EDN HWIP Technical Specification

Overview

This document specifies EDN hardware IP functionality. This module conforms to the Comportable guideline for peripheral functionality.

Features

The Entropy Distribution Network (EDN) block provides both hardware and software interfaces to the CSRNG IP module. A primary objective of the EDN block is to provide a simpler hardware interface for the peripheral blocks to use, in which case they do not have to directly interface with the CSRNG module.

  • The EDN block provides a set of registers for firmware to manage a CSRNG application interface port.
  • There are four request/acknowledge hardware interfaces.
  • Each hardware interface supports a parameterized data delivery width, for powers of 2 from 1 to 128-bit widths.
  • The EDN block has an “auto request mode” where generate and reseed CSRNG application commands can be programmed to be done continuously in hardware.
  • There is also a “boot-time request mode”, where a single TL-UL configuration write will trigger a proper CSRNG application command sequence to fetch the pre-FIPS entropy for tasks immediately at boot-time or after reset.
  • There are two interrupts that are supported:
    • CSRNG application command has completed.
    • An internal FIFO error has occurred.
  • No alerts are supported by this revision.

Description

This IP block acts as a gasket between peripheral hardware blocks and the CSRNG block. One function this IP block performs is to translate data transfer size. For example, CSRNG will return 128 bits on the genbits bus. A peripheral block may connect with a 32-bit data bus. The EDN block will move the first 32 bits from the returned genbits bus, and hold on to the remaining data until another request asks for more data. Furthermore, if data is not consumed immediately, the interface to the CSRNG will indicate back pressure to the CSRNG block. Each of the four interfaces can request data such that any genbits bus return can supply any requesting peripheral block.

At most one hardware peripheral block can connect to each EDN peripheral port. Hardware peripherals request more data from the EDN by asserting the req signal. When random values are available, the EDN transmits them on the bus and asserts an ack signal to signify the arrival of fresh values.

Application interface commands to the CSRNG block can be generated by either firmware or hardware.

Firmware can issue CSRNG commands on behalf of hardware peripherals, by writing the them to the SW_CMD_REQ register and the command status response is captured in the SW_CMD_STS register. Even when CRSNG generate commands are issued by firmware, all random values are distributed to the hardware peripherals.

If firmware applications require random values for their own use, they must issue the commands directly to the CSRNG, which maintains a dedicated CSRNG instance for firmware that is accessible through TL-UL.

There are two modes for EDN hardware to generate CSRNG commands. One is the “auto request mode”, where two FIFOs are used to send commands. The general operation of this mode is that the CSRNG instance is set up by firmware, then the FIFOs are preloaded with commands. One FIFO can be programmed to send generate commands. The other FIFO can be programmed to send reseed commands. The MAX_NUM_REQS_BETWEEN_RESEEDS register sets the number of generate commands allowed between reseed commands. Once this is done, the EDN block can request data from the CSRNG once firmware has instantiated the associated instance through the EDN command forwarding interface. Once in this mode, the EDN emits generate commands from the first FIFO to get more data. Once the MAX_NUM_REQS_BETWEEN_RESEEDS timer expires, the EDN block emits a reseed command from the second FIFO. The process of sending these two commands will repeat forever until the EDN_ENABLE bit is cleared, the AUTO_REQ_MODE bit is cleared, or the EDN is reset.

Any of the command FIFOs can be reset by asserting the CMD_FIFO_RST bit in the CTRL register.

The other mode is “boot-time request mode”, where only the hardware generates CSRNG application interface commands. In this mode a single instantiate command is sent, followed by a stream of generate commands. This sequence fetches the initial random values needed for the system to boot. Use of boot-time request mode, though simpler in operation, is only for applications which do not require FIPS-approved random values. Please see the entropy_src IP documentation for more information on trade-offs when creating CSRNG seeds before the completion of the FIPS-required health checks. In boot-time request mode the generate commands continue until EDN_ENABLE bit is cleared, the BOOT_REQ_DIS bit is set, or the EDN is reset. When the EDN_ENABLE bit is cleared or the BOOT_REQ_DIS bit is set an uninstantiate command is sent to destroy the instance. Note that the EDNs and CSRNG should always be reset together to ensure proper instantiation or uninstantiation of state variables.

