CryptoCurrency Security Standard (CCSS) – Key Storage Requirements

In this series we will review each of the core Aspects in the CCSS and provide our interpretation for each of the Aspect’s requirements and what possible evidence could provide assurance to the auditor that a requirement is in-place. Make sure to read our other in-depth articles on the CCSS Aspects:

• Key / Seed Generation (CCSS Aspect 1.01)
• Wallet Creation Requirements (CCSS Aspect 1.02)
• Key Usage Requirements (CCSS Aspect 1.03)

CCSS Aspect 1.03 Key Storage

In this article we will explore how an auditor could interpret the CCSS Aspect 1.03 Key Storage.

Aspect 1.03 Key Storage addresses the importance of protecting keys and seeds while at-rest. The Aspects objective defined within the CCSS is provided below.

“By separating the wallet’s keys across multiple locations, the risks associated with localized disruptions to business (e.g., fire, flood, earthquake, break-ins) do not affect the organization’s ability to spend funds.“

Aspect Controls

There are six controls to this Aspect. In this article we will address each control.

• 1.03.1 Primary keys are stored encrypted
• 1.03.2 Backup key exists
• 1.03.3 Backup key has environmental protection
• 1.03.4 Backup key is access-controlled
• 1.03.5 Backup key has tamper-evident seal
• 1.03.6 Backup key is encrypted

CCSS Levels

CCSS provides three levels of compliance – Level 1 being the base level of implementing CCSS requirements up to Level 3 being the most in-depth implementation of CCSS requirements. We shall review each compliance level and provide our thoughts on what evidence an auditor should seek to provide assurance that the requirements are in-place.

Level 1 Compliance

The CCSS committee has provided an official definition for “strong encryption”. The definition offers AES-256 as an acceptable encryption algorithm. However, the definition does not provide any reference standards that one can refer to for guidance if, for example, if the assessed entity is not using AES-256.

SP 800-57 Part 1 Rev. 5 – Recommendation for Key Management: Part 1 and SP 800-131A Rev. 2 – Transitioning the Use of Cryptographic Algorithms and Key Lengths in our opinion provide industry recognized and accepted guidance on encryption algorithms that are deemed to provide what is considered “strong” encryption.

The CCSSA should review the system(s) that provide the storage facility for the keys and/or seeds to ensure the encryption settings meet the CCSS requirements. This will involve reviewing the configuration settings of the storage facility and how the keys used to encrypt the storage facility are protected. It is no use storing keys and/or seeds within a storage facility providing encryption protection when the storage facilities own keys are weak or not protected from unauthorised access.

In order to confirm that there is a backup for each key or seed phrase a list that identifies in some way the current active keys and seed phases must be provided to the CCSSA. Using the inventory list, the CCSSA can then confirm each key or seed phase has at least one backup. Ensure evidence is provided by the assessed entity proving that there is a backup for each key or seed phase. This can be done by the CCSSA observing the location of the backup such as a physical safe or a virtual vault.

If the assessed entity states that the backup is managed by a third-party and can only be assessed in emergencies or key rotation events, then ensure documentation is provided from the third-party that officially lists the keys and seed phases under their care and the process to retrieve the keys and seed phrases from the third-party.

NOTE: we do not recommend that the CCSSA receive the actual keys or seed phases when asking for an inventory list! We would expect that there is a serial number or other identification that represents a particular key or seed phase.

If the backup media is physical such as removable electronic media, paper, wood or metal ensure that the backup is protected using a safe, safe deposit box, or locked cabinet review the following:

1. Review the assessed entities policies and standards regarding the certification requirements for the storage containers in regards to protection against environmental risks. A quick search online for fire ratings identified UL Classification for fire protection and ETL Verified for protection against water damage. Depending on the country in which the assessed entity resides there might be local standards that these types of storage containers must be compliant with.

As part of the CCSSA’s process of observing the location of each backup for a key or seed phase ensure that the backup is protected by access controls. An access control can be physical or logical depending on the type of media storing the key or seed phase.

