Repository: randombit/botan (cpp)

Issue:
DNS name matching for name constraints is case-sensitive
Hi,

I've noticed that when Botan checks name constraints during validation of X509 certificates, it performs a case-sensitive matching on DNS names. I think this is a bug, since [RFC 5280](https://www.rfc-editor.org/rfc/rfc5280.txt) (article 7.2) says _"When evaluating name constraints, conforming implementations MUST perform a case-insensitive exact match on a label-by-label basis."_.

I've included a [test_example_chain.zip](https://github.com/randombit/botan/files/6416038/test_example_chain.zip) which highlights precisely this behavior, such that:

_test_root_ca.pem_ specifies a single name constraint:

```
X509v3 Name Constraints: critical
    Permitted:
      DNS:example.com
```

_test_server_ok.pem_ has an alternative name:

```
X509v3 Subject Alternative Name:
    DNS:test.example.com
```

_test_server_error.pem_ has an alternative name:

```
X509v3 Subject Alternative Name:
    DNS:test.EXAMPLE.COM
```

and validation of chain _test_root_ca.pem_ -> _test_server_ok.pem_ returns code 0 (VERIFIED)
while validation of chain _test_root_ca.pem_ -> _test_server_error.pem_ returns code 4004 (NAME_CONSTRAINT_ERROR)

I've also included a [real_world_example_chain.zip](https://github.com/randombit/botan/files/6416144/real_world_example_chain.zip) for reference, where the problem is that _real_world_intermediate_ca.pem_ specifies a name constraint (in a permitted subtree) for `DNS:gov.it`, while _real_world_server.pem_ has an alternative name which contains `DNS:INDICEPA-COLLAUDO.GOV.IT`.

---

I am using Botan in version 2.17.2 through its Python binding.

The (test) example chain above was validated as follows:

```
import botan2 as botan

server = botan.X509Cert(filename="test_server_ok.pem")
print(server.verify(trusted_path="trust_store/", reference_time=1620061260))
```

Pull request description:
Name constraint fixes
Fixes #2735 #2736 and #2737

Patch:
diff --git a/doc/side_channels.rst b/doc/side_channels.rst
index 0448e1497..8ef448d54 100644
--- a/doc/side_channels.rst
+++ b/doc/side_channels.rst
@@ -75,8 +75,7 @@ by a technique based on the CRT - ``phi(n)`` is factored to ``2**e * z`` for
 some ``e`` > 1 and some odd ``z``. Then ``e`` is inverted modulo ``2**e`` and
 also modulo ``z``. The inversion modulo ``2**e`` is done via a specialized
 constant-time algoirthm which only works for powers of 2. Then the two
-inversions are combined using the CRT. This process does leak the value of
-``e``; to avoid problems ``p`` and ``q`` are chosen so that ``e`` is always 1.
+inversions are combined using the CRT.
 
 See blinding.cpp and rsa.cpp.
 
@@ -175,7 +174,7 @@ used in cases when the scalar is a secret.
 Both ``blinded_base_point_multiply`` and ``blinded_var_point_multiply`` apply
 side channel countermeasures. The scalar is masked by a multiple of the group
 order (this is commonly called Coron's first countermeasure [CoronDpa]),
-currently the mask is scaled to be half the bit length of the order of the group.
+currently the mask is an 80 bit random value.
 
 Botan stores all ECC points in Jacobian representation. This form allows faster
 computation by representing points (x,y) as (X,Y,Z) where x=X/Z^2 and
diff --git a/src/lib/utils/parsing.cpp b/src/lib/utils/parsing.cpp
index d44e59fc6..6b6fff967 100644
--- a/src/lib/utils/parsing.cpp
+++ b/src/lib/utils/parsing.cpp
@@ -195,8 +195,6 @@ std::string ipv4_to_string(uint32_t ip)
    return str;
    }
 
-namespace {
-
 std::string tolower_string(const std::string& in)
    {
    std::string s = in;
@@ -209,8 +207,6 @@ std::string tolower_string(const std::string& in)
    return s;
    }
 
-}
-
 bool host_wildcard_match(const std::string& issued_, const std::string& host_)
    {
    const std::string issued = tolower_string(issued_);
diff --git a/src/lib/utils/parsing.h b/src/lib/utils/parsing.h
index 49eacba81..699b7449b 100644
--- a/src/lib/utils/parsing.h
+++ b/src/lib/utils/parsing.h
@@ -92,6 +92,8 @@ std::map<std::string, std::string> read_kv(const std::string& kv);
 
 std::string clean_ws(const std::string& s);
 