Example Topology

In general, the OpenTitan random number subsystem consists of one entropy_src, one CSRNG, and one or more EDNs. The entropy_src only supports one connection to a CSRNG, but the CSRNG has multiple application interface ports for connecting to EDN’s or other hardware blocks. The diagram below shows an example topology where two EDN modules are used to distribute genbits from the CSRNG to peripheral modules.

EDN Example Topology Diagram

Theory of Operations

The EDN is for distributing random number streams to hardware blocks, via peripheral ports on on the EDN. Each block connected to a peripheral port is referred to as an endpoint.

To enable the EDN block, set the EDN_ENABLE bit in the CTRL register..

Interaction with CSRNG Application Interface Ports

The CSRNG application interface implements the “function envelopes” recommended by NIST SP 800-90A for random number generation, and these function envelopes establish certain requirements for the order of operations. For instance, the application interface port must receive an explicit instantiate command before receiving any generate commands. The sequences of commands generated by a particular EDN are either controlled by the EDN state machine or by commands forwarded from firmware through the SW_CMD_REQ register.

Whenever commands are directly forwarded from firmware to the CSRNG through the SW_CMD_REQ register, firmware must poll and clear the CMD_ACK bit of the SW_CMD_STS register before sending any further commands.

There are two broad modes for state machine control: auto request mode and boot-time request mode.

Boot-time Request Mode

Random values are needed by peripherals almost immediately after reset, so to simplify interactions with the boot ROM, boot-time request mode is the default mode.

In boot-time request mode, the command sequence is fully hardware-controlled and no command customization is possible. In this mode, the EDN automatically issues a special reduced-latency instantiate command followed by the default generate commands. This means, for instance, that no personalization strings or additional data may be passed to the CSRNG application interface port in this mode. On exiting, the EDN issues an uninstantiate command to destroy the associated CSRNG instance. Since boot-time request mode is enabled as the default mode on reset, a single write to the EDN_ENABLE bit will start the flow of entropy in this mode.

Once firmware initialization is complete, it is important to exit this mode if the endpoints ever need FIPS-approved random values. This is done by either clearing the EDN_ENABLE bit or setting the BOOT_REQ_DIS bit in CTRL to halt the boot-time request state machine. Firmware must then wait for successful the shutdown of the state machine by polling the REQ_MODE_SM_STS field of the SUM_STS register.

Note on Security Considerations when Using Boot-time Request Mode

Boot-time request mode is not intended for normal operation, as it tolerates the potential use of preliminary seeds for the attached CSRNG instance. These preliminary seeds are described as “pre-FIPS” since they are released from the entropy_src before the complete start-up health-checks recommended by FIPS have been completed. Thus pre-FIPS seeds have weaker guarantees on the amount of physical entropy included in their creation. As detailed in the entropy_src documentation, only the first CSRNG seed created after reset is pre-FIPS. All following seeds from the entropy_src are passed through the full FIPS-approved health checks. Therefore at most one EDN can receive a pre-FIPS seed after reset. Since boot-time request mode EDN streams may be FIPS non-compliant, firmware must at some point disable boot-time request mode and reinitialize the EDN for either firmware-driven operation or auto request mode.

Multiple EDNs in Boot-time Request Mode

If many endpoints require boot-time entropy multiple boot-time EDNs may be required, as the EDN has a fixed maximum number of peripheral ports. Since physical entropy generation takes time, there exists a mechanism to prioritize the EDNs, to match the boot priority of each group of attached endpoints. To establish an order to the instantiation of each EDN, enable them one at a time. To ensure that the most recently enabled EDN will get next priority for physical entropy, poll the BOOT_INST_ACK field in the SUM_STS register before enabling the following EDN.

If using boot-time request mode, the CSRNG seed material used for the first-activated EDN is the special pre-FIPS seed, which is specifically tested quickly to improve latency. The first random values distributed from this EDN will therefore be available roughly 2ms after reset. The entropy_src only creates only one pre-FIPS seed, so any other EDNs must wait for their seeds to pass the full FIPS-recommended health checks. This means that each subsequent EDN must wait an additional 5ms before it can start distributing data. For instance, if there are three boot-time request mode EDN’s in the system, the first will start distributing data 2ms after reset, the second will start distributing data 7ms after reset, and the third will start distributing data 12ms after reset.

