C++: add initial insufficient key size query

cpp/ql/src/Security/CWE/CWE-326/InsufficientKeySize.qhelp

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+<!DOCTYPE qhelp PUBLIC
+  "-//Semmle//qhelp//EN"
+  "qhelp.dtd">
+<qhelp>
+<overview>
+<p>Using cryptographic algorithms with a small key size can leave data vulnerable to being decrypted.</p>
+
+<p>Many cryptographic algorithms provided by cryptography libraries can be configured with key sizes that are
+vulnerable to brute force attacks. Using such a key size means that an attacker may be able to easily decrypt the
+encrypted data.</p>
+
+</overview>
+<recommendation>
+
+<p>Ensure that you use a strong, modern cryptographic algorithm. Use at least AES-128 or RSA-2048.</p>
+
+</recommendation>
+<example>
+
+
+
+</example>
+<references>
+
+<li>NIST, FIPS 140 Annex a: <a href="http://csrc.nist.gov/publications/fips/fips140-2/fips1402annexa.pdf">
+Approved Security Functions</a>.</li>
+<li>NIST, SP 800-131A: <a href="https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-131Ar2.pdf">
+Transitions: Recommendation for Transitioning the Use of Cryptographic Algorithms and Key Lengths</a>.</li>
+
+
+
+<!--  LocalWords:  CWE
+ -->
+
+</references>
+</qhelp>

cpp/ql/src/Security/CWE/CWE-326/InsufficientKeySize.ql

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+/**
+ * @name Use of a cryptographic algorithm with insufficient key size
+ * @description Using cryptographic algorithms with too small a key size can
+ *              allow an attacker to compromise security.
+ * @kind problem
+ * @problem.severity error
+ * @precision high
+ * @id cpp/insufficient-key-size
+ * @tags security
+ *       external/cwe/cwe-326
+ */
+
+import cpp
+import semmle.code.cpp.ir.dataflow.DataFlow
+import semmle.code.cpp.ir.IR
+
+int getMinimumKeyStrength(string func) {
+  func = "EVP_PKEY_CTX_set_dsa_paramgen_bits" and result = 2048
+  or
+  func = "EVP_PKEY_CTX_set_dh_paramgen_prime_len" and result = 2048
+  or
+  func = "EVP_PKEY_CTX_set_rsa_keygen_bits" and result = 2048
+}
+
+class KeyStrengthFlow extends DataFlow::Configuration {
+  KeyStrengthFlow() {
+    this = "KeyStrengthFlow"
+  }
+
+  override predicate isSource(DataFlow::Node node) {
+    node.asInstruction() instanceof IntegerConstantInstruction
+  }
+
+  override predicate isSink(DataFlow::Node node) {
+    exists(FunctionCall fc, string name|
+      node.asExpr() = fc.getArgument(1) and
+      fc.getTarget().hasGlobalName(name) and
+      exists(getMinimumKeyStrength(name))
+    )
+  }
+}
+
+from DataFlow::PathNode source, DataFlow::PathNode sink, KeyStrengthFlow conf, FunctionCall fc, string name, int bits
+where
+  conf.hasFlowPath(source, sink) and
+  sink.getNode().asExpr() = fc.getArgument(1) and
+  fc.getTarget().hasGlobalName(name) and
+  bits = getMinimumKeyStrength(name) and
+  source.getNode().asInstruction().(ConstantValueInstruction).getValue().toInt() <  bits
+select fc, source, sink, "The key size $@ is insufficient for security", source, source.toString()

cpp/ql/test/query-tests/Security/CWE/CWE-326/InsufficientKeySize.expected

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+| test.cpp:32:5:32:38 | call to EVP_PKEY_CTX_set_dsa_paramgen_bits | test.cpp:32:45:32:48 | 1024 | test.cpp:32:45:32:48 | 1024 | The key size $@ is insufficient for security | test.cpp:32:45:32:48 | 1024 | 1024 |
+| test.cpp:33:5:33:42 | call to EVP_PKEY_CTX_set_dh_paramgen_prime_len | test.cpp:33:49:33:52 | 1024 | test.cpp:33:49:33:52 | 1024 | The key size $@ is insufficient for security | test.cpp:33:49:33:52 | 1024 | 1024 |
+| test.cpp:35:5:35:36 | call to EVP_PKEY_CTX_set_rsa_keygen_bits | test.cpp:35:43:35:46 | 1024 | test.cpp:35:43:35:46 | 1024 | The key size $@ is insufficient for security | test.cpp:35:43:35:46 | 1024 | 1024 |

cpp/ql/test/query-tests/Security/CWE/CWE-326/InsufficientKeySize.qlref

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+Security/CWE/CWE-326/InsufficientKeySize.ql

cpp/ql/test/query-tests/Security/CWE/CWE-326/test.cpp

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+
+
+typedef int EVP_CIPHER;
+typedef int EVP_MD;
+
+const EVP_CIPHER *EVP_aes_128_ctr();
+const EVP_CIPHER *EVP_aes_192_ctr();
+const EVP_CIPHER *EVP_aes_256_ctr();
+
+const EVP_MD *EVP_sha224();
+const EVP_MD *EVP_sha256();
+const EVP_MD *EVP_sha384();
+const EVP_MD *EVP_sha512();
+
+
+class EVP_PKEY_CTX;
+
+ // int is a curve ID rather than a bit width
+int EVP_PKEY_CTX_set_ec_paramgen_curve_nid(EVP_PKEY_CTX*, int);
+
+int EVP_PKEY_CTX_set_dsa_paramgen_bits(EVP_PKEY_CTX*, int);
+int EVP_PKEY_CTX_set_dh_paramgen_prime_len(EVP_PKEY_CTX*, int);
+
+// RSA sets bits per-key rather than with parameters
+int EVP_PKEY_CTX_set_rsa_keygen_bits(EVP_PKEY_CTX*, int);
+
+void test1(EVP_PKEY_CTX *ctx) {
+    EVP_PKEY_CTX_set_dsa_paramgen_bits(ctx, 2048);
+    EVP_PKEY_CTX_set_dh_paramgen_prime_len(ctx, 2048);
+    // RSA sets bits per-key rather than with parameters
+    EVP_PKEY_CTX_set_rsa_keygen_bits(ctx, 2048);
+
+    // low key sizes
+    EVP_PKEY_CTX_set_dsa_paramgen_bits(ctx, 1024);
+    EVP_PKEY_CTX_set_dh_paramgen_prime_len(ctx, 1024);
+    // RSA sets bits per-key rather than with parameters
+    EVP_PKEY_CTX_set_rsa_keygen_bits(ctx, 1024);
+}