IOS stringByReplacingOccurrencesOfString coming back null - ios

Question

I've been banging my head against the wall on this, I must be missing something. This code is called just after I base64 encode some encrypted text. At the point where I call NSLog signature is always valid. However sometimes the rest works great and other times removePlus or finalSig come back null. Also I'm using Xcode 4.5, ios6 and I have ARC enabled for the project. I feel like maybe something is getting auto released before I want it to, or something like that. If anyone has any ideas any help is appreciated.
// Get the data out of the context
char *outputBuffer;
long outputLength = BIO_get_mem_data(context, &outputBuffer);
NSMutableString *signature = [[NSMutableString alloc] init];
[signature appendString:[NSMutableString stringWithCString:outputBuffer encoding:NSASCIIStringEncoding]];
NSLog(#"Base64 Pre Encoded: %#", signature);
signature = [[signature stringByAddingPercentEscapesUsingEncoding:NSASCIIStringEncoding] mutableCopy];
NSMutableString *removePlus = [[signature stringByReplacingOccurrencesOfString:#"+" withString:#"%2B"] mutableCopy];
NSString *finalSig = [removePlus stringByReplacingOccurrencesOfString:#"=" withString:#"%3D"];
//now we create the url request portion
NSMutableString *variables = [[NSMutableString alloc] init];
//set the variables we're going
[variables appendString:finalSig];

Answer 1

Try swapping your code for the following, as I imagine it will show you exactly where your problem is:
// Get the data out of the context
char *outputBuffer;
long outputLength = BIO_get_mem_data(context, &outputBuffer);
outputBuffer[outputLength] = '\0';
if(outputLength != strlen(outputBuffer)) NSLog(#"SOMETHING VERY WRONG");
NSString * signature = [NSString stringWithCString:outputBuffer encoding:NSASCIIStringEncoding]];
NSLog(#"Base64 Pre Encoded: %#", signature);
signature = [[signature stringByAddingPercentEscapesUsingEncoding:NSASCIIStringEncoding];
NSLog(#"Base64 Post Encoded: %#", signature);
signature = [signature stringByReplacingOccurrencesOfString:#"+" withString:#"%2B"];
NSLog(#"Base64 Remove '+': %#", signature);
signature = [removePlus stringByReplacingOccurrencesOfString:#"=" withString:#"%3D"];
NSLog(#"Base64 Remove '=': %#", signature);
//now we create the url request portion
NSMutableString *variables = [NSMutableString alloc] initWithString:signature];
…
//set the variables we're going
[variables appendString:finalSig];

Answer 2

Turns out that NSString * signature = [NSString stringWithCString:outputBuffer encoding:NSASCIIStringEncoding]]; requires a null terminated string and my string was not. Anyway this was a more effective way to create the string:
NSData *myRequestData = [ NSData dataWithBytes: [ variables UTF8String ] length: [ variables length ] ];