
         goto out_sval;
     }
 
-    /* "c=biws," + r + ",p=" + sign_b64 + '\0' */
+    auth_len = 3 + response_len + strlen(first_bare) + strlen(challenge);
-    response_len = 7 + strlen(r) + 3 + ((alg->digest_size + 2) / 3 * 4) + 1;
+    auth = strophe_alloc(ctx, auth_len);
-    response = strophe_alloc(ctx, response_len);
+    if (!auth) {
-    if (!response) {
+        goto out_response;
-        goto out_auth;
     }
 
-    strophe_snprintf(response, response_len, "c=biws,%s", r);
+    l = strophe_snprintf(response, response_len, "c=%s,%s", channel_binding, r);
-    strophe_snprintf(auth, auth_len, "%s,%s,%s", first_bare + 3, challenge,
+    if (l < 0 || (size_t)l >= response_len) {
-                     response);
+        goto out_auth;
+    }
+    l = strophe_snprintf(auth, auth_len, "%s,%s,%s", first_bare, challenge,
+                         response);
+    if (l < 0 || (size_t)l >= auth_len) {
+        goto out_auth;
+    }
 
     SCRAM_ClientKey(alg, (uint8_t *)password, strlen(password), (uint8_t *)sval,
                     sval_len, (uint32_t)ival, key);
     SCRAM_ClientSignature(alg, key, (uint8_t *)auth, strlen(auth), sign);
-    SCRAM_ClientProof(alg, sign, key, sign);
+    SCRAM_ClientProof(alg, key, sign, sign);
 
     sign_b64 = xmpp_base64_encode(ctx, sign, alg->digest_size);
     if (!sign_b64) {
-        goto out_response;
+        goto out_auth;
     }
 
     /* Check for buffer overflow */
     if (strlen(response) + strlen(sign_b64) + 3 + 1 > response_len) {
         strophe_free(ctx, sign_b64);
-        goto out_response;
+        goto out_auth;
     }
     strcat(response, ",p=");
     strcat(response, sign_b64);
@@ -467,14 +476,14 @@ char *sasl_scram(xmpp_ctx_t *ctx,
     response_b64 =
         xmpp_base64_encode(ctx, (unsigned char *)response, strlen(response));
     if (!response_b64) {
-        goto out_response;
+        goto out_auth;
     }
     result = response_b64;
 
-out_response:
-    strophe_free(ctx, response);
 out_auth:
     strophe_free(ctx, auth);
+out_response:
+    strophe_free(ctx, response);
 out_sval:
     strophe_free(ctx, sval);
 out:

src/sasl.h

@@ -29,6 +29,7 @@ char *sasl_digest_md5(xmpp_ctx_t *ctx,
                       const char *password);
 char *sasl_scram(xmpp_ctx_t *ctx,
                  const struct hash_alg *alg,
+                 const char *channel_binding,
                  const char *challenge,
                  const char *first_bare,
                  const char *jid,

src/scram.c

@@ -42,6 +42,15 @@ const struct hash_alg scram_sha1 = {
     (void (*)(void *, const uint8_t *, size_t))crypto_SHA1_Update,
     (void (*)(void *, uint8_t *))crypto_SHA1_Final};
 
+const struct hash_alg scram_sha1_plus = {
+    "SCRAM-SHA-1-PLUS",
+    SASL_MASK_SCRAMSHA1_PLUS,
+    SHA1_DIGEST_SIZE,
+    (void (*)(const uint8_t *, size_t, uint8_t *))crypto_SHA1,
+    (void (*)(void *))crypto_SHA1_Init,
+    (void (*)(void *, const uint8_t *, size_t))crypto_SHA1_Update,
+    (void (*)(void *, uint8_t *))crypto_SHA1_Final};
+
 const struct hash_alg scram_sha256 = {
     "SCRAM-SHA-256",
     SASL_MASK_SCRAMSHA256,
@@ -51,6 +60,15 @@ const struct hash_alg scram_sha256 = {
     (void (*)(void *, const uint8_t *, size_t))sha256_process,
     (void (*)(void *, uint8_t *))sha256_done};
 
+const struct hash_alg scram_sha256_plus = {
+    "SCRAM-SHA-256-PLUS",
+    SASL_MASK_SCRAMSHA256_PLUS,
+    SHA256_DIGEST_SIZE,
+    (void (*)(const uint8_t *, size_t, uint8_t *))sha256_hash,
+    (void (*)(void *))sha256_init,
+    (void (*)(void *, const uint8_t *, size_t))sha256_process,
+    (void (*)(void *, uint8_t *))sha256_done};
+
 const struct hash_alg scram_sha512 = {
     "SCRAM-SHA-512",
     SASL_MASK_SCRAMSHA512,
@@ -60,6 +78,32 @@ const struct hash_alg scram_sha512 = {
     (void (*)(void *, const uint8_t *, size_t))sha512_process,
     (void (*)(void *, uint8_t *))sha512_done};
 
