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+{{{
+#!htmlcomment
+
+This page is maintained automatically by a script.  Don't modify this page by hand,
+your changes will just be overwritten the next time the script runs.  Talk to your
+Friendly Neighborhood Repository Maintainer if you need to change something here.
+
+}}}
+
+{{{
+#!html
+<h1>libhal</h1>
+
+<h2>Overview</h2>
+
+<p>This library combines a set of low-level API functions which talk to
+the Cryptech FPGA cores with a set of higher-level functions providing
+various cryptographic services.</p>
+
+<p>There's some overlap between the low-level code here and the low-level
+code in core/platform/novena, which will need sorting out some day,
+but at the time this library forked that code, the
+core/platform/novena code was all written to support a test harness
+rather than a higher-level API.</p>
+
+<p>Current contents of the library:</p>
+
+<ul>
+<li><p>Low-level I/O code (FMC, EIM, and I2C).</p></li>
+<li><p>An implementation of AES Key Wrap using the Cryptech AES core.</p></li>
+<li><p>An interface to the Cryptech CSPRNG.</p></li>
+<li><p>An interface to the Cryptech hash cores, including HMAC.</p></li>
+<li><p>An implementation of PBKDF2.</p></li>
+<li><p>An implementation of RSA, optionally using the Cryptech ModExp core.</p></li>
+<li><p>An implementation of ECDSA, optionally using the Cryptech ECDSA base
+point multiplier cores.</p></li>
+<li><p>An implementation of HSS/LMS hash-based signatures.</p></li>
+<li><p>An interface to the Master Key Memory interface core on the Cryptech
+Alpha platform.</p></li>
+<li><p>A simple keystore implementation with drivers for RAM and flash
+storage on the Cryptech Alpha platform.</p></li>
+<li><p>A remote procedure call (RPC) interface.</p></li>
+<li><p>(Just enough) ASN.1 code to support a uniform interface to public
+(SubjectPublicKeyInformation (SPKI)) and private (PKCS #8) keys.</p></li>
+<li><p>A simple key backup mechanism, including a Python script to drive it
+from the client side.</p></li>
+<li><p>An RPC multiplexer to allow multiple clients (indepedent processes)
+to talk to the Cryptech Alpha at once.</p></li>
+<li><p>Client implenetations of the RPC mechanism in both C and Python.</p></li>
+<li><p>Test code for all of the above.</p></li>
+</ul>
+
+<p>Most of these are fairly well self-contained, although the PBKDF2
+implementation uses the hash-core-based HMAC implementation with
+fallback to a software implementation if the cores aren't available.</p>
+
+<p>The major exceptions are the RSA and ECDSA implementations, which uses
+an external bignum implementation (libtfm) to handle a lot of the
+arithmetic.  In the long run, much or all of this may end up being
+implemented in Verilog, but for the moment all of the RSA math except
+for modular exponentiation is happening in software, as is all of the
+math for ECDSA verification; ECDSA math for key generation and signing
+on the P-256 and P-384 curves is done in the ECDSA base point
+multiplier cores when those are available.</p>
+
+<h2>RSA</h2>
+
+<p>The RSA implementation includes a compile-time option to bypass the
+ModExp core and do everything in software, because the ModExp core is
+a tad slow at the moment (others are hard at work fixing this).</p>
+
+<p>The RSA implementation includes optional blinding (enabled by default).</p>
+
+<h2>ECDSA</h2>
+
+<p>The ECDSA implementation is specific to the NIST prime curves P-256,
+P-384, and P-521.</p>
+
+<p>The ECDSA implementation includes a compile-time option to allow test
+code to bypass the CSPRNG in order to test against known test vectors.
+Do <strong>NOT</strong> enable this in production builds, as ECDSA depends on good
+random numbers not just for private keys but for individual
+signatures, and an attacker who knows the random number used for a
+particular signature can use this to recover the private key.
+Arguably, this option should be removed from the code entirely.</p>
+
+<p>The ECDSA software implementation attempts to be constant-time, to
+reduce the risk of timing channel attacks.  The algorithms chosen for
+the point arithmetic are a tradeoff between speed and code complexity,
+and can probably be improved upon even in software; reimplementing at
+least the field arithmetic in hardware would probably also help.
