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Archived TD0089:  Updated Key Destruction SFR (FCS_CKM_EXT.4) for Server Virtualization PP

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Issue Description

FCS_CKM_EXT.4 did not take into account that software TOEs often contain 3rd-party modules, are sometimes programmed using memory-safe programming languages, and have little or no insight into storage areas outside the TOE.


Replacement of the FCS_CKM_EXT.4: Cryptographic Key Destruction with a new SFR.


The TOE vendor is responsible for everything in the TOE, but does not always have control over everything in the TOE. This revised SFR strikes a balance between what is possible and what is important for security.

The TC agreed on the following language:

FCS_CKM_EXT.4 Cryptographic Key Destruction

FCS_CKM_EXT.4.1 Destruction of Keys in TOE Volatile Memory

FCS_CKM_EXT.4.1 The TSF shall cause disused cryptographic keys in volatile memory to be destroyed or rendered unrecoverable.

Application Note:

The threat addressed by this element is the recovery of disused cryptographic keys from volatile memory by unauthorized processes.

The TSF is expected to destroy or cause to be destroyed all copies of cryptographic keys created and managed by the TOE once the keys are no longer needed. This requirement is the same for all instances of keys within TOE volatile memory regardless of whether the memory is controlled by TOE manufacturer software or by 3rd party TOE modules. The assurance activities are designed with flexibility to address cases where the TOE manufacturer has limited insight into the behavior of 3rd party TOE components.

The preferred method for destroying keys in TOE volatile memory is by direct overwrite of the memory occupied by the keys. The values used for overwriting can be all zeros, all ones, or any other pattern or combination of values significantly different than the value of the key itself such that the keys are rendered inaccessible to running processes.

Some implementations may find that direct overwriting of memory is not feasible or possible due to programming language constraints. Many memory- and type-safe languages provide no mechanism for programmers to specify that a particular memory location be accessed or written. The value of such languages is that it is much harder for a programming error to result in a buffer or heap overflow. The downside is that multiple copies of keys might be scattered throughout language-runtime memory. In such cases, the TOE should take whatever actions are feasible to cause the keys to become inaccessible—freeing memory, destroying objects, closing applications, programming using the minimum possible scope for variables containing keys.

Likewise, if keys reside in memory within the execution context of a third-party module, then the TOE should take whatever feasible actions it can to cause the keys to be destroyed.

Cryptographic keys in non-TOE volatile memory are not covered by this requirement. This expressly includes keys created and used by Guest VMs. The Guest is responsible for disposing of such keys.

FCS_CKM_EXT.4.2 Destruction of Keys in Non-Volatile Storage

FCS_CKM_EXT.4.2 The TSF shall cause disused cryptographic keys in non-volatile storage to be destroyed or rendered unrecoverable.

Application Note:

The ultimate goal of this element is to ensure that disused cryptographic keys are inaccessible not only to components of the running system, but are also unrecoverable through forensic analysis of discarded storage media. The element is designed to reflect the fact that the latter may not be wholly practical at this time due to the way some storage technologies are implemented (e.g. wear-leveling of flash storage).

Key storage areas in non-volatile storage can be overwritten with any value that renders the keys unrecoverable. The value used can be all zeros, all ones, or any other pattern or combination of values significantly different than the value of the key itself.

The TSF is expected to destroy all copies of cryptographic keys created and managed by the TOE once the keys are no longer needed. Since this is a software-only TOE, the hardware controllers that manage non-volatile storage media are necessarily outside the TOE boundary. Thus, the TOE manufacturer is likely to have little control over—or insight into—the functioning of these storage devices. The TOE is expected to make a “best-effort” to destroy disused cryptographic keys by invoking the appropriate platform interfaces—recognizing that the specific actions taken by the platform are out of the TOE’s control.

But in cases where the TOE has insight into the non-volatile storage technologies used by the platform, or where the TOE can specify a preference or method for destroying keys, the destruction should be executed by a single, direct overwrite consisting of pseudo-random data or a new key, by a repeating pattern of any static value, or by a block erase.

For keys stored on encrypted media, it is sufficient for the media encryption keys to be destroyed for all keys stored on the media to be considered destroyed.

Assurance Activities:

The evaluator shall check to ensure the TSS lists each type of key and its origin and location in memory or storage. The evaluator shall verify that the TSS describes when each type of key is cleared.

For each key clearing situation the evaluator shall perform one of the following activities:

·         The evaluator shall use appropriate combinations of specialized operational or development environments,  development tools (debuggers, emulators, simulators, etc.), or instrumented builds (developmental, debug, or release) to demonstrate that keys are cleared correctly, including all intermediate copies of the key that may have been created internally by the TOE during normal cryptographic processing.

·         In cases where testing reveals that 3rd-party software modules or programming language run-time environments do not properly overwrite keys, this fact must be documented. Likewise, it must be documented if there is no practical way to determine whether such modules or environments destroy keys properly.

·         In cases where it is impossible or impracticable to perform the above tests, the evaluator shall describe how keys are destroyed in such cases, to include:

o   Which keys are affected,

o   The reasons why testing is impossible or impracticable,

o   Evidence that keys are destroyed appropriately (e.g. citations to component documentation, component developer/vendor attestation, component vendor test results),

o   Aggravating and mitigating factors that may affect the timeliness or execution of key destruction (e.g. caching, garbage collection, operating system memory management).

Assurance Activity Note:

Using debug or instrumented builds of the TOE and TOE components is permitted in order to demonstrate that the TOE takes appropriate action to destroy keys. It is expected that these builds are based on the same source code as are release builds (of course, with instrumentation and debug-specific code added).

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