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fix(esp_wifi): Add some comments in github PR 15073

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Closes https://github.com/espressif/esp-idf/pull/15073
This commit is contained in:
Kapil Gupta 2025-01-08 11:37:58 +05:30
parent 03311c6867
commit e02e6fe5ca
3 changed files with 108 additions and 24 deletions
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8
components/wpa_supplicant/esp_supplicant/src/crypto/crypto_mbedtls.c
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@ -1,5 +1,5 @@
/*
* SPDX-FileCopyrightText: 2020-2024 Espressif Systems (Shanghai) CO LTD
* SPDX-FileCopyrightText: 2020-2025 Espressif Systems (Shanghai) CO LTD
*
* SPDX-License-Identifier: Apache-2.0
*/
@ -752,13 +752,13 @@ int pbkdf2_sha1(const char *passphrase, const u8 *ssid, size_t ssid_len,
int iterations, u8 *buf, size_t buflen)
{
#ifdef CONFIG_FAST_PBKDF2
# if CONFIG_IDF_TARGET_ESP32
#if CONFIG_IDF_TARGET_ESP32
fastpbkdf2_hmac_sha1((const u8 *) passphrase, os_strlen(passphrase),
ssid, ssid_len, iterations, buf, buflen);
return 0;
# else
#else
return esp_fast_psk(passphrase, os_strlen(passphrase), ssid, ssid_len, iterations, buf, buflen);
# endif
#endif
#else
int ret = mbedtls_pkcs5_pbkdf2_hmac_ext(MBEDTLS_MD_SHA1, (const u8 *) passphrase,
os_strlen(passphrase), ssid,

122
components/wpa_supplicant/esp_supplicant/src/crypto/fastpsk.c
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@ -1,5 +1,5 @@
/*
* SPDX-FileCopyrightText: 2015-2024 Espressif Systems (Shanghai) CO LTD
* SPDX-FileCopyrightText: 2025 Espressif Systems (Shanghai) CO LTD
*
* SPDX-License-Identifier: Apache-2.0
*/
@ -10,20 +10,61 @@
* Initially authored by Chien Wong(m@xv97.com).
*/
/*
* This implementation derives a Pre-Shared Key (PSK) for WPA/WPA2 using a custom PBKDF2-like approach.
*
* PBKDF2 (Password-Based Key Derivation Function 2) is a standard algorithm used to derive cryptographic keys
* from a password and salt. It relies on iteratively applying a pseudorandom function, such as HMAC, to the input.
* The derived key is designed to be computationally expensive to generate, making brute-force attacks more difficult.
*
* In standard PBKDF2, the process is as follows:
* 1. Combine the password and salt (SSID in WPA).
* 2. Compute HMAC for this combination, iteratively applying the previous HMAC output as input for the next iteration.
* 3. XOR all intermediate results to produce the final derived key.
*
* This implementation adapts PBKDF2 for WPA/WPA2 by leveraging the SHA1 hashing algorithm and fixed parameters:
* - The password is up to 63 characters long.
* - The SSID (salt) is up to 32 bytes.
* - The iteration count is fixed at 4096, as required by WPA.
* - The output key length is 32 bytes, suitable for WPA.
*
* Key Differences from Standard PBKDF2:
* - Instead of a general-purpose pseudorandom function, this implementation uses a fixed combination of SHA1 blocks.
* - The logic for handling HMAC is explicitly implemented to optimize for this specific use case.
* - Padding and block alignment are carefully managed to fit within hardware constraints (e.g., the ESP32 SHA1 hardware).
*
* How This Implementation Works:
* 1. The `fast_psk_f` function computes one segment of the derived key. It takes as input:
* - The password.
* - The SSID.
* - A counter value (`count`) that varies for each segment.
* 2. HMAC-SHA1 is implemented explicitly using:
* - An inner padding block (`ipad`) initialized with 0x36 XORed with the password.
* - An outer padding block (`opad`) initialized with 0x5C XORed with the password.
* 3. Intermediate hashes (`U1`, `U2`, ..., `Un`) are computed iteratively. Each `U` value depends on the previous one.
* - `U1` is derived from the password, SSID, and counter.
* - Subsequent `U` values are derived using SHA1 on the previous `U` value.
* 4. All intermediate values are XORed together to produce the final segment of the key.
* 5. The `esp_fast_psk` function combines two invocations of `fast_psk_f` to produce the complete 32-byte key.
* - The first invocation computes the first 16 bytes.
* - The second invocation computes the second 16 bytes.
