PIN

/pɪn/

n. "Shared numeric passcode used during legacy Bluetooth pairing generating 128-bit link key."

PIN, short for Personal Identification Number, authenticates initial Bluetooth device pairing by requiring identical 4-16 digit codes entered on both master/slave—combined with BD_ADDR and random challenge to derive 128-bit link key via SAFER+ hashing for subsequent authentication/encryption without re-entry. Legacy Bluetooth 2.0+ uses "0000"/"1234" defaults (security risk) while modern Secure Simple Pairing (SSP) replaces PINs with numeric comparison, passkey entry, or out-of-band (NFC) methods.

Key characteristics of PIN include: Shared Secret both devices input identical 4-16 alphanumeric codes; Link Key Generation PIN+BD_ADDR+challenge → SAFER+ → 128-bit K_AB; Challenge-Response prevents replay using 32-bit RAND per connection; Legacy Only replaced by LE Secure Connections (P-256 ECDH); Default Weakness "0000"/"1234" vulnerable to brute-force dictionary attacks.

Conceptual example of PIN usage:

/* Bluetooth Legacy PIN → Link Key derivation (simplified) */
uint8_t pin_code = "1234";  // User-entered PIN
uint8_t bd_addr;             // Remote device address  
uint8_t rand_challenge;     // 128-bit random number
uint8_t link_key;           // 128-bit result

void bluetooth_legacy_pairing() {
    // Step 1: User enters PIN on both devices
    
    // Step 2: IN_RAND + BD_ADDR → E22 (SAFER+ encryption)
    uint8_t in_rand;
    memcpy(in_rand, rand_challenge, 16);
    memcpy(in_rand + 8, bd_addr, 6);
    memcpy(in_rand + 14, pin_code, strlen(pin_code));
    
    // Step 3: E22(PIN, IN_RAND) → Key K_AB
    safer_plus_encrypt(pin_code, in_rand, link_key);
    
    // Step 4: Store link_key for future authentication
    store_link_key(bd_addr, link_key);
    
    // Authentication: challenge-response using K_AB
}

Conceptually, PIN seeds symmetric link key shared only after manual verification—both devices compute identical K_AB from PIN+device identity+race condition nonce, enabling encrypted TDMA slots within FHSS/AFH piconets. Weak defaults ("0000") enabled early eavesdropping attacks; modern Bluetooth LE Secure Connections use elliptic curve Diffie-Hellman eliminating shared secrets entirely.