Bleichenbacher Attack in Phoenix with Dynamodb

A Bleichenbacher attack is a cryptographic padding oracle that can allow an attacker to decrypt ciphertexts without knowing the key by repeatedly sending specially crafted requests and observing error behavior. When this pattern is applied in a Phoenix application that uses Amazon DynamoDB as a persistence layer, the interaction between application-level error handling and DynamoDB operations can unintentionally reveal information that facilitates the attack.

In Phoenix, developers often store encrypted values—such as authentication tokens or session blobs—in DynamoDB. If decryption is performed in application code and padding errors (e.g., :crypto_block_decrypt failures) are surfaced as distinguishable HTTP 500 responses or distinct timing differences, the endpoint becomes an oracle. An attacker can exploit this by submitting modified ciphertexts and observing whether each response indicates a padding validation failure versus other errors. Because DynamoDB is used as the storage backend, the attacker does not need to compromise the database; they only need to observe application behavior to infer validity of decrypted data.

Phoenix applications that perform decryption before validating request integrity are particularly at risk. For example, if an API endpoint retrieves an item from DynamoDB by a user-controlled identifier, decrypts a field, and then branches logic based on padding correctness, the timing and status code differences can be measurable. This measurable difference, combined with a predictable ciphertext format, maps directly to the classic Bleichenbacher threat model. The use of DynamoDB in this flow does not introduce the cryptographic weakness, but it standardizes the data layer and can make error handling patterns more consistent across requests, aiding an attacker’s ability to build an adaptive chosen-ciphertext oracle.

Real-world context includes known weaknesses around PKCS#1 v1.5 padding in RSA. Instances where Phoenix services decrypt data retrieved from DynamoDB without using constant-time checks or authenticated encryption are susceptible. Findings from a middleBrick scan targeting such an endpoint would highlight the absence of uniform error handling and the presence of timing variance tied to cryptographic operations, mapping to OWASP API Top 10 and relevant compliance frameworks.

Remediation focuses on ensuring that cryptographic operations do not leak distinguishable errors and that DynamoDB interactions do not amplify timing differences. Use authenticated encryption with associated data (AEAD) such as AES-GCM instead of raw RSA with PKCS#1 v1.5 padding. When RSA is required, employ OAEP with constant-time verification and avoid branching on padding correctness. Also, structure DynamoDB access patterns to be invariant regarding decryption outcomes.

Example: using AES-GCM for deterministic-safe encryption stored in DynamoDB via the AWS SDK for Erlang/Elixir. This pattern avoids padding-oracle conditions entirely because decryption either succeeds and yields plaintext or fails with a unified error path.

defmodule MyApp.Encryption do
  @aead_alg :aes_gcm
  @key <<128-bit key, 32 bytes>>

  def encrypt(plaintext) do
    iv = :crypto.strong_rand_bytes(12)
    {ciphertext, auth_tag} = :crypto.crypto_one_time(@aead_alg, @key, iv, plaintext, true)
    Base.encode64(iv <> auth_tag <> ciphertext)
  end

  def decrypt(blob) do
    data = Base.decode64!(blob)
    <> = data
    :crypto.crypto_one_time(@aead_alg, @key, iv, ciphertext, false, auth_tag)
  rescue
    _ -> {:error, :decryption_failed}
  end
end

If RSA-OAEP is required, ensure decryption errors are caught and mapped to a generic failure response with consistent timing. Avoid early exits based on padding checks.

def decrypt_rsa_oaep(ciphertext_blob) do
  private_key_pem = System.get_env("RSA_PRIVATE_PEM")
  {:ok, private_key} = :public_key.pem_decode(private_key_pem) |> List.keyfind(:RSAPrivateKey, 0)

  try do
    :public_key.decrypt_public(ciphertext_blob, private_key, [rsa_padding_type: :rsa_oaep_padding])
    {:ok, :decrypted}
  rescue
    _ ->
      # Always return a generic error and simulate constant-time work where possible
      :crypto.strong_rand_bytes(32)
      {:error, :generic_failure}
  end
end

In your Phoenix controller or context, ensure DynamoDB calls do not depend on decryption success for branching that changes response classification. Return a uniform error shape and use HTTP status 400 for client-side issues and 500 only for unexpected conditions, never for padding failures.

defmodule MyAppWeb.ApiController do
  use MyAppWeb, :controller

  def show(conn, %{"id" => id}) do
    with {:ok, item} <- Dynamo.get_item(MyApp.DynamoClient, "my_table", id),
         {:ok, plaintext} <- MyApp.Encryption.decrypt(item.ciphertext_blob) do
      json(conn, %{data: plaintext})
    else
      {:error, :decryption_failed} ->
        # Uniform response to avoid oracle behavior
        conn
        |> put_status(:bad_request)
        |> json(%{error: "invalid_request"})
      _ ->
        conn
        |> put_status(:internal_server_error)
        |> json(%{error: "internal_error"})
    end
  end
end

Additionally, consider storing metadata about encryption schemes in DynamoDB item attributes to avoid runtime negotiation that could introduce variability. middleBrick scans can validate that endpoints handling sensitive data do not exhibit timing-sensitive branching on cryptographic outcomes, ensuring remediation aligns with OWASP API Top 10 and compliance mappings.