How to add seek and position capabilities to CryptoStream

Question

I was trying to use CryptoStream with AWS .NET SDk it failed as seek is not supported on CryptoStream. I read somewhere with content length known we should be able to add th

Answer 1

Generally with encryption there isn't a 1:1 mapping between input bytes and output bytes, so in order to seek backwards (in particular) it would have to do a lot of work - perhaps even going right back to the start and moving forwards processing the data to consume [n] bytes from the decrypted stream. Even if it knew where each byte mapped to, the state of the encryption is dependent on the data that came before it (it isn't a decoder ring ;p), so again - it would either have to read from the start (and reset back to the initialisation-vector), or it would have to track snapshots of positions and crypto-states, and go back to the nearest snapshot, then walk forwards. Lots of work and storage.

This would apply to seeking relative to either end, too.

Moving forwards from the current position wouldn't be too bad, but again you'd have to process the data - not just jump the base-stream's position.

There isn't a good way to implement this that most consumers could use - normally if you get a true from CanSeek that means "random access", but that is not efficient in this case.

As a workaround - consider copying the decrypted data into a MemoryStream or a file; then you can access the fully decrypted data in a random-access fashion.