31. On-the-fly decryption engine (OTFDEC)

31.1 OTFDEC introduction

OTFDEC allows on-the-fly decryption of the AHB traffic based on the read request address information. Four independent and non-overlapping encrypted regions can be defined in OTFDEC.

OTFDEC uses AES-128 in counter mode to achieve the lowest possible latency. As a consequence, each time the content of one encrypted region is changed, the entire region must be re-encrypted with a different cryptographic context (key or initialization vector). This constraint makes OTFDEC suitable to decrypt read-only data or code, stored in external NOR flash.

Note: When OTFDEC is used in conjunction with OCTOSPI, it is mandatory to access the flash memory using the memory-mapped mode of the flash memory controller.

When security is enabled in the product, OTFDEC can be programmed only by a secure host.

31.2 OTFDEC main features

• • On-the-fly 128-bit decryption during the OCTOSPI Memory-mapped read operations (single or multiple).
  • – Use of AES in counter (CTR) mode, with two 128-bit keystream buffers
  • – Support for any read size
  • – Physical address of the reads used for the encryption/decryption
• • Up to four independent encrypted regions
  • – Granularity of the region definition: 4096 bytes
  • – Region configuration write-locking mechanism
  • – Each region has its own 128-bit key, two bytes firmware version, and eight bytes application-defined nonce. At least one of those must be changed each time an encryption is performed by the application.
• • Encryption keys confidentiality and integrity protection
  • – Write-only registers, with software locking mechanism
  • – Availability of 8-bit CRC as public key information
• • Support for OCTOSPI pre-fetching mechanism
• • Possibility to select an enhanced encryption mode to add a proprietary layer of protection on top of AES stream cipher (execute only)
• • Privileged-aware AMBA AHB slave peripheral, accessible through 32-bit word single accesses only (otherwise an AHB bus error is generated, and write accesses are ignored)
• • Secure only programming if TrustZone security is enabled in the product
• • Encryption mode

31.3 OTFDEC functional description

31.3.1 OTFDEC block diagram

Figure 225. OTFDEC block diagram

1. otfdect_tzen

31.3.2 OTFDEC internal signals

Table 241 describes a list of useful to know internal signals available at OTFDEC level, not at the product level (on pads).

Table 241. OTFDEC internal input/output signals

The TZEN option bit in FLASH is used to activate TrustZone in the device.

• • TZEN = 1: TrustZone security is enabled in the product.
• • TZEN = 0: TrustZone security is disabled in the product.

31.3.3 OTFDEC on-the-fly decryption

Introduction

Typical usage for OTFDEC is shown on Figure 226 .

Figure 226. Typical OTFDEC use in a SoC

Original purpose of OTFDEC is to protect the confidentiality of read-only firmware libraries stored in external SPI NOR flash devices.

A special locking scheme is available in OTFDEC in order to protect the integrity of the decryption keys and also to protect the other configurations against software denial of services attacks. OTFDEC access to most registers can be made privileged-only by setting PRIV bit in OTFDEC_PRIVCFG register. OTFDEC is only writeable by TrustZone CPU, when TrustZone security is activated.

When OTFDEC is used in conjunction with OCTOSPI, it is mandatory to read the flash memory using the Memory-mapped mode of the flash controller.

On top of decrypting on-the-fly, OTFDEC can also encrypt 32-bit word at a time (see Section 31.5.3: Encrypting for OTFDEC for more details).

OTFDEC architecture

OTFDEC analyzes all AHB read transfers on the associated AHB bus. If the read request is within one of the four regions programmed in OTFDEC, the control logic triggers a keystream computation based on AES algorithm in counter mode. This keystream is then used to decrypt on-the-fly the data present in the read transfer from the OCTOSPI AHB master, tying low the HREADYOUT signal of this master while the keystream information is being computed (this takes up to 11 cycles). Any accesses outside the enabled OTFDEC regions belong to a non-encrypted region.

