When discussing string comparison in programming, it’s crucial to move beyond the simplistic view of strings as mere sequences of bytes. Treating strings with nuance, especially in a world of Unicode and internationalized domain names (IDNs), is not just about academic correctness; it’s a practical necessity to prevent security vulnerabilities and improve user experience.
Consider the potential for scams arising from subtle differences in string representation. If a browser interprets “microsoftoffice.com” and “microsoftoffice.com” as distinct, a malicious actor could register the former – visually almost identical to the legitimate “microsoftoffice.com” – and deceive users into visiting a fraudulent site. This is more than a hypothetical scenario; it’s a tangible risk stemming from how strings are compared at a fundamental level in browsers and DNS systems.
Similarly, in DNS server code, if new domain registrations are checked against existing ones using a strict, byte-for-byte string comparison, deceptive domain names can slip through. Ligatures, for instance, which are single glyphs representing combinations of characters, can be treated as distinct from their component characters in naive string comparisons. This discrepancy allows for the registration of domain names that are visually misleading, opening doors for phishing and other malicious activities. The infamous IDN homograph attacks are a stark reminder of these real-world implications, highlighting failures even among experts in DNS internationalization and library development.
The core argument is that erring on the side of leniency in equality checks for strings is the safer default. Why? Because preventing the aforementioned problems with strict equality requires developers to perform extra, often overlooked, work. They must consciously implement additional layers of normalization and comparison logic to account for Unicode complexities like ligatures and case variations. If, however, string comparison were lenient by default – ignoring ligatures and perhaps even case – developers would have to actively choose stricter comparison methods if their specific use case demanded it. This shift in default would inherently reduce the likelihood of security oversights and simplify development in many common scenarios.
The Unicode Consortium’s sentiment regarding ligatures – they reportedly wish they had never been added – further underscores the potential problems they introduce. Discarding ligatures in string comparisons, or at least treating them as equivalent to their constituent characters, aligns with making string handling more robust and less error-prone.
Languages like Swift are already pushing the boundaries of string handling, emphasizing that strings are not just byte sequences. Swift’s design choices, such as making string indexing deliberately complex and computationally intensive, reflect a commitment to Unicode correctness and safety. However, the argument extends to suggesting that Swift, and other languages, might need to go even further by adopting more lenient default string comparison behaviors.
While the idea of default case-insensitive comparison (treating “A” and “a” as equal) might seem radical and a departure from traditional conventions, it’s worth considering in the context of evolving best practices for string handling. Just as the initial embrace of Unicode was met with resistance from developers accustomed to simpler byte-based string models, future perspectives on lenient string comparison might well view current hesitations as similarly short-sighted. Adopting a more nuanced approach to string comparison by default could significantly enhance security and streamline development, ultimately proving to be a valuable evolution in how we handle text in the digital age.