*
* - The code uses the ESP SHA1 hardware accelerator for faster computation.
*/
#include "fastpsk.h"
#include <string.h>
#include <sha/sha_dma.h>
#include <hal/sha_hal.h>
#ifndef PUT_UINT32_BE
#define PUT_UINT32_BE(n,b,i) \
#define PUT_UINT32_BE(n, b, i) \
{ \
(b)[(i) ] = (unsigned char) ( (n) >> 24 ); \
(b)[(i) + 1] = (unsigned char) ( (n) >> 16 ); \
(b)[(i) + 2] = (unsigned char) ( (n) >> 8 ); \
(b)[(i) + 3] = (unsigned char) ( (n) ); \
(b)[(i)] = (unsigned char)((n) >> 24); \
(b)[(i) + 1] = (unsigned char)((n) >> 16); \
(b)[(i) + 2] = (unsigned char)((n) >> 8); \
(b)[(i) + 3] = (unsigned char)((n)); \
}
#endif
@ -35,36 +76,45 @@
#define FAST_PSK_SHA1_BLOCKS_BUF_BYTES (FAST_PSK_SHA1_BLOCKS * SHA1_BLOCK_SZ)
#define FAST_PSK_SHA1_BLOCKS_BUF_WORDS (FAST_PSK_SHA1_BLOCKS * SHA1_BLOCK_SZ / 4)
/* Union to represent SHA1 HMAC blocks */
union hmac_block {
union {
uint32_t words[SHA1_BLOCK_SZ / 4];
uint8_t bytes[SHA1_BLOCK_SZ];
uint32_t words[SHA1_BLOCK_SZ / 4]; /* SHA1 block in words */
uint8_t bytes[SHA1_BLOCK_SZ]; /* SHA1 block in bytes */
} block[FAST_PSK_SHA1_BLOCKS];
uint8_t whole_bytes[FAST_PSK_SHA1_BLOCKS_BUF_BYTES];
uint32_t whole_words[FAST_PSK_SHA1_BLOCKS_BUF_WORDS];
uint8_t whole_bytes[FAST_PSK_SHA1_BLOCKS_BUF_BYTES]; /* Complete block as bytes */
uint32_t whole_words[FAST_PSK_SHA1_BLOCKS_BUF_WORDS]; /* Complete block as words */
};
_Static_assert(sizeof(union hmac_block) == 128, "Incorrect layout of hmac_block");
/* Structure to hold HMAC context */
struct fast_psk_context {
union hmac_block inner, outer;
uint32_t sum[SHA1_OUTPUT_SZ_WORDS];
union hmac_block inner, outer; /* Inner and outer padding */
uint32_t sum[SHA1_OUTPUT_SZ_WORDS]; /* Intermediate hash result */
};
/* Acquire SHA1 hardware for exclusive use */
static inline void sha1_setup(void)
{
esp_sha_acquire_hardware();
}
/* Release SHA1 hardware */
static inline void sha1_teardown(void)
{
esp_sha_release_hardware();
}
/*
* Pads the given HMAC block context with the appropriate SHA1 padding.
* Length is the number of bytes of actual data in the block.
*/
static void pad_blocks(union hmac_block *ctx, size_t len)
{
size_t bits = len << 3;
size_t bits = len << 3; /* Convert length to bits */
uint8_t *bytes = ctx->whole_bytes;
bytes[len] = 0x80;
bytes[len] = 0x80; /* Append 0x80 as per SHA1 padding rules */
// Set all remaining bytes to 0
memset(&bytes[len + 1], 0, FAST_PSK_SHA1_BLOCKS_BUF_BYTES - (len + 1));
@ -76,31 +126,53 @@ static void pad_blocks(union hmac_block *ctx, size_t len)
PUT_UINT32_BE(bits, bytes, FAST_PSK_SHA1_BLOCKS_BUF_BYTES - 4);
}
/*
* Performs SHA1 hash operation on two consecutive blocks.
* Input: blocks array (two blocks of 64 bytes each), output (20-byte digest).
*/
void sha1_op(uint32_t blocks[FAST_PSK_SHA1_BLOCKS_BUF_WORDS], uint32_t output[SHA1_OUTPUT_SZ_WORDS])
{
/* First block */
sha_hal_hash_block(SHA1, blocks, SHA1_BLOCK_SZ_WORDS, true);
/* Second block */
sha_hal_hash_block(SHA1, &blocks[SHA1_BLOCK_SZ_WORDS], SHA1_BLOCK_SZ_WORDS, false);
/* Read the final digest */
sha_hal_read_digest(SHA1, output);
}
/*
* Implements the PBKDF2-HMAC-SHA1 function for WPA key derivation.
