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feat: replace cosi with regular multisig #4

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Merged
merged 1 commit into from
Jul 21, 2025
Merged

feat: replace cosi with regular multisig #4

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109 changes: 54 additions & 55 deletions boot/bootutil/src/image_validate.c
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Original file line number Diff line number Diff line change
Expand Up @@ -170,12 +170,14 @@ bootutil_img_hash(struct enc_key_data *enc_state, int image_index,
/*
* The following TLVs are expected to be present in the image.
*
* EXPECTED_SIG_TLV contains the signature of the image.
* EXPECTED_SIG_1_TLV contains the signature 1 of the image.
* EXPECTED_SIG_2_TLV contains the signature 2 of the image.
* EXPECTED_SIGMASK_TLV contains the bitmask of the signers that signed the image, and is
* used to compute the public key against which the signature is verified.
*/
#define EXPECTED_SIG_TLV 0x00A0
#define EXPECTED_SIGMASK_TLV 0x00A1
#define EXPECTED_SIG_0_TLV 0x00A0
#define EXPECTED_SIG_1_TLV 0x00A1
#define EXPECTED_SIGMASK_TLV 0x00A2
#define SIG_BUF_SIZE 64
#define EXPECTED_SIG_LEN(x) ((x) == SIG_BUF_SIZE)

Expand Down Expand Up @@ -263,8 +265,8 @@ static const uint16_t allowed_unprot_tlvs[] = {
IMAGE_TLV_ENC_KW,
IMAGE_TLV_ENC_EC256,
IMAGE_TLV_ENC_X25519,
EXPECTED_SIG_TLV,
EXPECTED_SIGMASK_TLV,
EXPECTED_SIG_0_TLV,
EXPECTED_SIG_1_TLV,
/* Mark end with ANY. */
IMAGE_TLV_ANY,
};
Expand All @@ -287,42 +289,6 @@ static const uint8_t * const BOOTLOADER_KEYS[] = {
#error Production keys are not defined.
#endif

/*
Function that computes a combined public key for a multi-signature (multisig) cryptographic scheme using Ed25519 elliptic curve cryptography.

The function validates parameters and combines multiple Ed25519 public keys into a single aggregate public key based on which signers participated in signing.
Parameters

- sig_m: Minimum number of signatures required (threshold)
- sig_n: Total number of possible signers
- pub: Array of pointers to public keys from all possible signers
- sigmask: Bitmask indicating which signers actually participated
- res: Output buffer for the resulting combined public key
*/
static bool compute_pubkey(uint8_t sig_m, uint8_t sig_n,
const uint8_t *const *pub, uint8_t sigmask,
ed25519_public_key res) {
if (0 == sig_m || 0 == sig_n) return false;
if (sig_m > sig_n) return false;

// discard bits higher than sig_n
sigmask &= ((1 << sig_n) - 1);

// remove if number of set bits in sigmask is not equal to sig_m
if (__builtin_popcount(sigmask) != sig_m) return false;

ed25519_public_key keys[sig_m];
int j = 0;
for (int i = 0; i < sig_n; i++) {
if ((1 << i) & sigmask) {
memcpy(keys[j], pub[i], 32);
j++;
}
}

return (0 == ed25519_cosi_combine_publickeys(res, keys, sig_m));
}


/*
* Verify the integrity of the image.
Expand All @@ -335,7 +301,8 @@ bootutil_img_validate(struct enc_key_data *enc_state, int image_index,
int seed_len, uint8_t *out_hash)
{
int rc = 0;
bool sig_found = false;
bool sig_0_found = false;
bool sig_1_found = false;
uint16_t sigmask = 0;
FIH_DECLARE(fih_rc, FIH_FAILURE);
uint32_t off;
Expand All @@ -344,7 +311,8 @@ bootutil_img_validate(struct enc_key_data *enc_state, int image_index,
FIH_DECLARE(valid_signature, FIH_FAILURE);
struct image_tlv_iter it;
uint8_t buf[SIG_BUF_SIZE] = {0};
uint8_t sig[SIG_BUF_SIZE] = {0};
uint8_t sig0[SIG_BUF_SIZE] = {0};
uint8_t sig1[SIG_BUF_SIZE] = {0};
int image_hash_valid = 0;
uint8_t hash[IMAGE_HASH_SIZE] = {0};

