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fix(ota): Fix OTA decryption when HW acceleration is not available#12320

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lucasssvaz:fix/ota_mbedtls
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fix(ota): Fix OTA decryption when HW acceleration is not available#12320
lucasssvaz wants to merge 1 commit intoespressif:masterfrom
lucasssvaz:fix/ota_mbedtls

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@lucasssvaz lucasssvaz commented Feb 4, 2026 •
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Description of Change

This PR aims to fix decryption of images when using non-Hardware-Accelerated functions form mbedtls by removing a useless mbedtls_aes_setkey_dec call.

This pull request also improves the documentation and clarity of the ESP32 flash encryption and decryption logic in Updater.cpp, making the cryptographic implementation easier to understand and maintain. The changes focus on adding detailed comments explaining the key tweaking and decryption algorithms, clarifying the reasoning behind certain cryptographic choices, and cleaning up the code for readability.

Enhanced documentation and clarity for ESP32 flash encryption

  • Added a comprehensive comment block to the _cryptKeyTweak function, explaining the concept of address-based key tweaking, its security benefits, configuration options via _cryptCfg, and how it matches ESP32's hardware flash encryption scheme.
  • Improved inline comments throughout _cryptKeyTweak to clarify how address bits are mixed into the key, the role of the pattern array, and the logic for partial tweaking based on _cryptCfg.

Improved documentation and explanation for decryption logic

  • Added a detailed comment block to _decryptBuffer, describing the ESP32 flash encryption scheme, the involutory transform (using AES encryption for both encrypt and decrypt), the rationale for byte reversal, and how key tweaking is applied per flash region.
  • Enhanced step-by-step comments within _decryptBuffer to clarify the process: block byte reversal, key tweaking, AES encryption (for decryption), and reversing the result back to plaintext.

Code readability and maintainability improvements

  • Removed redundant calls to mbedtls_aes_setkey_dec and clarified that only mbedtls_aes_setkey_enc is needed due to the involutory nature of the encryption scheme.
  • Cleaned up and reorganized code blocks for better logical flow, making the cryptographic process easier to follow.

Test Scenarios

Tested locally with ESP32 and ESP32-C2

Related links

Closes #12217

@lucasssvaz lucasssvaz self-assigned this Feb 4, 2026
@lucasssvaz lucasssvaz requested a review from me-no-dev as a code owner February 4, 2026 20:59
@lucasssvaz lucasssvaz added the Area: Libraries Issue is related to Library support. label Feb 4, 2026
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Test Results

 90 files   90 suites   27m 37s ⏱️
 67 tests  67 ✅ 0 💤 0 ❌
683 runs  683 ✅ 0 💤 0 ❌

Results for commit ebadd1a.

♻️ This comment has been updated with latest results.

@lucasssvaz lucasssvaz added the Status: Review needed Issue or PR is awaiting review label Feb 4, 2026
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Memory usage test (comparing PR against master branch)

The table below shows the summary of memory usage change (decrease - increase) in bytes and percentage for each target.

MemoryFLASH [bytes]FLASH [%]RAM [bytes]RAM [%]
TargetDECINCDECINCDECINCDECINC
ESP32💚 -2000.000.00000.000.00
ESP32C3💚 -1400.000.00000.000.00
ESP32C5💚 -1400.000.00000.000.00
ESP32C6💚 -1400.000.00000.000.00
ESP32H2💚 -1400.000.00000.000.00
ESP32P4💚 -1600.000.00000.000.00
ESP32S2💚 -2000.000.00000.000.00
ESP32S3💚 -4400.000.00000.000.00
Click to expand the detailed deltas report [usage change in BYTES]
TargetESP32ESP32C3ESP32C5ESP32C6ESP32H2ESP32P4ESP32S2ESP32S3
ExampleFLASHRAMFLASHRAMFLASHRAMFLASHRAMFLASHRAMFLASHRAMFLASHRAMFLASHRAM
libraries/ArduinoOTA/examples/BasicOTA💚 -160💚 -140💚 -140💚 -140--💚 -140💚 -160💚 -200
libraries/ArduinoOTA/examples/SignedOTA💚 -160💚 -140💚 -140💚 -140--💚 -140💚 -160💚 -160
libraries/HTTPUpdate/examples/httpUpdate💚 -160💚 -140💚 -140💚 -140--💚 -140💚 -160💚 -160
libraries/HTTPUpdate/examples/httpUpdateSPIFFS💚 -160💚 -140💚 -140💚 -140--💚 -140💚 -160💚 -160
libraries/HTTPUpdate/examples/httpUpdateSecure💚 -160💚 -140💚 -140💚 -140--💚 -14000💚 -160
libraries/HTTPUpdateServer/examples/WebUpdater💚 -160💚 -140💚 -140💚 -140--💚 -140💚 -160💚 -160
libraries/Update/examples/AWS_S3_OTA_Update💚 -160💚 -140💚 -140💚 -140--💚 -140💚 -160💚 -160
libraries/Update/examples/HTTP_Client_AES_OTA_Update💚 -160💚 -140💚 -140💚 -140--💚 -140💚 -160💚 -160
libraries/Update/examples/HTTP_Server_AES_OTA_Update💚 -160💚 -140💚 -140💚 -140--💚 -140💚 -160💚 -160
libraries/Update/examples/OTAWebUpdater💚 -160💚 -140💚 -140💚 -140--💚 -140💚 -160💚 -120
libraries/Update/examples/SD_Update💚 -160💚 -140💚 -140💚 -140💚 -140💚 -160💚 -160💚 -160
libraries/Update/examples/Signed_OTA_Update💚 -160💚 -140💚 -140💚 -140--💚 -140💚 -160💚 -160
libraries/WebServer/examples/HttpAdvancedAuth💚 -200💚 -140💚 -140💚 -140--💚 -140💚 -160💚 -160
libraries/WebServer/examples/HttpAuthCallback💚 -160💚 -140💚 -140💚 -140--💚 -140💚 -160💚 -200
libraries/WebServer/examples/HttpAuthCallbackInline💚 -160💚 -140💚 -140💚 -140--💚 -140💚 -200💚 -440
libraries/WebServer/examples/HttpBasicAuth💚 -160💚 -140💚 -140💚 -140--💚 -140💚 -160💚 -160
libraries/WebServer/examples/HttpBasicAuthSHA1💚 -160💚 -140💚 -140💚 -140--💚 -14000💚 -120
libraries/WebServer/examples/HttpBasicAuthSHA1orBearerToken💚 -160💚 -140💚 -140💚 -140--💚 -140💚 -160💚 -160
libraries/WebServer/examples/WebUpdate💚 -160💚 -140💚 -140💚 -140--💚 -140💚 -160💚 -240

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ESP32-C2 OTA Update with AES not working

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