To minimize the risks of file format ambiguity, choose magic numbers for binary files that are uncommon and easily distinguishable. Favor longer magic numbers (at least 4 bytes) and incorporate asymmetry and randomness while avoiding printable ASCII characters. Consider including a version number within the magic to facilitate future evolution and potentially embedding the magic at both the beginning and end of the file for enhanced validation. This approach helps differentiate your file format from existing ones, reducing the likelihood of misidentification and improving long-term compatibility.
The post "Recommendations for designing magic numbers of binary file formats" discusses best practices for choosing and implementing magic numbers—the identifying byte sequences at the beginning of files that signal their type. The author emphasizes the importance of carefully selecting these magic numbers to minimize the risk of misidentification, ensuring robust and reliable software behavior.
The core recommendation revolves around incorporating human-readable ASCII characters within the magic number. This strategy makes it easier for developers and users to recognize the file type when inspecting the file's raw bytes, aiding in debugging and preventing accidental misinterpretation. This human-readable component should ideally be unique and relevant to the file format's purpose, clearly indicating its nature. The author suggests using a relevant abbreviation or acronym related to the file format, converted into ASCII characters.
Beyond the human-readable aspect, the author advises including non-ASCII bytes within the magic number to further reduce the chance of collision with other file formats or random data sequences. These non-printable characters increase the entropy of the magic number, making it more statistically distinct. The specific recommended non-ASCII bytes are 0x00 (null byte) and bytes with values above 0x7F (the highest ASCII value). These particular choices minimize the likelihood of accidental matches with common text files or other structured data.
Furthermore, the author recommends using a magic number of at least four bytes in length. This length provides a good balance between robust identification and minimizing overhead. Longer magic numbers offer stronger guarantees against collisions but can slightly increase processing time. Four bytes are generally considered a sweet spot, providing sufficient uniqueness without undue burden.
Finally, the post briefly touches on the practical implementation. It advises checking the entire magic number sequence before definitively identifying a file, avoiding partial matches that could lead to false positives. This rigorous checking ensures reliable file type identification, even in the presence of corrupted or incomplete data. In summary, the post provides a clear and concise set of guidelines for designing robust and easily identifiable magic numbers, advocating for a blend of human-readable ASCII and distinguishing non-ASCII bytes for optimal file format identification.
Summary of Comments ( 18 )
https://news.ycombinator.com/item?id=43366671
HN users discussed various strategies for handling magic numbers in binary file formats. Several commenters emphasized using longer, more unique magic numbers to minimize the chance of collisions with other file types. Suggestions included incorporating version numbers, checksums, or even reserved bytes within the magic number sequence. The use of human-readable ASCII characters within the magic number was debated, with some advocating for it for easier identification in hex dumps, while others prioritized maximizing entropy for more robust collision resistance. Using an initial "container" format with metadata and a secondary magic number for the embedded data was also proposed as a way to handle versioning and complex file structures. Finally, the discussion touched on the importance of registering new magic numbers to avoid conflicts and the practical reality that collisions can often be resolved contextually, even with shorter magic numbers.
The Hacker News post "Recommendations for designing magic numbers of binary file formats" sparked a discussion with several insightful comments focusing on practicality and real-world considerations when choosing magic numbers for file formats.
One of the most compelling comments highlights the importance of considering the encoding of the file when choosing a magic number. Specifically, it points out that using a UTF-8 BOM (Byte Order Mark) as a magic number can be problematic because it's valid UTF-8 and might appear within the data itself. This could lead to false positives when trying to identify the file type. The commenter suggests prioritizing human readability over relying solely on a BOM and proposes incorporating version numbers within the magic number for better future compatibility.
Another commenter expands on this idea by recommending a hybrid approach, combining a short magic number with a separate version field shortly thereafter. This approach balances quick identification with the ability to handle future format revisions. They further suggest using ASCII characters for the magic number to ensure straightforward identification and avoid encoding issues.
Several comments delve into the practical challenges of dealing with corrupted or truncated files. One user suggests incorporating checksums or other integrity checks alongside the magic number to avoid misinterpreting partial files. This preventative measure adds an extra layer of confidence when working with potentially damaged data.
Adding to the discussion of human readability, one commenter underscores its importance, especially for debugging. Being able to quickly recognize a file type by looking at its first few bytes in a hex editor can significantly speed up the debugging process. They suggest using memorable ASCII strings that clearly indicate the file's purpose.
Finally, a commenter reflects on the historical context of magic numbers, recalling how they were used in older systems for quick identification. They mention that, despite advancements in file systems, magic numbers still hold relevance, especially for low-level tools and when dealing with data from diverse sources. This historical perspective provides a valuable reminder of the enduring utility of magic numbers.
The overall sentiment in the comments leans toward practicality and robustness. The discussion emphasizes the need for clear, human-readable magic numbers, combined with versioning and integrity checks to ensure reliable file identification even in less-than-ideal circumstances.