Digital Signature

Digital signatures provide non-repudiable proof of origin. By signing a message with a private key, the sender cryptographically binds their identity to the data. The recipient uses the corresponding public key to verify the signature, confirming the message came from the claimed sender and not an imposter. This is fundamental for verifying the source of a TinyML model update or a command sent to a microcontroller fleet.

Signatures guarantee that the signed data has not been modified after signing. The signing process creates a cryptographic hash of the message, which is then encrypted with the private key. Any alteration to the message—even a single bit—will cause the hash verification to fail when the recipient recalculates it. This ensures a model artifact or firmware binary deployed to an edge device is bit-for-bit identical to the version released by the developer.

This property prevents the signer from later denying they created the signature. Because the signature is generated using a private key that should be exclusively controlled by the signer, successful verification with the associated public key serves as legally admissible evidence of the signer's action. In enterprise contexts, this is critical for audit trails and compliance, ensuring accountability for model deployments or configuration changes.

Digital signatures rely on Public Key Infrastructure to manage trust. PKI components include:

• Certificate Authorities (CAs): Trusted entities that issue and sign digital certificates.
• Digital Certificates: Documents that bind a public key to an identity (e.g., a company or device).
• Certificate Revocation Lists (CRLs): Lists of certificates that are no longer valid. This hierarchy establishes a chain of trust, allowing a device to verify a model's signature by trusting the CA that signed the developer's certificate.

The technical workflow for creating a digital signature involves three key steps:

1. Hashing: The original message (e.g., a .tflite model file) is processed by a cryptographic hash function (like SHA-256) to produce a fixed-size digest.
2. Encryption: This hash digest is then encrypted using the signer's private key. This encrypted hash is the digital signature.
3. Appending: The signature is appended to the original message or transmitted alongside it. The original data is not encrypted, preserving its usability.

To verify a signature, the recipient performs the inverse operations:

1. Decryption: The received signature is decrypted using the signer's public key, recovering the original hash digest.
2. Re-hashing: The recipient independently calculates the hash of the received message using the same hash function.
3. Comparison: The newly calculated hash is compared to the decrypted hash. If they match exactly, verification succeeds, proving both authenticity and integrity. A mismatch indicates the data was tampered with or the signature is invalid.

The process of applying a digital signature to software, firmware, or a machine learning model artifact. This signature:

• Authenticates the author/publisher.
• Verifies integrity, proving the code hasn't been altered since signing.
• In embedded contexts, this often involves signing the entire binary image (.bin or .hex file) that is flashed to the microcontroller's memory.

A deterministic algorithm that maps data of arbitrary size to a fixed-size bit string (a hash or digest). Hash functions like SHA-256 are a core component of digital signatures. The process:

1. The signer computes a hash of the message (e.g., a model file).
2. The hash is then encrypted with the signer's private key to create the signature.
3. The verifier recomputes the hash and decrypts the signature with the public key; a match confirms integrity and authenticity.