knownhosts: Introduced ssh_known_hosts_get_algorithms_names()

The added internal function obtain a newly allocated string containing a
list of the signature types that can be generated by the keys present in
the known_hosts files, separated by commas.

Signed-off-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>
(cherry picked from commit 65a38759ca)

src/knownhosts.c

@@ -42,6 +42,7 @@
 #include "libssh/pki.h"
 #include "libssh/dh.h"
 #include "libssh/knownhosts.h"
+#include "libssh/token.h"
 
 /**
  * @addtogroup libssh_session
@@ -451,6 +452,146 @@ error:
     return NULL;
 }
 
+/**
+ * @internal
+ *
+ * @brief   Returns a static string containing a list of the signature types the
+ * given key type can generate.
+ *
+ * @returns A static cstring containing the signature types the key is able to
+ * generate separated by commas; NULL in case of error
+ */
+static const char *ssh_known_host_sigs_from_hostkey_type(enum ssh_keytypes_e type)
+{
+    switch (type) {
+    case SSH_KEYTYPE_RSA:
+        return "rsa-sha2-512,rsa-sha2-256,ssh-rsa";
+    case SSH_KEYTYPE_ED25519:
+        return "ssh-ed25519";
+#ifdef HAVE_DSA
+    case SSH_KEYTYPE_DSS:
+        return "ssh-dss";
+#endif
+#ifdef HAVE_ECDH
+    case SSH_KEYTYPE_ECDSA_P256:
+        return "ecdsa-sha2-nistp256";
+    case SSH_KEYTYPE_ECDSA_P384:
+        return "ecdsa-sha2-nistp384";
+    case SSH_KEYTYPE_ECDSA_P521:
+        return "ecdsa-sha2-nistp521";
+#endif
+    case SSH_KEYTYPE_UNKNOWN:
+    default:
+        SSH_LOG(SSH_LOG_WARN, "The given type %d is not a base private key type "
+                "or is unsupported", type);
+        return NULL;
+    }
+}
+
+/**
+ * @internal
+ * @brief Get the host keys algorithms identifiers from the known_hosts files
+ *
+ * This expands the signatures types that can be generated from the keys types
+ * present in the known_hosts files
+ *
+ * @param[in]  session  The ssh session to use.
+ *
+ * @return A newly allocated cstring containing a list of signature algorithms
+ * that can be generated by the host using the keys listed in the known_hosts
+ * files, NULL on error.
+ */
+char *ssh_known_hosts_get_algorithms_names(ssh_session session)
+{
+    char methods_buffer[256 + 1] = {0};
+    struct ssh_list *entry_list = NULL;
+    struct ssh_iterator *it = NULL;
+    char *host_port = NULL;
+    size_t count;
+    bool needcomma = false;
+    char *names;
+
+    int rc;
+
+    if (session->opts.knownhosts == NULL ||
+        session->opts.global_knownhosts == NULL) {
+        if (ssh_options_apply(session) < 0) {
+            ssh_set_error(session,
+                          SSH_REQUEST_DENIED,
+                          "Can't find a known_hosts file");
+
+            return NULL;
+        }
+    }
+
+    host_port = ssh_session_get_host_port(session);
+    if (host_port == NULL) {
+        return NULL;
+    }
+
+    rc = ssh_known_hosts_read_entries(host_port,
+                                      session->opts.knownhosts,
+                                      &entry_list);
+    if (rc != 0) {
+        SAFE_FREE(host_port);
+        ssh_list_free(entry_list);
+        return NULL;
+    }
+
+    rc = ssh_known_hosts_read_entries(host_port,
+                                      session->opts.global_knownhosts,
+                                      &entry_list);
+    SAFE_FREE(host_port);
+    if (rc != 0) {
+        ssh_list_free(entry_list);
+        return NULL;
+    }
+
+    if (entry_list == NULL) {
+        return NULL;
+    }
+
+    count = ssh_list_count(entry_list);
+    if (count == 0) {
+        ssh_list_free(entry_list);
+        return NULL;
+    }
+
+    for (it = ssh_list_get_iterator(entry_list);
+         it != NULL;
+         it = ssh_list_get_iterator(entry_list))
+    {
+        struct ssh_knownhosts_entry *entry = NULL;
+        const char *algo = NULL;
+
+        entry = ssh_iterator_value(struct ssh_knownhosts_entry *, it);
+        algo = ssh_known_host_sigs_from_hostkey_type(entry->publickey->type);
+        if (algo == NULL) {
+            continue;
+        }
+
+        if (needcomma) {
+            strncat(methods_buffer,
+                    ",",
+                    sizeof(methods_buffer) - strlen(methods_buffer) - 1);
+        }
+
+        strncat(methods_buffer,
+                algo,
+                sizeof(methods_buffer) - strlen(methods_buffer) - 1);
+        needcomma = true;
+
+        ssh_knownhosts_entry_free(entry);
+        ssh_list_remove(entry_list, it);
+    }
+
+    ssh_list_free(entry_list);
+
+    names = ssh_remove_duplicates(methods_buffer);
+
+    return names;
+}
+
 /**
  * @brief Parse a line from a known_hosts entry into a structure
  *