RSA-AES Hybrid

Overview

RSA-AES Hybrid encryption combines RSA-2048 for secure key exchange and AES-256-CBC for high-performance data encryption, providing both the security of asymmetric encryption and the efficiency of symmetric encryption.

Key Features

RSA-2048 for encrypting session keys
AES-256-CBC with 256-bit session keys for data encryption
SHA256withRSA digital signatures for integrity verification
Per-request session keys for forward secrecy

How It Works

RSA-AES Hybrid encryption uses a two-layer approach — asymmetric encryption for securely exchanging a temporary key, and symmetric encryption for fast bulk data encryption. Below is a step-by-step breakdown of the entire process:

1. Key Exchange (One-Time Setup)

Both parties generate their own RSA-2048 key pairs and exchange only their public keys with each other. Private keys are never shared.

An RSA-2048 key pair is generated on the server side, and the server’s public key is shared with the client.
An RSA-2048 key pair is generated on the client side, and the client’s public key is shared with the server.

This one-time setup establishes the trust foundation for all subsequent encrypted communication.

2. Sending an Encrypted Request (Client → Server)

When an API request is sent:

A random session key is generated — A fresh 32-byte AES key is created for each request, ensuring every request uses a unique encryption key.
The request data is encrypted — The JSON payload is encrypted using AES-256-CBC with the session key. The first 16 bytes of the session key serve as the IV.
The session key is encrypted — The session key itself is encrypted using the server’s public key with RSA. Only the server can decrypt it using its private key.
The encrypted data is signed — The encrypted data is signed using the client’s private key. This proves the request originated from the client and has not been tampered with.
The payload is sent — The encrypted session key, encrypted data, and signature are combined into a single JSON payload and transmitted to the server.

3. Receiving an Encrypted Response (Server → Client)

When a response is sent back:

The response is encrypted by the server using an AES session key, and the session key is encrypted with the client’s public key — ensuring only the client can decrypt it.
The encrypted data is signed with the server’s private key — allowing the client to verify its authenticity.
The session key is decrypted by the client using the client’s private key.
The signature is verified using the server’s public key to confirm authenticity.
The response data is decrypted using the decrypted session key.

This ensures that every message is confidential (only the intended recipient can read it), authenticated (signed by the sender), and integrity-protected (any tampering invalidates the signature).

Key Setup

Step 1: Generate Client RSA Key Pair

An RSA-2048 key pair must be generated on the client side. The client’s public key is to be shared with the server. The client’s private key must be kept secret and securely stored.

import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.SecureRandom;
import java.util.Base64;

public class GenerateKeys {
    public static void main(String[] args) throws Exception {
        KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA");
        keyGen.initialize(2048, SecureRandom.getInstanceStrong());
        KeyPair keyPair = keyGen.generateKeyPair();

        String publicKey = Base64.getEncoder().encodeToString(
            keyPair.getPublic().getEncoded()
        );
        String privateKey = Base64.getEncoder().encodeToString(
            keyPair.getPrivate().getEncoded()
        );

        // Share publicKey with the server
        System.out.println("Client Public Key (share with server): " + publicKey);
        // Keep privateKey secret
        System.out.println("Client Private Key (keep secret): " + privateKey);
    }
}

Step 2: Exchange Public Keys

Action	Description
Provided by server	Server’s RSA public key — used by the client to encrypt session keys and verify server signatures
Provided by client	Client’s RSA public key — used by the server to encrypt response session keys and verify client signatures

Private keys must never be shared with anyone. Only public keys are exchanged between parties.

Algorithms & Parameters

Component	Detail
RSA Key Size	2048 bits
RSA Transformation	RSA/ECB/PKCS1Padding
RSA Purpose	Encrypting/decrypting the AES session key
AES Key Size	256 bits (32 bytes)
AES Transformation	AES/CBC/PKCS5Padding
AES IV	First 16 bytes of the session key
AES Purpose	Encrypting/decrypting the data payload
Signature Algorithm	SHA256withRSA
Signature Purpose	Signing and verifying data integrity
Base64 Encoding	Standard Base64 (not URL-safe)

Encrypted Payload Format

All encrypted requests and responses use the following JSON structure:

Request Payload

{
  "encrypted_session_key": "<Base64 encoded RSA-encrypted session key>",
  "encrypted_data": "<Base64 encoded AES-encrypted data>",
  "signature": "<Base64 encoded digital signature>"
}

