session-ios/SessionSnodeKit/OnionRequestAPI+Encryption.swift

import CryptoSwift
import PromiseKit
import SessionUtilities

internal extension OnionRequestAPI {

    static func encode(ciphertext: Data, json: JSON) throws -> Data {
        // The encoding of V2 onion requests looks like: | 4 bytes: size N of ciphertext | N bytes: ciphertext | json as utf8 |
        guard JSONSerialization.isValidJSONObject(json) else { throw HTTP.Error.invalidJSON }
        let jsonAsData = try JSONSerialization.data(withJSONObject: json, options: [ .fragmentsAllowed ])
        let ciphertextSize = Int32(ciphertext.count).littleEndian
        let ciphertextSizeAsData = withUnsafePointer(to: ciphertextSize) { Data(bytes: $0, count: MemoryLayout<Int32>.size) }
        return ciphertextSizeAsData + ciphertext + jsonAsData
    }

    /// Encrypts `payload` for `destination` and returns the result. Use this to build the core of an onion request.
    static func encrypt(_ payload: JSON, for destination: Destination) -> Promise<AESGCM.EncryptionResult> {
        let (promise, seal) = Promise<AESGCM.EncryptionResult>.pending()
        DispatchQueue.global(qos: .userInitiated).async {
            do {
                guard JSONSerialization.isValidJSONObject(payload) else { return seal.reject(HTTP.Error.invalidJSON) }
                // Wrapping isn't needed for file server or open group onion requests
                switch destination {
                case .snode(let snode):
                    let snodeX25519PublicKey = snode.publicKeySet.x25519Key
                    let payloadAsData = try JSONSerialization.data(withJSONObject: payload, options: [ .fragmentsAllowed ])
                    let plaintext = try encode(ciphertext: payloadAsData, json: [ "headers" : "" ])
                    let result = try AESGCM.encrypt(plaintext, for: snodeX25519PublicKey)
                    seal.fulfill(result)
                case .server(_, let serverX25519PublicKey):
                    let plaintext = try JSONSerialization.data(withJSONObject: payload, options: [ .fragmentsAllowed ])
                    let result = try AESGCM.encrypt(plaintext, for: serverX25519PublicKey)
                    seal.fulfill(result)
                }
            } catch (let error) {
                seal.reject(error)
            }
        }
        return promise
    }

    /// Encrypts the previous encryption result (i.e. that of the hop after this one) for this hop. Use this to build the layers of an onion request.
    static func encryptHop(from lhs: Destination, to rhs: Destination, using previousEncryptionResult: AESGCM.EncryptionResult) -> Promise<AESGCM.EncryptionResult> {
        let (promise, seal) = Promise<AESGCM.EncryptionResult>.pending()
        DispatchQueue.global(qos: .userInitiated).async {
            var parameters: JSON
            switch rhs {
            case .snode(let snode):
                let snodeED25519PublicKey = snode.publicKeySet.ed25519Key
                parameters = [ "destination" : snodeED25519PublicKey ]
            case .server(let host, _):
                parameters = [ "host" : host, "target" : "/loki/v2/lsrpc", "method" : "POST" ]
            }
            parameters["ephemeral_key"] = previousEncryptionResult.ephemeralPublicKey.toHexString()
            let x25519PublicKey: String
            switch lhs {
            case .snode(let snode):
                let snodeX25519PublicKey = snode.publicKeySet.x25519Key
                x25519PublicKey = snodeX25519PublicKey
            case .server(_, let serverX25519PublicKey):
                x25519PublicKey = serverX25519PublicKey
            }
            do {
                let plaintext = try encode(ciphertext: previousEncryptionResult.ciphertext, json: parameters)
                let result = try AESGCM.encrypt(plaintext, for: x25519PublicKey)
                seal.fulfill(result)
            } catch (let error) {
                seal.reject(error)
            }
        }
        return promise
    }
}