#End-to-end encryption

According to ReBIT spec, the FI data flow between the FIU and the FIP should be end-to-end encrypted. The FIU and the FIP should generate key pairs that are not permanent and are generated for every data fetch request that happens between them.

#Key generation and exchange

The FIU and the FIP should generate Curve25519 ephemeral key pairs comprising of public and private keys. Curve25519 is taken as default curve as per the ReBit spec.

The Elliptic Curve Diffie-Hellman (ECDH) key-exchange protocol is an anonymous key-agreement scheme. It is used by the FIU and the FIP to generate a shared secret key, using the ephemeral key pairs for encryption and decrytpion. Learn more about ECDH.

#KeyMaterial object

KeyMaterial is an object which contains information about the public key, encryption algorithm and nonce, as in the sample provided below.

{
    "cryptoAlg"   : "ECDH",
    "curve"       : "Curve25519",
    "params"      : "AES/GCM/NoPadding",
    "DHPublicKey" : {
        "expiry"     : "<PUBLIC_KEY_EXPIRY_DATE>",
        "Parameters" : "",
        "KeyValue"   : "<PUBLIC_KEY_VALUE>"
    },
    "Nonce"       : "<256_BIT_NONCE>"
}

#Encryption and decryption

Below is a list of steps that happen in a data flow session—

Encryption at FIP

• The FIU generates a ephemeral key pair, fiu_public_key & fiu_private_key and a 256-bit base64 encoded nonce, fiu_nonce. The FIU sends this information as KeyMaterial to the FIP in the data request for that session.
• The FIP generates a ephemeral key pair, fip_public_key & fip_private_key and a 256-bit base64 encoded nonce, fip_nonce.
• Using ECDH, the FIP generates a shared secret key, fip_shared_secret by using fip_private_key and fiu_public_key.
• The FIP generates a shared_nonce by XOR of fiu_nonce and fip_nonce
• The FIP generates a session_key by HKDF (HMAC based Key Derivation function) using the fip_shared_secret and shared_nonce. Learn more about HKDF.
• The FIP uses AES-GCM encryption algorithm with session_key used as the encryption key to encrypt the FI data requested by the FIU. According to spec and the library, AES-GCM is the default encryption algorithm. Learn more about AES.

Decryption at FIU

• Once the data is encrypted, the FIP sends the encrypted data along with fip_public_key and fip_nonce as KeyMaterial.
• Using ECDH, the FIU generates a shared secret key, fiu_shared_secret by using fiu_private_key and fip_public_key.
• Note that both the shared secret keys, fiu_shared_secret and fip_shared_secret will be the same.
• The FIU generates a shared_nonce by XOR of fiu_nonce and fip_nonce
• The FIU generates a session_key by HKDF (HMAC based Key Derivation function) using the fiu_shared_secret and shared_nonce.
• The FIU uses the same AES-GCM encryption algorithm with session_key used as the decryption key to decrypt the FI data sent by FIP.

#Rahasya helper library

To facilitate easier testing of our sandbox, Setu has deployed the official Sahamati library, Rahasya on our infra. This will help FIU to easily generate ephemeral keys and decrypting the FI data in sandbox.

Get the the Postman collection for key generation and decrypting data—the Rahasya helper library here.