Add decryption to DeviceNames.

SignalServiceKit/src/Util/DeviceNames.swift

@@ -8,6 +8,7 @@ import Curve25519Kit
 @objc
 public enum DeviceNameError: Int, Error {
     case assertionFailure
+    case invalidInput
 }
 
 @objc
@@ -15,6 +16,8 @@ public class DeviceNames: NSObject {
     // Never instantiate this class.
     private override init() {}
 
+    private static let syntheticIVLength: UInt = 16
+
     @objc
     public class func encryptDeviceName(plaintext: String,
                                         identityKeyPair: ECKeyPair) throws -> Data {
@@ -35,6 +38,39 @@ public class DeviceNames: NSObject {
             throw error
         }
 
+        // synthetic_iv = HmacSHA256(key=HmacSHA256(key=master_secret, input=“auth”), input=plaintext)[0:16]
+        let syntheticIV = try computeSyntheticIV(masterSecret: masterSecret,
+                                                 plaintextData: plaintextData)
+
+        // cipher_key = HmacSHA256(key=HmacSHA256(key=master_secret, “cipher”), input=synthetic_iv)
+        let cipherKey = try computeCipherKey(masterSecret: masterSecret, syntheticIV: syntheticIV)
+
+        // cipher_text = AES-CTR(key=cipher_key, input=plaintext, counter=0)
+        //
+        // TODO: Is this IV right?
+        let ciphertextIV = Data(count: Int(kAES256CTR_IVLength))
+        guard let ciphertextKey = OWSAES256Key(data: cipherKey) else {
+            owsFailDebug("Invalid cipher key.")
+            throw DeviceNameError.assertionFailure
+        }
+        guard let ciphertext: AES256CTREncryptionResult = Cryptography.encryptAESCTR(plaintextData: plaintextData, initializationVector: ciphertextIV, key: ciphertextKey) else {
+            owsFailDebug("Could not encrypt cipher text.")
+            throw DeviceNameError.assertionFailure
+        }
+
+        let protoBuilder = SignalIOSProtoDeviceName.builder(ephemeralPublic: ephemeralKeyPair.publicKey,
+                                                            syntheticIv: syntheticIV,
+                                                            ciphertext: ciphertext.ciphertext)
+        let protoData = try protoBuilder.buildSerializedData()
+
+        // TODO: This uses Data's foundation method rather than the NSData's SSK method.
+        let protoDataBase64 = protoData.base64EncodedData()
+
+        return protoDataBase64
+    }
+
+    private class func computeSyntheticIV(masterSecret: Data,
+                                          plaintextData: Data) throws -> Data {
         // synthetic_iv = HmacSHA256(key=HmacSHA256(key=master_secret, input=“auth”), input=plaintext)[0:16]
         guard let syntheticIVInput = "auth".data(using: .utf8) else {
             owsFail("Could not convert text to UTF-8.")
@@ -43,12 +79,15 @@ public class DeviceNames: NSObject {
             owsFailDebug("Could not compute synthetic IV key.")
             throw DeviceNameError.assertionFailure
         }
-        let ivLength: UInt = 16
-        guard let syntheticIV = Cryptography.truncatedSHA256HMAC(plaintextData, withHMACKey: syntheticIVKey, truncation: ivLength) else {
+        guard let syntheticIV = Cryptography.truncatedSHA256HMAC(plaintextData, withHMACKey: syntheticIVKey, truncation: syntheticIVLength) else {
             owsFailDebug("Could not compute synthetic IV.")
             throw DeviceNameError.assertionFailure
         }
+        return syntheticIV
+    }
 
+    private class func computeCipherKey(masterSecret: Data,
+                                        syntheticIV: Data) throws -> Data {
         // cipher_key = HmacSHA256(key=HmacSHA256(key=master_secret, “cipher”), input=synthetic_iv)
         guard let cipherKeyInput = "cipher".data(using: .utf8) else {
             owsFail("Could not convert text to UTF-8.")
@@ -61,27 +100,84 @@ public class DeviceNames: NSObject {
             owsFailDebug("Could not compute cipher key.")
             throw DeviceNameError.assertionFailure
         }
+        return cipherKey
+    }
 