+/* The order of this list defines the order in which the SCRAM algorithms are
+ * tried if the server supports them.
+ * Their order is derived from
+ * https://datatracker.ietf.org/doc/html/draft-ietf-kitten-password-storage
+ */
+const struct hash_alg *scram_algs[] = {
+    /* *1 */
+    &scram_sha256_plus,
+    /* *1 */
+    &scram_sha1_plus,
+    /* *1 */
+    &scram_sha512,
+    /* *1 */
+    &scram_sha256,
+    /* *1 */
+    &scram_sha1,
+};
+/* *1 - I want to use clang-format here, but by default it will put multiple
+ * elements per line if there's no comment. Currently clang-format also doesn't
+ * have an option to enforce it to behave like that, besides by putting comments
+ * between each element (or I couldn't find a way to do it). In order to prevent
+ * clang-format from re-formatting the array I added the comments above and
+ * wrote this lengthy description.
+ */
+const size_t scram_algs_num = sizeof(scram_algs) / sizeof(scram_algs[0]);
+
 union common_hash_ctx {
     SHA1_CTX sha1;
     sha256_context sha256;

src/scram.h

@@ -34,9 +34,8 @@ struct hash_alg {
     void (*final)(void *, uint8_t *);
 };
 
-extern const struct hash_alg scram_sha1;
+extern const struct hash_alg *scram_algs[];
-extern const struct hash_alg scram_sha256;
+extern const size_t scram_algs_num;
-extern const struct hash_alg scram_sha512;
 
 void SCRAM_ClientKey(const struct hash_alg *alg,
                      const uint8_t *password,

src/tls.h

@@ -45,6 +45,10 @@ unsigned int tls_id_on_xmppaddr_num(xmpp_conn_t *conn);
 
 xmpp_tlscert_t *tls_peer_cert(xmpp_conn_t *conn);
 int tls_set_credentials(tls_t *tls, const char *cafilename);
+int tls_init_channel_binding(tls_t *tls,
+                             const char **binding_prefix,
+                             size_t *binding_prefix_len);
+const void *tls_get_channel_binding_data(tls_t *tls, size_t *size);
 
 int tls_start(tls_t *tls);
 int tls_stop(tls_t *tls);

src/tls_dummy.c

@@ -75,6 +75,23 @@ int tls_set_credentials(tls_t *tls, const char *cafilename)
     return -1;
 }
 
+int tls_init_channel_binding(tls_t *tls,
+                             const char **binding_prefix,
+                             size_t *binding_prefix_len)
+{
+    UNUSED(tls);
+    UNUSED(binding_prefix);
+    UNUSED(binding_prefix_len);
+    return -1;
+}
+
+const void *tls_get_channel_binding_data(tls_t *tls, size_t *size)
+{
+    UNUSED(tls);
+    UNUSED(size);
+    return NULL;
+}
+
 int tls_start(tls_t *tls)
 {
     UNUSED(tls);

src/tls_gnutls.c

@@ -25,12 +25,26 @@
 #include "tls.h"
 #include "sock.h"
 
+#if GNUTLS_VERSION_NUMBER < 0x030702
+/* This is a very dirty workaround to make this file compile on older GnuTLS
+ * We simply define our own value for the later introduced
+ * `GNUTLS_CB_TLS_EXPORTER` which makes it compile, but it will fail on runtime
+ * when the combination of the following options happen:
+ * * GnuTLS < 3.7.2
+ * * TLS 1.3
+ * * A SCRAM PLUS variant to authenticate
+ * We will think about fixing this once a user pops up who has that problem.
+ */
+#define GNUTLS_CB_TLS_EXPORTER 3
+#endif
+
 struct _tls {
     xmpp_ctx_t *ctx; /* do we need this? */
     xmpp_conn_t *conn;
     gnutls_session_t session;
     gnutls_certificate_credentials_t cred;
     gnutls_x509_crt_t client_cert;
+    gnutls_datum_t channel_binding;
     int lasterror;
 };
 
@@ -507,8 +521,8 @@ error_out:
 
 void tls_free(tls_t *tls)
 {
-    if (tls->client_cert)
+    gnutls_free(tls->channel_binding.data);
-        gnutls_x509_crt_deinit(tls->client_cert);
+    gnutls_x509_crt_deinit(tls->client_cert);
     gnutls_deinit(tls->session);
     gnutls_certificate_free_credentials(tls->cred);
     strophe_free(tls->ctx, tls);
@@ -555,6 +569,55 @@ int tls_set_credentials(tls_t *tls, const char *cafilename)
     return err == GNUTLS_E_SUCCESS;
 }
 
