diff --git a/src/halmos/bytevec.py b/src/halmos/bytevec.py
index 99cedfdd..c060c979 100644
--- a/src/halmos/bytevec.py
+++ b/src/halmos/bytevec.py
@@ -4,7 +4,7 @@
 from typing import Any, ForwardRef
 
 from sortedcontainers import SortedDict
-from z3 import BitVecRef, If, eq, is_bool, is_bv, is_bv_value, simplify
+from z3 import BitVecRef, If, eq, is_bool, is_bv, is_bv_value, simplify, substitute
 
 from .utils import (
     Byte,
@@ -142,6 +142,33 @@ def __getitem__(self, key) -> Byte | ForwardRef("Chunk"):
     def __len__(self) -> int:
         return self.length
 
+    def concretize(self, substitution: dict[BitVecRef, BitVecRef]) -> "Chunk":
+        """
+        Replace all symbols in the data with their corresponding concrete values,
+        if they exist in the given substitution mapping.
+
+        Return a new object with concrete values, or self if no substitution is made.
+        """
+
+        if not isinstance(self, SymbolicChunk):
+            return self
+
+        data = self.data
+        # TODO: call substitute() only when the data contains variables to be substituted
+        new_data = substitute(data, *substitution.items())
+
+        # NOTE: `new_data is data` doesn't work, because substitute() always returns a new object even if no substitution is made
+        if eq(new_data, data):
+            return self
+
+        new_data = simplify(new_data)
+
+        return (
+            ConcreteChunk(bv_value_to_bytes(new_data), self.start, self.length)
+            if is_bv_value(new_data)
+            else SymbolicChunk(new_data, self.start, self.length)
+        )
+
     @abstractmethod
     def get_byte(self, offset) -> Byte:
         raise NotImplementedError
@@ -610,6 +637,21 @@ def set_word(self, offset: int, value: Word) -> None:
 
         self.set_slice(offset, offset + 32, value)
 
+    def concretize(self, substitution: dict[BitVecRef, BitVecRef]) -> "ByteVec":
+        """
+        Replace all symbols in the chunks with their corresponding concrete values,
+        if they exist in the given substitution mapping.
+
+        Return a new ByteVec object even if no substitution is made.
+        """
+
+        result = ByteVec()
+
+        for chunk in self.chunks.values():
+            result.append(chunk.concretize(substitution))
+
+        return result
+
     ### read operations
 
     def slice(self, start: int, stop: int) -> "ByteVec":
diff --git a/src/halmos/config.py b/src/halmos/config.py
index fad3b0d3..c9d0f963 100644
--- a/src/halmos/config.py
+++ b/src/halmos/config.py
@@ -188,7 +188,7 @@ class Config:
 
     default_bytes_lengths: str = arg(
         help="set the default length candidates for bytes and string not specified in --array-lengths",
-        global_default="0,32,1024,65",  # 65 is ECDSA signature size
+        global_default="0,1024,65",  # 65 is ECDSA signature size
         metavar="LENGTH1,LENGTH2,...",
         action=ParseCSV,
     )
@@ -549,7 +549,19 @@ def parse_dict(self, parsed: dict, source: str = "halmos.toml") -> dict:
                 )
                 sys.exit(2)
 
-        return {k.replace("-", "_"): v for k, v in data.items()}
+        # gather custom actions
+        actions = {
+            field.name: field.metadata["action"]
+            for field in fields(Config)
+            if "action" in field.metadata
+        }
+
+        result = {}
+        for key, value in data.items():
+            key = key.replace("-", "_")
+            action = actions.get(key)
+            result[key] = action.parse(value) if action else value
+        return result
 
 
 def _create_default_config() -> "Config":
diff --git a/src/halmos/sevm.py b/src/halmos/sevm.py
index 28a731c6..cfd7da4d 100644
--- a/src/halmos/sevm.py
+++ b/src/halmos/sevm.py
@@ -644,8 +644,10 @@ class Concretization:
     In the future, it may be used for other purposes, such as concrete hash reasoning.
     """
 
-    substitution: dict = field(default_factory=dict)  # symbol -> constant
-    candidates: dict = field(default_factory=dict)  # symbol -> set of constants
+    # symbol -> constant
+    substitution: dict[BitVecRef, BitVecRef] = field(default_factory=dict)
+    # symbol -> set of constants
+    candidates: dict[BitVecRef, list[int]] = field(default_factory=dict)
 
     def process_cond(self, cond):
         if not is_eq(cond):
@@ -2864,6 +2866,7 @@ def finalize(ex: Exec):
 
                     if size > 0:
                         data: ByteVec = ex.calldata().slice(offset, offset + size)
+                        data = data.concretize(ex.path.concretization.substitution)
                         ex.st.memory.set_slice(loc, end_loc, data)
 
                 elif opcode == EVM.CODECOPY: