Modify article about `Cryptography`

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1.3.2. Secure Boot

Secure Boot is a security feature that ensures only signed and trusted code is executed during a device's boot process, preventing unauthorized or malicious software from running at startup.

Secure Boot is typically implemented using cryptographic methods. These methods collectively contribute to the overall security of the boot process, safeguarding against unauthorized code execution.

Secure Boot, providing security features to protect the boot process from unauthorized or malicious code. The choice of tools depends on the specific hardware, software, and security requirements of the system.

Representation and Diagrams:

This diagram outlines the two scenarios: firmware update and the regular boot process. In both cases, the device verifies the signature of the received data using its public key.

sequenceDiagram
  participant Device
  participant Bootloader
  participant Updater

  Note over Device: Generate key pair (Private, Public)
  Note over Updater: Generate key pair (Private, Public)

  alt Firmware Update
    Updater->>Updater: Generate Firmware Data
    Updater->>Updater: Sign Firmware Data
    Updater-->>Device: Firmware Data, Signature
    Device->>Device: Verify Signature
  else Boot Process
    Bootloader->>Bootloader: Load Firmware
    Bootloader->>Device: Firmware Data, Signature
    Device->>Device: Verify Signature
  end

Example and Explanation:

Go example demonstrating how digital signatures can be implemented using the crypto package. This example generates an RSA key pair, signs the data (e.g. firmware or bootloader code) with the private key, and then verifies the signature using the corresponding public key.

package main

import (
    "crypto"
    "crypto/rand"
    "crypto/rsa"
    "crypto/sha256"
    "fmt"
)

func generateKeyPair() (*rsa.PrivateKey, *rsa.PublicKey, error) {
    privateKey, err := rsa.GenerateKey(rand.Reader, 2048)
    if err != nil {
        return nil, nil, err
    }
    return privateKey, &privateKey.PublicKey, nil
}

func signData(data []byte, privateKey *rsa.PrivateKey) ([]byte, error) {
    hashed := sha256.Sum256(data)
    signature, err := rsa.SignPKCS1v15(rand.Reader, privateKey, crypto.SHA256, hashed[:])
    if err != nil {
        return nil, err
    }
    return signature, nil
}

func verifySignature(data, signature []byte, publicKey *rsa.PublicKey) error {
    hashed := sha256.Sum256(data)
    return rsa.VerifyPKCS1v15(publicKey, crypto.SHA256, hashed[:], signature)
}

func main() {
    // Generate key pair
    privateKey, publicKey, err := generateKeyPair()
    if err != nil {
        fmt.Println("Error generating key pair:", err)
        return
    }

    // Data to be signed
    data := []byte("Example data to be signed")

    // Sign the data
    signature, err := signData(data, privateKey)
    if err != nil {
        fmt.Println("Error signing data:", err)
        return
    }

    // Verify the signature
    err = verifySignature(data, signature, publicKey)
    if err != nil {
        fmt.Println("Signature verification failed:", err)
        return
    }

    fmt.Println("Signature verified successfully.")
}

Conventions and Standards:

1. Digital Signatures

Firmware and bootloader code are signed with cryptographic keys. During boot, the signatures are verified to ensure the code hasn't been tampered with.

Digital signatures in the context of Secure Boot involve using asymmetric cryptography. By employing digital signatures, Secure Boot ensures that only code signed by trusted entities with the correct private key can be executed during the boot sequence, enhancing the overall security of the system.

• Key Generation

  Two cryptographic keys are generated – a private key and a corresponding public key. The private key is kept secure, while the public key is distributed widely.

• Signing

  The firmware or bootloader code is hashed using a cryptographic hash function (e.g., SHA-256) to generate a unique fixed-size hash value.

  The private key is then used to create a digital signature of the hash. This signature is essentially a unique cryptographic representation of the hash.

• Distribution

  The signed code along with the digital signature is distributed to devices.

• Verification during Boot

  During the boot process, the bootloader or firmware checks the integrity of the code.

  It uses the corresponding public key, obtained from a trusted source, to verify the digital signature.

  If the signature is valid and matches the computed hash of the received code, it confirms that the code has not been altered since it was signed.

• Trust Chain

  This process creates a trust chain. The bootloader is trusted because its signature is verified using a trusted key. If the bootloader is trusted, it can then verify the signature of the next stage in the boot process, and so on.

2. Key Management

Secure Boot relies on a chain of trust established through key pairs. The private key signs the code, and the corresponding public key verifies it. Keys are securely managed to prevent unauthorized access.

3. Hash Functions

The code's hash is generated, and the hash value is signed. During boot, the bootloader checks the hash integrity of the loaded code to confirm its authenticity.

4. Hardware Root of Trust

Establishing a secure root, often through hardware-based mechanisms, ensures the trustworthiness of the initial boot components.

Tools and Frameworks:

Implementing Secure Boot in embedded systems, especially considering resource constraints, microcontroller or processor architecture, and overall system requirements, a tailored set of tools and frameworks proves essential.

1. U-Boot:

   • An open-source bootloader widely used in embedded systems, supporting Secure Boot features, including signature verification.

2. Trusted Firmware-A (TF-A):

   • An open-source project providing a reference implementation of secure world software for Arm platforms, often used in conjunction with U-Boot.

3. ARM mbed TLS:

   • A lightweight, open-source cryptographic library suitable for implementing cryptographic functions in resource-constrained embedded systems.

4. OP-TEE (Open Portable Trusted Execution Environment):

   • An open-source TEE project enhancing the security of embedded systems.

5. Mcuboot:

   • An open-source bootloader designed for small embedded systems, featuring Secure Boot and image signing.

6. Arm TrustZone:

   • Hardware-based security on Arm processors, isolating secure and non-secure code in embedded systems.

7. WOLFSSL:

   • A lightweight cryptographic library suitable for resource-constrained embedded systems.

8. Coreboot:

   • An open-source firmware project customizable for embedded systems, supporting various payloads, including Secure Boot capabilities.

9. TianoCore EDK II:

   • An open-source development environment for UEFI-based firmware, providing a platform for building embedded system firmware with Secure Boot.

10. RIOT OS:

    • An open-source operating system designed for IoT and embedded systems, including features for secure bootstrapping.

Considering broader Secure Boot implementations with digital signatures.

11. Trusted Platform Module (TPM):

    • A hardware-based security solution providing a root of trust for the system and secure storage of cryptographic keys.

12. OpenSSL:

    • A widely used open-source cryptographic library, supporting functions for key generation, data signing, and signature verification.

13. GNU GRUB:

    • A popular bootloader supporting Secure Boot and configurable for digital signature verification.

14. Keylime:

    • A project extending TPM capabilities for remote attestation, enhancing overall system security.

15. Microsoft Secure Boot:

    • A technology commonly used in Windows systems, ensuring that only signed and authorized code is executed during the boot process.

16. Yocto Project:

    • Providing tools for building custom Linux distributions for embedded systems, configurable to include Secure Boot features.

17. UEFI (Unified Extensible Firmware Interface):

    • A specification ensuring the integrity of the boot process, including Secure Boot as a feature.

18. Open Attestation (OpenAttest):

    • An open-source project for remote attestation, ensuring the integrity of the system's firmware and software components.