[ot] hw/opentitan: ot_hmac: Restore correct msg_length with HMAC_EN #151
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HMAC can operate either with HMAC_EN, using HMAC algorithms, or without, using standard SHA algorithms. The HMAC algorithms introduce additional logic surrounding a key, and inner and outer padding. Relevant to this commit is that when computing HMAC, we first process a block of inner padding XORed with the key. This means that the message length reported to software in the msg_length register diverges from the message length reported by the tomcrypt cryptographic library's state. Specifically, with HMAC_EN=1, it undercounts by a block. This caused an error where, when saving and restoring context with HMAC_EN=1, the hash length would be undercounted by a block and thus the incorrect digest would be computed. This meant that save/restore and streaming operations were not working properly with HMAC_EN. This commit introduces the additional logic to fix this edge case. Signed-off-by: Alex Jones <[email protected]>
jwnrt
approved these changes
Mar 20, 2025
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Fixes a bug introduced to HMAC with the save/restore additions. All tests that can currently be run for HMAC are passing, except:
hmac_kattests, which require a test harness.clkmgr_off_hmac_trans_test, which is reliant on other blocks (e.g. clkmgr).hmac_multistream_functest, which this PR aims to address to get the HMAC as functional as possible.HMAC can operate either with HMAC_EN, using HMAC algorithms, or without, using standard SHA algorithms. The HMAC algorithms introduce additional logic surrounding a key, and inner and outer padding. Relevant to this PR is that when computing HMAC, we first process a block of inner padding XORed with the key. This means that the message length reported to software in the MSG_LENGTH register diverges from the message length reported by the tomcrypt cryptographic library's state. Specifically, with HMAC_EN=1, it undercounts by a block. See relevant RTL.
This caused an error where, when saving and restoring context with HMAC_EN=1, the hash length would be undercounted by a block, leading to different hash computations. This meant that save/restore and streaming operations were not working properly with HMAC_EN.
This PR introduces additional logic to fix this edge case. All existing HMAC tests appear to continue to pass, and the
hmac_multistream_functestis now passing with this change.