* - password: The passphrase (up to 63 bytes).
* - password_len: Length of the passphrase.
* - ssid: The SSID (up to 32 bytes).
* - ssid_len: Length of the SSID.
* - count: The iteration counter.
* - digest: Output buffer for the resulting digest (20 bytes).
*/
void fast_psk_f(const char *password, size_t password_len, const uint8_t *ssid, size_t ssid_len, uint32_t count, uint8_t digest[SHA1_OUTPUT_SZ])
{
struct fast_psk_context ctx_, *ctx = &ctx_;
size_t i;
/* Clear the context */
memset(ctx, 0, sizeof(*ctx));
/* Initialize inner and outer padding */
memset(ctx->outer.block[0].bytes, 0x5c, SHA1_BLOCK_SZ);
memset(ctx->inner.block[0].bytes, 0x36, SHA1_BLOCK_SZ);
/* XOR the password into the padding */
for (i = 0; i < password_len; ++i) {
ctx->outer.block[0].bytes[i] ^= password[i];
ctx->inner.block[0].bytes[i] ^= password[i];
}
// U1 = PRF(P, S || i)
/* Prepare the first input block for HMAC (S || i) */
/* Copy SSID */
memcpy(ctx->inner.block[1].bytes, ssid, ssid_len);
/* Append the counter */
PUT_UINT32_BE(count, ctx->inner.block[1].bytes, ssid_len);
/* Pad the block */
pad_blocks(&ctx->inner, SHA1_BLOCK_SZ + ssid_len + 4);
sha1_setup();
@ -108,6 +180,7 @@ void fast_psk_f(const char *password, size_t password_len, const uint8_t *ssid,
uint32_t *pi, *po;
pi = ctx->inner.whole_words;
po = ctx->outer.whole_words;
// T1 = SHA1(K ^ ipad, S || i)
sha1_op(pi, ctx->outer.block[1].words);
@ -118,16 +191,19 @@ void fast_psk_f(const char *password, size_t password_len, const uint8_t *ssid,
uint32_t *sum = ctx->sum;
sha1_op(po, inner_blk1);
/* Copy result to the sum */
memcpy(sum, inner_blk1, SHA1_OUTPUT_SZ);
pad_blocks(&ctx->inner, SHA1_BLOCK_SZ + SHA1_OUTPUT_SZ);
/* Iterate for remaining 4096 - 1 times */
for (i = 1; i < 4096; ++i) {
/* Compute Tn and Un */
// Tn = SHA1(K ^ ipad, Un-1)
sha1_op(pi, outer_blk1);
// Un = SHA1(K ^ opad, Tn)
sha1_op(po, inner_blk1);
/* XOR the results to accumulate into F */
// F = U1 ^ U2 ^ ... Un
for (size_t j = 0; j < SHA1_OUTPUT_SZ_WORDS; ++j) {
sum[j] ^= inner_blk1[j];
@ -136,19 +212,27 @@ void fast_psk_f(const char *password, size_t password_len, const uint8_t *ssid,
sha1_teardown();
/* Copy the final result to the output digest */
memcpy(digest, sum, SHA1_OUTPUT_SZ);
/* Clear sensitive data */
memset(ctx, 0, sizeof(*ctx));
}
int esp_fast_psk(const char *password, size_t password_len, const uint8_t *ssid, size_t ssid_len, size_t iterations, uint8_t *output, size_t output_len)
{
if (!(ssid_len <= 32 && password_len <= 63 && iterations == 4096 && output_len == 32)) {
return -1;
return -1; /* Invalid input parameters */
}
/* Compute the first 16 bytes of the PSK */
fast_psk_f(password, password_len, ssid, ssid_len, 2, output);
/* Replicate the first 16 bytes to form the second half temporarily */
memcpy(output + SHA1_OUTPUT_SZ, output, 32 - SHA1_OUTPUT_SZ);
/* Compute the second 16 bytes of the PSK */
fast_psk_f(password, password_len, ssid, ssid_len, 1, output);
return 0;
return 0; /* Success */
}

2
components/wpa_supplicant/esp_supplicant/src/crypto/fastpsk.h
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@ -1,5 +1,5 @@
/*
* SPDX-FileCopyrightText: 2015-2024 Espressif Systems (Shanghai) CO LTD
* SPDX-FileCopyrightText: 2025 Espressif Systems (Shanghai) CO LTD
*
* SPDX-License-Identifier: Apache-2.0
*/