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[certs] add attestation key ID salts
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The attestation key IDs are generated via a KMAC KDF operation. The key
is a static salt. This adds the salts.

Signed-off-by: Tim Trippel <[email protected]>
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@timothytrippel
timothytrippel committed Feb 12, 2024
1 parent f9f33f2 commit f336a1e
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Showing 2 changed files with 50 additions and 44 deletions.
63 changes: 42 additions & 21 deletions sw/device/silicon_creator/lib/cert/util.c
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Original file line number Diff line number Diff line change
Expand Up @@ -11,38 +11,59 @@
#include "sw/device/lib/crypto/include/datatypes.h"
#include "sw/device/lib/crypto/include/mac.h"

/**
* Salts for generating attestation key IDs (for generating certificates).
*
* Generated on calling the following, three separate times:
* `hexdump -vn132 -e'33/4 "0x%08x,\n" 1 "\n"' /dev/urandom`
*/
const uint32_t kUdsKeyIdSalt[kOtCertPubkeyIdSaltSizeIn32BitWords] = {
0x24c80b8a, 0xdeae1ac1, 0x6abf2278, 0xb878d9b4,
0x8b0bdc90, 0x90084245, 0xbb5b6e3c, 0x41eb9d0f,
};
const uint32_t kCdi0KeyIdSalt[kOtCertPubkeyIdSaltSizeIn32BitWords] = {
0x55aeb248, 0xaee2b11d, 0xc28623a3, 0xeb1c34a0,
0xacccf2b2, 0x89f3fa7d, 0xb557465a, 0x31a1dd9a,
};
const uint32_t kCdi1KeyIdSalt[kOtCertPubkeyIdSaltSizeIn32BitWords] = {
0x5df3082a, 0x645a5f95, 0x527812eb, 0x24607172,
0x45717a6a, 0xa34add96, 0x107eeab3, 0x8e40c9f5,
};

status_t ot_cert_gen_key_id(const attestation_public_key_t *pubkey,
const ot_cert_pubkey_id_salt_t *salt,
ot_cert_pubkey_id_t *id) {
const uint32_t *salt, uint32_t *id) {
uint32_t blinded_key_buf[kOtCertPubkeyIdSaltSizeIn32BitWords * 2] = {0};
memcpy(blinded_key_buf, salt, kOtCertPubkeyIdSaltSizeInBytes);
const otcrypto_key_config_t kKmacKeyConfig = {
.version = kOtcryptoLibVersion1,
.key_mode = kOtcryptoKeyModeKmac256,
.key_length = kOtCertPubkeyIdSaltSizeInBytes,
.hw_backed = kHardenedBoolFalse,
.exportable = kHardenedBoolTrue,
.security_level = kOtcryptoKeySecurityLevelLow,
};
const otcrypto_blinded_key_t kSalt = {
.config =
{
.version = kOtcryptoLibVersion1,
.key_mode = kOtcryptoKeyModeKdfKmac256,
.key_length = kOtCertPubkeyIdSaltSizeInBytes,
.hw_backed = kHardenedBoolFalse,
.exportable = kHardenedBoolTrue,
.security_level = kOtcryptoKeySecurityLevelLow,
},
.keyblob_length = kOtCertPubkeyIdSaltSizeInBytes,
.keyblob = salt->data,
.config = kKmacKeyConfig,
.keyblob_length = kOtCertPubkeyIdSaltSizeInBytes * 2,
.keyblob = blinded_key_buf,
.checksum = 0,
};
uint8_t str_buffer[2] = "ID";
otcrypto_const_byte_buf_t str = {
.data = "ID",
.data = str_buffer,
.len = 2,
};
uint8_t attestation_pubkey_byte_buf[kAttestationPublicKeyCoordBytes * 2];
memcpy(attestation_pubkey_byte_buf, pubkey.x,
kAttestationPublicKeyCoordBytes);
memcpy(attestation_pubkey_byte_buf[kAttestationPublicKeyCoordBytes], pubkey.y,
memcpy(attestation_pubkey_byte_buf, pubkey->x,
kAttestationPublicKeyCoordBytes);
memcpy(&attestation_pubkey_byte_buf[kAttestationPublicKeyCoordBytes],
pubkey->y, kAttestationPublicKeyCoordBytes);
otcrypto_const_byte_buf_t attestation_pubkey = {
.data = attestation_pubkey,
.data = attestation_pubkey_byte_buf,
.len = kAttestationPublicKeyCoordBytes * 2,
};
otcrypto_word32_buf_t id = {
.data = id->data,
otcrypto_word32_buf_t tag = {
.data = id,
.len = kOtCertPubkeyIdSaltSizeIn32BitWords,
};

