Bleichenbacher Attack in Chi with Dynamodb

A Bleichenbacher attack is a padding oracle attack originally described against RSA encryption with PKCS#1 v1.5 padding. In Chi, when an application uses Dynamodb to store encrypted data and relies on RSA decryption in an API endpoint without proper integrity checks, the combination can expose a padding oracle. An attacker can send manipulated ciphertexts to the endpoint and observe distinct error responses—such as “bad padding” versus “valid decryption but invalid data”—which allows iterative decryption of ciphertexts without knowing the private key.

With Dynamodb, this often arises when encrypted fields (e.g., secrets or tokens) are stored as attributes and decrypted per-request in application logic. If the decryption routine returns different HTTP status codes or response bodies for padding failures versus other errors, the unauthenticated attack surface becomes enumerable. middleBrick scans such endpoints among its 12 parallel security checks, including Input Validation and Unsafe Consumption, and can detect disproportionate error messages or timing differences that suggest an oracle condition. Attack patterns like CVE-2016-2183 (the ROBOT attack family) illustrate this class of vulnerability when public-key operations leak via error handling.

In Chi, if an API uses RSA-OAEP or RSA-PKCS1 with Dynamodb as the persistence layer, ensure decryption errors are normalized. Do not let the API surface low-level crypto exceptions or allow timing variance based on padding correctness. middleBrick’s LLM/AI Security checks are not applicable here, but its Authentication, BOLA/IDOR, and Input Validation checks can help identify endpoints that process attacker-controlled ciphertexts and return distinguishable responses.

Remediation focuses on making decryption constant-time and returning uniform error responses regardless of padding validity. In Chi, handle decryption failures inside your handler and return a generic error payload with a consistent HTTP status code, such as 400, for all client-side decryption problems. Avoid branching logic that reveals which step failed (padding vs. integrity vs. business rule).

Use authenticated encryption (e.g., AES-GCM) where possible, or use an HMAC to verify ciphertext integrity before decryption. If you must use RSA, decrypt with OAEP and catch all exceptions, mapping them to a single, opaque failure response. Never expose stack traces or internal messages to the client.

Example in Chi with Dynamodb get-item and decryption wrapper:

// Chi endpoint example with constant-time error handling
import chisel.core._
import chisel.http._
import chisel.db._
import software.amazon.awssdk.services.dynamodb.DynamoDbClient
import software.amazon.awssdk.services.dynamodb.model.GetItemRequest
import javax.crypto.Cipher
import javax.crypto.spec.SecretKeySpec
import java.util.Base64

object Crypto {
  private val cipher = Cipher.getInstance("RSA/ECB/OAEPWITHSHA-256ANDMGF1PADDING")

  def decryptOAEP(ciphertext: Array[Byte], privateKey: PrivateKey): Either[String, Array[Byte]] = {
    try {
      cipher.init(Cipher.DECRYPT_MODE, privateKey)
      Right(cipher.doFinal(ciphertext))
    } catch {
      case _: Exception => Left("decryption_error")
    }
  }
}

object App {
  val ddb: DynamoDbClient = DynamoDbClient.create()

  val getSecretRoute = get("secret") { req: Request =>
    val id = req.params.getOrElse("id", "")
    val request = GetItemRequest.builder
      .tableName("secrets")
      .key(Map("id" -> AttributeValue.builder().s(id).build()).asJava)
      .build()

    val item = ddb.getItem(request).item()
    val encB64 = item.get("encrypted_data").s()
    val ciphertext = Base64.getDecoder.decode(encB64)

    // Use a secure key management approach in production
    val privateKey = loadPrivateKey()
    Crypto.decryptOAEP(ciphertext, privateKey) match {
      case Right(plaintext) => Ok(plaintext.utf8String)
      case Left(_) => // Always return the same status and body shape to avoid oracle
                       BadRequest(Map("error" -> "invalid_request").asJava)
    }
  }
}

Key points:

• Always catch exceptions from decryption and map to a generic error response.
• Use OAEP with SHA-256 and MGF1 rather than PKCS1 v1.5 padding to avoid known padding oracle weaknesses.
• Consider encrypting at rest with Dynamodb server-side encryption and adding an application-level AEAD envelope for highly sensitive fields.
• Validate and encode all inputs that become DynamoDB keys or attribute names to prevent NoSQL injection or traversal, which could amplify an oracle by altering query paths.

middleBrick’s CLI can scan your Chi endpoints to detect non-uniform error handling or inputs that reach Dynamodb in unexpected ways. Run middlebrick scan <url> to get a report on findings related to Authentication, Input Validation, and BOLA/IDOR that may contribute to an exploitable condition.