feat: seal stored records with HMAC-SHA256 instead of CRC-16 - #14
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SolidSyslogMbedTlsHmacSha256Policy replaces SolidSyslogCrc16Policy on the store. Records at rest are tamper-evident rather than checksummed: an edit without the key fails verification. Flash +13,432 B (+348 on the previous stage) RAM +37,468 B (+20) Log stack +680 B (unchanged) Service +3,768 B (unchanged) Twenty bytes, and it is the policy's pool entry. The mechanism for holding a named symmetric key and handing it out is the device's own, so the key slot, the loader and the accessor all sit below the line. SHA-256 was already linked, and the policy hashes into a caller-owned buffer, so there is no stack movement and no heap. The key is fetched per seal and per verify rather than held, so it never sits on the policy instance. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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Replace the CRC-16 with a keyed HMAC. The checksum established that a record came back the way it
went in; the HMAC establishes that nobody has changed it since. An edit made without the key fails
verification, so stored records become tamper-evident rather than merely intact.
The key is fetched per seal and per verify rather than held, so it never sits on the policy
instance. Key custody, rotation and provisioning are yours; the library consumes a key you supply
and never stores one.
Holding a named symmetric key and handing it out is the device's own mechanism — a device already
doing mTLS has provisioned secrets and somewhere to keep them, so the key slot, the loader and the
accessor all sit below the line. What SolidSyslog is charged for is the policy and the callback that
reaches for the key.
When you need it. If an attacker could reach the medium — removable, unattended, or stealable —
and stored records must be provably unaltered.