gistfile1.m

-(AVVideoComposition *)squareCropperVideoComposition:(AVAssetTrack *)clipVideoTrack {
    AVMutableVideoComposition * videoComposition = [AVMutableVideoComposition videoComposition];
    
    CGSize originVideoSize = clipVideoTrack.naturalSize;

    CGFloat shortSideLength = MIN(originVideoSize.width, originVideoSize.height);
    CGFloat longSideLength  = MAX(originVideoSize.width, originVideoSize.height);
    CGFloat renderSideLength = 640.0; // MIN(640.0, shortSideLength);
    CGFloat scale = renderSideLength / shortSideLength;
    
    videoComposition.renderSize = CGSizeMake(renderSideLength, renderSideLength);
    videoComposition.frameDuration = CMTimeMake(1, 30);
    
    AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
    instruction.timeRange = clipVideoTrack.timeRange;
    
    AVMutableVideoCompositionLayerInstruction* cropper = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:clipVideoTrack];

    CGAffineTransform t = CGAffineTransformIdentity; //CGAffineTransformMakeRotation(M_PI_2);
    
    if (_cameraPosition == AVCaptureDevicePositionFront) {
        t = CGAffineTransformTranslate(t, renderSideLength, 0);
        t = CGAffineTransformScale(t, -1, 1);
    }

    t = CGAffineTransformRotate(t, M_PI_2);
    t = CGAffineTransformScale(t, scale, scale);
    t = CGAffineTransformTranslate(t, -(longSideLength - shortSideLength) / 2, -shortSideLength);
    [cropper setTransform:t atTime:kCMTimeZero];
    
    instruction.layerInstructions = @[cropper];
    videoComposition.instructions = @[instruction];
    
    return videoComposition;
}


    AVMutableComposition * mutableComposition = [AVMutableComposition composition];
    
    AVMutableCompositionTrack * videoTrack = [mutableComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    AVMutableCompositionTrack * audioTrack = [mutableComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];

    AVAssetTrack * vt = [asset tracksWithMediaType:AVMediaTypeVideo][0];
    AVAssetTrack * at = [asset tracksWithMediaType:AVMediaTypeAudio][0];
    
    [videoTrack insertTimeRange:vt.timeRange ofTrack:vt atTime:kCMTimeZero error:nil];
    [audioTrack insertTimeRange:at.timeRange ofTrack:at atTime:kCMTimeZero error:nil];
    
    AVAssetExportSession *_exporter = [[AVAssetExportSession alloc] initWithAsset:mutableComposition presetName:AVAssetExportPresetHighestQuality];
    _exporter.videoComposition = [self squareCropperVideoComposition:videoTrack];
    _exporter.outputFileType = AVFileTypeMPEG4;
    _exporter.outputURL = ... some file url

    [_exporter exportAsynchronouslyWithCompletionHandler:^{
    
    }];