The Principle Behind Changing a File Hash
Typical cryptographic hashes include MD5, SHA-1/256, etc. The core feature of cryptographic hash functions is the "avalanche effect"—any tiny change in the input leads to a huge change in the output. This means that even if only one byte is modified, the entire hash value will be completely different. For example, if you write a 2,000-word document and save it as a Word file, changing just one character, such as adding a space, will completely change the file's MD5. Therefore, the essence of "changing a file hash" is changing the file content itself, not external attributes such as the file name. This characteristic makes file-level hash modification relatively easy to achieve.
Different systems and formats define "metadata" differently: if metadata is embedded in the file data, such as tags inside a JPEG or video container, changing it will change the hash; if metadata is stored in external data streams or attributes of the file system, changing it will not affect the hash calculated from the file content. This explains the common phenomenon where "changing the file name does not change the hash, but changing embedded metadata does."
Video Platform Transcoding and "Detecting Whether the Hash Has Been Changed"
After a user uploads a video to a video platform, the platform will perform the following on the video:
Receive the original file
Format inspection and validation
Transcoding processing (format conversion, resolution adjustment, compression)
Keyframe extraction
Multi-layer hash calculation
This is a transcoding pipeline of "demuxing → decoding → processing → re-encoding → muxing" to produce multiple versions for adaptive playback at different bitrates and resolutions. This process itself changes the file byte sequence and any cryptographic hash from before upload.
At the file hash level: the platform regenerates the file during transcoding, so the original file hash naturally changes. Therefore, any file hash modified by the user in advance becomes meaningless after transcoding. So the platform will not check whether your file's MD5 or similar hash has been modified, because it has no significance!
Deduplication/copyright detection does not rely on the uploader's file hash. Instead, the platform calculates its own "audio/video fingerprint" or perceptual hash based on the decoded content and compares it with reference fingerprints in its database. For example, YouTube's Content ID "fingerprints" uploaded content and matches it against references provided by rights holders.
Therefore, the platform is not "detecting whether the hash has been changed," but rather "ignoring the uploaded hash, recalculating the content fingerprint, and matching it." This means that simply modifying file bytes or container metadata to change MD5/SHA cannot bypass content-based detection.
