From fefad1e7814d37fe6565cbd5f3b50613c95b560c Mon Sep 17 00:00:00 2001
From: Matteo Pace <pace.matteo96@gmail.com>
Date: Fri, 15 Dec 2023 10:59:11 +0100
Subject: [PATCH] better comment about second DecodeString call

---
 internal/transformations/base64decode.go      |  6 ++++--
 internal/transformations/base64decode_test.go | 15 ++++++++++-----
 2 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/internal/transformations/base64decode.go b/internal/transformations/base64decode.go
index 52bb86baa..42eb0b5f3 100644
--- a/internal/transformations/base64decode.go
+++ b/internal/transformations/base64decode.go
@@ -21,8 +21,10 @@ func base64decode(data string) (string, bool, error) {
 		// and perform a partial decoding up to that point
 		if corrErr, ok := err.(base64.CorruptInputError); ok {
 			illegalCharPos := int(corrErr)
-			// Forgiving call to DecodeString, decoding is performed up to the illegal characther
-			// If an error occurs, dec will still contain the decoded string up to the error
+			// Forgiving call (no error check) to DecodeString. Decoding is performed truncating
+			// the input string to the first error index. If a new decoding error occurs,
+			// it will not be about an illegal character but a malformed encoding of the trailing
+			// character because of the truncation. The dec will still contain a best effort decoded string
 			dec, _ = base64.RawStdEncoding.DecodeString(dataNoPadding[:illegalCharPos])
 		} else {
 			return data, false, nil
diff --git a/internal/transformations/base64decode_test.go b/internal/transformations/base64decode_test.go
index 65b43adbf..cd5ef9213 100644
--- a/internal/transformations/base64decode_test.go
+++ b/internal/transformations/base64decode_test.go
@@ -41,27 +41,32 @@ var b64DecodeTests = []struct {
 		expected: "<TEST>",
 	},
 	{
-		name:     "decoded up to the space (invalid caracter)",
+		name:     "Malformed base64 encoding",
+		input:    "PHNjcmlwd",
+		expected: "<scrip",
+	},
+	{
+		name:     "decoded up to the space (invalid character)",
 		input:    "PFR FU1Q+",
 		expected: "<T",
 	},
 	{
 		name:     "decoded up to the dot (invalid caracter)",
 		input:    "P.HNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==",
-		expected: "",
+		expected: "", // Only the P character does not result in a printable character conversion.
 	},
 	{
-		name:     "decoded up to the dot (invalid caracter)",
+		name:     "decoded up to the dot (invalid character)",
 		input:    "PHNjcmlwd.D5hbGVydCgxKTwvc2NyaXB0Pg==",
 		expected: "<scrip",
 	},
 	{
-		name:     "decoded up to the dot (invalid caracter)",
+		name:     "decoded up to the dot (invalid character)",
 		input:    "PHNjcmlwdD.5hbGVydCgxKTwvc2NyaXB0Pg==",
 		expected: "<script",
 	},
 	{
-		name:     "decoded up to the dash (invalid caracter for base64.RawStdEncoding)",
+		name:     "decoded up to the dash (invalid character for base64.RawStdEncoding)",
 		input:    "PFRFU1Q-",
 		expected: "<TEST",
 	},