+int tls_init_channel_binding(tls_t *tls,
+                             const char **binding_prefix,
+                             size_t *binding_prefix_len)
+{
+    gnutls_channel_binding_t binding_type;
+    gnutls_protocol_t tls_version = gnutls_protocol_get_version(tls->session);
+
+    switch (tls_version) {
+    case GNUTLS_SSL3:
+    case GNUTLS_TLS1_0:
+    case GNUTLS_TLS1_1:
+    case GNUTLS_TLS1_2:
+        *binding_prefix = "tls-unique";
+        *binding_prefix_len = strlen("tls-unique");
+        binding_type = GNUTLS_CB_TLS_UNIQUE;
+        break;
+    case GNUTLS_TLS1_3:
+        *binding_prefix = "tls-exporter";
+        *binding_prefix_len = strlen("tls-exporter");
+        binding_type = GNUTLS_CB_TLS_EXPORTER;
+        break;
+    default:
+        strophe_error(tls->ctx, "tls", "Unsupported TLS Version: %s",
+                      gnutls_protocol_get_name(tls_version));
+        return -1;
+    }
+
+    if (tls->channel_binding.data) {
+        gnutls_free(tls->channel_binding.data);
+        tls->channel_binding.data = NULL;
+    }
+    int ret = gnutls_session_channel_binding(tls->session, binding_type,
+                                             &tls->channel_binding);
+    if (ret) {
+        strophe_error(tls->ctx, "tls", "could not get channel binding: %s",
+                      gnutls_strerror(ret));
+    }
+    return ret;
+}
+
+const void *tls_get_channel_binding_data(tls_t *tls, size_t *size)
+{
+    if (!tls->channel_binding.data || !tls->channel_binding.size) {
+        strophe_error(tls->ctx, "tls", "No channel binding data available");
+    }
+    *size = tls->channel_binding.size;
+    return tls->channel_binding.data;
+}
+
 int tls_start(tls_t *tls)
 {
     sock_set_blocking(tls->conn->sock);

src/tls_openssl.c

@@ -107,6 +107,8 @@ struct _tls {
     SSL_CTX *ssl_ctx;
     SSL *ssl;
     X509 *client_cert;
+    void *channel_binding_data;
+    size_t channel_binding_size;
     int lasterror;
 };
 
@@ -715,6 +717,7 @@ err:
 
 void tls_free(tls_t *tls)
 {
+    strophe_free(tls->ctx, tls->channel_binding_data);
     SSL_free(tls->ssl);
     X509_free(tls->client_cert);
     SSL_CTX_free(tls->ssl_ctx);
@@ -741,6 +744,65 @@ int tls_set_credentials(tls_t *tls, const char *cafilename)
     return -1;
 }
 
+int tls_init_channel_binding(tls_t *tls,
+                             const char **binding_prefix,
+                             size_t *binding_prefix_len)
+{
+    const char *label = NULL;
+    size_t labellen = 0;
+
+    switch (SSL_version(tls->ssl)) {
+    case SSL3_VERSION:
+        *binding_prefix = "tls-unique";
+        *binding_prefix_len = strlen("tls-unique");
+        tls->channel_binding_size = 36;
+        break;
+    case TLS1_VERSION:
+    case TLS1_1_VERSION:
+    case TLS1_2_VERSION:
+        *binding_prefix = "tls-unique";
+        *binding_prefix_len = strlen("tls-unique");
+        tls->channel_binding_size = 12;
+        break;
+#ifdef TLS1_3_VERSION
+    case TLS1_3_VERSION:
+        label = "EXPORTER-Channel-Binding";
+        labellen = 24;
+        *binding_prefix = "tls-exporter";
+        *binding_prefix_len = strlen("tls-exporter");
+        tls->channel_binding_size = 32;
+        break;
+#endif
+    default:
+        strophe_error(tls->ctx, "tls", "Unsupported TLS Version: %s",
+                      SSL_get_version(tls->ssl));
+        return -1;
+    }
+
+    strophe_free_and_null(tls->ctx, tls->channel_binding_data);
+    tls->channel_binding_data =
+        strophe_alloc(tls->ctx, tls->channel_binding_size);
+    if (!tls->channel_binding_data)
+        return -1;
+
+    if (SSL_export_keying_material(tls->ssl, tls->channel_binding_data,
+                                   tls->channel_binding_size, label, labellen,
+                                   NULL, 0, 0) != 1) {
+        strophe_error(tls->ctx, "tls", "Could not get channel binding data");
+        return -1;
+    }
+    return 0;
+}
+
+const void *tls_get_channel_binding_data(tls_t *tls, size_t *size)
+{
+    if (!tls->channel_binding_data || !tls->channel_binding_size) {
+        strophe_error(tls->ctx, "tls", "No channel binding data available");
+    }
+    *size = tls->channel_binding_size;
+    return tls->channel_binding_data;
+}
+
 int tls_start(tls_t *tls)
 {
     int error;

src/tls_schannel.c

@@ -237,6 +237,23 @@ int tls_set_credentials(tls_t *tls, const char *cafilename)
     return -1;
 }
 
+int tls_init_channel_binding(tls_t *tls,
+                             const char **binding_prefix,
+                             size_t *binding_prefix_len)
+{
+    UNUSED(tls);
+    UNUSED(binding_prefix);
+    UNUSED(binding_prefix_len);
+    return -1;
+}
+
+const void *tls_get_channel_binding_data(tls_t *tls, size_t *size)
+{
+    UNUSED(tls);
+    UNUSED(size);
+    return NULL;
+}
+
 int tls_start(tls_t *tls)
 {
     ULONG ctxtreq = 0, ctxtattr = 0;