fix(kms-keyring): validate content algorithm matches selected suite on decrypt

src/main/java/software/amazon/encryption/s3/materials/KmsKeyring.java

@@ -26,6 +26,7 @@
 import software.amazon.awssdk.services.s3.model.S3Request;
 import software.amazon.encryption.s3.S3EncryptionClient;
 import software.amazon.encryption.s3.S3EncryptionClientException;
+import software.amazon.encryption.s3.algorithms.AlgorithmSuite;
 import software.amazon.encryption.s3.internal.ApiNameVersion;
 
 //= specification/s3-encryption/materials/s3-kms-keyring.md#interface
@@ -251,6 +252,17 @@ public byte[] decryptDataKey(DecryptionMaterials materials, byte[] encryptedData
                 throw new S3EncryptionClientException("Provided encryption context does not match information retrieved from S3");
             }
 
+            // The content encryption algorithm is part of the KMS-authenticated encryption context, while the
+            // algorithm suite is selected from the (unauthenticated) content metadata. The two must agree; if
+            // they do not, the content metadata does not correspond to how the object was sealed, so the data
+            // key is not released for a mismatched suite.
+            String storedContentAlgorithm = materials.encryptionContext().get(ENCRYPTION_CONTEXT_ALGORITHM_KEY);
+            if (storedContentAlgorithm != null
+                    && !storedContentAlgorithm.equals(expectedContentAlgorithm(materials.algorithmSuite()))) {
+                throw new S3EncryptionClientException("The content encryption algorithm in the KMS-authenticated "
+                        + "encryption context does not match the algorithm suite selected for decryption.");
+            }
+
             //= specification/s3-encryption/materials/s3-kms-keyring.md#kms-context
             //# To attempt to decrypt a particular [encrypted data key](../structures.md#encrypted-data-key), the KmsKeyring
             //# MUST call [AWS KMS Decrypt](https://docs.aws.amazon.com/kms/latest/APIReference/API_Decrypt.html) with the configured AWS KMS client.
@@ -313,6 +325,23 @@ protected Map<String, DecryptDataKeyStrategy> decryptDataKeyStrategies() {
         return decryptDataKeyStrategies;
     }
 
+    // The canonical value the content-encryption algorithm (aws:x-amz-cek-alg) holds in the stored encryption
+    // context for a given selected algorithm suite. The committing GCM/CTR pair share the committing suite ID,
+    // and the non-committing GCM/CTR pair share GCM's cipher name, because a ranged get selects the CTR variant
+    // of an object that was sealed under its GCM counterpart.
+    private static String expectedContentAlgorithm(AlgorithmSuite algorithmSuite) {
+        switch (algorithmSuite) {
+            case ALG_AES_256_GCM_HKDF_SHA512_COMMIT_KEY:
+            case ALG_AES_256_CTR_HKDF_SHA512_COMMIT_KEY:
+                return AlgorithmSuite.ALG_AES_256_GCM_HKDF_SHA512_COMMIT_KEY.idAsString();
+            case ALG_AES_256_GCM_IV12_TAG16_NO_KDF:
+            case ALG_AES_256_CTR_IV16_TAG16_NO_KDF:
+                return AlgorithmSuite.ALG_AES_256_GCM_IV12_TAG16_NO_KDF.cipherName();
+            default:
+                return algorithmSuite.cipherName();
+        }
+    }
+
     public static class Builder extends S3Keyring.Builder<KmsKeyring, Builder> {
         private KmsClient _kmsClient;
         private String _wrappingKeyId;

src/test/java/software/amazon/encryption/s3/materials/KmsKeyringTest.java

@@ -2,13 +2,29 @@
 // SPDX-License-Identifier: Apache-2.0
 package software.amazon.encryption.s3.materials;
 
+import org.junit.jupiter.api.Test;
 import org.junitpioneer.jupiter.RetryingTest;
+import software.amazon.awssdk.core.SdkBytes;
+import software.amazon.awssdk.services.kms.KmsClient;
+import software.amazon.awssdk.services.kms.model.DecryptRequest;
+import software.amazon.awssdk.services.kms.model.DecryptResponse;
+import software.amazon.awssdk.services.s3.model.GetObjectRequest;
 import software.amazon.encryption.s3.S3EncryptionClientException;
+import software.amazon.encryption.s3.algorithms.AlgorithmSuite;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
 
 import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.mockito.ArgumentMatchers.argThat;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
 
 public class KmsKeyringTest {
 
+    private static final String KMS_KEY_ID = "arn:aws:kms:us-west-2:111122223333:key/example";
+
     @RetryingTest(3)
     public void buildKmsKeyringWithNullSecureRandomFails() {
         assertThrows(S3EncryptionClientException.class, () -> KmsKeyring.builder().secureRandom(null));
@@ -19,4 +35,44 @@ public void buildKmsKeyringWithNullDataKeyGeneratorFails() {
         assertThrows(S3EncryptionClientException.class, () -> KmsKeyring.builder().dataKeyGenerator(null));
     }
 
+    // The data key was wrapped for a GCM object, so the KMS-authenticated encryption context carries
+    // aws:x-amz-cek-alg = AES/GCM/NoPadding. The decryption materials select the CBC suite (as they would
+    // if the content-algorithm metadata read CBC). The authenticated content algorithm and the selected
+    // suite disagree, so onDecrypt must fail rather than return the data key.
+    @Test
+    public void kmsContextDecryptFailsWhenAuthenticatedCekAlgDoesNotMatchSelectedSuite() {
+        Map<String, String> storedEncryptionContext = new HashMap<>();
+        storedEncryptionContext.put("kms_cmk_id", KMS_KEY_ID);
+        storedEncryptionContext.put("aws:x-amz-cek-alg", AlgorithmSuite.ALG_AES_256_GCM_IV12_TAG16_NO_KDF.cipherName());
+
+        // Releases the data key only for the encryption context it was wrapped under.
+        KmsClient kmsClient = mock(KmsClient.class);
+        when(kmsClient.decrypt(argThat((DecryptRequest request) ->
+                storedEncryptionContext.equals(request.encryptionContext()))))
+                .thenReturn(DecryptResponse.builder()
+                        .keyId(KMS_KEY_ID)
+                        .plaintext(SdkBytes.fromByteArray(new byte[32]))
+                        .build());
+
+        KmsKeyring keyring = KmsKeyring.builder()
+                .kmsClient(kmsClient)
+                .wrappingKeyId(KMS_KEY_ID)
+                .build();
+
+        DecryptionMaterials materials = DecryptionMaterials.builder()
+                .s3Request(GetObjectRequest.builder().bucket("bucket").key("key").build())
+                .algorithmSuite(AlgorithmSuite.ALG_AES_256_CBC_IV16_NO_KDF)
+                .encryptionContext(storedEncryptionContext)
+                .build();
+
+        EncryptedDataKey edk = EncryptedDataKey.builder()
+                .keyProviderId(S3Keyring.KEY_PROVIDER_ID)
+                .keyProviderInfo("kms+context")
+                .encryptedDataKey(new byte[]{1, 2, 3})
+                .build();
+
+        assertThrows(S3EncryptionClientException.class,
+                () -> keyring.onDecrypt(materials, Collections.singletonList(edk)));
+    }
+
 }