Expand Down Expand Up @@ -423,18 +391,18 @@ bootutil_img_validate(struct enc_key_data *enc_state, int image_index,
break;
}
case EXPECTED_SIGMASK_TLV:
if (len != 2) {
if (len != 1) {
rc = -1;
goto out;
}
rc = LOAD_IMAGE_DATA(hdr, fap, off, buf, sizeof(hash));
if (rc) {
goto out;
}
sigmask = (buf[0] << 8) | buf[1];
sigmask = buf[0];
break;

case EXPECTED_SIG_TLV:
case EXPECTED_SIG_0_TLV:
{
if (!EXPECTED_SIG_LEN(len) || len > sizeof(buf)) {
rc = -1;
Expand All @@ -445,36 +413,67 @@ bootutil_img_validate(struct enc_key_data *enc_state, int image_index,
goto out;
}

if (sig_found) {
if (sig_0_found) {
/* We already found a signature, but we should only have one. */
rc = -1;
goto out;
}

sig_found = true;
sig_0_found = true;

memcpy(sig, buf, len);
memcpy(sig0, buf, len);
break;
}
case EXPECTED_SIG_1_TLV:
{
if (!EXPECTED_SIG_LEN(len) || len > sizeof(buf)) {
rc = -1;
goto out;
}
rc = LOAD_IMAGE_DATA(hdr, fap, off, buf, len);
if (rc) {
goto out;
}

if (sig_1_found) {
/* We already found a signature, but we should only have one. */
rc = -1;
goto out;
}

sig_1_found = true;

memcpy(sig1, buf, len);
break;
}
}
}

if (sigmask == 0 || !sig_found) {
if (sigmask == 0 || !sig_0_found || !sig_1_found) {
rc = -1;
goto out;
}

ed25519_public_key pub;
if (true != compute_pubkey(BOOTLOADER_KEY_M, BOOTLOADER_KEY_N, BOOTLOADER_KEYS, sigmask, pub))
{
int sig0_idx = sigmask & (1 << 0) ? 0 : 1;
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This sigmask handling is wrong: it assumes that there are exactly two out of the first three bits signed and nothing else.
sigmask is part of the signed data so we really have to honor what the signers are commiting to.

the code as written still works if we verify the assumptions, that is:

  • __builtin_popcount(sigmask) == BOOTLOADER_KEY_M
  • bits above BOOTLOADER_KEY_N are zero, so ... sigmask & ~((1 << BOOTLOADER_KEY_N) - 1) == 0 ? i think

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(of course we shouldn't use BOOTLOADER_KEY_M because we're hardcoding the data structures for exactly two signatures; we should still use BOOTLOADER_KEY_N and _Static_assert(BOOTLOADER_KEY_N >= 2))

int sig1_idx = sigmask & (1 << 2) ? 2 : 1;

// There must be two different signatures to verify
if (sig0_idx == sig1_idx) {
rc = -1;
goto out;
} else {
valid_signature = FIH_SUCCESS;
}

if (FIH_NOT_EQ(0, ed25519_sign_open(hash, sizeof(hash), pub,
*(const ed25519_signature *)sig))){

if (FIH_NOT_EQ(0, ed25519_sign_open(hash, sizeof(hash), BOOTLOADER_KEYS[sig0_idx],
*(const ed25519_signature *)sig0))){
rc = -1;
goto out;
}

if (FIH_NOT_EQ(0, ed25519_sign_open(hash, sizeof(hash), BOOTLOADER_KEYS[sig1_idx],
*(const ed25519_signature *)sig1))){
rc = -1;
goto out;
}
Expand Down
1 change: 1 addition & 0 deletions scripts/imgtool/image.py
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Original file line number Diff line number Diff line change
Expand Up @@ -94,6 +94,7 @@
'DECOMP_SIZE': 0x70,
'DECOMP_SHA': 0x71,
'DECOMP_SIGNATURE': 0x72,
'SIGMASK': 0xA2,
}

TLV_SIZE = 4
Expand Down
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