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fix(rsa_ds): make RSA-OAEP unpadding constant-time

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This commit is contained in:
Ashish Sharma
2026-06-16 14:46:24 +08:00
parent c313d339ab
commit f92ccb1348
1 changed files with 71 additions and 63 deletions
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components/mbedtls/port/psa_driver/esp_rsa_ds/psa_crypto_driver_esp_rsa_ds_utilities.c
+71 -63
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@@ -9,9 +9,7 @@
#include "mbedtls/asn1.h"
#include "mbedtls/psa_util.h"
#include "esp_log.h"
/* The unpadding routines below borrow the audited constant-time primitives
* from upstream mbedtls (the same header is already used by the esp_aes
* driver in this tree). */
#include "mbedtls/constant_time.h"
#include "constant_time_internal.h"
typedef struct {
@@ -449,81 +447,91 @@ psa_status_t esp_rsa_ds_pad_oaep_unpad(unsigned char *input,
size_t *olen,
psa_algorithm_t hash_alg)
{
/* This mirrors mbedtls_rsa_rsaes_oaep_decrypt() in upstream rsa.c.
* Below the public-input sanity check, the unpadding scan operates
* only through mbedtls_ct primitives, so its time and memory trace
* depend solely on ilen and hash_alg. The single branch on `bad`
* at the end leaks no more than the return value already does,
* which matches the upstream design and is acceptable for OAEP
* (the relevant side-channel attack relies on distinguishing
* failure modes, not on timing the success path). */
unsigned int hlen = PSA_HASH_LENGTH(hash_alg);
unsigned char bad = 0;
size_t msg_len = 0;
/* Validate input length */
bad |= (ilen < 2 * hlen + 2);
/* Apply MGF masks */
bad |= esp_rsa_ds_mgf_mask(input + 1, hlen, input + hlen + 1, ilen - hlen - 1, hash_alg) != PSA_SUCCESS;
bad |= esp_rsa_ds_mgf_mask(input + hlen + 1, ilen - hlen - 1, input + 1, hlen, hash_alg) != PSA_SUCCESS;
/* Check first byte (should be 0x00) */
bad |= input[0];
/* Skip the first byte and maskSeed */
unsigned char *db = input + 1 + hlen;
size_t db_len = ilen - hlen - 1;
/* Compute hash, label is NULL and label_len is 0 */
mbedtls_ct_condition_t bad;
mbedtls_ct_condition_t in_padding;
size_t pad_len;
unsigned char *p;
unsigned char lhash[PSA_HASH_MAX_SIZE];
memset(lhash, 0, sizeof(lhash));
/* All lengths checked here are public. */
if (hlen == 0 || hlen > PSA_HASH_MAX_SIZE || ilen < 2 * hlen + 2) {
return PSA_ERROR_INVALID_ARGUMENT;
}
/* Unmask seed and DB. A failure here is an internal hash error,
* not a ciphertext-dependent condition, so an early return is
* safe. */
if (esp_rsa_ds_mgf_mask(input + 1, hlen,
input + hlen + 1, ilen - hlen - 1, hash_alg) != PSA_SUCCESS ||
esp_rsa_ds_mgf_mask(input + hlen + 1, ilen - hlen - 1,
input + 1, hlen, hash_alg) != PSA_SUCCESS) {
return PSA_ERROR_INVALID_ARGUMENT;
}
/* lHash of the empty label, recomputed each call. */
size_t lhen = 0;
bad |= psa_hash_compute(hash_alg, NULL, 0, lhash, sizeof(lhash), &lhen) != PSA_SUCCESS;
bad |= (lhen != hlen);
/* Verify hash portion of db against lhash */
for (size_t i = 0; i < hlen && i < db_len; i++) {
bad |= db[i] ^ lhash[i];
if (psa_hash_compute(hash_alg, NULL, 0, lhash, sizeof(lhash), &lhen) != PSA_SUCCESS
|| lhen != hlen) {
mbedtls_platform_zeroize(lhash, sizeof(lhash));
return PSA_ERROR_INVALID_ARGUMENT;
}
/* Skip past lhash in DB */
unsigned char *p = db + hlen;
size_t remaining = db_len - hlen;
/* Constant-time padding check. */
p = input;
bad = mbedtls_ct_bool(*p++); /* First byte must be 0x00 */
p += hlen; /* Skip seed */
bad = mbedtls_ct_bool_or(bad,
mbedtls_ct_bool(mbedtls_ct_memcmp(lhash, p, hlen)));
p += hlen;
/*
* Scan PS || 0x01 || M
*/
unsigned char seen_one = 0;
size_t msg_index = 0;
for (size_t i = 0; i < remaining; i++) {
unsigned char is_zero = (p[i] == 0);
unsigned char is_one = (p[i] == 1);
/* Before delimiter, only 0x00 allowed */
bad |= (seen_one == 0) & !(is_zero | is_one);
/* Record first 0x01 */
msg_index |= (seen_one == 0 && is_one) * i;
seen_one |= is_one;
/* Count leading zeros in DB (between lHash and the 0x01 delimiter).
* The loop scans the full DB region every time; the latch via
* in_padding keeps pad_len from incrementing once a non-zero byte
* is seen. */
pad_len = 0;
in_padding = MBEDTLS_CT_TRUE;
for (size_t i = 0; i < ilen - 2 * hlen - 2; i++) {
in_padding = mbedtls_ct_bool_and(in_padding, mbedtls_ct_uint_eq(p[i], 0));
pad_len += mbedtls_ct_uint_if_else_0(in_padding, 1);
}
p += pad_len;
bad = mbedtls_ct_bool_or(bad, mbedtls_ct_uint_ne(*p++, 0x01));
/* Must see exactly one delimiter */
bad |= (seen_one == 0);
mbedtls_platform_zeroize(lhash, sizeof(lhash));
/* Calculate message length */
msg_len = remaining - msg_index - 1;
bad |= (msg_len == 0);
/* Check output buffer size */
bad |= (output_max_len < msg_len);
if (bad) {
/* Single decision point on the accumulated bit. */
if (bad != MBEDTLS_CT_FALSE) {
*olen = 0;
return PSA_ERROR_INVALID_ARGUMENT;
}
/* Copy message in constant time */
*olen = msg_len;
if (*olen > 0) {
memcpy(output, p + msg_index + 1, msg_len);
/* Padding is valid; from here the plaintext length is no longer
* secret (it is returned to the caller via *olen). */
size_t plaintext_size = ilen - (size_t) (p - input);
if (plaintext_size > output_max_len) {
*olen = 0;
return PSA_ERROR_INVALID_ARGUMENT;
}
*olen = plaintext_size;
if (plaintext_size != 0) {
/* memmove (not memcpy): the driver's caller aliases input and
* output to the same buffer, so the source range [p, p+plaintext_size)
* overlaps the destination range [output, output+plaintext_size).
* Both pointers and the length are public, so memmove's access
* pattern stays fixed by public values -- CT property preserved. */
memmove(output, p, plaintext_size);
}
return PSA_SUCCESS;
}
#endif /* CONFIG_MBEDTLS_SSL_PROTO_TLS1_3 */