securechip: replace sign interface function with native Rust impl

The securechip unsafe sign key slot in the ATECC608 was only used as
an alternative to adding firmware code for signing using hte NIST
P-256 curve.

The Optiga Trust M does not support the same functionality - one can't
write a private key to this chip and extract the corresponding pubkey.

Since we want one interface for both chips, we remove the unsafe sign
functionality and replace it with the Rust p256 crate.

It is only used in U2F.

As an alternative, one could use the native MCU PUKCC/PUKCL feature to
derive a pubkey and sign using this curve (see pukcc.c), but I could
not get it to work. Using the Rust crate is much easier to implement /
use.

This adds 10592 bytes of binary space. If we need it back, we should
try again to use PUKCC instead and offload these operations to the
MCU.

Author: benma

src/atecc/atecc.c

@@ -34,7 +34,8 @@ typedef enum {
     ATECC_SLOT_ROLLKEY = 3,
     ATECC_SLOT_KDF = 4,
     ATECC_SLOT_ATTESTATION = 5,
-    ATECC_SLOT_ECC_UNSAFE_SIGN = 6,
+    // Deprecated as the equivalent does not exist in the Optiga chip.
+    ATECC_SLOT_ECC_UNSAFE_SIGN_DEPRECATED = 6,
     ATECC_SLOT_DATA0 = 9,
     // The other slots are currently not in use.
 } atecc_slot_t;
@@ -631,74 +632,6 @@ bool atecc_random(uint8_t* rand_out)
     return false;
 }
 
-// Length of priv_key must be 32 bytes
-static bool _ecc_write_priv_key(const uint8_t* priv_key)
-{
-    uint8_t atca_priv_key[36] = {0};
-    memcpy(atca_priv_key + 4, priv_key, 32);
-
-    uint8_t encryption_key[32] = {0};
-    UTIL_CLEANUP_32(encryption_key);
-    _interface_functions->get_encryption_key(encryption_key);
-
-    uint8_t nonce_contribution[32] = {0};
-    UTIL_CLEANUP_32(nonce_contribution);
-    _interface_functions->random_32_bytes(nonce_contribution);
-#if NONCE_NUMIN_SIZE > 32
-#error "size mismatch"
-#endif
-
-    ATCA_STATUS result = _authorize_key();
-    if (result != ATCA_SUCCESS) {
-        return false;
-    }
-
-    return atcab_priv_write(
-               ATECC_SLOT_ECC_UNSAFE_SIGN,
-               atca_priv_key,
-               ATECC_SLOT_ENCRYPTION_KEY,
-               encryption_key,
-               nonce_contribution) == ATCA_SUCCESS;
-}
-
-bool atecc_ecc_generate_public_key(uint8_t* priv_key, uint8_t* pub_key)
-{
-    if (!_ecc_write_priv_key(priv_key)) {
-        return false;
-    }
-
-    ATCA_STATUS result = _authorize_key();
-    if (result != ATCA_SUCCESS) {
-        return false;
-    }
-
-    result = atcab_get_pubkey(ATECC_SLOT_ECC_UNSAFE_SIGN, pub_key);
-    if (result != ATCA_SUCCESS) {
-        return false;
-    }
-
-    return true;
-}
-
-bool atecc_ecc_unsafe_sign(const uint8_t* priv_key, const uint8_t* msg, uint8_t* sig)
-{
-    if (!_ecc_write_priv_key(priv_key)) {
-        return false;
-    }
-
-    ATCA_STATUS result = _authorize_key();
-    if (result != ATCA_SUCCESS) {
-        return false;
-    }
-
-    result = atcab_sign(ATECC_SLOT_ECC_UNSAFE_SIGN, msg, sig);
-    if (result != ATCA_SUCCESS) {
-        return false;
-    }
-
-    return true;
-}
-
 #if APP_U2F == 1 || FACTORYSETUP == 1
 // Read a "standard" sized block from a data slot (must be 32 bytes)
 static bool _read_data_slot_block(uint8_t* bytes, uint16_t slot, uint8_t block)

src/atecc/atecc.h

@@ -98,24 +98,6 @@ USE_RESULT bool atecc_monotonic_increments_remaining(uint32_t* remaining_out);
  */
 USE_RESULT bool atecc_random(uint8_t* rand_out);
 
-/**
- * Generates the matching public key to the provided private key. Will put private key in unsafe
- * ECC slot.
- * @param[in] priv_key Private key (32 bytes).
- * @param[out] pub_key Public key. Format will be the X and Y coordinates in big-endian (64 bytes).
- * @return True if success
- */
-USE_RESULT bool atecc_ecc_generate_public_key(uint8_t* priv_key, uint8_t* pub_key);
-
-/**
- * Sign hash with private key. Will put private key in unsafe ECC slot.
- * @param[in] priv_key Private key to use for signing (32 bytes)
- * @param[in] msg Message to sign (32 bytes)
- * @param[out] sig Signature (64 bytes)
- * @return True if success
- */
-USE_RESULT bool atecc_ecc_unsafe_sign(const uint8_t* priv_key, const uint8_t* msg, uint8_t* sig);
-
 #if APP_U2F == 1 || FACTORYSETUP == 1
 /**
  * Set the u2f counter to `counter`. Should only be used for initialization.

src/rust/Cargo.lock

@@ -28,6 +28,7 @@ bitbox02-rust = { path = "../bitbox02-rust", optional = true }
 bitbox02 = { path = "../bitbox02", optional = true }
 bitbox02-noise = { path = "../bitbox02-noise", optional = true }
 util = { path = "../util" }
+p256 = { version = "0.13.2", default-features = false, features = ["arithmetic", "ecdsa"], optional = true }
 hex = { workspace = true }
 sha2 = { workspace = true, optional = true }
 sha3 = { workspace = true, optional = true }
@@ -62,7 +63,7 @@ target-c-unit-tests = [
 platform-bitbox02 = []
 
 bootloader = []
-firmware = ["bitbox02-rust", "bitbox02", "bitbox02-noise", "sha2"]
+firmware = ["bitbox02-rust", "bitbox02", "bitbox02-noise", "sha2", "p256"]
 
 # Only to be enabled in Rust unit tests.
 testing = ["bitbox02-rust/testing", "bitbox02/testing"]

src/rust/bitbox02-rust-c/Cargo.toml

@@ -31,6 +31,8 @@ mod async_usb;
 #[cfg(feature = "firmware")]
 mod noise;
 #[cfg(feature = "firmware")]
+mod p256;
+#[cfg(feature = "firmware")]
 mod sha2;
 #[cfg(feature = "firmware")]
 mod workflow;