From bc9155fd8476f011ecf3a03847afe91242155c06 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tam=C3=A1s=20Michelberger?= <tomi@secretsaucepartners.com>
Date: Wed, 20 Dec 2017 19:29:17 +0100
Subject: [PATCH] Fix compatibility issue with credstash >=1.13.1 fixes #8

---
 credstash/secret.go      | 18 ++++++++++++++++++
 credstash/secret_test.go | 18 ++++++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/credstash/secret.go b/credstash/secret.go
index 912d5bc..156f43e 100644
--- a/credstash/secret.go
+++ b/credstash/secret.go
@@ -170,6 +170,16 @@ func keyMaterialFromDBItem(item map[string]*dynamodb.AttributeValue) (keyMateria
 	if material.HMAC, err = getStringAndDecode(item, "hmac", hex.DecodeString); err != nil {
 		return keyMaterial{}, err
 	}
+	// In credstash < 1.13.1 HMAC was store as a hex encoded string. After
+	// version 1.13.1 credstash started storing the value in a hex encoded
+	// binary value. To keep compatibility with both versions when the HMAC is
+	// empty after trying to decode it from a string field we try the binary
+	// field.
+	if len(material.HMAC) == 0 {
+		if material.HMAC, err = getBinaryStringAndDecode(item, "hmac", hex.DecodeString); err != nil {
+			return keyMaterial{}, err
+		}
+	}
 
 	if material.Key, err = getStringAndDecode(item, "key", base64.StdEncoding.DecodeString); err != nil {
 		return keyMaterial{}, err
@@ -207,3 +217,11 @@ func getStringAndDecode(item map[string]*dynamodb.AttributeValue, key string, f
 	}
 	return f(s)
 }
+
+func getBinaryStringAndDecode(item map[string]*dynamodb.AttributeValue, key string, f func(string) ([]byte, error)) ([]byte, error) {
+	value, ok := item[key]
+	if !ok {
+		return nil, fmt.Errorf("missing key: %s", key)
+	}
+	return f(string(value.B))
+}
diff --git a/credstash/secret_test.go b/credstash/secret_test.go
index 54eb6bb..ff49813 100644
--- a/credstash/secret_test.go
+++ b/credstash/secret_test.go
@@ -115,6 +115,18 @@ func TestKeyMaterialFromDDBResult(t *testing.T) {
 			item:       dummyItemWithMissingKey(),
 			shouldFail: true,
 		},
+		{
+			desc: "binary HMAC field",
+			item: dummyItemWithBinaryHMAC("01020304"),
+			km: keyMaterial{
+				Name:    "test_key",
+				Version: "0000000000000000001",
+				Digest:  "SHA256",
+				Key:     []byte{1, 2, 3, 4},
+				Content: []byte{1, 2, 3, 4},
+				HMAC:    []byte{1, 2, 3, 4},
+			},
+		},
 	}
 
 	for _, tt := range testCases {
@@ -172,6 +184,12 @@ func dummyItemWithAllFields() map[string]*dynamodb.AttributeValue {
 	}
 }
 
+func dummyItemWithBinaryHMAC(hmac string) map[string]*dynamodb.AttributeValue {
+	item := dummyItemWithAllFields()
+	item["hmac"] = &dynamodb.AttributeValue{B: []byte(hmac)}
+	return item
+}
+
 func dummyItemWithWrongKey() map[string]*dynamodb.AttributeValue {
 	item := dummyItemWithAllFields()
 	item["key"] = attrValueString("not base64")