security: E2EE and transport correctness (issue #147)

T04: promote security events to warnings — SASL auth failure, TLS
handshake failure, cert-failure details, see-other-host redirect;
DISABLE_TLS, TRUST_TLS and LEGACY_AUTH get a log warning plus a
console notice (REQ-LOG-01, REQ-LOG-02, REQ-AUTH-03)

T05: warn when a session ends up unencrypted without the user having
asked for it; refuse in-band registration on an unencrypted stream;
warn on each HTTP transfer with certificate verification disabled
(REQ-CRY-03, REQ-CFG-01)

T06: pin the update check to https with peer/host verification and no
redirects; strict N.N.N parser for the fetched version, which is
untrusted network input (REQ-VUL-02)

T09: no plaintext logging on failed MUC OMEMO sends; OTR opportunistic
first message passes allow_unencrypted_message(); get_random_string()
draws from a CSPRNG without modulo bias; guard the identity-key length
decrement against unsigned underflow (REQ-CRY-01, REQ-CRY-02,
REQ-CRY-07, REQ-MEM-05)

REQ-VUL-03 and REQ-CRY-09 are already satisfied on master and are
left unchanged.

The console warnings use cons_show_warning() from #87, so this needs
that change in master first.

tests/unittests/test_common.c

@@ -1089,6 +1089,45 @@ release_is_new__tests__various(void** state)
     assert_false(release_is_new("0.16.0", ""));
     assert_false(release_is_new(NULL, "1.0.0"));
     assert_false(release_is_new("1.0.0", NULL));
+
+    // the found version is untrusted network input: nothing but strictly N.N.N may parse
+    assert_false(release_is_new("0.16.0", "  1.0.0"));       // leading whitespace
+    assert_false(release_is_new("0.16.0", "+1.0.0"));        // explicit sign
+    assert_false(release_is_new("0.16.0", "-1.0.0"));        // negative major
+    assert_false(release_is_new("0.16.0", "1.0.0-rc1"));     // trailing garbage
+    assert_false(release_is_new("0.16.0", "1.0.0\n"));       // trailing newline
+    assert_false(release_is_new("0.16.0", "1..0"));          // empty component
+    assert_false(release_is_new("0.16.0", "0x10.0.0"));      // hex is not accepted
+    assert_false(release_is_new("0.16.0", "99999999999.0.0")); // out of int range, must not wrap
+    assert_false(release_is_new("0.16.0", "1.0.0 malicious"));
+
+    // Boundary: the largest still-parsable component is accepted
+    assert_true(release_is_new("0.16.0", "2147483647.0.0"));
+}
+
+void
+get_random_string__generates_valid_ids(void** state)
+{
+    const gchar* alphabet = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
+
+    for (size_t len = 1; len <= 40; len++) {
+        gchar* id = get_random_string(len);
+        assert_non_null(id);
+        assert_int_equal(len, strlen(id));
+        for (size_t i = 0; i < len; i++) {
+            assert_non_null(strchr(alphabet, id[i]));
+        }
+        g_free(id);
+    }
+
+    // a collision here would mean a dead random source
+    gchar* a = get_random_string(20);
+    gchar* b = get_random_string(20);
+    assert_non_null(a);
+    assert_non_null(b);
+    assert_string_not_equal(a, b);
+    g_free(a);
+    g_free(b);
 }
 
 void