Objective-C Hash Functions

HashFunctions.m

+ (NSString*) sha1:(NSString*)input
{
	const char *cstr = [input cStringUsingEncoding:NSUTF8StringEncoding];
	NSData *data = [NSData dataWithBytes:cstr length:input.length];
	
	uint8_t digest[CC_SHA1_DIGEST_LENGTH];
	
	CC_SHA1(data.bytes, data.length, digest);
	
	NSMutableString* output = [NSMutableString stringWithCapacity:CC_SHA1_DIGEST_LENGTH * 2];
	
	for(int i = 0; i < CC_SHA1_DIGEST_LENGTH; i++)
		[output appendFormat:@"%02x", digest[i]];
	
	return output;
	
}

+ (NSString *) md5:(NSString *) input
{
	const char *cStr = [input UTF8String];
	unsigned char digest[16];
	CC_MD5( cStr, strlen(cStr), digest ); // This is the md5 call
	
	NSMutableString *output = [NSMutableString stringWithCapacity:CC_MD5_DIGEST_LENGTH * 2];
	
	for(int i = 0; i < CC_MD5_DIGEST_LENGTH; i++)
		[output appendFormat:@"%02x", digest[i]];
	
	return  output;
	
}

why CC_MD5_DIGEST_LENGTH * 2 is required? why cant we use CC_MD5_DIGEST_LENGTH