Cryptography in Embedded Systems

• Lecture 12 focuses on cryptography in embedded systems for hardware platforms and programming.

Motivation for Cryptography

• Cryptography serves as a fundamental security building block for many applications.
• Applications of cryptography include user or device authentication to a server or other user.
• Cryptography ensures data integrity and authenticity for documents and software.
• Cryptography provides data confidentiality for documents and passwords.
• Cryptographic applications are categorized into encryption and digital signatures.
• Encryption hides the content of a message from unauthorized parties.
• Digital signatures ensure message authenticity or prove entity identity.

Encryption

• Encryption encodes a message so only authorized parties can access it.
• Encryption denies message content to the interceptor, but it won't prevent interception.
• Encryption protects sensitive information, like personal data and intellectual property.

Digital Signatures

• Digital signatures are mathematical schemes verifying digital message authenticity.
• Valid digital signatures provide confidence of the message's origin without alterations during transit.
• Digital signatures are used to detect forgery or tampering in software updates, authentication, and data consistency ensuring software integrity.
• Cryptographic authentication methods often rely on digital signatures.

Cryptographic Hash Functions

• Functions convert a message to a fixed-length digit string (hash value).
• The hash value is a message's digital fingerprint, making it practically impossible to derive the original message from the hash value.
• Cryptographic hash functions make it impossible to find two different messages with the same hash value (collision resistance).
• MD5 (Message Digest 5) is insecure but was once very common.
• Secure Hash Algorithm (SHA) function families include SHA-1 (insecure), SHA-2 (most common like SHA-256), and SHA-3 (recently standardized).
• Cryptographic hash functions are used in integrity verification, preprocessing for digital signatures, and password storage.

Symmetric Cryptography

• Uses the same key to encrypt and decrypt a message.
• The key must be known to and kept secret by both parties.
• AES (Advanced Encryption Standard), DES (Data Encryption Standard), and Enigma are the best-known algorithms.
• It is fast but key distribution can be challenging.

Asymmetric Cryptography

• Encryption where two different keys are used to encrypt and decrypt a message.
• RSA (Rivest-Shamir-Adleman) and ECC (Elliptic Curve Cryptography) are the most common algorithms.
• Slower than symmetric cryptography but has key distribution advantages.
• One key can be distributed publicly, and the other key never needs to leave the device where it was created.
• Digital signatures are based on asymmetric encryption,

Private and Public Keys

• All asymmetric cryptography concepts use two different keys: A private key and a public key derived from the private key.
• The private key must be kept secret.
• The public key can be shared.
• The sender uses the recepient's public key to encrypt messages.
• The recepient uses their own private key to decrypt messages.
• Senders use their own private key and reciepients use sender's public key to verify the e signature.

Public-Key Authentication

• Cryptographically verifies the identity of a device or user, proving access to a secret private key.
• Public-key authentication is also known as challenge-response authentication.
• Encryption and Decryption method involves Bob sending Alice a a message encrypted with Alice's public key, and Alicia decrypts the message with her private key and sends it back to Bob in the form of a hash.
• Signature and verification involves, Bob sends alice random message and Alice signs it with her private key, Bob can then verify the signature.

Generation of Private and Public Keys

• Generated using a key pair generator based on one-way functions.
• A one-way function example is the multiplication of two large prime numbers; finding the two prime numbers from the result is possible but not feasible in a reasonable time.
• The individual prime numbers are the private key, and the result is the public key which can be used for encryption.
• This approach is used in the RSA cryptographic system.

Limits of Asymmetric Cryptography

• Public-key cryptography can be compromised if the private key is not kept secret.
• Algorithms are considered secure under the condition that the keys are long enough and generated from good random numbers.
• One-way functions may become reversible with quantum computers. Quantum comptuers have not broken the foundation of asymmetric cryptography so far.
• Slower than symmetric cryptography

Side-Channel Attacks

• Attempts to get secret knowledge based on physical implementation of cryptosystems.
• Examples include timing attacks, power analysis, and electromagnetic analysis.
• Countermeasures can be taken by reducing side-channel leakage, or reducing correlation between the leakage and the secret information.

Risk by Quantum Computing

• Makes one-way functions reversible, potentially breaking asymmetric cryptography like RSA and ECC.
• Symmetric cryptography should not be affected by quantum computers because they aren't based one-way functions.

Combination with Symmetric Cryptography

• Asymmetric cryptography can be used to exchange a common session-specific key, which can then be used for symmetric cryptography (like AES).
• In one implementation, the sender generates a random symmetric key, encrypts it with the receiver's public key, and sends it to the receiver.
• In a more complex Diffie-Hellman key exchange the sender and receiver exchange public keys, and use them to generate a common key, which is then used for symmetric cryptography.

Applications of Cryptography in Embedded Systems

• Including Secure Boot, Secure Firmware Updates and Secure Communication

Secure Boot

• Restricts the software that can be run on the device.
• Used to protect private data or intellectual property and prevent unintended uses.
• It requires a bootloader and/or a kernel to be verified by the processor before execution, based on asymmetric cryptography.
• The bootloader/kernel is signed with a private key, and the corresponding public key is immutably stored in hte processor, and the private key is kept secret by the devicd manufacturer

Secure Firmware Updates

• Firmware updates provide security to the end-users of electronic products and the update process should be secured from unauthorized access and the update should be signed
• Encryption of the update files the update usually keeps information that the manufacturer keeps secret.
• Digital signatures are used to prevent the device from accepting a malicious update, and it is signed with a private key.

Secure Communication

• Involves a use of Secure Shell (SSH) and Transport Layer Security (TLS)

Secure Shell (SSH)

• Network protocol for accessing a remote computer securely.
• Used for secure remote login, command execution, and other network services.
• Used for logging in to Linux-based devices via a network.
• SSH is a server-client protocol based on public-key authentication and symmetric encryption.

Transport Layer Security (TLS)

• Transport Layer Security (TLS) works similarly to SSH, but it is used for communication between a client (e.g. web browser) and a (web) server.
• TLS is used for encrypting the data traffic, and for authentication of the server.

Pretty Good Privacy (PGP)

• The OpenPGP standard for encryption of data and digital signatures.
• Designed for encrypting and signing e-mails but can be used for other applications.
• The OpenPGP standard is implemented by PGP and GNU Privacy Guard (GnuPG, free software).

Trust in Known Public Keys

• All asymmetric cryptography applications require the verifier or sender to trusts that the public key belongs to the entity.
• Trust can be established when humans meet, OEMs stores the key, use logs in with a password

Trust in UNknown Public Keys

• If there is no secure channel the public key must be verified by a trusted third party
• Few root certificate authorites in relation to authenticating the certificate path is used.

Cryptographic APIs

• Using common cryptographic libraries is important to use security correctly and avoid introducing implementation vulnerabilities.
• Common APIs include OpenSSL, Crypto++, and mbed TLS.