Each OTFDEC region is programmed through OTFDEC_RxCFGR, OTFDEC_RxSTARTADDR, OTFDEC_RxENDADDR, OTFDEC_RxNONCER and

OTFDEC_RxKEYR registers, where \( x = 1 \) to 4. In OTFDEC_RxCFGR, the MODE bits define the OTFDEC operating mode (standard or enhanced encryption).

Granularity for the region determination is 4096 bytes.

Note: Although OTFDEC does not prevent region overlapping, it is not a valid programming and it must be avoided by application software.

OTFDEC can decrypt incremental or wrap bursts only if they do not cross the 4096-byte aligned address boundaries.

31.3.4 OTFDEC usage of AES in counter mode decryption

Figure 227 shows how OTFDEC uses industry standard Advanced Encryption Standard (AES) algorithm in counter chaining mode. This mode is specified by NIST in Special Publication 800-38A, Recommendation for Block Cipher Modes of Operation .

Figure 227. AES CTR decryption flow

Every 128-bit data block, a special keystream information is computed using AES block cipher, as defined below:

• initialization vector \( AES\_IV[127:0] = RxNONCER1[31:0] \parallel RxNONCER0[31:0] \parallel 0b0000\ 0000\ 0000\ 0000 \parallel RxCFGR[31:16] \parallel 0b00 \parallel (x-1) \parallel ReadAddress[31:4] \)
• key material \( AES\_KEY[127:0] = RxKEYR3[31:0] \parallel RxKEYR2[31:0] \parallel RxKEYR1[31:0] \parallel RxKEYR0[31:0] \)

Note: Above \( x \) is the RegionID of the selected encrypted region ( \( x=1 \) to 4). ReadAddress is the AHB address of the encrypted data block, modulo 128-bit.

Resulting 128-bit keystream is XORed with 128-bit cipher text data to produce the 128-bit clear text data.

• AES_DIN and AES_DOUT data blocks are constructed following the rule below (“ \( \parallel \) ” represents a binary concatenation):
  \( AES\_Dx[127:0] = AHB\_word(@ \mid 0xC)[31:0] \mid AHB\_word(@ \mid 0x8)[31:0] \mid AHB\_word(@ \mid 0x4)[31:0] \mid AHB\_word(@ \mid 0x0)[31:0] \) , where \( @ \) is the hexadecimal address used to compute the keystream (ReadAddress[31:4] above).

When the read request is not within an encrypted region, or the decryption is not enabled in this region, the AHB data is not changed.

Note: When the application sets the MODE bitfield to 11 in OTFDEC_RxCFGR, an additional layer of protection is added on top of the AES stream cipher. This enhanced encryption mode can only be used with instructions (execute-only region).

31.3.5 Flow control management

Figure 228 shows how OTFDEC manages one INCR4 AHB burst that corresponds to one 128-bit AES data block.

Figure 228. OTFDEC flow control overview (dual burst read request)

with the following notes:

1. 1. OTFDEC enforces HREADY signal from the AHB master low as it is not ready to decrypt data (keystream computation).
2. 2. Thanks to the keystream buffer, OTFDEC can be ready to process a new batch of data within 12 cycles in this configuration (120 MHz AHB clock, 104 MHz SPI bus delivering 2 bytes per SPI clock).

31.3.6 OTFDEC error management

OTFDEC automatically manages errors defined as below:

• • Illegal read to OTFDEC_RxKEYR registers
• • Illegal write to OTFDEC_RxKEYR registers while CONFIGLOCK or KEYLOCK = 1 in OTFDEC_RxCFGR, while the access is secure. If the security is disabled in the product, the same error occurs when the access is nonsecure.
• • Illegal write to OTFDEC_RxCFGR, OTFDEC_RxSTARTADDR, OTFDEC_RxENDADDR or OTFDEC_RxNONCER registers while CONFIGLOCK = 1 in OTFDEC_RxCFGR (x = 1 to 4), while the access is secure. If the security is disabled in the product the same error occurs when the access is nonsecure.
• • Illegal read to an execute-only region (MODE[1:0] = 11). Such illegal request returns 0x0, without bus error.
• • Execution request to a region while encryption is enabled (ENC = 1). The request returns 0x0, without bus error.
• • Key error: read request to an encrypted region while its key registers are null or not properly initialized (KEYCRC = 0x0). Source of the error can be an incorrect key loading sequence (see KEYCRC in OTFDEC_RxCFGR) or it can be an abort event

(tamper detection, unauthorized debug connection, untrusted boot, RDP level regression). Such read request returns 0x0, without bus error.

