Buffered input to SecTransform - swift

Question

I'm reading data from an audio file and computing a hash of it, as in the style of the ffmpeg MD5 muxer, except I'm doing SHA2 and using AVFoundation and the OS X SecTransform API.
What this does is it opens an audio file, converts it into it's native PCM format if it's compressed, and then hashes the interleaved samples byte-by-byte.
When I read the audio from my files, I would normally read the samples into a buffer in a for or while loop.
extension AVAudioFile {
func sha2() throws -> NSData {
let bufSize = AVAudioFrameCount(0x1000)
let buffer = AVAudioPCMBuffer(PCMFormat: self.processingFormat,
frameCapacity: bufSize)
// initialize digest algo
for(;;) {
try readIntoBuffer(buffer)
if buffer.frameLength > 0 {
// read buffer into digest
} else {
break
}
}
// finalize digest and return...
return NSData()
}
}
The issue I'm having is, the only way I can see of loading data into a SecTransform is either handing it all the data at once in a CFData, or as a CFReadStream. How do I feed my data buffer-by-buffer into a SecTransform?

Answer 1

I figured it out, you create bound input and output streams with NSStream.getBoundStreamsWithBufferSize() and then you feed the output stream with an asynchronous loop
The complete implementation is like this:
func writeSamplesFromBuffer(buffer: AVAudioPCMBuffer, toStream : NSOutputStream) {
assert(buffer.format.interleaved == true)
var rawBuffer = UnsafePointer<UInt8>(buffer.int32ChannelData.memory)
var toWrite = sizeof(Int32) *
Int(buffer.format.channelCount) * Int(buffer.frameLength)
while toWrite > 0 {
let written = toStream.write(rawBuffer, maxLength: toWrite)
rawBuffer = rawBuffer.advancedBy(written)
toWrite -= written
}
}
func writeAudioDataFromURL(url : NSURL,
usingFormat format: AVAudioCommonFormat,
toStream: NSOutputStream) throws {
let audioFile = try AVAudioFile(forReading: url,
commonFormat: format,
interleaved: true)
let pcmBuffer = AVAudioPCMBuffer(PCMFormat:
audioFile.processingFormat,
frameCapacity: 0x1000)
toStream.open()
let writerQueue = dispatch_get_global_queue(QOS_CLASS_UTILITY, 0)
dispatch_async(writerQueue) {
while true {
do {
try audioFile.readIntoBuffer(pcmBuffer)
if pcmBuffer.frameLength > 0 {
writeSamplesFromBuffer(pcmBuffer, toStream: toStream)
} else {
break
}
} catch let error {
fatalError("Fatal error: \(error) while reading audio file \(audioFile) at URL \(url)")
}
}
toStream.close()
}
}
func sha256DigestForStream(stream : NSInputStream) throws -> NSData {
let transform = SecTransformCreateGroupTransform().takeRetainedValue()
let readXform = SecTransformCreateReadTransformWithReadStream(stream as CFReadStreamRef).takeRetainedValue()
var error : Unmanaged<CFErrorRef>? = nil
let digestXform = SecDigestTransformCreate(kSecDigestSHA2, 256, &error).takeRetainedValue()
SecTransformConnectTransforms(readXform, kSecTransformOutputAttributeName,
digestXform, kSecTransformInputAttributeName,
transform, &error)
if let e = error { throw e.takeUnretainedValue() }
if let output = SecTransformExecute(transform, &error) as? NSData {
return output
} else {
throw error!.takeRetainedValue()
}
}
func sha256DigestForAudioFile(url : NSURL,
convertedToSampleFormat sampleFormat: AVAudioCommonFormat) throws -> NSData {
let streamBufSize = 0x1000
var rs : NSInputStream? = nil
var ws : NSOutputStream? = nil
NSStream.getBoundStreamsWithBufferSize(streamBufSize,
inputStream: &rs, outputStream: &ws)
guard let readStream = rs, writeStream = ws else {
fatalError("Failed to create file read streams")
}
try writeAudioDataFromURL(url,
usingFormat: sampleFormat, toStream: writeStream)
return try sha256DigestForStream(readStream)
}