Subclass of NSOutputStreamer that transparently decrypts data - ios

Question

I have an encrypted stream of data and I have implemented a function more or less similar to:
NSInteger DecryptContent(NSInputStream *inputStream,
NSOutputStream *outputStream,
NSData *key)
{
NSInteger totalNumberOfWrittenBytes = 0;
uint32_t recordSequenceNumber = 0;
NSMutableData *ciphertextInput = [NSMutableData dataWithLength:recordSize];
NSData *plaintextOutput = nil;
NSInteger recordDelimiterIndex = -1;
do {
CodingHeader *codingHeader = ReadCodingHeaderFromInoutStream(inputStream);
NSInteger numberOfReadBytes = [inputStream read:ciphertextInput.mutableBytes maxLength:codingHeader.recordSize];
if (numberOfReadBytes <= 0) {
LogError(#"Error: Stream should not have ended");
return -1;
}
NSData *actualCiphertextInput = ciphertextInput;
// Last chunk
if (numberOfReadBytes != ciphertextInput.length) {
actualCiphertextInput = [ciphertextInput subdataWithRange:NSMakeRange(0, numberOfReadBytes)];
}
NSData *scrambledKey = ScrambleKeyWithRecordSequenceNumberAndSalt(recordSequenceNumber, codingHeader.salt);
plaintextOutput = Decrypt(actualCiphertextInput, scrambledKey);
recordDelimiterIndex = FindRecordDelimiterIndex(plaintextOutput);
if (recordDelimiterIndex < 0) {
LogError(#"Error: Delimiter not found");
return -2;
}
NSInteger numberOfWrittenBytes = [outputStream write:plaintextOutput.bytes maxLength:recordDelimiterIndex];
if (numberOfWrittenBytes == -1) {
LogError(#"Error writing bytes: %#", outputStream.streamError);
return -3;
}
totalNumberOfWrittenBytes += numberOfWrittenBytes;
recordSequenceNumber++;
} while (((uint8_t *)plaintextOutput.bytes)[recordDelimiterIndex] != LastRecordDelimiterByte);
return totalNumberOfWrittenBytes;
}
This is not ideal because it's a blocking function that uses polling on the streams. What's a good approach for adapting this code into an NSOutputStream subclass that transparently decrypts data on the fly? Any other async alternatives?
Do I have to override - (NSInteger)write:(const uint8_t *)buffer maxLength:(NSUInteger)length and just manage the decryption using my own intermediary buffer, or is there a better/simpler approach?
If I have to manage my own buffer, not being able to use NSInputStream to conveniently read data (and having to use buffer offsets, concatenate several reads into one encrypted record, etc.) seems like a huge pain.