diff --git a/src/lib/base/String.cpp b/src/lib/base/String.cpp
index 389ca8aa..dcf7318a 100644
--- a/src/lib/base/String.cpp
+++ b/src/lib/base/String.cpp
@@ -185,8 +185,7 @@ removeFileExt(std::string filename)
     return filename.substr(0, dot);
 }
 
-void
-toHex(std::string& subject, int width, const char fill)
+std::string to_hex(const std::string& subject, int width, const char fill)
 {
     std::stringstream ss;
     ss << std::hex;
@@ -194,7 +193,7 @@ toHex(std::string& subject, int width, const char fill)
         ss << std::setw(width) << std::setfill(fill) << (int)(unsigned char)subject[i];
     }
 
-    subject = ss.str();
+    return ss.str();
 }
 
 void
diff --git a/src/lib/base/String.h b/src/lib/base/String.h
index 047b6e16..a543cf86 100644
--- a/src/lib/base/String.h
+++ b/src/lib/base/String.h
@@ -75,7 +75,7 @@ std::string removeFileExt(std::string filename);
 /*!
 Convert each character in \c subject into hexdecimal form with \c width
 */
-void toHex(std::string& subject, int width, const char fill = '0');
+std::string to_hex(const std::string& subject, int width, const char fill = '0');
 
 //! Convert to all uppercase
 /*!
diff --git a/src/lib/net/SecureUtils.cpp b/src/lib/net/SecureUtils.cpp
index 72d4fbf0..7c4a52da 100644
--- a/src/lib/net/SecureUtils.cpp
+++ b/src/lib/net/SecureUtils.cpp
@@ -23,7 +23,7 @@ std::string format_ssl_fingerprint(const std::string& fingerprint, bool hex, boo
     std::string result = fingerprint;
     if (hex) {
         // to hexadecimal
-        barrier::string::toHex(result, 2);
+        result = barrier::string::to_hex(result, 2);
     }
 
     // all uppercase
@@ -36,4 +36,5 @@ std::string format_ssl_fingerprint(const std::string& fingerprint, bool hex, boo
             result.insert(i * 3 - 1, ":");
         }
     }
+    return result;
 }
diff --git a/src/test/unittests/base/StringTests.cpp b/src/test/unittests/base/StringTests.cpp
index 5643aa53..fd771f42 100644
--- a/src/test/unittests/base/StringTests.cpp
+++ b/src/test/unittests/base/StringTests.cpp
@@ -59,9 +59,7 @@ TEST(StringTests, toHex_plaintext_hexString)
     String subject = "foobar";
     int width = 2;
 
-    string::toHex(subject, width);
-
-    EXPECT_EQ("666f6f626172", subject);
+    EXPECT_EQ("666f6f626172", string::to_hex(subject, width));
 }
 
 TEST(StringTests, uppercase_lowercaseInput_uppercaseOutput)