Auto Request Mode

Before entering auto request mode, it is the responsibility of firmware to first generate an instantiate command for the EDN-associated instance via the SW_CMD_REQ register. The required generate and reseed commands must also be custom generated by firmware and loaded into the respective command replay FIFOs via the GENERATE_CMD and RESEED_CMD registers. These generate commands will be issued as necessary to meet the bandwidth requirements of the endpoints. The reseed commands will be issued once every MAX_NUM_REQS_BETWEEN_RESEEDS generate requests. For details on the options for application interface commands please see the CSRNG IP Documentation. Once the CSRNG instance has been instantiated, and the generate and reseed commands have been loaded, auto request mode can be entered by programming the CTRL register with EDN_ENABLE, AUTO_REQ_MODE and BOOT_REQ_DIS all asserted. Note that if BOOT_REQ_DIS is deasserted the state machine will enter boot-time request mode, even if AUTO_REQ_MODE is asserted.

To issue any new commands other than those stored in the generate or reseed FIFOs, it is important to disable auto request mode, by deasserting the AUTO_REQ_MODE field in the CTRL register. Firmware must then wait until the current command is completed by polling the REQ_MODE_SM_STS bit in the SUM_STS register Once REQ_MODE_SM_STS reads zero, the state machine is idle and new firmware-driven commands can be passed to the CSRNG via the SW_CMD_REQ register.

Note on State Machine Shutdown Delays

When leaving boot-time request mode or auto request mode, the EDN state machine waits for completion of the last command, before sending a shutdown acknowledgement to firmware. The longest possible commands are the instantiate or reseed requests, which typically take about 5ms, due to the time required to gather the necessary physical entropy. By contrast, the largest possible generate command allowed by NIST SP 800-90A is for 219 bits (or 4096 AES codewords). Assuming an AES encryption delay of 16 clocks, and a 100 MHz clock frequency, the longest allowable generate command would take only 0.7 ms to complete.

Note on Sharing of CSRNG Instance State Variables

Once an application interface port has received an instantiate command, that application interface port then has access to a unique CSRNG instance, which is shared by all endpoints on the same EDN. Therefore from a security perspective, an attack to that particular CSRNG instance is an attack on all the endpoints that share the same EDN. Meanwhile, seeds and other state variables specific to a particular CSRNG instance are not shared between endpoints on separate EDN instances, or with any hardware devices with direct connections to dedicated CSRNG application interface ports.

Interactions with Peripheral Devices

Peripheral ports distribute data to the endpoints using four signals: req, ack, bus, and fips.

Fresh (i.e. previously unseen) random values are distributed to the endpoints via the parametrizable-width bus signal, in response to an req signal. Whenever new values are placed on the bus, the ack is asserted until the values are consumed by the endpoint, as indicated by simultaneous assertion of the req and ack signals in the same cycle. Otherwise ack is deasserted until enough fresh bits are received from CSRNG.

The fips signal is used to identify whether the values received on the bus have been prepared with complete adherence to the recommendations in NIST SP 800-90. If the fips signal is deasserted, it means the associated CSRNG instance has been instantiated with a pre-FIPS seed.

Block Diagram

EDN Block Diagram

Hardware Interfaces

Referring to the Comportable guideline for peripheral device functionality, the module edn has the following hardware interfaces defined.

Primary Clock: clk_i

Other Clocks: none

Bus Device Interface: tlul

Bus Host Interface: none

Peripheral Pins for Chip IO: none

Interrupts:

Interrupt NameDescription
edn_cmd_req_doneAsserted when a software CSRNG request has completed.
edn_fifo_errAsserted when a FIFO error occurs.

Security Alerts: none

Design Details

EDN Initialization

After power-up, the EDN block is disabled. A single TL-UL configuration write to the CTRL register will start random-number streams processing in boot-time request mode. CSRNG application commands will be sent immediately. Once these commands have completed, a status bit will be set. At this point, firmware can later come and reconfigure the EDN block for a different mode of operation.

