Fix KMS encryption context handling (#435)

* Fix KMS encryption context handling

The code copying encryption context value strings to a map
containing string pointers was incorrectly getting a pointer to a
string variable which is being re-used by the for loop, causing
all keys to point to the same value string.

* Extract helper method for KmsKey to KMS MasterKey conversion

* Add test for kmsKeyToMasterKey helper function

keyservice/server.go

@@ -29,16 +29,7 @@ func (ks *Server) encryptWithPgp(key *PgpKey, plaintext []byte) ([]byte, error)
 }
 
 func (ks *Server) encryptWithKms(key *KmsKey, plaintext []byte) ([]byte, error) {
-	ctx := make(map[string]*string)
-	for k, v := range key.Context {
-		ctx[k] = &v
-	}
-	kmsKey := kms.MasterKey{
-		Arn:               key.Arn,
-		Role:              key.Role,
-		EncryptionContext: ctx,
-		AwsProfile:        key.AwsProfile,
-	}
+	kmsKey := kmsKeyToMasterKey(key)
 	err := kmsKey.Encrypt(plaintext)
 	if err != nil {
 		return nil, err
@@ -78,16 +69,7 @@ func (ks *Server) decryptWithPgp(key *PgpKey, ciphertext []byte) ([]byte, error)
 }
 
 func (ks *Server) decryptWithKms(key *KmsKey, ciphertext []byte) ([]byte, error) {
-	ctx := make(map[string]*string)
-	for k, v := range key.Context {
-		ctx[k] = &v
-	}
-	kmsKey := kms.MasterKey{
-		Arn:               key.Arn,
-		Role:              key.Role,
-		EncryptionContext: ctx,
-		AwsProfile:        key.AwsProfile,
-	}
+	kmsKey := kmsKeyToMasterKey(key)
 	kmsKey.EncryptedKey = string(ciphertext)
 	plaintext, err := kmsKey.Decrypt()
 	return []byte(plaintext), err
@@ -249,3 +231,17 @@ func (ks Server) Decrypt(ctx context.Context,
 	}
 	return response, nil
 }
+
+func kmsKeyToMasterKey(key *KmsKey) kms.MasterKey {
+	ctx := make(map[string]*string)
+	for k, v := range key.Context {
+		value := v // Allocate a new string to prevent the pointer below from referring to only the last iteration value
+		ctx[k] = &value
+	}
+	return kms.MasterKey{
+		Arn:               key.Arn,
+		Role:              key.Role,
+		EncryptionContext: ctx,
+		AwsProfile:        key.AwsProfile,
+	}
+}

keyservice/server_test.go

@@ -0,0 +1,81 @@
+package keyservice
+
+import (
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"testing"
+)
+
+func TestKmsKeyToMasterKey(t *testing.T) {
+
+	cases := []struct {
+		description        string
+		expectedArn        string
+		expectedRole       string
+		expectedCtx        map[string]string
+		expectedAwsProfile string
+	}{
+		{
+			description:        "empty context",
+			expectedArn:        "arn:aws:kms:eu-west-1:123456789012:key/d5c90a06-f824-4628-922b-12424571ed4d",
+			expectedRole:       "ExampleRole",
+			expectedCtx:        map[string]string{},
+			expectedAwsProfile: "",
+		},
+		{
+			description:  "context with one key-value pair",
+			expectedArn:  "arn:aws:kms:eu-west-1:123456789012:key/d5c90a06-f824-4628-922b-12424571ed4d",
+			expectedRole: "",
+			expectedCtx: map[string]string{
+				"firstKey": "first value",
+			},
+			expectedAwsProfile: "ExampleProfile",
+		},
+		{
+			description:  "context with three key-value pairs",
+			expectedArn:  "arn:aws:kms:eu-west-1:123456789012:key/d5c90a06-f824-4628-922b-12424571ed4d",
+			expectedRole: "",
+			expectedCtx: map[string]string{
+				"firstKey":  "first value",
+				"secondKey": "second value",
+				"thirdKey":  "third value",
+			},
+			expectedAwsProfile: "",
+		},
+	}
+
+	for _, c := range cases {
+
+		t.Run(c.description, func(t *testing.T) {
+
+			inputCtx := make(map[string]string)
+			for k, v := range c.expectedCtx {
+				inputCtx[k] = v
+			}
+
+			key := &KmsKey{
+				Arn:        c.expectedArn,
+				Role:       c.expectedRole,
+				Context:    inputCtx,
+				AwsProfile: c.expectedAwsProfile,
+			}
+
+			masterKey := kmsKeyToMasterKey(key)
+			foundCtx := masterKey.EncryptionContext
+
+			for k, _ := range c.expectedCtx {
+				require.Containsf(t, foundCtx, k, "Context does not contain expected key '%s'", k)
+			}
+			for k, _ := range foundCtx {
+				require.Containsf(t, c.expectedCtx, k, "Context contains an unexpected key '%s' which cannot be found from expected map", k)
+			}
+			for k, expected := range c.expectedCtx {
+				foundVal := *foundCtx[k]
+				assert.Equalf(t, expected, foundVal, "Context key '%s' value '%s' does not match expected value '%s'", k, foundVal, expected)
+			}
+			assert.Equalf(t, c.expectedArn, masterKey.Arn, "Expected ARN to be '%s', but found '%s'", c.expectedArn, masterKey.Arn)
+			assert.Equalf(t, c.expectedRole, masterKey.Role, "Expected Role to be '%s', but found '%s'", c.expectedRole, masterKey.Role)
+			assert.Equalf(t, c.expectedAwsProfile, masterKey.AwsProfile, "Expected AWS profile to be '%s', but found '%s'", c.expectedAwsProfile, masterKey.AwsProfile)
+		})
+	}
+}