+std::string tolower_string(const std::string& s);
+
 /**
 * Check if the given hostname is a match for the specified wildcard
 */
diff --git a/src/lib/x509/name_constraint.cpp b/src/lib/x509/name_constraint.cpp
index 2b102ecac..cd7ece6d1 100644
--- a/src/lib/x509/name_constraint.cpp
+++ b/src/lib/x509/name_constraint.cpp
@@ -166,17 +166,19 @@ bool GeneralName::matches_dns(const std::string& nam) const
    {
    if(nam.size() == name().size())
       {
-      return nam == name();
+      return tolower_string(nam) == tolower_string(name());
       }
    else if(name().size() > nam.size())
       {
+      // The constraint is longer than the issued name: not possibly a match
       return false;
       }
    else // name.size() < nam.size()
       {
-      std::string constr = name().front() == '.' ? name() : "." + name();
       // constr is suffix of nam
-      return constr == nam.substr(nam.size() - constr.size(), constr.size());
+      const std::string constr = name().front() == '.' ? name() : "." + name();
+      const std::string substr = nam.substr(nam.size() - constr.size(), constr.size());
+      return tolower_string(constr) == tolower_string(substr);
       }
    }
 
diff --git a/src/lib/x509/x509_ext.cpp b/src/lib/x509/x509_ext.cpp
index 82633af5e..c811ba6c9 100644
--- a/src/lib/x509/x509_ext.cpp
+++ b/src/lib/x509/x509_ext.cpp
@@ -555,20 +555,17 @@ void Name_Constraints::validate(const X509_Certificate& subject, const X509_Cert
    {
    if(!m_name_constraints.permitted().empty() || !m_name_constraints.excluded().empty())
       {
-      if(!subject.is_CA_cert() || !subject.is_critical("X509v3.NameConstraints"))
+      if(!subject.is_CA_cert())
+         {
          cert_status.at(pos).insert(Certificate_Status_Code::NAME_CONSTRAINT_ERROR);
+         }
 
       const bool issuer_name_constraint_critical =
          issuer.is_critical("X509v3.NameConstraints");
 
-      const bool at_self_signed_root = (pos == cert_path.size() - 1);
-
       // Check that all subordinate certs pass the name constraint
-      for(size_t j = 0; j <= pos; ++j)
+      for(size_t j = 0; j < pos; ++j)
          {
-         if(pos == j && at_self_signed_root)
-            continue;
-
          bool permitted = m_name_constraints.permitted().empty();
          bool failed = false;
 


Test patch:
diff --git a/src/tests/data/x509/bsi/expected.txt b/src/tests/data/x509/bsi/expected.txt
index 9099ab58a..a3f957a81 100644
--- a/src/tests/data/x509/bsi/expected.txt
+++ b/src/tests/data/x509/bsi/expected.txt
@@ -48,7 +48,7 @@ cert_path_ext_12$Certificate contains duplicate policy
 cert_path_ext_13$Unknown critical extension encountered
 cert_path_ext_14$Unknown critical extension encountered
 cert_path_ext_15$Certificate does not pass name constraint
-cert_path_ext_16$Certificate does not pass name constraint
+cert_path_ext_16$Verified
 #cert_path_ext_17$
 cert_path_ext_18$Unknown critical extension encountered
 cert_path_ext_19$Unknown critical extension encountered
diff --git a/src/tests/data/x509/misc/name_constraint_ci/int.pem b/src/tests/data/x509/misc/name_constraint_ci/int.pem
new file mode 100644
index 000000000..e72186198
--- /dev/null
+++ b/src/tests/data/x509/misc/name_constraint_ci/int.pem
@@ -0,0 +1,101 @@
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----
diff --git a/src/tests/data/x509/misc/name_constraint_ci/leaf.pem b/src/tests/data/x509/misc/name_constraint_ci/leaf.pem
new file mode 100644
index 000000000..8dbba412e
--- /dev/null
+++ b/src/tests/data/x509/misc/name_constraint_ci/leaf.pem
@@ -0,0 +1,45 @@
+-----BEGIN CERTIFICATE-----
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diff --git a/src/tests/data/x509/misc/name_constraint_ci/root.pem b/src/tests/data/x509/misc/name_constraint_ci/root.pem
new file mode 100644
index 000000000..5762f37e6
--- /dev/null
+++ b/src/tests/data/x509/misc/name_constraint_ci/root.pem
@@ -0,0 +1,33 @@
+-----BEGIN CERTIFICATE-----
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diff --git a/src/tests/data/x509/misc/nc_skip_self/int.pem b/src/tests/data/x509/misc/nc_skip_self/int.pem
new file mode 100644
index 000000000..209bda324
--- /dev/null
+++ b/src/tests/data/x509/misc/nc_skip_self/int.pem
@@ -0,0 +1,30 @@
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----
diff --git a/src/tests/data/x509/misc/nc_skip_self/leaf.pem b/src/tests/data/x509/misc/nc_skip_self/leaf.pem
new file mode 100644
index 000000000..dd1991acd
--- /dev/null
+++ b/src/tests/data/x509/misc/nc_skip_self/leaf.pem
@@ -0,0 +1,29 @@
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----
diff --git a/src/tests/data/x509/misc/nc_skip_self/root.pem b/src/tests/data/x509/misc/nc_skip_self/root.pem
new file mode 100644
index 000000000..27f99ee35
--- /dev/null
+++ b/src/tests/data/x509/misc/nc_skip_self/root.pem
@@ -0,0 +1,29 @@
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----
diff --git a/src/tests/test_x509_path.cpp b/src/tests/test_x509_path.cpp
index bd113a6f4..e777f85fe 100644
--- a/src/tests/test_x509_path.cpp
+++ b/src/tests/test_x509_path.cpp
@@ -643,6 +643,98 @@ std::vector<Test::Result> Validate_Name_Constraint_SAN_Test::run()
 
