Rebex.Net.Sftp - Key exchange failed, Unable to acquire context

Question

Hi,

Using Rebex.Net.Sftp on a Windows Mobile device it works most of the time. We are calling this (cut down example):-

 Sftp sftp = new Sftp();
 sftp.Connect(remoteHost, Sftp.DefaultPort, sshparams);

but often we get this error:-

Rebex.Net.SftpException: Key exchange failed. ---> Rebex.Net.SshException: Key exchange failed. ---> System.Security.Cryptography.CryptographicException: Unable to acquire context (-2146893795).
at fSAA8Z.4J1Sh.guGYU()
at Rebex.Security.Cryptography.CryptoHelper.242mil(1ZVxoO , 1ZVxoO , 1ZVxoO )
at Rebex.Security.Cryptography.CryptoHelper.GenerateDiffieHellmanKeys(Byte[] G, Byte[] P, Byte[]& privateKey, Byte[]& publicKey)
at gbMKS.qyK1..ctor(Byte[] G, Byte[] P)
at gbMKS.1iQOSNZ..ctor(Byte[] G, Byte[] P)

We repeatly try and connect and it does not work but the next day it seems to work.

Can you help with what is causing this error?

Thanks very much.

Answer 1

To answer my own question, it looks like -2146893795 = 0x8009001D which is documented here:-

https://msdn.microsoft.com/en-us/library/windows/desktop/aa379886(v=vs.85).aspx

NTE_PROVIDER_DLL_FAIL 0x8009001D
The provider DLL file could not be loaded or failed to initialize.

So I am thinking that the device has run out of memory available for loading dlls.

Comments

Yes, this definitely looks like a possible explanation. I don't remember any other customer reporting this error before, although NTE_PROV_DLL_NOT_FOUND errors do sometimes occur on Windows CE devices whose vendor forgot to include Microsoft Enhanced DSS and Diffie-Hellman Cryptographic Provider" in the OS image (but we have an automated workaround for that). On the other hand, the error you got indicates that the DSS and Diffie-Hellman CSP was in fact found, but failed to load. And if this is caused by lack of free memory, applying the workaround (=use a managed but slow implementation of Diffie-Hellman) is not a proper solution. We will at least enhance the error message to make it easier to identify the issue in the future.