Unexplained Decryption failure due to "Unable to decrypt attachment: Mismatched SHA-256 digest"

[richvdh]

A user reported a decryption failure on a message containing an image.

The logs contain no megolm decryption failure, but they do have:

2024-10-21T09:53:11.961Z E Unable to decrypt attachment:  Mismatched SHA-256 digest
./node_modules/matrix-encrypt-attachment/lib/browser-encrypt-attachment.js/</<[1]</decryptAttachment/</<@https://develop.element.io/bundles/08f5e67c245583e16087/7530.js:41217:31
step@https://develop.element.io/bundles/08f5e67c245583e16087/7530.js:41152:23
./node_modules/matrix-encrypt-attachment/lib/browser-encrypt-attachment.js/</<[1]</__generator</verb/<@https://develop.element.io/bundles/08f5e67c245583e16087/7530.js:41133:53
fulfilled@https://develop.element.io/bundles/08f5e67c245583e16087/7530.js:41124:58

I am at a loss to explain why the digest would be a mismatch.

The logs are unhelpful; they lack key information such as:

• The URL of the media that could not be decrypted
• The event for which the error occurred
• Whether it was the thumbnail or full image (see also https://github.com/element-hq/element-web/issues/28311)
• it might also be helpful to record the size of the file (to see if it was truncated in transit), the expected mac, and the actual mac.

[uhoreg]

In one instance of this, the downloaded file was binary data that ended with

{"errcode":"M_UNKNOWN","error":"unexpected error","mr_errcode":"M_UNKNOWN"}

So something went wrong on the server.