secretive/SecretAgent/Agent.swift

import Foundation
import CryptoKit
import OSLog
import SecretKit
import SecretAgentKit

class Agent<StoreType: SecretStore> {

    fileprivate let store: StoreType
    fileprivate let notifier: Notifier

    public init(store: StoreType, notifier: Notifier) {
        os_log(.debug, "Agent is running")
        self.store = store
        self.notifier = notifier
    }
    
}

extension Agent {

    func handle(fileHandle: FileHandle) {
        os_log(.debug, "Agent handling new data")
        let data = fileHandle.availableData
        guard !data.isEmpty else { return }
        let requestTypeInt = data[4]
        guard let requestType = SSHAgent.RequestType(rawValue: requestTypeInt) else { return }
        os_log(.debug, "Agent handling request of type %@", requestType.debugDescription)
        let subData = Data(data[5...])
        handle(requestType: requestType, data: subData, fileHandle: fileHandle)
    }

    func handle(requestType: SSHAgent.RequestType, data: Data, fileHandle: FileHandle) {
        var response = Data()
        do {
            switch requestType {
            case .requestIdentities:
                response.append(SSHAgent.ResponseType.agentIdentitiesAnswer.data)
                response.append(try identities())
                os_log(.debug, "Agent returned %@", SSHAgent.ResponseType.agentIdentitiesAnswer.debugDescription)
            case .signRequest:
                response.append(SSHAgent.ResponseType.agentSignResponse.data)
                response.append(try sign(data: data))
                os_log(.debug, "Agent returned %@", SSHAgent.ResponseType.agentSignResponse.debugDescription)
            }
        } catch {
            response.removeAll()
            response.append(SSHAgent.ResponseType.agentFailure.data)
            os_log(.debug, "Agent returned %@", SSHAgent.ResponseType.agentFailure.debugDescription)
        }
        let full = OpenSSHKeyWriter().lengthAndData(of: response)
        fileHandle.write(full)
    }

}

extension Agent {

    func identities() throws -> Data {
        var count = UInt32(store.secrets.count).bigEndian
        let countData = Data(bytes: &count, count: UInt32.bitWidth/8)
        var keyData = Data()
        let writer = OpenSSHKeyWriter()
        for secret in store.secrets {
            let keyBlob = writer.data(secret: secret)
            keyData.append(writer.lengthAndData(of: keyBlob))
            let curveData = OpenSSHKeyWriter.Constants.curveType.data(using: .utf8)!
            keyData.append(writer.lengthAndData(of: curveData))
        }
        os_log(.debug, "Agent enumerated %@ identities", store.secrets.count as NSNumber)
        return countData + keyData
    }

    func sign(data: Data) throws -> Data {
        let reader = OpenSSHReader(data: data)
        let writer = OpenSSHKeyWriter()
        let hash = try reader.readNextChunk()
        let matching = store.secrets.filter { secret in
            hash == writer.data(secret: secret)
        }
        guard let secret = matching.first else {
            throw AgentError.noMatchingKey
        }
        let dataToSign = try reader.readNextChunk()
        let derSignature = try store.sign(data: dataToSign, with: secret)
        // TODO: Move this
        notifier.notify(accessTo: secret)
        let curveData = OpenSSHKeyWriter.Constants.curveType.data(using: .utf8)!

        // Convert from DER formatted rep to raw (r||s)
        let signature = try CryptoKit.P256.Signing.ECDSASignature(derRepresentation: derSignature)
        let rawLength = signature.rawRepresentation.count/2
        let r = signature.rawRepresentation[0..<rawLength]
        let s = signature.rawRepresentation[rawLength...]

        var signatureChunk = Data()
        signatureChunk.append(writer.lengthAndData(of: r))
        signatureChunk.append(writer.lengthAndData(of: s))

        var signedData = Data()
        var sub = Data()
        sub.append(writer.lengthAndData(of: curveData))
        sub.append(writer.lengthAndData(of: signatureChunk))
        signedData.append(writer.lengthAndData(of: sub))

        os_log(.debug, "Agent signed request")

        return signedData
    }

}


extension Agent {

    enum AgentError: Error {
        case unhandledType
        case noMatchingKey
    }

}

extension SSHAgent.ResponseType {

    var data: Data {
        var raw = self.rawValue
        return  Data(bytes: &raw, count: UInt8.bitWidth/8)
    }

}