Bleichenbacher Attack in Echo Go with Dynamodb

A Bleichenbacher attack is a chosen-ciphertext attack that exploits adaptive chosen-ciphertext vulnerabilities in RSA encryption, particularly when padding validation leaks information via timing or error messages. In the context of an Echo Go service that interacts with AWS Dynamodb, the risk arises when encrypted or signed tokens, API keys, or session identifiers are decrypted or verified server-side before data is stored or retrieved from Dynamodb.

Echo Go is a lightweight HTTP framework; if it accepts encrypted inputs (e.g., a JWT or an encrypted record identifier) and passes them to Dynamodb queries without strict validation, an attacker can iteratively craft ciphertexts that probe the padding verification routine. Each request to an Echo Go endpoint that calls Dynamodb can reveal whether the server accepted or rejected the ciphertext, based on response differences such as HTTP status codes, latency, or error messages. Over many requests, the attacker can decrypt data without possessing the private key.

When the Dynamodb table stores sensitive items like encrypted user secrets or signed tokens, and the Echo Go application performs decryption or signature verification before using those items in queries, the combination creates a side-channel amenable to Bleichenbacher-style exploitation. For example, an endpoint that accepts an encrypted parameter, decrypts it in Go, and then uses the plaintext to query Dynamodb for a user record can inadvertently signal validity through timing or error behavior, enabling the adaptive attack.

To illustrate, consider an Echo Go route that receives an encrypted user identifier, decrypts it using RSA-OAEP, and then queries Dynamodb:

import (
    "crypto/rand"
    "crypto/rsa"
    "crypto/x509"
    "encoding/pem"
    "net/http"
    "github.com/labstack/echo/v4"
)

func decryptHandler(c echo.Context) error {
    encryptedPEM := c.FormValue("token")
    block, _ := pem.Decode([]byte(encryptedPEM))
    if block == nil || block.Type != "RSA PRIVATE KEY" {
        return c.String(http.StatusBadRequest, "invalid key")
    }
    priv, err := x509.ParsePKCS1PrivateKey(block.Bytes)
    if err != nil {
        return c.String(http.StatusBadRequest, "parse failed")
    }
    ciphertext := []byte(c.FormValue("data"))
    plaintext, err := rsa.DecryptOAEP(rand.Reader, nil, priv, ciphertext, nil)
    if err != nil {
        return c.String(http.StatusBadRequest, "decrypt error")
    }
    // Use plaintext as a Dynamodb key
    svc := getDynamoClient()
    out, err := svc.GetItem(c.Request().Context(), &dynamodb.GetItemInput{
        TableName: aws.String("users"),
        Key: map[string]types.AttributeValue{
            "user_id": &types.AttributeValueMemberS{Value: string(plaintext)},
        },
    })
    if err != nil {
        return c.String(http.StatusInternalServerError, "db error")
    }
    return c.JSON(http.StatusOK, out.Item)
}

If the error path for invalid padding or decryption failures is distinguishable from a missing item in Dynamodb (e.g., different status codes or timing), an attacker can perform a Bleichenbacher attack to recover the plaintext or forge valid ciphertexts. This is especially concerning when the plaintext is used directly as a key in Dynamodb queries, because the application surface includes both cryptographic verification and database lookup, providing multiple observable behaviors for the attacker to measure.

Remediation focuses on ensuring that cryptographic operations do not leak distinguishable errors to the caller and that Dynamodb interactions remain constant-time and error-agnostic where possible. In Echo Go, avoid returning different HTTP status codes or messages based on decryption or padding validity. Instead, use a constant-time validation flow and treat all decryption failures as generic authentication failures.

Always prefer authenticated encryption with associated data (AEAD) such as AES-GCM rather than RSA-OAEP for session tokens, but if RSA is required, ensure decryption errors are indistinguishable. Use crypto/rsa with constant-time practices and avoid branching on secret-dependent data. When querying Dynamodb, do not expose internal errors; map them to a uniform response.

Below is a revised example that mitigates timing and error-channel leaks:

import (
    "crypto/aes"
    "crypto/cipher"
    "crypto/rand"
    "crypto/rsa"
    "crypto/x509"
    "encoding/pem"
    "io"
    "net/http"
    "time"
    "github.com/labstack/echo/v4"
)

// Constant-time byte comparison to avoid timing leaks
func subtleEquals(a, b []byte) bool {
    if len(a) != len(b) {
        return false
    }
    var equal byte
    for i := 0; i < len(a); i++ {
        equal |= a[i] ^ b[i]
    }
    return equal == 0
}

// Prefer AES-GCM for token encryption/decryption
func decryptTokenAESGCM(key, ciphertext []byte) ([]byte, error) {
    block, err := aes.NewCipher(key)
    if err != nil {
        return nil, err
    }
    if len(ciphertext) < aes.GCMNonceSize {
        return nil, io.ErrUnexpectedEOF
    }
    nonce, ciphertext := ciphertext[:aes.GCMNonceSize], ciphertext[aes.GCMNonceSize:]
    aesgcm, err := cipher.NewGCM(block)
    if err != nil {
        return nil, err
    }
    return aesgcm.Open(nil, nonce, ciphertext, nil)
}

// Generic error response to avoid leaking failure reasons
genericErr := `{"error":"invalid_request"}`

func secureDecryptHandler(c echo.Context) error {
    // Example using AES-GCM; RSA-OAEP should follow strict constant-time practices
    encToken := c.FormValue("token")
    encData := c.FormValue("data")
    // Decode base64 inputs as needed; omitted for brevity
    key := []byte("32-byte-long-key-for-aes-256-gcm-encryption-key")
    plaintext, err := decryptTokenAESGCM(key, []byte(encData))
    if err != nil {
        // Always return the same status and message
        return c.String(http.StatusUnauthorized, genericErr)
    }
    // Ensure plaintext is safe for use as a key; validate format, length, charset
    if len(plaintext) == 0 {
        return c.String(http.StatusUnauthorized, genericErr)
    }
    // Constant-time check: simulate work to obscure timing if needed
    // (not shown; ensure no early returns based on secret data)
    _ = subtleEquals([]byte("dummy"), plaintext[:8]) // example placeholder

    // Query Dynamodb with the validated identifier
    svc := getDynamoClient()
    out, err := svc.GetItem(c.Request().Context(), &dynamodb.GetItemInput{
        TableName: aws.String("users"),
        Key: map[string]types.AttributeValue{
            "user_id": &types.AttributeValueMemberS{Value: string(plaintext)},
        },
    })
    if err != nil {
        // Log the internal error; return generic response
        return c.String(http.StatusInternalServerError, genericErr)
    }
    if out.Item == nil {
        return c.String(http.StatusUnauthorized, genericErr)
    }
    return c.JSON(http.StatusOK, out.Item)
}

Additional recommendations:

• Use short-lived, randomly generated identifiers for Dynamodb keys instead of exposing decrypted secrets directly.
• Enable AWS CloudTrail and monitor unusual GetItem patterns that might indicate probing behavior.
• Apply rate limiting at the Echo Go layer to reduce the feasibility of iterative adaptive attacks.
• If you must use RSA, ensure decryption errors are swallowed and replaced with a uniform process, and consider hybrid encryption (RSA to encrypt a symmetric key, AES-GCM for data).

These steps reduce the attack surface by removing observable differences between cryptographic and database errors, making Bleichenbacher-style adaptive attacks significantly harder to execute against an Echo Go service backed by Dynamodb.