Response Payload

{
  "encrypted_session_key": "c6+Ma3Crcqr3VGRGseTB9xQIS8fqQxgzUlLpXGEcBO005fKbx+/giMXbvrSPwf7bHHQeTfw6+Iz6TSRUW7Q/W2cqr4SpBvKl6nXqzvaQLYZQ8LPXIFmYuJgjsHUvUqMnbHKTuoxlw1EhCXH/RkutXjTOGUZAOQyqwmnr4iiVxWffI4Agfpc7wKaaobyBLKOYP5shh9YN08qmr9SF1v4ryNs+4fAD9WQIHZKnAf/9wDYf9tCo3wtPFk5tI6sAlZMSBRA0gKGnsv0mHbEh7YJhJghDA2RuN5soVdAe8ZfcgWIbcysJ5xy9YsXxHVwjAH7BBhAyD5XXOhe/yvuFCgzkUA==",
  "encrypted_data": "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",
  "signature": "gw5WO2U03M7hyE6+PluUWeo18B7lQCB7hWwwkum4C/ZtQboKwv51kv8KfK9xIQmL6dMKDnQObGHZtfHE0y2IpxlKekms0t6VjJCSagEdDU9NMOBQtaTuWxU4WExjGUbOgqR/nTA63x03/B70NEWx1mfVR5/4Ekr1orIWuDpKeqkzU0xPVbRDCGYU3OZLrRm1NsPS8gy39/0KV8BWRIvjjsKacXFCMg5pj8ORBYCBLnyXnJem025f8p7XKdpz/31SLYh6ilyght+07SMQwYOKLNPYnYCRNaZTzaYiC2vn8+m02NWSKnGfGqTOk/ShR8ShTP0W2sh0Zd3QXIuoGevGYQ=="
}

Field	Description
`encrypted_session_key`	The AES session key, encrypted with the client’s public key using RSA. Decrypted using the client’s private key.
`encrypted_data`	The response data, encrypted with AES-256-CBC using the session key.
`signature`	Digital signature of the `encrypted_data` string, signed with the server’s private key. Verified using the server’s public key.

Encryption Flow

Implementation Examples

Encrypting a Request (Client → Server)

import javax.crypto.Cipher;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;
import java.nio.charset.StandardCharsets;
import java.security.*;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;
import java.util.Arrays;
import java.util.Base64;

public class EncryptRequest {

    private static final String RSA_ALGORITHM = "RSA";
    private static final String RSA_TRANSFORMATION = "RSA/ECB/PKCS1Padding";
    private static final String AES_ALGORITHM = "AES";
    private static final String AES_TRANSFORMATION = "AES/CBC/PKCS5Padding";
    private static final String SIGNATURE_ALGORITHM = "SHA256withRSA";
    private static final int AES_KEY_SIZE = 32; // 256 bits
    private static final int IV_SIZE = 16; // 128 bits

    private static PublicKey loadPublicKey(String base64Key) throws Exception {
        byte[] keyBytes = Base64.getDecoder().decode(base64Key);
        X509EncodedKeySpec spec = new X509EncodedKeySpec(keyBytes);
        KeyFactory keyFactory = KeyFactory.getInstance(RSA_ALGORITHM);
        return keyFactory.generatePublic(spec);
    }

    private static PrivateKey loadPrivateKey(String base64Key) throws Exception {
        byte[] keyBytes = Base64.getDecoder().decode(base64Key);
        PKCS8EncodedKeySpec spec = new PKCS8EncodedKeySpec(keyBytes);
        KeyFactory keyFactory = KeyFactory.getInstance(RSA_ALGORITHM);
        return keyFactory.generatePrivate(spec);
    }

    public static void main(String[] args) {
        // Server's public key (provided by server during key exchange)
        String serverPublicKeyBase64 = "<SERVER_PUBLIC_KEY>";
        // Client's private key (generated by client, kept secret)
        String clientPrivateKeyBase64 = "<CLIENT_PRIVATE_KEY>";
        String requestJson = "{\"your\": \"request data\"}";

        try {
            // 1. Generate random AES-256 session key
            SecureRandom secureRandom = new SecureRandom();
            byte[] sessionKey = new byte[AES_KEY_SIZE];
            secureRandom.nextBytes(sessionKey);

            // 2. Extract IV from first 16 bytes
            byte[] iv = Arrays.copyOfRange(sessionKey, 0, IV_SIZE);

            // 3. Encrypt data with AES-256-CBC
            SecretKeySpec aesKeySpec = new SecretKeySpec(sessionKey, AES_ALGORITHM);
            IvParameterSpec ivSpec = new IvParameterSpec(iv);
            Cipher aesCipher = Cipher.getInstance(AES_TRANSFORMATION);
            aesCipher.init(Cipher.ENCRYPT_MODE, aesKeySpec, ivSpec);
            byte[] encryptedDataBytes = aesCipher.doFinal(
                requestJson.getBytes(StandardCharsets.UTF_8)
            );
            String encryptedData = Base64.getEncoder().encodeToString(encryptedDataBytes);

