Signed OTA

Introduction

Deploying embedded devices with (automatic) OTA functionality introduces new risks to local networks and the whole internet. If an attacker takes over the update server or runs a MITM attack, he might be able to turn the devices into zombies.

To prevent this, you can either provide a secure connection between device and update server (e. g. VPN or TLS) or add a cryptographic signature to all OTA files. [pr893] (https://github.com/SmingHub/Sming/pull/893) provides hooks to the OTA functionality to allow checking of such signatures.

A proven method for this is, for example, ECDSA in conjunction with SHA-256. For both steps libraries are available (micro-ecc and Arduino Cryptosuite).

To use it, you can subclass rBootHttpUpdate like this:

#define PREFIX_MAGIC    0x54, 0x49, 0x55, 0x53
#define PREFIX_TYPE     0x00, 0x01
#define PREFIX_SIZE     sizeof(_my_prefix)
#define SIGNATURE_SIZE  64

const u8 _my_prefix[6] = { PREFIX_MAGIC, PREFIX_TYPE };    

typedef struct {
    u8  prefix[PREFIX_SIZE];
    u8  signature[SIGNATURE_SIZE];
} MyHdr;

//-----------------------------------------------------------------------------
class MyUpdate : public rBootHttpUpdate {

protected:
    virtual void writeInit();
    virtual bool writeFlash(const u8 *data, u16 size);
    virtual bool writeEnd();
    
private:
    Sha256 *sha256;
    u8      hdr_len;
    MyHdr   hdr;
};

//-----------------------------------------------------------------------------
void MyUpdate::writeInit() {
    rBootHttpUpdate::writeInit();
    sha256  = new Sha256;
    hdr_len = 0;
}

bool MyUpdate::writeFlash(const u8 *data, u16 size) {
    //  store header
    u8 missing = sizeof(hdr) - hdr_len;
    if (missing) {
        if (size < missing) missing = size;
        memcpy( &hdr, data, missing );
        size    -= missing;
        data    += missing;
        hdr_len += missing;
        
        //  check prefix
        if ( hdr_len >= PREFIX_SIZE ) {
            if ( memcmp(hdr.prefix, _my_prefix, PREFIX_SIZE) ) {
                debugf("invalid prefix");
                return 0;
            }
        }
    }

    //  update hash
    sha256->update(data, size);

    //  save data
    return rBootHttpUpdate::writeFlash(data, size);
}

bool MyUpdate::writeEnd() {
    if (!rBootHttpUpdate::writeEnd()) return 0;

    u8 hash[SHA256_BLOCK_SIZE];
    sha256->final( hash );

    bool sig_ok = /* add signature check here */;
    if (!sig_ok) {
        debugf("wrong signature");
        return 0;
    }
    return 1;
}