diff --git a/.jules/sentinel.md b/.jules/sentinel.md
new file mode 100644
index 000000000..ec2af4f1d
--- /dev/null
+++ b/.jules/sentinel.md
@@ -0,0 +1,4 @@
+## 2026-06-16 - Unhandled IndexError Hardening in Cryptography Routines
+**Vulnerability:** Amber's `MessageVerifier` and `MessageEncryptor` attempted to slice arrays and decrypt payloads before checking the bounds of the provided data (`IndexError` due to missing `split` size check and negative offsets in slices). While top-level middleware rescues and returns a 500 status code, these classes should gracefully reject malformed payloads themselves with custom exceptions.
+**Learning:** Crystal's array slicing and destructuring raise `IndexError` when bounds are not met, which propagates to an uncaught exception instead of a graceful rejection. Always check payload sizes before executing cryptography functions.
+**Prevention:** Always validate size and structure before extracting signatures, IVs, or payload data from untrusted inputs in cryptography routines. Add `size` bounds checks and safely handle destructuring, raising typed exceptions like `InvalidSignature` and `InvalidMessage`.
diff --git a/spec/amber/support/message_encryptor_spec.cr b/spec/amber/support/message_encryptor_spec.cr
new file mode 100644
index 000000000..7addb153a
--- /dev/null
+++ b/spec/amber/support/message_encryptor_spec.cr
@@ -0,0 +1,31 @@
+require "../../spec_helper"
+
+module Amber::Support
+  describe MessageEncryptor do
+    secret = "a" * 32
+    encryptor = MessageEncryptor.new(secret)
+
+    it "encrypts and decrypts valid messages successfully" do
+      message = "Hello, world!"
+      encrypted = encryptor.encrypt_and_sign(message)
+      decrypted = encryptor.verify_and_decrypt(encrypted)
+      String.new(decrypted).should eq message
+    end
+
+    it "raises Exceptions::InvalidMessage for undersized payloads in verify_and_decrypt" do
+      # signature_size is 32, block_size is 16. Total required: >= 48 bytes
+      undersized = Bytes.new(20)
+      expect_raises(Exceptions::InvalidMessage, "Invalid message size") do
+        encryptor.verify_and_decrypt(undersized)
+      end
+    end
+
+    it "raises Exceptions::InvalidMessage for undersized payloads in decrypt" do
+      # block_size is 16. decrypt expects >= 16 bytes
+      undersized = Bytes.new(10)
+      expect_raises(Exceptions::InvalidMessage, "Invalid message size") do
+        encryptor.decrypt(undersized)
+      end
+    end
+  end
+end
diff --git a/spec/amber/support/message_verifier_spec.cr b/spec/amber/support/message_verifier_spec.cr
new file mode 100644
index 000000000..32ba25076
--- /dev/null
+++ b/spec/amber/support/message_verifier_spec.cr
@@ -0,0 +1,27 @@
+require "../../spec_helper"
+
+module Amber::Support
+  describe MessageVerifier do
+    secret = "a" * 32
+    verifier = MessageVerifier.new(secret)
+
+    it "signs and verifies valid messages successfully" do
+      message = "Hello, world!"
+      signed = verifier.generate(message)
+      verified = verifier.verified(signed)
+      verified.should eq message
+    end
+
+    it "returns nil for malformed/no-delimiter payloads in verified" do
+      malformed = "invalidpayload"
+      verifier.verified(malformed).should be_nil
+    end
+
+    it "raises Exceptions::InvalidSignature for malformed/no-delimiter payloads in verify_raw" do
+      malformed = "invalidpayload"
+      expect_raises(Exceptions::InvalidSignature) do
+        verifier.verify_raw(malformed)
+      end
+    end
+  end
+end
diff --git a/src/amber/support/message_encryptor.cr b/src/amber/support/message_encryptor.cr
index 8ebbb1133..ed31ebbd7 100644
--- a/src/amber/support/message_encryptor.cr
+++ b/src/amber/support/message_encryptor.cr
@@ -18,6 +18,10 @@ module Amber::Support
     # Verify and Decrypt a message. We need to verify the message in order to
     # avoid padding attacks. Reference: http://www.limited-entropy.com/padding-oracle-attacks.
     def verify_and_decrypt(value : Bytes) : Bytes
+      if value.size < @signature_size + @block_size
+        raise Exceptions::InvalidMessage.new("Invalid message size")
+      end
+
       signature = value[value.size - @signature_size, @signature_size]
       data_iv = value[0, value.size - @signature_size]
       if Crypto::Subtle.constant_time_compare(sign_bytes(data_iv), signature)
@@ -48,6 +52,10 @@ module Amber::Support
     end
 
     def decrypt(value : Bytes)
+      if value.size < @block_size
+        raise Exceptions::InvalidMessage.new("Invalid message size")
+      end
+
       cipher = OpenSSL::Cipher.new(@cipher_algorithm)
       data = value[0, value.size - @block_size]
       iv = value[value.size - @block_size, @block_size]
diff --git a/src/amber/support/message_verifier.cr b/src/amber/support/message_verifier.cr
index 0d40f5b12..c78b47f10 100644
--- a/src/amber/support/message_verifier.cr
+++ b/src/amber/support/message_verifier.cr
@@ -11,7 +11,10 @@ module Amber::Support
     end
 
     def verified(signed_message : String)
-      data, digest = signed_message.split("--")
+      parts = signed_message.split("--", 2)
+      return unless parts.size == 2
+      data, digest = parts[0], parts[1]
+
       if valid_message?(data, digest)
         String.new(decode(data))
       end
@@ -25,7 +28,10 @@ module Amber::Support
     end
 
     def verify_raw(signed_message : String) : Bytes
-      data, digest = signed_message.split("--")
+      parts = signed_message.split("--", 2)
+      raise(Exceptions::InvalidSignature.new) unless parts.size == 2
+      data, digest = parts[0], parts[1]
+
       if valid_message?(data, digest)
         decode(data)
       else