            // 4. Encrypt session key with server's public key
            PublicKey serverPublicKey = loadPublicKey(serverPublicKeyBase64);
            Cipher rsaCipher = Cipher.getInstance(RSA_TRANSFORMATION);
            rsaCipher.init(Cipher.ENCRYPT_MODE, serverPublicKey);
            byte[] encryptedSessionKey = rsaCipher.doFinal(sessionKey);
            String sessionKeyBase64 = Base64.getEncoder().encodeToString(encryptedSessionKey);

            // 5. Sign encrypted_data with client's private key
            PrivateKey clientPrivateKey = loadPrivateKey(clientPrivateKeyBase64);
            Signature signature = Signature.getInstance(SIGNATURE_ALGORITHM);
            signature.initSign(clientPrivateKey);
            signature.update(encryptedData.getBytes(StandardCharsets.UTF_8));
            byte[] signatureBytes = signature.sign();
            String signBase64 = Base64.getEncoder().encodeToString(signatureBytes);

            // 6. Build the encrypted payload
            // {
            //   "encrypted_session_key": sessionKeyBase64,
            //   "encrypted_data": encryptedData,
            //   "signature": signBase64
            // }

        } catch (Exception e) {
            System.err.println("Encryption failed: " + e.getMessage());
            e.printStackTrace();
        }
    }
}

Steps Breakdown

Generate Session Key: 32 random bytes are generated using a secure random generator
Extract IV: The first 16 bytes of the session key are used as the IV
Encrypt Data: The request JSON is encrypted with AES/CBC/PKCS5Padding using the session key and IV
Encrypt Session Key: The session key is encrypted with the server’s public key using RSA/ECB/PKCS1Padding
Sign: The encrypted_data string (Base64 string, as UTF-8 bytes) is signed with the client’s private key using SHA256withRSA
Send: All three fields are combined into the JSON payload

Decrypting a Response (Server → Client)

import javax.crypto.Cipher;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;
import java.nio.charset.StandardCharsets;
import java.security.*;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;
import java.util.Arrays;
import java.util.Base64;

public class DecryptResponse {

    private static final String RSA_ALGORITHM = "RSA";
    private static final String RSA_TRANSFORMATION = "RSA/ECB/PKCS1Padding";
    private static final String AES_ALGORITHM = "AES";
    private static final String AES_TRANSFORMATION = "AES/CBC/PKCS5Padding";
    private static final String SIGNATURE_ALGORITHM = "SHA256withRSA";
    private static final int IV_SIZE = 16;

    private static PublicKey loadPublicKey(String base64Key) throws Exception {
        byte[] keyBytes = Base64.getDecoder().decode(base64Key);
        X509EncodedKeySpec spec = new X509EncodedKeySpec(keyBytes);
        KeyFactory keyFactory = KeyFactory.getInstance(RSA_ALGORITHM);
        return keyFactory.generatePublic(spec);
    }

    private static PrivateKey loadPrivateKey(String base64Key) throws Exception {
        byte[] keyBytes = Base64.getDecoder().decode(base64Key);
        PKCS8EncodedKeySpec spec = new PKCS8EncodedKeySpec(keyBytes);
        KeyFactory keyFactory = KeyFactory.getInstance(RSA_ALGORITHM);
        return keyFactory.generatePrivate(spec);
    }

    public static void main(String[] args) {
        // Server's public key (provided by server during key exchange)
        String serverPublicKeyBase64 = "<SERVER_PUBLIC_KEY>";
        // Client's private key (generated by client, kept secret)
        String clientPrivateKeyBase64 = "<CLIENT_PRIVATE_KEY>";

        // Response fields received from server
        String encryptedSessionKey = "<encrypted_session_key from response>";
        String encryptedData = "<encrypted_data from response>";
        String signatureStr = "<signature from response>";

        try {
            // 1. Decrypt session key with client's private key
            PrivateKey clientPrivateKey = loadPrivateKey(clientPrivateKeyBase64);
            Cipher rsaCipher = Cipher.getInstance(RSA_TRANSFORMATION);
            rsaCipher.init(Cipher.DECRYPT_MODE, clientPrivateKey);
            byte[] sessionKey = rsaCipher.doFinal(
                Base64.getDecoder().decode(encryptedSessionKey)
            );

            // 2. Verify signature with server's public key
            PublicKey serverPublicKey = loadPublicKey(serverPublicKeyBase64);
            Signature signature = Signature.getInstance(SIGNATURE_ALGORITHM);
            signature.initVerify(serverPublicKey);
            signature.update(encryptedData.getBytes(StandardCharsets.UTF_8));
            boolean isValid = signature.verify(
                Base64.getDecoder().decode(signatureStr)
            );

            if (!isValid) {
                throw new SecurityException("Signature verification failed!");
            }

