diff --git a/moto/cognitoidp/models.py b/moto/cognitoidp/models.py
index 7078583fa..13da5d41d 100644
--- a/moto/cognitoidp/models.py
+++ b/moto/cognitoidp/models.py
@@ -944,8 +944,8 @@ class CognitoIdpBackend(BaseBackend):
                 "ChallengeName": "PASSWORD_VERIFIER",
                 "Session": session,
                 "ChallengeParameters": {
-                    "SALT": str(uuid.uuid4()),
-                    "SRP_B": str(uuid.uuid4()),
+                    "SALT": uuid.uuid4().hex,
+                    "SRP_B": uuid.uuid4().hex,
                     "USERNAME": user.id,
                     "USER_ID_FOR_SRP": user.id,
                     "SECRET_BLOCK": session,
diff --git a/tests/test_cognitoidp/test_cognitoidp.py b/tests/test_cognitoidp/test_cognitoidp.py
index c61be4aa4..f45f7c079 100644
--- a/tests/test_cognitoidp/test_cognitoidp.py
+++ b/tests/test_cognitoidp/test_cognitoidp.py
@@ -1294,7 +1294,7 @@ def user_authentication_flow(conn):
         AuthFlow="USER_SRP_AUTH",
         AuthParameters={
             "USERNAME": username,
-            "SRP_A": str(uuid.uuid4()),
+            "SRP_A": uuid.uuid4().hex,
             "SECRET_HASH": secret_hash,
         },
     )
@@ -1341,7 +1341,7 @@ def user_authentication_flow(conn):
         AuthFlow="USER_SRP_AUTH",
         AuthParameters={
             "USERNAME": username,
-            "SRP_A": str(uuid.uuid4()),
+            "SRP_A": uuid.uuid4().hex,
             "SECRET_HASH": secret_hash,
         },
     )
@@ -1638,7 +1638,7 @@ def test_initiate_auth_USER_SRP_AUTH():
         AuthFlow="USER_SRP_AUTH",
         AuthParameters={
             "USERNAME": username,
-            "SRP_A": str(uuid.uuid4()),
+            "SRP_A": uuid.uuid4().hex,
             "SECRET_HASH": secret_hash,
         },
     )
@@ -1688,7 +1688,7 @@ def test_initiate_auth_for_unconfirmed_user():
             AuthFlow="USER_SRP_AUTH",
             AuthParameters={
                 "USERNAME": username,
-                "SRP_A": str(uuid.uuid4()),
+                "SRP_A": uuid.uuid4().hex,
                 "SECRET_HASH": secret_hash,
             },
         )
@@ -1724,7 +1724,7 @@ def test_initiate_auth_with_invalid_secret_hash():
             AuthFlow="USER_SRP_AUTH",
             AuthParameters={
                 "USERNAME": username,
-                "SRP_A": str(uuid.uuid4()),
+                "SRP_A": uuid.uuid4().hex,
                 "SECRET_HASH": invalid_secret_hash,
             },
         )
@@ -1782,7 +1782,7 @@ def test_respond_to_auth_challenge_with_invalid_secret_hash():
         AuthFlow="USER_SRP_AUTH",
         AuthParameters={
             "USERNAME": result["username"],
-            "SRP_A": str(uuid.uuid4()),
+            "SRP_A": uuid.uuid4().hex,
             "SECRET_HASH": valid_secret_hash,
         },
     )