• • Write to any registers while the access is nonsecure, if TrustZone security is enabled in the product.

This last error is managed and cleared through TrustZone interrupt controller, as described in the GTZC section of the product reference manual.

For these errors (except the last one), an interrupt can be generated if the SEIE, XONEIE or KEIE bit is set in OTFDEC_IER register (see Section 31.4 ).

Note: After a key error, OTFDEC keys must be properly initialized again, and a reset of OTFDEC may be needed if registers are locked.

31.4 OTFDEC interrupts

There are three independent maskable interrupt sources generated by the OTFDEC, signaling following security events:

• • Illegal read or write access to keys (SEIF flag), see Section 31.3.6
• • Illegal write to a region configuration while CONFIGLOCK = 1 (SEIF flag), see Section 31.3.6
• • Read access to an execute-only region (MODE[1:0] = 11), triggering the XONEIF flag
• • Executing while encryption is enabled (XONEIF flag)
• • Key error (encrypted regions read as zero) triggering the KEIF flag, see Section 31.3.6 .

Interrupt sources are connected to the same global interrupt request signal.

OTFDEC interrupt sources can be enabled/disabled by setting the corresponding SEIE, XONEIE or KEIE bit in OTFDEC_IER, as described in Table 242 . Status of the interrupt event is found in OTFDEC_ISR, and this event can be cleared using OTFDEC_ICR.

Table 242. OTFDEC interrupt requests

1. The event flags are found in the OTFDEC_ISR register.

31.5 OTFDEC application information

31.5.1 OTFDEC initialization process

Introduction

One key aspect of OTFDEC is the trusted initialization of its registers, as it involves secret keys. Two trusted initialization schemes are recommended here below.

Note: Those sequences are for production code, as during firmware development, it is not always recommended to lock the key or the region configuration.

Writes to configuration registers are effective when the configuration locks allow it, even if the region is enabled.

Initialization scheme 1: one key for all regions

In this scheme, one entity owns the secret key used to decrypt the four protected regions. The recommended OTFDEC configuration sequence is described below:

1. 1. For \( x = 1 \) to 4, write the correct MODE[1:0] value in OTFDEC_RxCFGR.
2. 2. For \( x = 1 \) to 4, program OTFDEC_RxKEYR registers using the sequence described in KEYCRC (to have a valid CRC). Warning as key registers are write only.
3. 3. For \( x = 1 \) to 4, check the key CRC. If OK, set KEYLOCK bit in OTFDEC_RxCFGR. This bit cannot be cleared (key registers in this region \( x \) are no more writable).
4. 4. To do to decrypt a region \( x \) (task that does not necessarily have to be performed by the entity that owns the decryption keys):
   1. a) Verify if the key CRC corresponds to the encrypted binary stored in the region.
   2. b) Fill the detailed information corresponding to this binary (nonce, start address, end address, version number).
   3. c) Enable decryption of this region using REG_EN.
   4. d) Set CONFIGLOCK bit in OTFDEC_RxCFGR. This bit cannot be cleared (the region configuration is no more writable).

Caution: For a given region, when MODE bits are changed, the key registers and associated CRC are cleared by hardware. As a consequence, step 1 above must be done before step 2, and MODE bits must not be modified after step 2.

Initialization scheme 2: one key per region

In this scheme, one entity can own the secret used to decrypt one (or more) protected region. The recommended OTFDEC configuration sequence is described below:

1. 1. To do to decrypt a region \( x \) (this task must be performed by the entity that owns the corresponding key):
   1. a) Write the correct MODE[1:0] value in OTFDEC_RxCFGR.
   2. b) Program OTFDEC_RxKEYR registers using the sequence described in KEYCRC (to have a valid CRC). Warning as key registers are write only.
   3. c) Check the key CRC. If OK, set KEYLOCK bit in OTFDEC_RxCFGR. This bit cannot be cleared (key registers are no more writable).
   4. d) Fill the detailed information corresponding to the protected firmware (nonce, start address, end address, version number).
   5. e) Enable decryption of this region using REG_EN.
   6. f) Set CONFIGLOCK bit in OTFDEC_RxCFGR. This bit cannot be cleared (the region configuration is no more writable).