-        // cipher_text = AES-CTR(key=cipher_key, input=plaintext, counter=0)
+    @objc
+    public class func decryptDeviceName(input: Data,
+                                        identityKeyPair: ECKeyPair) throws -> String {
+
+        guard let protoData = Data(base64Encoded: input) else {
+            // Not necessarily an error; might be a legacy device name.
+            throw DeviceNameError.invalidInput
+        }
+
+        let proto: SignalIOSProtoDeviceName
+        do {
+            proto = try SignalIOSProtoDeviceName.parseData(protoData)
+        } catch {
+            // Not necessarily an error; might be a legacy device name.
+            Logger.error("failed to parse proto")
+            throw DeviceNameError.invalidInput
+        }
+
+        let ephemeralPublic = proto.ephemeralPublic
+        let receivedSyntheticIV = proto.syntheticIv
+        let ciphertext = proto.ciphertext
+
+        guard ephemeralPublic.count > 0 else {
+            owsFailDebug("Invalid ephemeral public.")
+            throw DeviceNameError.assertionFailure
+        }
+        guard receivedSyntheticIV.count == syntheticIVLength else {
+            owsFailDebug("Invalid synthetic IV.")
+            throw DeviceNameError.assertionFailure
+        }
+        guard ciphertext.count > 0 else {
+            owsFailDebug("Invalid cipher text.")
+            throw DeviceNameError.assertionFailure
+        }
+
+        // master_secret = ECDH(identity_private, ephemeral_public)
+        let masterSecret: Data
+        do {
+            masterSecret = try Curve25519.generateSharedSecret(fromPublicKey: ephemeralPublic,
+                                                               privateKey: identityKeyPair.privateKey)
+        } catch {
+            Logger.error("Could not generate shared secret: \(error)")
+            throw error
+        }
+
+        // cipher_key = HmacSHA256(key=HmacSHA256(key=master_secret, input=“cipher”), input=synthetic_iv)
+        let cipherKey = try computeCipherKey(masterSecret: masterSecret, syntheticIV: receivedSyntheticIV)
+
+        // plaintext = AES-CTR(key=cipher_key, input=ciphertext, counter=0)
         //
-        // TODO: Is this right?
-        let cipherTextIV = Data(count: Int(kAES256CTR_IVLength))
-        guard let cipherTextKey = OWSAES256Key(data: cipherKey) else {
+        // TODO: Is this IV right?
+        let ciphertextIV = Data(count: Int(kAES256CTR_IVLength))
+        guard let ciphertextKey = OWSAES256Key(data: cipherKey) else {
             owsFailDebug("Invalid cipher key.")
             throw DeviceNameError.assertionFailure
         }
-        guard let cipherText: AES256CTREncryptionResult = Cryptography.encryptAESCTR(plaintextData: plaintextData, initializationVector: cipherTextIV, key: cipherTextKey) else {
-            owsFailDebug("Could not compute cipher text.")
+        guard let plaintextData = Cryptography.decryptAESCTR(cipherText: ciphertext, initializationVector: ciphertextIV, key: ciphertextKey) else {
+            owsFailDebug("Could not decrypt cipher text.")
             throw DeviceNameError.assertionFailure
         }
 
-        let protoBuilder = SignalIOSProtoDeviceName.builder(ephemeralPublic: ephemeralKeyPair.publicKey,
-                                                            syntheticIv: syntheticIV,
-                                                            ciphertext: cipherText.ciphertext)
-        let protoData = try protoBuilder.buildSerializedData()
+        // Verify the synthetic IV was correct.
+        // constant_time_compare(HmacSHA256(key=HmacSHA256(key=master_secret, input=”auth”), input=plaintext)[0:16], synthetic_iv) == true
+        let computedSyntheticIV = try computeSyntheticIV(masterSecret: masterSecret,
+                                                         plaintextData: plaintextData)
+        guard receivedSyntheticIV.ows_constantTimeIsEqual(to: computedSyntheticIV) else {
+            owsFailDebug("Synthetic IV did not match.")
+            throw DeviceNameError.assertionFailure
+        }
 
-        let protoDataBase64 = protoData.base64EncodedData()
+        guard let plaintext = String(bytes: plaintextData, encoding: .utf8) else {
+            owsFailDebug("Invalid plaintext.")
+            throw DeviceNameError.invalidInput
+        }
 
-        return protoDataBase64
+        return plaintext
     }
 }