TinyCrypt File Transfer using PKI

Implementing secure file transfer using TinyCrypt and PKI (Public Key Infrastructure) involves a combination of encryption, digital signatures, and key management. Below is a step-by-step guide on how to achieve this:

Overview of Secure File Transfer

For secure file transfer using TinyCrypt and PKI, we will:

Use Public Key Encryption (RSA) for confidentiality: Encrypt the file using the recipient's public key.
Use Digital Signatures for Integrity and Authenticity: Sign the file with the sender's private key.
Use a Certificate to Validate Identities: The recipient verifies the sender's public key using a trusted certificate.

High-Level Steps

Key Generation: Each participant generates their own public/private key pairs.
Certificate Creation: Each participant gets a digital certificate signed by a trusted Certificate Authority (CA).
Encrypt the File: The sender encrypts the file using the recipient's public key.
Sign the File: The sender signs the file to ensure its integrity and authenticity.
Send the File: The file (along with its signature) is transferred to the recipient.
Verify the Signature: The recipient uses the sender’s public key (found in the certificate) to verify the file’s integrity and authenticity.
Decrypt the File: The recipient decrypts the file using their private key.

Step-by-Step Implementation

Step 1: Set Up TinyCrypt for Key Generation

Start by setting up TinyCrypt in your project to support RSA key generation. You need to generate RSA key pairs for both the sender and recipient.

#include <tinycrypt/rsa.h>

// Function to generate RSA key pairs (public and private)
void generate_key_pair(rsa_key_pair_t *key_pair, unsigned key_size) {
    rsa_generate_key_pair(key_pair, key_size);  // key_size is 2048 or 3072 bits
}

Generate the keys for both the sender and recipient:

rsa_key_pair_t sender_key_pair;
rsa_key_pair_t recipient_key_pair;
generate_key_pair(&sender_key_pair, 2048);  // Sender's key pair
generate_key_pair(&recipient_key_pair, 2048);  // Recipient's key pair

Step 2: Create a Digital Certificate

A digital certificate associates the public key with an identity (e.g., the sender's or recipient's name). The certificate is signed by a trusted Certificate Authority (CA). Since TinyCrypt doesn't provide built-in certificate management, you'll need to manually create and manage certificates.

The CA signs the sender's public key to create a certificate.
The recipient uses the CA's public key to verify the sender's certificate.

For simplicity, we will skip the implementation of a full certificate authority and instead assume that you have a trusted certificate for each participant.

Step 3: Encrypt the File (Using the Recipient’s Public Key)

Use the recipient's public key to encrypt the file. Here’s an example of encrypting the file content:

#include <tinycrypt/rsa.h>

void encrypt_file(const uint8_t *file_data, size_t file_size, rsa_key_t *recipient_public_key, uint8_t *encrypted_data) {
    rsa_encrypt(recipient_public_key, file_data, file_size, encrypted_data);
}

file_data: The raw content of the file.
recipient_public_key: The recipient’s public key.
encrypted_data: The encrypted file data.

Encrypt the file content:

uint8_t encrypted_file[2048];  // Size of the encrypted data
encrypt_file(file_data, file_size, &recipient_key_pair.public_key, encrypted_file);

Step 4: Sign the File (Using the Sender’s Private Key)

Sign the file using the sender’s private key to ensure its authenticity and integrity.

#include <tinycrypt/rsa.h>

void sign_file(const uint8_t *file_data, size_t file_size, rsa_key_t *sender_private_key, uint8_t *signature) {
    rsa_sign(sender_private_key, file_data, file_size, signature);
}

signature: The digital signature of the file, which will be used for verification.

Sign the file:

uint8_t signature[256];  // Signature length depends on the key size
sign_file(file_data, file_size, &sender_key_pair.private_key, signature);

Step 5: Transfer the Encrypted File and Signature

Now, you can transfer the encrypted file and its signature to the recipient. You could transfer the file over a network, save it to disk, or use any other method of communication.

The transferred data might look like this:

Encrypted file content (encrypted_file).
Digital signature (signature).
Optionally, include the sender’s certificate to allow the recipient to verify the sender’s identity.

Step 6: Verify the File’s Signature (Using the Sender’s Public Key)

Upon receiving the file, the recipient needs to verify the sender’s signature using the sender's public key (contained in the sender’s certificate). The recipient also needs to ensure that the certificate is valid.

bool verify_signature(const uint8_t *file_data, size_t file_size, uint8_t *signature, rsa_key_t *sender_public_key) {
    return rsa_verify(sender_public_key, signature, file_data, file_size);
}

file_data: The original file content (before encryption).
signature: The signature to be verified.
sender_public_key: The sender's public key to verify the signature.

Verify the signature:

if (verify_signature(encrypted_file, encrypted_file_size, signature, &sender_key_pair.public_key)) {
    printf("Signature is valid.\n");
} else {
    printf("Signature is invalid.\n");
}

Step 7: Decrypt the File (Using the Recipient’s Private Key)

Once the file’s authenticity and integrity have been verified, the recipient can decrypt the file using their private key.

#include <tinycrypt/rsa.h>

void decrypt_file(const uint8_t *encrypted_data, size_t encrypted_size, rsa_key_t *recipient_private_key, uint8_t *decrypted_data) {
    rsa_decrypt(recipient_private_key, encrypted_data, encrypted_size, decrypted_data);
}

Decrypt the file:

uint8_t decrypted_file[2048];  // Size of the decrypted data
decrypt_file(encrypted_file, encrypted_file_size, &recipient_key_pair.private_key, decrypted_file);

Final Steps

At this point, the recipient has successfully decrypted the file and verified its authenticity. Here’s a summary of the steps:

Sender: Encrypts the file using the recipient’s public key and signs it with their private key.
Recipient: Verifies the signature using the sender’s public key (from the certificate) and decrypts the file using their private key.

Conclusion

By following these steps, you can implement secure file transfer using TinyCrypt and PKI. Although TinyCrypt provides the basic cryptographic functions (RSA key generation, signing, and encryption), you may need to build some additional components, such as certificate management and verification, depending on the complexity of your PKI.

Note that TinyCrypt is minimalistic, so while it can serve well in constrained environments, it lacks some of the advanced features and optimizations found in full-fledged libraries like OpenSSL. For production systems requiring robust PKI, consider using a more comprehensive library if resources allow.

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