Trusted Execution Environment

Full Title or Meme

For any digital Entity to make trustworthy statements, it must have fully protected cryptographic means to sign any statement that it issues and evaluate any command that it receives.

Context

Early History

The origin of computers built with any sort of trusted execution is an IBM 7094 with two separate memory banks with a switch that was designed and built at MIT to support CTSS, the Compatible Time Sharing System. One bank ran kernel code and the other user code, although those were not the terms used at that time. The following quote describes the changes made for MIT.

• The hardware RPQs that made CTSS run on MIT's 7094s added an interval timer (B/M 570220) and memory boundary and relocation registers (RPQ E007291) to the 7094 processor. In addition, the MIT machines had two 32K core [36 bit word] memory banks instead of one (RPQ E02120). Core bank A held the CTSS supervisor, and B Core was used for user programs. (RPQ stood for Request Price Quotation. IBM, in those days, would engineer and sell features not part of the standard product, for a price.) More importantly, the RPQ made the 7094 a two-mode machine. When the machine was running a program in B-core, many instructions were forbidden: executing them caused the machine to trap and take its next instruction from A-core. In particular, I/O instructions were forbidden, as well as attempts to switch core banks.

The following quote is a description of the method to switch between user (slave) and kernel (master) modes in the GE 645. Which was designed by MIT & GE specifically to take over operation of the CTSS. It was later replaced with the Honeywell 6180 that had modest success running the Multics operating system.

• Because it was felt desirable to make it possible to branch easily between various programs including between slave and master programs, a certain degree of insurance has to be built into the hardware to guarantee that spurious branches would not take place into the middle of master mode programs from slave programs. As a consequence, a master mode procedure when viewed from a slave mode procedure appears to be a segment which can neither be written nor read. Further, the only method of addressing this segment that is permitted is a branch to the 0th location. Any attempt to get at other locations by branch, execute, return or any other instructions will result in an improper procedure fault causing an appropriate interrupt.

IBM did not want to be left out of the time-sharing market and so built the 360/67 to support time-sharing. It did not fit into their normal marketing program and was not successful in the same way that the rest of the 360 line of computers were.

A1 classification

The Orange Book defines Class (A1): Verified Design as meeting the highest level of security of other classes with the "distinguishing feature that an analysis derived from formal design specification and verification techniques and the resulting high degree of assurance that the TCB is correctly implemented."

Co-processors

Co-processors or management processor are not a new idea. Even the Data General Eclipse mini-computer system released in 1974 came with a management co-processor on the mother board. What is most interesting is that the Eclipse served as a Maintenance and Control Unit (MCU) for the Cray-1 supercomputer. Layers upon layers.

Security Boundaries

• The original security boundary was that between the core operating system (here called the kernel of the operating system) and the application programs. This was effectuated in the CTSS version of the IBM 7094 and the GE 645 by creating boundaries between different memory spaces that could only be breached with special traps from the application area to the kernel area. This was an absolute boundary in the beginning, but various performance additions to be hardware and its operating systems (like direct access to memory from optional hardware components) has made the boundary less secure. Attempts to re-secure this boundary have consistently be found to be inadequate.
• Virtual Machine boundaries have been available since virtualization was introduced in the IBM 360. These virtual images have good security boundaries between each other, but very little with the host o/s. Still good security features have been based on them in Windows and Unix based machines. Some attacks have be created directly against the underlying computers, but those are limited to highly incentive actors.
• The co-processor boundary is the best available and is recommended whenever that option is available. There are general co-processors available from ARM TrustZone and Intel SGX, but the security of them, particularly against timing attacks is not well known. The TPM 2.0 design resides in one of these areas and have had good threat analyses developed. In particular timing attacks and side channel attacks are mitigated with the TPM. So the TPM 2.0 approach is the best known security boundary available today for commonly available processors.

Solutions Today

Portable Tokens

The most common TEE of all is the chip or smart card which has an serial interface via those 6=8 contacts visible on the face of the card. These are an excellent and portable way for a user to carry a TEE in their wallet. Many Enterprises use these today as authentication factors for employee access. Unfortunately the interface to user's other devices, like Smart Phones is difficult and the user credential problem was never solved in a way that ordinary users could accept.

FIDO U2F (and now web authentication) devices are simple key chain fobs that connect to computers via USB. With the advent or Smart Phones with USB C connects on the power cord, there is an easy way to connect these to Smart Phones. It seems though that the time has passed for user tokens, but time will tell if these gain any traction.

Common TEE Platform APIs

The Global Platform standard for a Trusted Execution Environment (TEE) is designed to reside alongside the normal smartphone or other Mobile Device Rich Execution Environment (REE) (where normal applications execute) and to provide a safe area of the Mobile Device to protect assets and execute trusted code. At the highest level, a Trusted Execution Environment (TEE) is an environment where the following are true: (a) any code executing inside the TEE is trusted in authenticity and integrity; (b) the other assets are also protected in confidentiality; (c) the TEE resists by design all known remote and software attacks, and a set of external hardware attacks; and (c) both assets and code are protected from unauthorized tracing and control through debug and test features

Access APIs

• Before any of the functionality of the Trusted Execution Environment or Trusted Computing can have a measurable effect on user security, the functionality must be exposed to the programmers that code the applications that the users want. There are a few APIs, like Open SSL which are accessible to the programmers, but the key problem of protecting keys used by applications programs is still fragmented. The following are the most important APIs addressing that issue.

Windows

• Fairly early in the adoption of Public Key Cryptography Microsoft licensed the RSA API library and exposed it to programmers as a part of the Crypto API (CAPI). This had the advantage of abstracting all of the underlying key protection methods and so was quite successful. Unfortunately it was strongly oriented towards the Public Key Infrastructure (PKI) build around the X.509 Certificate and its binding between the user and the private key. This clumsy design based on bindings has frustrated users and programmers for decades. It was partially corrected by the Crypto Next Generation (CNG) API which was more closely bound to the important part, the public key itself.
• When the user sets up Windows Hello on their machine, it generates a new public–private key pair on the device. The trusted platform module (TPM) generates and protects this private key. If the device does not have a TPM chip, the private key is encrypted and protected by software. In addition TPM-enabled devices generate a block of data that can be used to attest that a key is bound to TPM. This attestation information can be used in your solution to decide if the user is granted a different authorization level for example. To enable Windows Hello on a device, the user must have either their Azure Active Directory account or Microsoft Account connected in Windows settings.

This table shows the security features available in recent versions of Windows UWP.

Build	Version	Security Features	Recent News
10.0.18362	1903	twist
10.0.17763	1809	problems with roll-out deleting user files
10.0.17134	1803	t400
10.0.	xx
10.0.10240	1507	KeyCredentialManager Class

Android

There used to be a secure version of Android supported by the NSA. That has since been rolled into the current releases which makes the full security of the TEE integrated into Android, but the APIs available to the Native App are only slowly catching up.

This table shows the security features available in recent versions of Android.

Ver	API	Security Features	Recent News
9	28	StrongBox KeyStore APIs will generate and store private keys in Titan M
8.1	27
8.0	26
7.1	25
7.0	24
6.0	23	KeyInfo.isInsideSecureHardware()
	18	AndroidKeyStore

• Android KeyStore: what is the difference between “StrongBox” and “hardware-backed” keys?

References

External Material

1. OWASP TrustZone and Mobile Security (2015-10-15) https://www.owasp.org/images/c/c8/OWASP_Security_Tapas_-_TrustZone%2C_TEE_and_Mobile_Security_final.pdf
2. Android Trusty is a set of software components supporting a Trusted Execution Environment on mobile devices. https://source.android.com/security/trusty/