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fix(rsa_ds): make PKCS#1 v1.5 unpadding constant-time

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This commit is contained in:
Ashish Sharma
2026-06-25 10:40:37 +08:00
parent a6928be465
commit 9a703650cd
1 changed files with 74 additions and 42 deletions
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components/mbedtls/port/psa_driver/esp_rsa_ds/psa_crypto_driver_esp_rsa_ds_utilities.c
+74 -42
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@@ -9,6 +9,10 @@
#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 "constant_time_internal.h"
typedef struct {
psa_algorithm_t md_alg;
@@ -167,63 +171,91 @@ psa_status_t esp_rsa_ds_pad_v15_unpad(unsigned char *input,
size_t output_max_len,
size_t *olen)
{
/* This implementation mirrors mbedtls_ct_rsaes_pkcs1_v15_unpadding()
* in upstream rsa.c. Below the public-input length check, every
* operation must be constant-time w.r.t. the plaintext contents,
* the position of the 0x00 separator, and padding validity. Failing
* that opens a Bleichenbacher-style padding oracle. The function is
* deliberately longer than the project guideline because each step
* carries an audit comment that has to survive intact. */
/* ilen and output_max_len are public; this branch is safe. */
if (ilen < MIN_V15_PADDING_LEN) {
return PSA_ERROR_INVALID_ARGUMENT;
}
unsigned char bad = 0;
size_t msg_len = 0;
size_t msg_max_len = 0;
unsigned char pad_done = 0;
#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C)
size_t pad_count = 0;
size_t plaintext_size = 0;
size_t plaintext_max_size;
mbedtls_ct_condition_t bad;
mbedtls_ct_condition_t pad_done;
mbedtls_ct_condition_t output_too_large;
msg_max_len = (output_max_len > ilen - MIN_V15_PADDING_LEN) ? ilen - MIN_V15_PADDING_LEN : output_max_len;
plaintext_max_size = (output_max_len > ilen - MIN_V15_PADDING_LEN)
? ilen - MIN_V15_PADDING_LEN : output_max_len;
/* Check the first byte (0x00) */
bad |= input[0];
/* EME-PKCS1-v1_5: 0x00 || 0x02 || PS (>= 8 non-zero) || 0x00 || M */
bad = mbedtls_ct_bool(input[0]);
bad = mbedtls_ct_bool_or(bad, mbedtls_ct_uint_ne(input[1], 2 /* MBEDTLS_RSA_CRYPT */));
/* Check the padding type */
bad |= input[1] ^ 2; // MBEDTLS_RSA_CRYPT;
/* Scan for separator (0x00) and count padding bytes in constant time */
/* Scan the full buffer; pad_done latches at the first 0x00 found. */
pad_done = MBEDTLS_CT_FALSE;
for (size_t i = 2; i < ilen; i++) {
unsigned char found = (input[i] == 0x00);
pad_done = pad_done | found;
pad_count += (pad_done == 0) ? 1 : 0;
mbedtls_ct_condition_t found = mbedtls_ct_uint_eq(input[i], 0);
pad_done = mbedtls_ct_bool_or(pad_done, found);
pad_count += mbedtls_ct_uint_if_else_0(mbedtls_ct_bool_not(pad_done), 1);
}
/* Check if we found a separator and padding is long enough */
bad |= (pad_done == 0); /* No separator found */
bad |= (pad_count < 8); /* Padding too short (need at least 8 non-zero bytes) */
/* No separator found, or PS too short. */
bad = mbedtls_ct_bool_or(bad, mbedtls_ct_bool_not(pad_done));
bad = mbedtls_ct_bool_or(bad, mbedtls_ct_uint_gt(8, pad_count));
/* Calculate message length */
msg_len = ilen - pad_count - 3;
/* If invalid, substitute plaintext_max_size so the remaining cache
* and timing trace matches the good case. */
plaintext_size = mbedtls_ct_uint_if(bad,
(unsigned) plaintext_max_size,
(unsigned) (ilen - pad_count - 3));
output_too_large = mbedtls_ct_uint_gt(plaintext_size, plaintext_max_size);
plaintext_size = mbedtls_ct_uint_if(output_too_large,
(unsigned) plaintext_max_size,
(unsigned) plaintext_size);
/* Check if separator is not at the very end */
bad |= (msg_len > output_max_len);
if (bad) {
msg_len = msg_max_len;
/* On any failure path (bad padding, or plaintext doesn't fit) zero
* the post-header region of `input` BEFORE the memmove_left + memcpy
* below. Those two operations execute unconditionally to keep the
* memory access trace fixed; this step ensures they propagate zeros
* rather than a failed-decryption plaintext attempt into the
* caller-visible output buffer. Mirrors upstream rsa.c. */
mbedtls_ct_zeroize_if(mbedtls_ct_bool_or(bad, output_too_large),
input + MIN_V15_PADDING_LEN,
ilen - MIN_V15_PADDING_LEN);
/* Slide the plaintext to a fixed in-buffer position, then read
* from that fixed position. The slide is CT in the secret offset. */
mbedtls_ct_memmove_left(input + ilen - plaintext_max_size,
plaintext_max_size,
plaintext_max_size - plaintext_size);
if (output_max_len != 0) {
/* memmove handles input/output aliasing (callers may pass the
* same buffer for both). The length is the public bound, so
* the access pattern reveals nothing secret. */
memmove(output, input + ilen - plaintext_max_size, plaintext_max_size);
}
/* Verify padding bytes are non-zero in constant time */
#if defined(__clang__) && defined(__xtensa__)
#pragma clang loop vectorize(disable)
#endif
for (size_t i = 2; i < ilen; i++) {
unsigned char in_padding = (i < pad_count + 2);
unsigned char is_zero = (input[i] == 0x00);
bad |= in_padding & is_zero;
}
*olen = plaintext_size;
if (bad) {
return PSA_ERROR_INVALID_ARGUMENT;
}
*olen = msg_len;
if (*olen > 0) {
memcpy(output, input + ilen - msg_len, msg_len);
}
return PSA_SUCCESS;
/* Collapse both error conditions into the single status we already
* return (PSA_ERROR_INVALID_ARGUMENT); distinguishing them would
* give a Bleichenbacher attacker a finer oracle. */
return (psa_status_t) mbedtls_ct_error_if_else_0(
mbedtls_ct_bool_or(bad, output_too_large),
PSA_ERROR_INVALID_ARGUMENT);
#else
/* PKCS#1 v1.5 padding is not configured; the driver should not be
* dispatched for this algorithm in the first place. */
(void) input; (void) output; (void) output_max_len; (void) olen;
return PSA_ERROR_NOT_SUPPORTED;
#endif /* MBEDTLS_PKCS1_V15 && MBEDTLS_RSA_C */
}
#if CONFIG_MBEDTLS_SSL_PROTO_TLS1_3