Zephyr ECDSA P-256 support #11
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The ECDSA signature is written as two DER-encoded INTEGERS. Although the values are always 256 bits, the encoding ends up being variable length, because the encoding is signed, and therefore needs an extra zero byte to keep the number positive. This means that the length can vary by up to two bytes. The 'newt' tool handles this for signature by allowing space for the largest encoding, and padding with one or two zeros. However, the bootutil image check code insists that the length is exact, resulting in a decoding error on about 3/4 signatures. Fix this by only verifying that we have at least enough payload to hold the signature. There are later checks that will fail if the integers themselves are too large.
Make it clear in the top-level Makefile how to configure mcuboot for Zephyr for a particular signing algorithm. Currently supported, are the RSA signatures, and ECDSA with the P-256 curve. These configuration lines will select the code built in the bootloader, as well as which public key gets included with the image. This also adds a demo public key for the P-256 signatures.
Separate the signature selection blocks, so that it is clearer to add another config option for the boot verification.
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Looks good to me. Thanks, David. |
ccollins476ad
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Apr 26, 2024
ref: mcu-tools#599 which is still not mainlined ref: grandcentrix/mcuboot@d7c58b62 part 3 of a 3 commit series
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Add Zephyr support for ECDSA P-256 signatures.