Expand All @@ -51,7 +72,7 @@ status_t ot_cert_gen_key_id(const attestation_public_key_t *pubkey,
/*kmac_mode=*/kOtcryptoKmacModeKmac256,
/*customization_string=*/str,
/*required_output_len=*/kOtCertPubkeyIdSizeInBytes,
/*tag=*/id));
/*tag=*/tag));

return OK_STATUS();
}
31 changes: 8 additions & 23 deletions sw/device/silicon_creator/lib/cert/util.h
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Original file line number Diff line number Diff line change
Expand Up @@ -18,40 +18,26 @@ enum {
* See [RFC 5280](https://datatracker.ietf.org/doc/html/rfc5280).
*/
kOtCertPubkeyIdSizeInBytes = 20,
kOtCertPubkeyIdSaltSizeIn32BitWords =
kOtCertPubkeyIdSaltSizeInBytes / sizeof(uint32_t),
kOtCertPubkeyIdSizeIn32BitWords =
kOtCertPubkeyIdSizeInBytes / sizeof(uint32_t),

/**
* Size of the salt used to produce OpenTitan attestation certificate pubkey
* IDs via a KMAC.
*
* See NIST SP800-56C, Rev. 2, Section 4.2, Table 3.
*/
kOtCertPubkeyIdSaltSizeInBytes = 132,
kOtCertPubkeyIdSaltSizeInBytes = 32,
kOtCertPubkeyIdSaltSizeIn32BitWords =
kOtCertPubkeyIdSaltSizeInBytes / sizeof(uint32_t),
};

/**
* Certificate pubkey ID.
*
* Used as the serial number and subject key identifier fields in the X.509
* attestation certificates. See
* [RFC5280](https://datatracker.ietf.org/doc/html/rfc5280).
*/
typedef struct ot_cert_pubkey_id {
uint32_t data[kOtCertPubkeyIdSaltSizeIn32BitWords];
} ot_cert_pubkey_id_t;

/**
* Certificate pubkey ID salt.
*
* Used as the key to a KMAC operation to produce OpenTitan attestation
* certificate pubkey IDs.
* Salts for generating attestation key IDs (for generating certificates).
*/
typedef struct ot_cert_pubkey_id_salt {
uint32_t data[kOtCertPubkeyIdSaltLengthIn32BitWords];
} ot_cert_pubkey_id_salt_t;
extern const uint32_t kUdsKeyIdSalt[kOtCertPubkeyIdSaltSizeIn32BitWords];
extern const uint32_t kCdi0KeyIdSalt[kOtCertPubkeyIdSaltSizeIn32BitWords];
extern const uint32_t kCdi1KeyIdSalt[kOtCertPubkeyIdSaltSizeIn32BitWords];

/**
* Generates an OpenTitan attestation certificate pubkey ID via KMAC operation.
Expand All @@ -63,7 +49,6 @@ typedef struct ot_cert_pubkey_id_salt {
*/
OT_WARN_UNUSED_RESULT
status_t ot_cert_gen_key_id(const attestation_public_key_t *pubkey,
const ot_cert_pubkey_id_salt_t *salt,
ot_cert_pubkey_id_t *id);
const uint32_t *salt, uint32_t *id);

#endif // OPENTITAN_SW_DEVICE_SILICON_CREATOR_LIB_CERT_UTIL_H_

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