Caution: For a given region, when MODE bits are changed, the key registers and associated CRC are cleared by hardware. As a consequence step a) above must be done before step b), and MODE bits must not be modified after step b).

31.5.2 OTFDEC and power management

Each time OTFDEC is reset, the correct key loading sequence described in Section 31.5.1 must be performed (in this case KEYCRC = 0 in OTFDEC_RxCFGGR).

It is recommended for application software to verify this point each time OTFDEC is reset by hardware.

31.5.3 Encrypting for OTFDEC

Code and data standard encryption

OTFDEC uses standard AES in counter mode when processing a binary stored in a protected region with MODE[1:0] = 10. When this mode is selected, any AES compatible hardware accelerator or library can be used to encrypt those protected libraries. OTFDEC can be used as well, as described in enhanced encryption section below (with MODE[1:0] = 10).

Definition and endianness of the AES inputs and outputs are defined in Section 31.3.4: OTFDEC usage of AES in counter mode decryption .

Enhanced encryption with OTFDEC

OTFDEC uses a proprietary layer of protection on top of the standard AES in counter mode when processing a code stored in a protected region with MODE[1:0] = 11.

Enhanced encryption mode can be used to increase the robustness against tampering.

Recommended sequence to encrypt using OTFDEC is described below:

1. 1. The application in charge of the encryption sets the ENC bit in OTFDEC_CR. This application must run in TrustZone secure mode when TrustZone security is enabled in the product. If PRIV bit is set in OTFDEC_PRIVCFGGR, this application must be privileged.
2. 2. Encryption application initializes OTFDEC as described in Section 31.5.1: OTFDEC initialization process . OCTOSPI must also be properly clocked, so that OTFDEC is fully functional in encryption mode. This step can also be done before step 1.
3. 3. Encryption application writes 32-bit of clear-text data at the expected protected address, then reads it back encrypted at the same address to store it in RAM. Note that this data stays inside the device, as it is intercepted by OTFDEC in encryption mode.
4. 4. Encryption application goes back to previous step (changing the address) until the whole binary is processed.
5. 5. Encryption application clears the ENC bit in OTFDEC_CR. Another application can then take the encrypted binary and flash it to the correct address in external flash.

There are few important notes about this procedure:

• • Encryption granularity is 32-bit (single 32-bit access is mandatory).
• • While ENC bit is set, reads to non-encrypted regions return normal data (such as no encryption nor decryption). While in encryption mode, no access to OCTOSPI (including registers) must be done. This is because the OTFDEC cuts the communication with OCTOSPI while ENC bit is set.
• • OTFDEC does not support execution while ENC = 1 (only encrypted data reads). Upon illegal execution detection a XONEIF flag is raised and zero is returned.

31.5.4 OTFDEC key CRC source code

Below is the CRC source code that can be used to compare with the result of the computation provided by OTFDEC in KEYCRC bitfield after loading the keys in OTFDEC_RxKEYR registers.

uint8_t getCRC(uint32_t * keyin)
{
    const uint8_t CRC7_POLY = 0x7;
    const uint32_t key_strobe[4] = {0xAA55AA55, 0x3, 0x18, 0xC0};
    uint8_t i, j, k, crc = 0x0;
    uint32_t keyval;

    for (j = 0; j < 4; j++)
    {
        keyval = *(keyin+j);
        if (j == 0)
        {
            keyval ^= key_strobe[0];
        }
        else
        {
            keyval ^= (key_strobe[j] << 24) | (crc << 16) | (key_strobe[j] << 8)
            | crc;
        }

        for (i = 0, crc = 0; i < 32; i++)
        {
            k = (((crc >> 7) ^ (keyval >> (31-i))&0xF)) & 1;
            crc <<= 1;
            if (k)
            {
                crc ^= CRC7_POLY;
            }
        }
        crc ^= 0x55;
    }
    return crc;
}

31.6 OTFDEC registers

31.6.1 OTFDEC control register (OTFDEC_CR)

Address offset: 0x0

Reset value: 0x0000 0000

Nonsecure AHB write access (HNONSEC = 1) is discarded if the TrustZone security is enabled in the product.

