EFS - “Element not found” error


Hello

It’s Rafal Sosnowski from Microsoft Dubai Security PFE Team. Today I will troubleshoot with you one error related to EFS (Encrypting File System).

EFS (Encrypting File System) is a component of the NTFS file system present in Windows starting from Windows 2000. EFS enables transparent encryption and decryption of files by using advanced, standard cryptographic algorithms. EFS uses hybrid cryptography: symmetric key is used to encrypt data; asymmetric key is used to protect the symmetric one. To take advantage of asymmetric cryptography EFS uses certificates: self-signed or issued by Certification Authority (CA).

To encrypt the file just right click on it, select Properties> Advanced > Encrypt contents to secure data

 

 

On rare occasion when we try to encrypt file or folder, we might get an error “An error occurred applying attribute to the file: Element not found”

 

 

Troubleshooting:

 

So let’s start with enabling verbose EFS\Debug log in Event Viewer. Go to the Eventvwr.msc and select Application and Services Logs > Show Analytic and Debug Logs. This will add dozens of new and very interesting logs to your MMC, including EFS\debug log.

 

 

Enable EFS\Debug log and try to encrypt file again to reproduce an error.

 

 

Instantly in the log we will see repetitive errors:

event 277:  try and locate the matching key based on cert hash

event 1:  An API call failed at 2.93.  Error code: 2148081668

2148081668 translates to - CRYPT_E_NOT_FOUND

 

So at the first glance it looks like certificate or proper private key cannot be found. However, when we look at the personal store using command certmgr.msc we see that multiple EFS certificates are delivered from Certification Authority:

clip_image005

 

Note: when you don’t see certificate delivered to the user personal store, you can take a look at “Failed Requests” container on the CA server to verify why the certificate request was denied:

clip_image006

 

In our case certificate was successfully issued and delivered by the CA. Moreover we got more than one certificate because File Properties UI cannot find proper certificate and send additional requests to the CA.

 

Now verify our certificate in the user store using command:

Certutil – verifystore -v -user my

Among the lines we see that certificate has been issued with next generation cryptographic provider called Key Storage Provider (KSP) and public/private key pair has been generated with RSA algorithm. The problem is that EFS doesn’t support keys from KSP based on RSA.

 

Provider = Microsoft Software Key Storage Provider

ProviderType = 0

Unique container name: a58f5c5c98486fdb30e11dee7877ce02_042a9558-f6ac-4d95-a6ab-6e99a8c2e2a7

RSA

UI Policy = 0

CERT_KEY_PROV_INFO_PROP_ID(2):

Key Container = te-ContosoEFS-cc8788c8-f9bc-4ab1-a784-7266cb7cb673

Unique container name: a58f5c5c98486fdb30e11dee7877ce02_042a9558-f6ac-4d95-a6ab-6e99a8c2e2a7

Provider = Microsoft Software Key Storage Provider

ProviderType = 0

Flags = 0

KeySpec = 0 -- XCN_AT_NONE

 

EFS will work with keys generated from KSP based on Diffie-Hellman Algorithm (ECDH):

 

Provider = Microsoft Software Key Storage Provider

ProviderType = 0

Unique container name: b0e837b6acbbb65ddf5142a288e107ef_042a9558-f6ac-4d95-a6ab-6e99a8c2e2a7

ECDH_P521

ECDH

UI Policy = 0

CERT_KEY_PROV_INFO_PROP_ID(2):

Key Container = te-ContosoEFS-42d4a65d-e55f-411d-9f8e-a026bbbabcd3

Unique container name: b0e837b6acbbb65ddf5142a288e107ef_042a9558-f6ac-4d95-a6ab-6e99a8c2e2a7

Provider = Microsoft Software Key Storage Provider

ProviderType = 0

Flags = 0

KeySpec = 0 -- XCN_AT_NONE

 

Also EFS will work fine with keys generated in legacy cryptographic providers (CSP) and using RSA algorithm.

 

Provider = Microsoft Strong Cryptographic Provider

ProviderType = 1

Simple container name: te-ContosoEFS-2adf345e-263e-490a-af3b-d1a86ce54018

RSA

PP_KEYSTORAGE = 1

CERT_KEY_PROV_INFO_PROP_ID(2):

Key Container = 4d85da42c5c12b17ccd2c4833db0e856_042a9558-f6ac-4d95-a6ab-6e99a8c2e2a7

Simple container name: te-ContosoEFS-2adf345e-263e-490a-af3b-d1a86ce54018

Provider = Microsoft Strong Cryptographic Provider

ProviderType = 1

Flags = 0

KeySpec = 1 -- AT_KEYEXCHANGE

 

So where shall we configure proper cryptographic providers and algorithms for EFS? The answer is of course on the certificate template. Remember don’t select RSA while Key Storage Provider is your cryptography provider or you will face “Element not found” error.

 

 

Hope this was informative for you and happy encrypting!

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