 BOTAN_REGISTER_TEST("x509", "x509_name_constraint_san", Validate_Name_Constraint_SAN_Test);
 
+class Validate_Name_Constraint_CaseInsensitive final : public Test
+   {
+   public:
+      std::vector<Test::Result> run() override;
+   };
+
+std::vector<Test::Result> Validate_Name_Constraint_CaseInsensitive::run()
+   {
+   if(Botan::has_filesystem_impl() == false)
+      {
+      return {Test::Result::Note("Path validation",
+                                 "Skipping due to missing filesystem access")};
+      }
+
+   std::vector<Test::Result> results;
+
+   const std::string root_crt = Test::data_file("/x509/misc/name_constraint_ci/root.pem");
+   const std::string int_crt  = Test::data_file("/x509/misc/name_constraint_ci/int.pem");
+   const std::string ee_crt   = Test::data_file("/x509/misc/name_constraint_ci/leaf.pem");
+
+   auto validation_time =
+      Botan::calendar_point(2021, 5, 8, 1, 0, 0).to_std_timepoint();
+
+   Botan::X509_Certificate root(root_crt);
+   Botan::X509_Certificate intermediate(int_crt);
+   Botan::X509_Certificate ee_cert(ee_crt);
+
+   Botan::Certificate_Store_In_Memory trusted;
+   trusted.add_certificate(root);
+
+   std::vector<Botan::X509_Certificate> chain = { ee_cert, intermediate };
+
+   Botan::Path_Validation_Restrictions restrictions;
+   Botan::Path_Validation_Result validation_result =
+      Botan::x509_path_validate(chain, restrictions, trusted, "",
+                                Botan::Usage_Type::UNSPECIFIED, validation_time);
+
+   Test::Result result("DNS name constraints are case insensitive");
+   result.test_eq("Path validation succeeded",
+                  validation_result.successful_validation(), true);
+
+   return {result};
+   }
+
+BOTAN_REGISTER_TEST("x509", "x509_name_constraint_ci", Validate_Name_Constraint_CaseInsensitive);
+
+class Validate_Name_Constraint_NoCheckSelf final : public Test
+   {
+   public:
+      std::vector<Test::Result> run() override;
+   };
+
+std::vector<Test::Result> Validate_Name_Constraint_NoCheckSelf::run()
+   {
+   if(Botan::has_filesystem_impl() == false)
+      {
+      return {Test::Result::Note("Path validation",
+                                 "Skipping due to missing filesystem access")};
+      }
+
+   std::vector<Test::Result> results;
+
+   const std::string root_crt = Test::data_file("/x509/misc/nc_skip_self/root.pem");
+   const std::string int_crt  = Test::data_file("/x509/misc/nc_skip_self/int.pem");
+   const std::string ee_crt   = Test::data_file("/x509/misc/nc_skip_self/leaf.pem");
+
+   auto validation_time =
+      Botan::calendar_point(2021, 5, 8, 1, 0, 0).to_std_timepoint();
+
+   Botan::X509_Certificate root(root_crt);
+   Botan::X509_Certificate intermediate(int_crt);
+   Botan::X509_Certificate ee_cert(ee_crt);
+
+   Botan::Certificate_Store_In_Memory trusted;
+   trusted.add_certificate(root);
+
+   std::vector<Botan::X509_Certificate> chain = { ee_cert, intermediate };
+
+   Botan::Path_Validation_Restrictions restrictions;
+   Botan::Path_Validation_Result validation_result =
+      Botan::x509_path_validate(chain, restrictions, trusted, "",
+                                Botan::Usage_Type::UNSPECIFIED, validation_time);
+
+   Test::Result result("Name constraints do not apply to the certificate which includes them");
+   result.test_eq("Path validation succeeded",
+                  validation_result.successful_validation(), true);
+
+   return {result};
+   }
+
+BOTAN_REGISTER_TEST("x509", "x509_name_constraint_no_check_self", Validate_Name_Constraint_NoCheckSelf);
+
 class BSI_Path_Validation_Tests final : public Test
 
    {