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@@ -30,6 +30,8 @@ Current contents of the library: * An implementation of ECDSA, optionally using the Cryptech ECDSA base point multiplier cores. +* An implementation of HSS/LMS hash-based signatures. + * An interface to the Master Key Memory interface core on the Cryptech Alpha platform. @@ -115,6 +117,55 @@ the coordinates themselves in Montgomery form; final mapping back to affine coordinates also handles the final Montgomery reduction. +## Hash-Based Signatures ## + +A hashsig private key is a Merkle tree of one-time signing keys, which can +be used to sign a finite number of messages. Since they don't rely on +"hard math" for security, hashsig schemes are quantum-resistant. + +This hashsig code is a clean-room implementation of draft-mcgrew-hash-sigs. +It has been shown to interoperate with the Cisco reference code (each can +verify the other's signatures). + +Following the recommendations of the draft, we only store the topmost hash +tree (the "root tree") in the token keystore; lower-level trees are stored +in the volatile keystore, and are regenerated upon a system restart. + +This implementation has limitations on the number of keys, size of OTS +keys, and size of signatures, because of the design of the keystore and of +the RPC mechanism: + +1. The token keystore is a fairly small flash, partitioned into 2048 +8096-byte blocks. Therefore, we can't support LMS algorithm types > +lms_sha256_n32_h10 (a.k.a. h=10, or 1024 keys per tree). In this case, +keygen will return HAL_ERROR_NO_KEY_INDEX_SLOTS. + +Additionally, the 8KB key storage size means that we can't support LM-OTS +algorithm type lmots_sha256_n32_w1, which has an OTS key size of 8504 +bytes. In this case, keygen will return HAL_ERROR_UNSUPPORTED_KEY. + +2. The volatile keystore is currently limited to 1280 keys, so only 2 +levels at h=10, but more levels at h=5. One could easily increase the size +of the volatile keystore, but L=2/h=10 gives us a key that can sign 1M +messages, which is sufficient for development and testing purposes. + +3. The RPC mechanism currently limits request and response messages to +16KB, so we can't generate or verify signatures greater than that size. +In this case, keygen will return HAL_ERROR_UNSUPPORTED_KEY. + +Because the hashsig private key consists of a large number of one-time +signing keys, and because only the root tree is stored in flash, it can +take several minutes to reconstruct the full tree on system restart. +During this time, attempts to generate a hashsig key, delete a hashsig +key, or sign with a hashsig key will return HAL_ERROR_NOT_READY. + +A hashsig private key can sign at most 2^(L*h) messages. (System restarts +will cause the lower-level trees to be regenerated, which will need to be +signed with by the root tree, so frequent restarts will rapidly exhaust +the root tree.) When a hashsig key is exhausted, any attempt to use it for +signing will return HAL_ERROR_HASHSIG_KEY_EXHAUSTED. + + ## Keystore ## The keystore is basically a light-weight database intended to be run |