            // 3. Extract IV from session key
            byte[] iv = Arrays.copyOfRange(sessionKey, 0, IV_SIZE);

            // 4. Decrypt data with AES-256-CBC
            SecretKeySpec aesKeySpec = new SecretKeySpec(sessionKey, AES_ALGORITHM);
            IvParameterSpec ivSpec = new IvParameterSpec(iv);
            Cipher aesCipher = Cipher.getInstance(AES_TRANSFORMATION);
            aesCipher.init(Cipher.DECRYPT_MODE, aesKeySpec, ivSpec);
            byte[] decryptedBytes = aesCipher.doFinal(
                Base64.getDecoder().decode(encryptedData)
            );
            String decryptedJson = new String(decryptedBytes, StandardCharsets.UTF_8);

            System.out.println(decryptedJson);

        } catch (Exception e) {
            System.err.println("Decryption failed: " + e.getMessage());
            e.printStackTrace();
        }
    }
}

Steps Breakdown

Decrypt Session Key: The encrypted_session_key is decrypted with the client’s private key using RSA/ECB/PKCS1Padding
Verify Signature: The signature is verified against the encrypted_data string using the server’s public key with SHA256withRSA. The response should be rejected if verification fails.
Extract IV: The first 16 bytes of the decrypted session key are used as the IV
Decrypt Data: The encrypted_data is decrypted with AES/CBC/PKCS5Padding using the session key and IV

Important Notes

All Base64 encoding uses standard Base64 (not URL-safe)
The signature is computed over the Base64-encoded encrypted_data string, not over the raw encrypted bytes
The IV is derived from the session key (first 16 bytes) — it is not sent separately in the payload
The session key is re-encrypted with the client’s public key in the response, allowing the client to decrypt it with its private key
For POST requests, the server reuses the same AES session key from the request for encrypting the response
For GET requests/callbacks, a new session key is generated by the server

Security Best Practices

Key Management

Keys should be generated using cryptographically secure random number generators
Keys must be stored securely and never logged or transmitted in plain text
A request can be placed with the tech team to rotate keys

Implementation Guidelines

The digital signature should always be verified before decrypting the data
Proper error handling should be implemented to avoid information leakage
Secure key storage mechanisms must be in place
Private keys must never be shared or exposed

PFM Overview

Architecture

Analytics Data Dictionary

Integration Guide

API Encryption Guide

PFM APIs

Scheduled Data Sharing

Overview

Key Features

How It Works

1. Key Exchange (One-Time Setup)

2. Sending an Encrypted Request (Client → Server)

3. Receiving an Encrypted Response (Server → Client)

Key Setup

Step 1: Generate Client RSA Key Pair

Step 2: Exchange Public Keys

Algorithms & Parameters

Encrypted Payload Format

Request Payload

Response Payload

Encryption Flow

Implementation Examples

Encrypting a Request (Client → Server)

Steps Breakdown

Decrypting a Response (Server → Client)

Steps Breakdown

Important Notes

Security Best Practices

Key Management

Implementation Guidelines

PFM Overview

Architecture

Analytics Data Dictionary

Integration Guide

API Encryption Guide

PFM APIs

Scheduled Data Sharing

​Overview

​Key Features

​How It Works

​1. Key Exchange (One-Time Setup)

​2. Sending an Encrypted Request (Client → Server)

​3. Receiving an Encrypted Response (Server → Client)

​Key Setup

​Step 1: Generate Client RSA Key Pair

​Step 2: Exchange Public Keys

​Algorithms & Parameters

​Encrypted Payload Format

​Request Payload

​Response Payload

​Encryption Flow

​Implementation Examples

​Encrypting a Request (Client → Server)

​Steps Breakdown

​Decrypting a Response (Server → Client)

​Steps Breakdown

​Important Notes

​Security Best Practices

​Key Management

​Implementation Guidelines

Overview

Key Features

How It Works

1. Key Exchange (One-Time Setup)

2. Sending an Encrypted Request (Client → Server)

3. Receiving an Encrypted Response (Server → Client)

Key Setup

Step 1: Generate Client RSA Key Pair

Step 2: Exchange Public Keys

Algorithms & Parameters

Encrypted Payload Format

Request Payload

Response Payload

Encryption Flow

Implementation Examples

Encrypting a Request (Client → Server)

Steps Breakdown

Decrypting a Response (Server → Client)

Steps Breakdown

Important Notes

Security Best Practices

Key Management

Implementation Guidelines