fix nip04 decryption padding and add tests.

nip04/nip04.go

@@ -51,10 +51,12 @@ func Encrypt(message string, key []byte) (string, error) {
 	}
 	mode := cipher.NewCBCEncrypter(block, iv)
 
-	// PKCS5 padding
-	padding := block.BlockSize() - len([]byte(message))%block.BlockSize()
-	padtext := bytes.Repeat([]byte{byte(padding)}, padding)
-	paddedMsgBytes := append([]byte(message), padtext...)
+	plaintext := []byte(message)
+
+	// add padding
+	padding := block.BlockSize() - len(plaintext)%block.BlockSize() // will be a number between 1 and 16 (inc), never 0
+	padtext := bytes.Repeat([]byte{byte(padding)}, padding)         // encode the padding in all the padding bytes
+	paddedMsgBytes := append(plaintext, padtext...)
 
 	ciphertext := make([]byte, len(paddedMsgBytes))
 	mode.CryptBlocks(ciphertext, paddedMsgBytes)
@@ -87,5 +89,9 @@ func Decrypt(content string, key []byte) (string, error) {
 	plaintext := make([]byte, len(ciphertext))
 	mode.CryptBlocks(plaintext, ciphertext)
 
-	return string(plaintext[:]), nil
+	// remove padding
+	padding := int(plaintext[len(plaintext)-1]) // the padding amount is encoded in the padding bytes themselves
+	message := string(plaintext[0 : len(plaintext)-padding])
+
+	return message, nil
 }

nip04/nip04_test.go

@@ -0,0 +1,47 @@
+package nip04
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestEncryptionAndDecryption(t *testing.T) {
+	sharedSecret := make([]byte, 32)
+	message := "hello hellow"
+
+	ciphertext, err := Encrypt(message, sharedSecret)
+	if err != nil {
+		t.Errorf("failed to encrypt: %s", err.Error())
+	}
+
+	plaintext, err := Decrypt(ciphertext, sharedSecret)
+	if err != nil {
+		t.Errorf("failed to decrypt: %s", err.Error())
+	}
+
+	if message != plaintext {
+		t.Errorf("original '%s' and decrypted '%s' messages differ", message, plaintext)
+	}
+}
+
+func TestEncryptionAndDecryptionWithMultipleLengths(t *testing.T) {
+	sharedSecret := make([]byte, 32)
+
+	for i := 0; i < 150; i++ {
+		message := strings.Repeat("a", i)
+
+		ciphertext, err := Encrypt(message, sharedSecret)
+		if err != nil {
+			t.Errorf("failed to encrypt: %s", err.Error())
+		}
+
+		plaintext, err := Decrypt(ciphertext, sharedSecret)
+		if err != nil {
+			t.Errorf("failed to decrypt: %s", err.Error())
+		}
+
+		if message != plaintext {
+			t.Errorf("original '%s' and decrypted '%s' messages differ", message, plaintext)
+		}
+	}
+}