The recommended write sequence for the entire entropy system is one configuration write to ENTROPY_SRC, then CSRNG, and finally to EDN.

Interrupts

The EDN module has two interrupts: edn_cmd_req_done and edn_fifo_err.

The es_cmd_req_done interrupt should be used when a CSRNG command is issued and firmware is waiting for completion.

The es_fifo_err interrupt will fire when a FIFO has a malfunction. The conditions that cause this to happen are either when there is a push to a full FIFO or a pull from an empty FIFO.

Waveforms

See the CSRNG IP waveform section for the CSRNG application interface commands.

Peripheral Hardware Interface - Req/Ack

The following waveform shows an example of how the peripheral hardware interface works. This example shows the case where the boot-time mode in the ENTROPY_SRC block is enabled.

Programmers Guide

Initialization

The following code snippet demonstrates initializing the EDN block.


void csrng_init(unsigned int enable) {

  // set the control register enable bit
  *CTRL_REG = enable; // should be 0x1 by default

  // the EDN interrupts can optionally be enabled
}

Error conditions

Need to alert the system of a FIFO overflow condition.

Register Table

edn.INTR_STATE @ + 0x0
Interrupt State Register
Reset default = 0x0, mask 0x3
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  edn_fifo_err edn_cmd_req_done
BitsTypeResetNameDescription
0rw1c0x0edn_cmd_req_doneAsserted when a software CSRNG request has completed.
1rw1c0x0edn_fifo_errAsserted when a FIFO error occurs.


edn.INTR_ENABLE @ + 0x4
Interrupt Enable Register
Reset default = 0x0, mask 0x3
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  edn_fifo_err edn_cmd_req_done
BitsTypeResetNameDescription
0rw0x0edn_cmd_req_doneEnable interrupt when INTR_STATE.edn_cmd_req_done is set
1rw0x0edn_fifo_errEnable interrupt when INTR_STATE.edn_fifo_err is set


edn.INTR_TEST @ + 0x8
Interrupt Test Register
Reset default = 0x0, mask 0x3
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  edn_fifo_err edn_cmd_req_done
BitsTypeResetNameDescription
0wo0x0edn_cmd_req_doneWrite 1 to force INTR_STATE.edn_cmd_req_done to 1
1wo0x0edn_fifo_errWrite 1 to force INTR_STATE.edn_fifo_err to 1


edn.REGEN @ + 0xc
Register write enable for all control registers
Reset default = 0x1, mask 0x1
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  REGEN
BitsTypeResetNameDescription
0rw1c0x1REGENWhen true, the BOOT_REQ_DIS and BCAST_MODE_DIS fields within the CTRL registers can be modified. When false, this field read-only. Defaults true, write one to clear. Note that this needs to be cleared after initial configuration at boot in order to lock in the listed register settings.


edn.CTRL @ + 0x10
EDN control register
Reset default = 0x0, mask 0xf
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  BOOT_REQ_DIS AUTO_REQ_MODE CMD_FIFO_RST EDN_ENABLE
BitsTypeResetNameDescription
0rw0x0EDN_ENABLE Setting this bit enables the EDN module.
1rw0x0CMD_FIFO_RSTSetting this bit clears the three command FIFOs: the SW_CMD_REQ FIFO, the RESEED_CMD FIFO, and the GENERATE_CMD FIFO. This bit must be cleared by software before any further commands can be issued to these FIFOs.
2rw0x0AUTO_REQ_MODESetting this bit will enable the EDN block to automatically send another request to CSRNG application interface. It is assumed that the command has been set up ahead of time, such that when the previous command has returned ack, then a new command will be send out again without software intervention. It is expected that the generate command will be sent repeatedly so that a continuous supply of entropy can be delivered to the endpoints.
3rw0x0BOOT_REQ_DISSetting this bit will disable the feature where the EDN block will automatically send a boot-time request to the CSRNG application interface. The purpose of this feature is to request entropy as fast as possible after reset, and during chip boot-time.


