Ruby: weak crypto: do not report weak hash algorithms

Weak hash algorithms such as MD5 and SHA1 are often
used in non security sensitive contexts and reporting
all uses is far too noisy.

ruby/ql/src/change-notes/2022-11-04-weak-crypto-hash.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* The `rb/weak-cryptographic-algorithm` has been updated to no longer report uses of hash functions such as `MD5` and `SHA1` even if they are known to be weak. These hash algorithms are used very often in non-sensitive contexts, making the query too imprecise in practice.

ruby/ql/src/queries/security/cwe-327/BrokenCryptoAlgorithm.ql

@@ -15,8 +15,12 @@ import codeql.ruby.Concepts
 
 from Cryptography::CryptographicOperation operation, string msgPrefix
 where
-  operation.getAlgorithm().isWeak() and
-  msgPrefix = "The cryptographic algorithm " + operation.getAlgorithm().getName()
+  exists(Cryptography::CryptographicAlgorithm algorithm |
+    algorithm = operation.getAlgorithm() and
+    algorithm.isWeak() and
+    msgPrefix = "The cryptographic algorithm " + algorithm.getName() and
+    not algorithm instanceof Cryptography::HashingAlgorithm
+  )
   or
   operation.getBlockMode().isWeak() and msgPrefix = "The block mode " + operation.getBlockMode()
 select operation, msgPrefix + " is broken or weak, and should not be used."

ruby/ql/test/query-tests/security/cwe-327/BrokenCryptoAlgorithm.expected

@@ -17,13 +17,3 @@
 | broken_crypto.rb:75:1:75:24 | call to new | The cryptographic algorithm RC4 is broken or weak, and should not be used. |
 | broken_crypto.rb:77:1:77:29 | call to new | The cryptographic algorithm RC4 is broken or weak, and should not be used. |
 | broken_crypto.rb:79:1:79:35 | call to new | The cryptographic algorithm RC4 is broken or weak, and should not be used. |
-| broken_crypto.rb:81:1:81:28 | call to hexdigest | The cryptographic algorithm MD5 is broken or weak, and should not be used. |
-| broken_crypto.rb:84:1:84:31 | call to base64digest | The cryptographic algorithm MD5 is broken or weak, and should not be used. |
-| broken_crypto.rb:87:1:87:20 | call to digest | The cryptographic algorithm MD5 is broken or weak, and should not be used. |
-| broken_crypto.rb:89:1:89:21 | call to update | The cryptographic algorithm MD5 is broken or weak, and should not be used. |
-| broken_crypto.rb:90:1:90:17 | ... << ... | The cryptographic algorithm MD5 is broken or weak, and should not be used. |
-| broken_crypto.rb:95:1:95:34 | call to bubblebabble | The cryptographic algorithm MD5 is broken or weak, and should not be used. |
-| broken_crypto.rb:97:11:97:37 | call to file | The cryptographic algorithm MD5 is broken or weak, and should not be used. |
-| broken_crypto.rb:103:1:103:21 | call to digest | The cryptographic algorithm SHA1 is broken or weak, and should not be used. |
-| broken_crypto.rb:104:1:104:17 | ... << ... | The cryptographic algorithm SHA1 is broken or weak, and should not be used. |
-| broken_crypto.rb:106:1:106:37 | call to digest | The cryptographic algorithm SHA1 is broken or weak, and should not be used. |

ruby/ql/test/query-tests/security/cwe-327/broken_crypto.rb

@@ -78,30 +78,30 @@
 # BAD: weak encryption algorithm
 OpenSSL::Cipher::RC4.new 'hmac-md5'
 
-Digest::MD5.hexdigest('foo') # BAD: weak hash algorithm
+Digest::MD5.hexdigest('foo') # OK: don't report hash algorithm even if it is weak
 Digest::SHA256.hexdigest('foo') # GOOD: strong hash algorithm
 
-Digest::MD5.base64digest('foo') # BAD: weak hash algorithm
+Digest::MD5.base64digest('foo') # OK: don't report hash algorithm even if it is weak
 
 md5 = Digest::MD5.new
-md5.digest 'message' # BAD: weak hash algorithm
+md5.digest 'message' # OK: don't report hash algorithm even if it is weak
 
-md5.update 'message1' # BAD: weak hash algorithm
+md5.update 'message1' # # OK: don't report hash algorithm even if it is weak
 md5 << 'message2' # << is an alias for update
 
 sha256 = Digest::SHA256.new
 sha256.digest 'message' # GOOD: strong hash algorithm
 
-Digest::MD5.bubblebabble 'message' # BAD: weak hash algorithm
+Digest::MD5.bubblebabble 'message' # OK: don't report hash algorithm even if it is weak
 
-filemd5 = Digest::MD5.file 'testfile'
+filemd5 = Digest::MD5.file 'testfile' # OK: don't report hash algorithm even if it is weak
 filemd5.hexdigest
 
-Digest("MD5").hexdigest('foo') # BAD: weak hash algorithm
+Digest("MD5").hexdigest('foo') # OK: don't report hash algorithm even if it is weak
 
 sha1 = OpenSSL::Digest.new('SHA1')
-sha1.digest 'message' # BAD: weak hash algorithm
+sha1.digest 'message' # OK: don't report hash algorithm even if it is weak
 sha1 << 'message' # << is an alias for update
 
-OpenSSL::Digest.digest('SHA1', "abc") # BAD: weak hash algorithm
+OpenSSL::Digest.digest('SHA1', "abc") # OK: don't report hash algorithm even if it is weak
 OpenSSL::Digest.digest('SHA3-512', "abc") # GOOD: strong hash algorithm