If the backup media is physical such as removable electronic media, paper, wood or metal, ensure that the backup is protected using a safe, safe deposit box, or locked cabinet review the following:

1. The policies, standards and procedures that define the requirements for protecting the key or seed phase backups. Ensure that the currently implemented access controls meet the requirements stated in the documentation.
2. The list of personnel who have access to the keys and/or combination locks of the backup media (safe, locked draw etc…). Ensure that each person who has access to the backup requires access based on their role.
3. The access log that records all access to the backup. The access log should contain: (a) the person’s full name who access the backup, (b) the date and time of access and (c) reason for accessing the backup.
4. Check if the key or seed phase backups can be sent to another premise such as premises controlled by the assessed entity or any third-party premises. If backups are sent then review the process for sending the backups – there should be a log recording the movement and authorization to transport the backup. The transportation of the backup should allow for complete tracking of the backup media while in transit.
5. Access controls can also include CCTV. If CCTV is used to monitor the location of the backup media then review the CCTV video footage to ensure the CCTV is working correctly.

If the backup is stored virtually review the following:

1. The policies, standards and procedures that define the requirements for protecting the key or seed phase backups. Ensure that the current access controls meet the requirements stated in the documentation.
2. The list of personnel who have access to the passwords and other authentication factors such as devices generating One Time Passwords (OTP). Ensure that each person who has access to the backup requires access based on their role.
3. The systems that provide access control to the backups are configured correctly, hardened using industry recognized hardening standards and the systems are patched based on the assessed entities patch management requirements.
4. The access log that records all access to the backup. The access log should contain: (a) the person’s full name or another unique identification token such as user ID who access the backup, (b) the date and time of access and (c) reason for accessing the backup.

NOTE: The CCSS Aspect that addresses audit logs (2.04 Audit Logs) does not define how long access logs or CCTV video records are to be retained for. Therefore, the CCSSA must reviewed the assessed entities data retention policy for their data which the CCSSA can refer to for guidance. The CCSSA is seeking assurance regarding the retention of access logs and CCTV records, that during an incident, the log entries and CCTV records will assist forensic examination tasks and police enquires when needed.

Level 2 Compliance

Ensure that there is documentation which defines the number of keys required to spend funds. The documentation should provide detailed information as to the number of keys required from the pool of available keys.

Once the documentation has been reviewed ensure that the defined number of keys required for spending funds have backups. The CCSS requirement provides examples as to how many keys are to have backups for a M-of-N wallet setup.

There is not much to add or break down for this requirement. The only consideration we can see is using the home as a backup location of a production key/seed is not very secure unless there are multiple access controls in-place such as a safe and CCTV camera recording all access to the safe.

For auditing, it is always preferable that a control is compliant or certified to an industry standard. This provides assurance to the auditor that the control has undertaken a certification process that provides some consistency in review of the control and meets a standard’s requirements. There appears to be standards for tamper evident mechanisms, one of which is German Federal Institute for Materials Research and Testing (BAM). This web page provides a good overview of what features a tamper evident envelope could provide as a guide for the CCSSA.

NOTE: we do not endorse nor have any relationship with the product owners or the website – this is merely a website that was located during research and provides criteria that could guide the CCSSA.

Level 3 Compliance

As with the tamper evident bags above, the auditor should research and identify standards and certifications for mechanisms providing a faraday capability and ensure that the mechanism implemented is certified or complies to those standards.

As described above, SP 800-57 Part 1 Rev. 5 – Recommendation for Key Management: Part 1 – General #2 and SP 800-131A Rev. 2 – Transitioning the Use of Cryptographic Algorithms and Key Lengths #3 provide industry recognized and accepted guidance on encryption algorithms that are deemed to provide what is considered “strong” encryption. This requirement states that the backups of the key/seed must be protected by “strong” encryption while at rest to at least the same encryption strength as the production key/seed version.

Summary

In this post we explored aspect 1.03 Key Storage of the CCSS with our auditors hat on to determine how an auditor could approach the evidence gathering required so that an opinion could be reached as to if the assessed entity is compliant to Level 1, Level 2 or Level 3 for this aspect.