Decrypt Media Files in chunks and play via AVPlayer - ios

Question

I have a mp4 video file which i am encrypting to save and decrypting to play via AVPlayer. Using CRYPTOSWIFT Library for encrypting/decrypting
Its working fine when i am decrypting whole file at once but my file is quite big and taking 100% CPU usage and lot of memory. So, I need to decrypt encrypted file in chunks.
I tried to decrypt file in chunks but its not playing video as AVPlayer is not recognizing decrypted chunk data maybe data is not stored sequentially while encrypting file. I have tried chacha20, AES, AES.CTR & AES.CBC protocols to encrypt and decrypt files but to no avail.
extension PlayerController: AVAssetResourceLoaderDelegate {
func resourceLoader(resourceLoader: AVAssetResourceLoader, shouldWaitForLoadingOfRequestedResource loadingRequest: AVAssetResourceLoadingRequest) -> Bool {
let request = loadingRequest.request
guard let path = request.URL?.path where request.URL?.scheme == Constants.customVideoScheme else { return true }
if let contentRequest = loadingRequest.contentInformationRequest {
do {
let fileAttributes = try NSFileManager.defaultManager().attributesOfItemAtPath(path)
if let fileSizeNumber = fileAttributes[NSFileSize] {
contentRequest.contentLength = fileSizeNumber.longLongValue
}
} catch { }
if fileHandle == nil {
fileHandle = NSFileHandle(forReadingAtPath: (request.URL?.path)!)!
}
contentRequest.contentType = "video/mp4"
contentRequest.byteRangeAccessSupported = true
}
if let data = decryptData(loadingRequest, path: path), dataRequest = loadingRequest.dataRequest {
dataRequest.respondWithData(data)
loadingRequest.finishLoading()
return true
}
return true
}
func decryptData(loadingRequest: AVAssetResourceLoadingRequest, path: String) -> NSData? {
print("Current OFFSET: \(loadingRequest.dataRequest?.currentOffset)")
print("requested OFFSET: \(loadingRequest.dataRequest?.requestedOffset)")
print("Current Length: \(loadingRequest.dataRequest?.requestedLength)")
if loadingRequest.contentInformationRequest != nil {
var data = fileHandle!.readDataOfLength((loadingRequest.dataRequest?.requestedLength)!)
fileHandle!.seekToFileOffset(0)
data = decodeVideoData(data)!
return data
} else {
fileHandle?.seekToFileOffset(UInt64((loadingRequest.dataRequest?.currentOffset)!))
let data = fileHandle!.readDataOfLength((loadingRequest.dataRequest?.requestedLength)!)
// let data = fileHandle!.readDataOfLength(length!) ** When I use this its not playing video but play fine when try with requestedLength **
return decodeVideoData(data)
}
}
}
Decode code to decode nsdata :
func decodeVideoData(data: NSData) -> NSData? {
if let cha = ChaCha20(key: Constants.Encryption.SecretKey, iv: Constants.Encryption.IvKey) {
let decrypted: NSData = try! data.decrypt(cha)
return decrypted
}
return nil
}
I need help regarding this issue, Kindly guide me to the right way to achieve this.

Answer 1

For in depth and a more complete CommonCrypto wrapper, check out my CommonCrypto wrapper. I've extracted bits and pieces for this answer.
First of all, we need to define some functions that will do the encryption/decryption. I'm assuming, for now, you use AES(256) CBC with PKCS#7 padding. Summarising the snippet below: we have an update function, that can be called repeatedly to consume the chunks. There's also a final function that will wrap up any left overs (usually deals with padding).
import CommonCrypto
import Foundation
enum CryptoError: Error {
case generic(CCCryptorStatus)
}
func getOutputLength(_ reference: CCCryptorRef?, inputLength: Int, final: Bool) -> Int {
CCCryptorGetOutputLength(reference, inputLength, final)
}
func update(_ reference: CCCryptorRef?, data: Data) throws -> Data {
var output = [UInt8](repeating: 0, count: getOutputLength(reference, inputLength: data.count, final: false))
let status = data.withUnsafeBytes { dataPointer -> CCCryptorStatus in
CCCryptorUpdate(reference, dataPointer.baseAddress, data.count, &output, output.count, nil)
}
guard status == kCCSuccess else {
throw CryptoError.generic(status)
}
return Data(output)
}
func final(_ reference: CCCryptorRef?) throws -> Data {
var output = [UInt8](repeating: 0, count: getOutputLength(reference, inputLength: 0, final: true))
var moved = 0
let status = CCCryptorFinal(reference, &output, output.count, &moved)
guard status == kCCSuccess else {
throw CryptoError.generic(status)
}
output.removeSubrange(moved...)
return Data(output)
}
Next up, for the purpose of demonstration, the encryption.
let key = Data(repeating: 0x0a, count: kCCKeySizeAES256)
let iv = Data(repeating: 0, count: kCCBlockSizeAES128)
let bigFile = (0 ..< 0xffff).map { _ in
return Data(repeating: UInt8.random(in: 0 ... UInt8.max), count: kCCBlockSizeAES128)
}.reduce(Data(), +)
var encryptor: CCCryptorRef?
CCCryptorCreate(CCOperation(kCCEncrypt), CCAlgorithm(kCCAlgorithmAES), CCOptions(kCCOptionPKCS7Padding), Array(key), key.count, Array(iv), &encryptor)
do {
let ciphertext = try update(encryptor, data: bigFile) + final(encryptor)
print(ciphertext) // 1048576 bytes
} catch {
print(error)
}
That appears to me as quite a large file. Now decrypting, would be done in a similar fashion.
var decryptor: CCCryptorRef?
CCCryptorCreate(CCOperation(kCCDecrypt), CCAlgorithm(kCCAlgorithmAES), CCOptions(kCCOptionPKCS7Padding), Array(key), key.count, Array(iv), &decryptor)
do {
var plaintext = Data()
for i in 0 ..< 0xffff {
plaintext += try update(decryptor, data: ciphertext[i * kCCBlockSizeAES128 ..< i * kCCBlockSizeAES128 + kCCBlockSizeAES128])
}
plaintext += try final(decryptor)
print(plaintext == bigFile, plaintext) // true 1048560 bytes
} catch {
print(error)
}
The encryptor can be altered for different modes and should also be released once it's done, and I'm not too sure how arbitrary output on the update function will behave, but this should be enough to give you an idea of how it can be done using CommonCrypto.