Unprivileged reads return zero and unprivileged writes are ignored if PRIV bit is set in OTFDEC_PRIVCFGR.

Bits 31:1 Reserved, must be kept at reset value.

Bit 0 ENC : Encryption mode bit

When this bit is set, OTFDEC is used in encryption mode, during which application can write clear text data then read back encrypted data. When this bit is cleared (default), OTFDEC is used in decryption mode, during which application only read back decrypted data. For both modes, cryptographic context (keys, nonces, firmware versions) must be properly initialized. When this bit is set, only data accesses are allowed (zeros are returned otherwise, and XONEIF is set). When MODE = 11, enhanced encryption mode is automatically selected.

0: OTFDEC working in decryption mode
1: OTFDEC working in encryption mode

Note: When ENC bit is set, no access to OCTOSPI must be done (registers and Memory-mapped region).

31.6.2 OTFDEC privileged access control configuration register (OTFDEC_PRIVCFGR)

Address offset: 0x10

Reset value: 0x0000 0000

Nonsecure AHB write access (HNONSEC = 1) is discarded if the TrustZone security is enabled in the product.

Bits 31:1 Reserved, must be kept at reset value.

Bit 0 PRIV : Privileged access protection.

0: No additional protection is added on OTFDEC register accesses.

1: An additional protection is added when accessing all registers except OTFDEC_PRIVCFGR:

• – Unprivileged read accesses to registers return zeros
• – Unprivileged write accesses to registers are ignored.

Note: This bit can only be written in privileged mode. There is no limitations on reads.

31.6.3 OTFDEC region x configuration register (OTFDEC_RxCFGR)

Address offset: 0x20 + 0x30 * (x - 1), (x = 1 to 4)

Reset value: 0x0000 0000

Nonsecure AHB write access (HNONSEC = 1) is discarded if the TrustZone security is enabled in the product.

Unprivileged reads return zero and unprivileged writes are ignored if PRIV bit is set in OTFDEC_PRIVCFGR.

Writes are ignored if CONFIGLOCK bit is set to 1.

Bits 31:16 REG_VERSION[15:0] : region firmware version

This 16-bit bitfield must be correctly initialized before the region corresponding REG_EN bit is set in OTFDEC_RxCFGR.

Bits 15:8 KEYCRC[7:0] : region key 8-bit CRC

When KEYLOCK = 0, KEYCRC bitfield is automatically computed by hardware while loading the key of this region in this exact sequence: KEYR0 then KEYR1 then KEYR2 then finally KEYR3 (all written once). A new computation starts as soon as a new valid sequence is initiated, and KEYCRC is read as zero until a valid sequence is completed.

When KEYLOCK = 1, KEYCRC remains unchanged until the next reset.

CRC computation is an 8-bit checksum using the standard CRC-8-CCITT algorithm \( X^8 + X^2 + X + 1 \) (according to the convention). Source code is available in Section 31.5.4 .

This field is read only.

Note: CRC information is updated only after the last bit of the key has been written.

Bits 7:6 Reserved, must be kept at reset value.

Bits 5:4 MODE[1:0] : operating mode

This bitfield selects the OTFDEC operating mode for this region:

• 10: All read accesses are decrypted (instruction or data).
• 11: Enhanced encryption mode is activated, and only instruction accesses are decrypted
• Others: Reserved

When MODE ≠ 11, the standard AES encryption mode is activated.

When either of the MODE bits are changed, the region key and associated CRC are zeroed.

Bit 3 Reserved, must be kept at reset value.

Bit 2 KEYLOCK : region key lock

• 0: Writes to this region KEYRx registers are allowed.
• 1: Writes to this region KEYRx registers are ignored until next OTFDEC reset. KEYCRC bitfield is locked.

Note: This bit is set once: if this bit is set, it can only be reset to 0 if the OTFDEC is reset.

Bit 1 CONFIGLOCK : region config lock

• 0: Writes to this region OTFDEC_RxCFGR, OTFDEC_RxSTARTADDR, OTFDEC_RxENDADDR and OTFDEC_RxNONCERY registers are allowed.
• 1: Writes to this region OTFDEC_RxCFGR, OTFDEC_RxSTARTADDR, OTFDEC_RxENDADDR and OTFDEC_RxNONCERY registers are ignored until next OTFDEC reset.

Note: This bit is set once. If this bit is set, it can only be reset to 0 if OTFDEC is reset. Setting this bit forces KEYLOCK bit to 1.

Bit 0 REG_EN : region on-the-fly decryption enable

• 0: On-the-fly decryption is disabled for this region.
• 1: On-the-fly decryption is enabled for this region. Data are XORed with the corresponding keystream.

Note: Garbage is decrypted if region context (version, key, nonce) is not valid when this bit is set.

31.6.4 OTFDEC region x start address register (OTFDEC_RxSTARTADDR)

Address offset: \( 0x24 + 0x30 \times (x - 1) \) , ( \( x = 1 \) to \( 4 \) )

Reset value: 0x0000 0000

Nonsecure AHB write access (HNONSEC = 1) is discarded if the TrustZone security is enabled in the product.

Unprivileged reads return zero and unprivileged writes are ignored if PRIV bit is set in OTFDEC_PRIVCFGR.

Bits 31:0 REG_START_ADDR[31:0] : Region AHB start address

This register must be written before the region corresponding REG_EN bit in the OTFDEC_RxCFGFR register is set.

Writing to this register is discarded if performed while the region CONFIGLOCK bit in the OTFDEC_RxCFGFR register is set.

Note: When determining the region the first 12 bits (LSB) and the last 4 bits (MSB) are ignored.

When this register is accessed in read the 4 MSB bits and the 12 LSB bits return zeros .

31.6.5 OTFDEC region x end address register (OTFDEC_RxENDADDR)

Address offset: \( 0x28 + 0x30 \times (x - 1) \) , ( \( x = 1 \) to \( 4 \) )

Reset value: 0x0000 0FFF

Nonsecure AHB write access (HNONSEC = 1) is discarded if the TrustZone security is enabled in the product.

Unprivileged reads return zero and unprivileged writes are ignored if PRIV bit is set in OTFDEC_PRIVCFGR.

Bits 31:0 REG_END_ADDR[31:0] : Region AHB end address

This register must be written before the region corresponding REG_EN bit in the OTFDEC_RxCFGR register is set, and OTFDEC_RxENDADDR must be strictly greater than OTFDEC_RxSTARTADDR to be valid.

Writing to this register is discarded if performed while the region CONFIGLOCK bit in OTFDEC_RxCFGR is set.

Note: When determining the region the first 12 bits (LSB) and the last 4 bits (MSB) are ignored.

When this register is accessed in read the 4 MSB bits return zeros and the 12 LSB bits return ones.

31.6.6 OTFDEC region x nonce register 0 (OTFDEC_RxNONCER0)

Address offset: \( 0x2C + 0x30 * (x - 1) \) , ( \( x = 1 \) to 4)

Reset value: 0x0000 0000

Nonsecure AHB write access (HNONSEC = 1) is discarded if the TrustZone security is enabled in the product.

Unprivileged reads return zero and unprivileged writes are ignored if PRIV bit is set in OTFDEC_PRIVCFGR.

Bits 31:0 REG_NONCE[31:0] : Region nonce, bits [31:0]

This register must be written before the region corresponding REG_EN bit in OTFDEC_RxCFGR is set.

Writing is discarded in this register if performed while the region CONFIGLOCK bit in the OTFDEC_RxCFGR is set.

31.6.7 OTFDEC region x nonce register 1 (OTFDEC_RxNONCER1)

Address offset: \( 0x30 + 0x30 * (x - 1) \) , ( \( x = 1 \) to \( 4 \) )

Reset value: 0x0000 0000

Nonsecure AHB write access (HNONSEC = 1) is discarded if the TrustZone security is enabled in the product.

Unprivileged reads return zero and unprivileged writes are ignored if PRIV bit is set in OTFDEC_PRIVCFGR.

Bits 31:0 REG_NONCE[63:32] : Region nonce, bits [63:32]

Refer to the OTFDEC_RxNONCER0 register for description of the NONCE[63:0] bitfield.

31.6.8 OTFDEC region x key register 0 (OTFDEC_RxKEYR0)

Address offset: \( 0x34 + 0x30 * (x - 1) \) , ( \( x = 1 \) to \( 4 \) )

Reset value: 0x0000 0000

Nonsecure AHB write access (HNONSEC = 1) is discarded if the TrustZone security is enabled in the product.

Unprivileged writes are ignored if PRIV bit is set in OTFDEC_PRIVCFGR.

Bits 31:0 REG_KEY[31:0] : Region key, bits [31:0]

This register must be written before the region corresponding REG_EN bit in OTFDEC_RxCFGR is set.

Reading this register returns a zero value. Writing to this register is discarded if performed while the region CONFIGLOCK or KEYLOCK bit is set in the OTFDEC_RxCFGR.

Note: When application successfully changes MODE bits in OTFDEC_RxCFGR and OTFDEC_RxKEYR, and associated KEYCRC are erased.

31.6.9 OTFDEC region x key register 1 (OTFDEC_RxKEYR1)

Address offset: 0x38 + 0x30 * (x - 1), (x = 1 to 4)

Reset value: 0x0000 0000

Nonsecure AHB write access (HNONSEC = 1) is discarded if the TrustZone security is enabled in the product.

Unprivileged writes are ignored if PRIV bit is set in OTFDEC_PRIVCFGR.

Bits 31:0 REG_KEY[63:32] : Region key, bits [63:32]

Refer to the OTFDEC_RxKEYR0 register for description of the KEY[127:0] bitfield.

31.6.10 OTFDEC region x key register 2 (OTFDEC_RxKEYR2)

Address offset: 0x3C + 0x30 * (x - 1), (x = 1 to 4)

Reset value: 0x0000 0000

Nonsecure AHB write access (HNONSEC = 1) is discarded if the TrustZone security is enabled in the product.

Unprivileged writes are ignored if PRIV bit is set in OTFDEC_PRIVCFGR.

Bits 31:0 REG_KEY[95:64] : Region key, bits [95:64]

Refer to the OTFDEC_RxKEYR0 register for description of the KEY[127:0] bitfield.

31.6.11 OTFDEC region x key register 3 (OTFDEC_RxKEYR3)

Address offset: \( 0x40 + 0x30 * (x - 1) \) , ( \( x = 1 \) to 4)

Reset value: 0x0000 0000

Nonsecure AHB write access (HNONSEC = 1) is discarded if the TrustZone security is enabled in the product.

Unprivileged writes are ignored if PRIV bit is set in OTFDEC_PRIVCFGR.

Bits 31:0 REG_KEY[127:96] : Region key, bits [127:96]

Refer to the OTFDEC_RxKEYR0 register for description of the KEY[127:0] bitfield.

31.6.12 OTFDEC interrupt status register (OTFDEC_ISR)

Address offset: 0x300

Reset value: 0x0000 0000

Unprivileged reads return zero if PRIV bit is set in OTFDEC_PRIVCFGR.

Bits 31:3 Reserved, must be kept at reset value.

Bit 2 KEIF : Key error interrupt flag status

This bit is set by hardware and read only by application. The bit is set when a read access occurs on an encrypted region, while its key registers is null or not properly initialized (KEYCRC = 0x0).

This bit is cleared when the application sets in OTFDEC_ICR the corresponding bit to 1.

0: OTFDEC operates properly.

1: Read access detected on an enabled encrypted region with its key registers null or not properly initialized (KEYCRC = 0x0). OTFDEC returns a zeroed value for the read, and an optional interrupt is generated if bit KEIE is set to 1 in OTFDEC_IER.

After KEIF is set any subsequent read to the region with bad key registers returns a zeroed value. This state remains until those key registers are properly initialized (KEYCRC not zero).

Bit 1 XONEIF : Execute-only execute-never error interrupt flag status

This bit is set by hardware and read only by application. This bit is set when a read access and not an instruction fetch is detected on any encrypted region with MODE bits set to 11. Lastly, XONEIF is also set when an execute access is detected while encryption mode is enabled.

This bit is cleared when application sets in OTFDEC_ICR the corresponding bit to 1.

0: No execute-only error status. No interrupt pending.

1: Read access detected on one region with MODE bits set to 11 or execute access detected while ENC = 1. OTFDEC returns a zeroed value for the illegal access, and an optional interrupt is generated if bit XONEIE is set to 1 in OTFDEC_IER.

Bit 0 SEIF : Security error interrupt flag status

This bit is set by hardware and read only by application. This bit is set when at least one security error has been detected.

This bit is cleared when application sets in OTFDEC_ICR the corresponding bit to 1.

0: No security error status. No interrupt pending.

1: Security error flag status, with interrupt pending. Actual interrupt generation is dependent on OTFDEC_IER corresponding bit SEIE.

31.6.13 OTFDEC interrupt clear register (OTFDEC_ICR)

Address offset: 0x304

Reset value: 0x0000 0000

Nonsecure AHB write access (HNONSEC = 1) is discarded if the TrustZone security is enabled in the product.

Unprivileged writes are ignored if PRIV bit is set in OTFDEC_PRIVCFGGR.

Bits 31:3 Reserved, must be kept at reset value.

Bit 2 KEIF : Key error interrupt flag clear

This bit is written by application, and always read as 0.

0: KEIF flag status is not affected.

1: KEIF flag status is cleared in OTFDEC_ISR.

Note: Clearing KEIF does not solve the source of the problem (bad key registers). To be able to access again any encrypted region, OTFDEC key registers must be properly initialized again.

Bit 1 XONEIF : Execute-only execute-never error interrupt flag clear

This bit is written by application, and always read as 0.

0: XONEIF flag status is not affected.

1: XONEIF flag status is cleared in OTFDEC_ISR.

Bit 0 SEIF : Security error interrupt flag clear

This bit is written by application, and always read as 0.

0: SEIF flag status is not affected.

1: SEIF flag status is cleared in OTFDEC_ISR.

31.6.14 OTFDEC interrupt enable register (OTFDEC_IER)

Address offset: 0x308

Reset value: 0x0000 0000

Nonsecure AHB write access (HNONSEC = 1) is discarded if the TrustZone security is enabled in the product.

Unprivileged reads return zero and unprivileged writes are ignored if PRIV bit is set in OTFDEC_PRIVCFGR.

Bits 31:3 Reserved, must be kept at reset value.

Bit 2 KEIE : Key error interrupt enable

This bit is read and written by application. It controls the OTFDEC interrupt generation when KEIF flag status is set.

• 0: Interrupt generation on key error flag KEIF is disabled (masked).
• 1: Interrupt generation on key error flag KEIF is enabled (not masked).

Bit 1 XONEIE : Execute-only execute-never error interrupt enable

This bit is read and written by application. It controls the OTFDEC interrupt generation when XONEIF flag status is set.

• 0: Interrupt generation on execute-only error XONEIF is disabled (masked).
• 1: Interrupt generation on execute-only error XONEIF is enabled (not masked).

Bit 0 SEIE : Security error interrupt enable

This bit is read and written by application. It controls the OTFDEC interrupt generation when SEIF flag status is set.

• 0: Interrupt generation on security error SEIF is disabled (masked).
• 1: Interrupt generation on security error SEIF is enabled (not masked).

31.6.15 OTFDEC register map

Table 243. OTFDEC register map and reset values

Table 243. OTFDEC register map and reset values (continued)

Table 243. OTFDEC register map and reset values (continued)

Refer to Section 2.3: Memory organization for the register boundary addresses.