How to resolve an error when uploading an image with AWS on iOS

I am following this tutorial which involves uploading a UIImage to an s3 bucket using cognito for authentication. I am able to connect to cognito because my device is showing in the identity pool. However, when I try to upload the image to a bucket, I get this error:

Error Domain=com.amazonaws.AWSGeneralErrorDomain Code=3 "The request signature we calculated does not match the signature you provided. Check your key and signing method." UserInfo=0x17dc0f40 {NSLocalizedDescription=The request signature we calculated does not match the signature you provided. Check your key and signing method.}

The cognito authentication policy looks like this:

    {
"Version": "2012-10-17",
"Statement": [{
    "Action": [
        "mobileanalytics:PutEvents",
        "cognito-sync:*",
        "s3:*"
    ],
    "Effect": "Allow",
    "Resource": [
        "*"
    ]
}]
}

The code for setting up the credentials looks like this:

    AWSCognitoCredentialsProvider *credentialsProvider = [AWSCognitoCredentialsProvider
                                                      credentialsWithRegionType:AWSRegionUSEast1
                                                      accountId:@"#######"
                                                      identityPoolId:@"######"
                                                      unauthRoleArn:@"#####"
                                                      authRoleArn:@"######"];

AWSServiceConfiguration *configuration = [AWSServiceConfiguration configurationWithRegion:AWSRegionUSEast1
                                                                      credentialsProvider:credentialsProvider]

and the code for uploading the image to s3 looks like this:

    NSString *tempPath = [NSTemporaryDirectory() stringByAppendingPathComponent:@"image.png"];
NSData *imageData = UIImagePNGRepresentation(image);
[imageData writeToFile:tempPath atomically:YES];


NSURL *url = [[NSURL alloc] initFileURLWithPath:tempPath];

AWSS3TransferManagerUploadRequest *uploadRequest = [AWSS3TransferManagerUploadRequest new];
uploadRequest.bucket = @"##########";
//uploadRequest.ACL = AWSS3ObjectCannedACLPublicRead;
uploadRequest.key = @"image.png";
//uploadRequest.contentType = @"image/png";
uploadRequest.body = url;

uploadRequest.uploadProgress =^(int64_t bytesSent, int64_t totalBytesSent, int64_t totalBytesExpectedToSend){
    dispatch_sync(dispatch_get_main_queue(), ^{

    });
};

AWSS3TransferManager *transferManager = [AWSS3TransferManager defaultS3TransferManager];
[[transferManager upload:uploadRequest] continueWithExecutor:[BFExecutor mainThreadExecutor] withBlock:^id(BFTask *task) {
    if (task.error) {
        NSLog(@"%@", task.error);
    }else{
        //success
        NSLog(@"success");
        [[NSFileManager defaultManager] removeItemAtURL:url error:nil];
    }
    return nil;
}];