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[cryptolib] Use hardened_* functions in KMAC driver #29155

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Merged
nasahlpa merged 2 commits into from
Feb 10, 2026
Merged

[cryptolib] Use hardened_* functions in KMAC driver #29155

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159e3d4
[cryptolib] Use `hardened_xor` when unmasking the digest
nasahlpa Jan 19, 2026
5067454
[cryptolib] Use hardened_memcpy() in KMAC
nasahlpa Jan 20, 2026
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1 change: 1 addition & 0 deletions sw/device/lib/crypto/drivers/BUILD
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Original file line number Diff line number Diff line change
Expand Up @@ -128,6 +128,7 @@ cc_library(
"//sw/device/lib/base:abs_mmio",
"//sw/device/lib/base:bitfield",
"//sw/device/lib/base:hardened",
"//sw/device/lib/base:hardened_memory",
"//sw/device/lib/base:macros",
"//sw/device/lib/crypto/impl:status",
],
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78 changes: 32 additions & 46 deletions sw/device/lib/crypto/drivers/kmac.c
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,7 @@
#include "hw/top/dt/kmac.h"
#include "sw/device/lib/base/abs_mmio.h"
#include "sw/device/lib/base/bitfield.h"
#include "sw/device/lib/base/hardened_memory.h"
#include "sw/device/lib/base/memory.h"
#include "sw/device/lib/crypto/drivers/entropy.h"
#include "sw/device/lib/crypto/drivers/rv_core_ibex.h"
Expand Down Expand Up @@ -566,27 +567,18 @@ static status_t kmac_write_key_block(kmac_blinded_key_t *key) {
KMAC_KEY_LEN_REG_RESVAL, KMAC_KEY_LEN_LEN_FIELD, key_len_enum);
abs_mmio_write32(kBase + KMAC_KEY_LEN_REG_OFFSET, key_len_reg);

// Write random words to the key registers first for SCA defense.
for (size_t i = 0; i * sizeof(uint32_t) < key->len; i++) {
abs_mmio_write32(
kBase + KMAC_KEY_SHARE0_0_REG_OFFSET + i * sizeof(uint32_t),
ibex_rnd32_read());
}
for (size_t i = 0; i * sizeof(uint32_t) < key->len; i++) {
abs_mmio_write32(
kBase + KMAC_KEY_SHARE0_0_REG_OFFSET + i * sizeof(uint32_t),
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@nasahlpa nasahlpa Jan 20, 2026

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Hm no I don't think so, we want to access byte 0 of share0:
https://opentitan.org/book/hw/ip/kmac/doc/registers.html#key_share0

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@johannheyszl johannheyszl Jan 20, 2026 •
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thanks sorry

key->share0[i]);
}
for (size_t i = 0; i * sizeof(uint32_t) < key->len; i++) {
abs_mmio_write32(
kBase + KMAC_KEY_SHARE1_0_REG_OFFSET + i * sizeof(uint32_t),
ibex_rnd32_read());
}
for (size_t i = 0; i * sizeof(uint32_t) < key->len; i++) {
abs_mmio_write32(
kBase + KMAC_KEY_SHARE1_0_REG_OFFSET + i * sizeof(uint32_t),
key->share1[i]);
}
// Write random words to the key registers and use hardened_memcpy
// for SCA defense. Using the hardened_mem* functions is fine as
// we are always operating on multiples of 32-bit words.
uint32_t share0_addr = kBase + KMAC_KEY_SHARE0_0_REG_OFFSET;
uint32_t share1_addr = kBase + KMAC_KEY_SHARE1_0_REG_OFFSET;
size_t key_len_words = key->len / sizeof(uint32_t);
HARDENED_TRY(hardened_memshred((uint32_t *)share0_addr, key_len_words));
HARDENED_TRY(
hardened_memcpy((uint32_t *)share0_addr, key->share0, key_len_words));
HARDENED_TRY(hardened_memshred((uint32_t *)share1_addr, key_len_words));
HARDENED_TRY(
hardened_memcpy((uint32_t *)share1_addr, key->share1, key_len_words));

return OTCRYPTO_OK;
}
Expand Down Expand Up @@ -706,41 +698,35 @@ static status_t kmac_process_msg_blocks(kmac_operation_t operation,
// Read words from the state registers (either `digest_len_words` or the
// maximum number of words available).
size_t offset = 0;
uint32_t offset_share0 = kBase + KMAC_STATE_REG_OFFSET;
uint32_t offset_share1 =
kBase + KMAC_STATE_REG_OFFSET + kKmacStateShareSize;
size_t read_len_words = keccak_rate_words;
if (idx + read_len_words >= digest_len_words) {
read_len_words = digest_len_words - idx;
}
if (launder32(masked_digest) == kHardenedBoolTrue) {
HARDENED_CHECK_EQ(masked_digest, kHardenedBoolTrue);
// Read the digest into each share in turn. Do this in separate loops so
// corresponding shares aren't handled close together.
for (offset = 0; launder32(idx + offset) < digest_len_words &&
offset < keccak_rate_words;
offset++) {
digest[idx + offset] = abs_mmio_read32(kBase + KMAC_STATE_REG_OFFSET +
offset * sizeof(uint32_t));
}
for (offset = 0; launder32(idx + offset) < digest_len_words &&
offset < keccak_rate_words;
offset++) {
digest[idx + offset + digest_len_words] =
abs_mmio_read32(kBase + KMAC_STATE_REG_OFFSET +
kKmacStateShareSize + offset * sizeof(uint32_t));
}
idx += offset;
// Read the digest into each share in turn. Do this by using the SCA
// resilient hardened_memcpy function.
HARDENED_TRY(hardened_memcpy(
&digest[idx], (const uint32_t *)offset_share0, read_len_words));
HARDENED_TRY(hardened_memcpy(&digest[idx + digest_len_words],
(const uint32_t *)offset_share1,
read_len_words));
} else {
// Skip right to the hardened check here instead of returning
// `OTCRYPTO_BAD_ARGS` if the value is not `kHardenedBoolFalse`; this
// value always comes from within the cryptolib, so we expect it to be
// valid and should be suspicious if it's not.
HARDENED_CHECK_EQ(masked_digest, kHardenedBoolFalse);
// Unmask the digest as we read it.
for (; launder32(idx) < digest_len_words && offset < keccak_rate_words;
offset++) {
digest[idx] = abs_mmio_read32(kBase + KMAC_STATE_REG_OFFSET +
offset * sizeof(uint32_t));
digest[idx] ^=
abs_mmio_read32(kBase + KMAC_STATE_REG_OFFSET +
kKmacStateShareSize + offset * sizeof(uint32_t));
idx++;
}
HARDENED_TRY(hardened_xor((const uint32_t *)offset_share0,
(const uint32_t *)offset_share1, read_len_words,
&digest[idx]));
}
offset += read_len_words;
idx += read_len_words;

// If we read all the remaining words and still need more digest, issue
// `CMD.RUN` to generate more state.
Expand Down
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