diff --git a/src/requests/auth.py b/src/requests/auth.py
index 2af481dbf5..fb4b672345 100644
--- a/src/requests/auth.py
+++ b/src/requests/auth.py
@@ -218,7 +218,16 @@ def KD(s: str, d: str) -> str:
         if p_parsed.query:
             path += f"?{p_parsed.query}"
 
-        A1 = f"{self.username}:{realm}:{self.password}"
+        if isinstance(self.username, bytes):
+            username = self.username.decode("utf-8")
+        else:
+            username = self.username
+        if isinstance(self.password, bytes):
+            password = self.password.decode("utf-8")
+        else:
+            password = self.password
+
+        A1 = f"{username}:{realm}:{password}"
         A2 = f"{method}:{path}"
 
         HA1 = hash_utf8(A1)
@@ -250,8 +259,12 @@ def KD(s: str, d: str) -> str:
         self._thread_local.last_nonce = nonce
 
         # XXX should the partial digests be encoded too?
+        if isinstance(self.username, bytes):
+            username_header = self.username.decode("utf-8")
+        else:
+            username_header = self.username
         base = (
-            f'username="{self.username}", realm="{realm}", nonce="{nonce}", '
+            f'username="{username_header}", realm="{realm}", nonce="{nonce}", '
             f'uri="{path}", response="{respdig}"'
         )
         if opaque: