Cleanup of agent (#58)

* Extract key selection.

* Moving agent and socket stuff to SecretAgentKit

* Cleanup of agent

SecretAgentKit/Agent.swift

@@ -0,0 +1,161 @@
+import Foundation
+import CryptoKit
+import OSLog
+import SecretKit
+
+public class Agent {
+
+    fileprivate let storeList: SecretStoreList
+    fileprivate let witness: SigningWitness?
+    fileprivate let writer = OpenSSHKeyWriter()
+
+    public init(storeList: SecretStoreList, witness: SigningWitness? = nil) {
+        os_log(.debug, "Agent is running")
+        self.storeList = storeList
+        self.witness = witness
+    }
+    
+}
+
+extension Agent {
+
+    public 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 {
+        // TODO: RESTORE ONCE XCODE 11.4 IS GM
+        let secrets = storeList.stores.flatMap { $0.secrets }
+//        let secrets = storeList.stores.flatMap(\.secrets)
+        var count = UInt32(secrets.count).bigEndian
+        let countData = Data(bytes: &count, count: UInt32.bitWidth/8)
+        var keyData = Data()
+        let writer = OpenSSHKeyWriter()
+        for secret in secrets {
+            let keyBlob = writer.data(secret: secret)
+            keyData.append(writer.lengthAndData(of: keyBlob))
+            let curveData = writer.curveType(for: secret.algorithm, length: secret.keySize).data(using: .utf8)!
+            keyData.append(writer.lengthAndData(of: curveData))
+        }
+        os_log(.debug, "Agent enumerated %@ identities", secrets.count as NSNumber)
+        return countData + keyData
+    }
+
+    func sign(data: Data) throws -> Data {
+        let reader = OpenSSHReader(data: data)
+        let hash = try reader.readNextChunk()
+        guard let (store, secret) = secret(matching: hash) else {
+            os_log(.debug, "Agent did not have a key matching %@", hash as NSData)
+            throw AgentError.noMatchingKey
+        }
+
+        if let witness = witness {
+            try witness.witness(accessTo: secret)
+        }
+
+        let dataToSign = try reader.readNextChunk()
+        let derSignature = try store.sign(data: dataToSign, with: secret)
+
+        let curveData = writer.curveType(for: secret.algorithm, length: secret.keySize).data(using: .utf8)!
+
+        // Convert from DER formatted rep to raw (r||s)
+
+        let rawRepresentation: Data
+        switch (secret.algorithm, secret.keySize) {
+        case (.ellipticCurve, 256):
+            rawRepresentation = try CryptoKit.P256.Signing.ECDSASignature(derRepresentation: derSignature).rawRepresentation
+        case (.ellipticCurve, 384):
+            rawRepresentation = try CryptoKit.P384.Signing.ECDSASignature(derRepresentation: derSignature).rawRepresentation
+        default:
+            fatalError()
+        }
+
+
+        let rawLength = rawRepresentation.count/2
+        let r = rawRepresentation[0..<rawLength]
+        let s = 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 {
+
+    func secret(matching hash: Data) -> (AnySecretStore, AnySecret)? {
+        storeList.stores.compactMap { store -> (AnySecretStore, AnySecret)? in
+            let allMatching = store.secrets.filter { secret in
+                hash == writer.data(secret: secret)
+            }
+            if let matching = allMatching.first {
+                return (store, matching)
+            }
+            return nil
+        }.first
+    }
+
+}
+
+
+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)
+    }
+
+}