edn.SUM_STS @ + 0x14
EDN summary status register
Reset default = 0x0, mask 0x80000003
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INTERNAL_USE  
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  BOOT_INST_ACK REQ_MODE_SM_STS
BitsTypeResetNameDescription
0rw0x0REQ_MODE_SM_STSThis bit reflects the status of the EDN command state machine. This bit is asserted whenever the the state machine is actively managing commands in either auto request mode, or boot-time request mode. After the state machine has been suspended, this status bit should be polled to know when the last remaining state machine command has been completed, at which point is safe to issue custom commands via the software command forwarding interface. A value of zero indicates that all commands have completed, and the state machine is idle.
1rw0x0BOOT_INST_ACKThis bit indicates that the EDN state machine has started the instantiation process. If this bit is asserted it means the state machine has started in boot-time request mode and that the CSRNG has confirmed that this EDN will be the next application to receive a seed from the entropy_src IP.
30:2Reserved
31rw0x0INTERNAL_USEThis bit is for internal use only.


edn.SW_CMD_REQ @ + 0x18
EDN csrng app command request register
Reset default = 0x0, mask 0x0
Register enable = REGEN
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SW_CMD_REQ...
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...SW_CMD_REQ
BitsTypeResetNameDescription
31:0woxSW_CMD_REQAny CSRNG action can be initiated by writing a CSRNG command to this register. The application interface must wait for the "ack" to return before issuing new commands. This interface is intended to be controlled solely by software. If AUTO_REQ_MODE is set, this register will have no effect on operation.


edn.SW_CMD_STS @ + 0x1c
EDN command status register
Reset default = 0x0, mask 0x13
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  CMD_STS   CMD_ACK CMD_RDY
BitsTypeResetNameDescription
0rw0x0CMD_RDYThis bit indicates when the command interface is ready to accept commands.
1rw0x0CMD_ACKThis bit is set when the command request has completed. This bit must be cleared by software before a new request is made. (CMD_RDY will not be asserted until this bit has been cleared).
3:2Reserved
4rw0x0CMD_STSThis one bit field is the status code returned with the application command ack. 0b0: Request completed successfully 0b1: Request completed with an error


edn.RESEED_CMD @ + 0x20
EDN csrng reseed command register
Reset default = 0x0, mask 0x0
Register enable = REGEN
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RESEED_CMD...
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...RESEED_CMD
BitsTypeResetNameDescription
31:0woxRESEED_CMDWriting this register will fill a FIFO with up to 13 command words (32b words). This FIFO will be used to automatically send out a reseed command to the CSRNG application interface when in AUTO_REQ_MODE. This command will be sent only after the MAX_NUM_REQS_BETWEEN_RESEEDS counter value has reached zero. On overflow this FIFO pops the oldest word from the previous command. Updated commands can be inserted by pushing 13 command words into the FIFO, pushing the previous command out.


edn.GENERATE_CMD @ + 0x24
EDN csrng generate command register
Reset default = 0x0, mask 0x0
Register enable = REGEN
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GENERATE_CMD...
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...GENERATE_CMD
BitsTypeResetNameDescription
31:0woxGENERATE_CMDWriting this register will fill a FIFO with up to 13 command words (32b words). This FIFO will be used to automatically send out a generate command to the CSRNG appl interface when in AUTO_REQ_MODE. This command will be sent only after receiving a command ack from the previous command. On overflow this FIFO pops the oldest word from the previous command. Updated commands can be inserted by pushing 13 command words into the FIFO, pushing the previous command out.


edn.MAX_NUM_REQS_BETWEEN_RESEEDS @ + 0x28
EDN maximum number of requests between reseeds register
Reset default = 0x0, mask 0xffffffff
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MAX_NUM_REQS_BETWEEN_RESEEDS...
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...MAX_NUM_REQS_BETWEEN_RESEEDS
BitsTypeResetNameDescription
31:0rw0x0MAX_NUM_REQS_BETWEEN_RESEEDSSetting this field will set the number of generate requests that can be made to CSRNG before a reseed request is made. This value only has meaning when in AUTO_REQ_MODE. This register supports a maximum of 2^32 requests between reseeds. This register will be used by a counter that counts down, triggering an automatic reseed when it reaches zero.