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Clarify cryptographic magic numbers #4

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Clarify cryptographic magic numbers #4

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bfe9c1c
replace crypto magic constants
Jun 6, 2024
26dd544
replace magic constants
Jun 6, 2024
1e5abf8
update digest size checks before signing
Jun 8, 2024
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36 changes: 24 additions & 12 deletions src/crypto.c
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Original file line number Diff line number Diff line change
Expand Up @@ -69,7 +69,7 @@ zxerr_t crypto_extractPublicKey(const hd_path_t path,
return zxerr_invalid_crypto_settings;
}

if (pubKeyLen < 65) {
if (pubKeyLen < PUB_KEY_SIZE) {
zemu_log_stack("extractPublicKey: zxerr_buffer_too_small");
return zxerr_buffer_too_small;
}
Expand All @@ -85,13 +85,10 @@ zxerr_t crypto_extractPublicKey(const hd_path_t path,
}

typedef struct {
uint8_t r[32];
uint8_t s[32];
uint8_t r[CURVE_ORDER_SIZE];
uint8_t s[CURVE_ORDER_SIZE];
uint8_t v;

// DER signature max size should be 73
// https://bitcoin.stackexchange.com/questions/77191/what-is-the-maximum-size-of-a-der-encoded-ecdsa-signature#77192
uint8_t der_signature[73];
uint8_t der_signature[DER_SIG_MAX_SIZE];
} __attribute__((packed)) signature_t;

void sha256(const uint8_t *message, uint16_t messageLen, uint8_t message_digest[CX_SHA256_SIZE]) {
Expand Down Expand Up @@ -132,18 +129,18 @@ zxerr_t digest_message(const uint8_t *message,
return zxerr_ok;
}
case HASH_SHA3_256: {
if (digestMax < 32) {
if (digestMax < CX_SHA3_256_SIZE) {
return zxerr_buffer_too_small;
}
zemu_log_stack("sha3_256");
cx_sha3_t sha3;
cx_err = cx_sha3_init_no_throw(&sha3, 256);
cx_err = cx_sha3_init_no_throw(&sha3, CX_SHA3_256_SIZE * 8);
if (cx_err != CX_OK) return zxerr_invalid_crypto_settings;
cx_err =
cx_hash_no_throw((cx_hash_t *) &sha3, 0, domainTag, DOMAIN_TAG_LENGTH, NULL, 0);
if (cx_err != CX_OK) return zxerr_invalid_crypto_settings;
cx_err =
cx_hash_no_throw((cx_hash_t *) &sha3, CX_LAST, message, messageLen, digest, 32);
cx_hash_no_throw((cx_hash_t *) &sha3, CX_LAST, message, messageLen, digest, CX_SHA3_256_SIZE);
if (cx_err != CX_OK) return zxerr_invalid_crypto_settings;
*digest_size = cx_hash_get_size((cx_hash_t *) &sha3);
return zxerr_ok;
Expand Down Expand Up @@ -174,7 +171,7 @@ zxerr_t crypto_sign(const hd_path_t path,

const digest_type_e cx_hash_kind = get_hash_type(options);

uint8_t messageDigest[32];
uint8_t messageDigest[CURVE_ORDER_SIZE];
uint16_t messageDigestSize = 0;

CHECK_ZXERR(digest_message(message,
Expand All @@ -185,11 +182,26 @@ zxerr_t crypto_sign(const hd_path_t path,
sizeof(messageDigest),
&messageDigestSize));

if (messageDigestSize != CX_SHA256_SIZE) {

// check the message worked properly
if (cx_hash_kind == HASH_SHA2_256 && messageDigestSize != CX_SHA256_SIZE) {
zemu_log_stack("crypto_sign: zxerr_out_of_bounds");
return zxerr_out_of_bounds;
}

if (cx_hash_kind == HASH_SHA3_256 && messageDigestSize != CX_SHA3_256_SIZE) {
zemu_log_stack("crypto_sign: zxerr_out_of_bounds");
return zxerr_out_of_bounds;
}

// check that the hashing choice doesn't reduce the curve security.
// it's enough to check that the digest size is larger or equal to the group order
// but we check the equality to make sure the algos are fully compatible.
if (messageDigestSize != CURVE_ORDER_SIZE) {
zemu_log_stack("crypto_sign: zxerr_invalid_crypto_settings");
return zxerr_invalid_crypto_settings;
}

size_t signatureLength;
uint32_t info = 0;

Expand Down
11 changes: 11 additions & 0 deletions src/crypto.h
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Original file line number Diff line number Diff line change
Expand Up @@ -32,7 +32,18 @@ typedef enum { CURVE_UNKNOWN, CURVE_SECP256K1, CURVE_SECP256R1 } curve_e;

#if defined(TARGET_NANOS) || defined(TARGET_NANOX) || defined(TARGET_NANOS2)
#else
/* output sizes of the supported hash algorithms */
#define CX_SHA256_SIZE 32
#define CX_SHA3_256_SIZE 32

/* constants related to the supported elliptic curves */
/* the currently supported curves have the same group order byte size and prime field byte size */
#define CURVE_ORDER_SIZE 32
#define CURVE_FIELD_SIZE 32
#define PUB_KEY_SIZE ((2*CURVE_FIELD_SIZE)+1)
// DER signature max size should be 73
// https://bitcoin.stackexchange.com/questions/77191/what-is-the-maximum-size-of-a-der-encoded-ecdsa-signature#77192
#define DER_SIG_MAX_SIZE (2*(CURVE_ORDER_SIZE+3)+3)
#endif

void sha256(const uint8_t *message, uint16_t messageLen, uint8_t message_digest[CX_SHA256_SIZE]);
Expand Down