+Signing and key generation performance is significantly better when
+the ECDSA base point multiplier cores are available.</p>
+
+<p>The point addition and point doubling algorithms in the current ECDSA
+software implementation come from the <a href="http://www.hyperelliptic.org/EFD/g1p/auto-shortw-jacobian-3.html">EFD</a>.  At least at the
+moment, we're only interested in ECDSA with the NIST prime curves, so
+we use algorithms optimized for a=-3.</p>
+
+<p>The point multiplication algorithm is a straightforward double-and-add
+loop, which is not the fastest possible algorithm, but is relatively
+easy to confirm by inspection as being constant-time within the limits
+imposed by the NIST curves.  Point multiplication could probably be
+made faster by using a non-adjacent form (NAF) representation for the
+scalar, but the author doesn't understand that well enough to
+implement it as a constant-time algorithm.  In theory, changing to a
+NAF representation could be done without any change to the public API.</p>
+
+<p>Points stored in keys and curve parameters are in affine format, but
+point arithmetic is performed in Jacobian projective coordinates, with
+the coordinates themselves in Montgomery form; final mapping back to
+affine coordinates also handles the final Montgomery reduction.</p>
+
+<h2>Hash-Based Signatures</h2>
+
+<p>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.</p>
+
+<p>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).</p>
+
+<p>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.</p>
+
+<p>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:</p>
+
+<ol>
+<li>The token keystore is a fairly small flash, partitioned into 2048
+8096-byte blocks. Therefore, we can't support LMS algorithm types &gt;
+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.</li>
+</ol>
+
+<p>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.</p>
+
+<ol>
+<li><p>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.</p></li>
+<li><p>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.</p></li>
+</ol>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<h2>Keystore</h2>
+
+<p>The keystore is basically a light-weight database intended to be run
+directly over some kind of block-access device, with an internal
+low-level driver interface so that we can use the same API for
+multiple keystore devices (eg, flash for "token objects" and RAM for
+"session objects", in the PKCS #11 senses of those terms).</p>
+
+<p>The available storage is divided up into "blocks" of a fixed size; for
+simplicity, the block size is a multiple of the subsector size of the
+flash chip on the Alpha platform, since that's the minimum erasable
+unit.  All state stored in the keystore itself follows the conventions
+needed for flash devices, whether the device in question is flash or
+not.  The basic rule here is that one can only clear bits, never set
+them: the only way to set a bit is to erase the whole block and start
+over.  So blocks progress from an initial state ("erased") where all
+bits are set to one, through several states where the block contains
+useful data, and ending in a state where all bits are set to zero
+("zeroed"), because that's the way that flash hardware works.</p>
+
+<p>The keystore implementation also applies a light-weight form of wear
+leveling to all keystore devices, whether they're flash devices or
+not.  The wear-leveling mechanism is not particularly sophisticated,
+but should suffice.  The wear-leveling code treats the entirety of a
+particular keystore device as a ring buffer of blocks, and keeps track
+of which blocks have been used recently by zeroing blocks upon freeing
+them rather than erasing them immediately, while also always keeping
+the block at the current head of the free list in the erased state.
+Taken together, this is enough to recover location of the block at the
+head of the free list after a reboot, which is sufficient for a
+round-robin wear leveling strategy.</p>
+
+<p>The block format includes a field for a CRC-32 checksum, which covers
+the entire block except for a few specific fields which need to be
+left out.  On reboot, blocks with bad CRC-32 values are considered
+candidates for reuse, but are placed at the end of the free list,
+preserve their contents for as long as possible in case the real
+problem is a buggy firmware update.</p>
+
+<p>At the moment, the decision about whether to use the CRC-32 mechanism
+is up to the individual driver: the flash driver uses it, the RAM
+driver (which never stores anything across reboots anyway) does not.</p>
+
+<p>Since the flash-like semantics do not allow setting bits, updates to a
+block always consist of allocating a new block and copying the
+modified data.  The keystore code uses a trivial lock-step protocol
+for this: first:</p>
+
+<ol>
+<li>The old block is marked as a "tombstone";</li>
+<li>The new block (with modified data) is written;</li>
+<li>The old block is erased.</li>
+</ol>
+
+<p>This protocol is deliberately as simple as possible, so that there is
+always a simple recovery path on reboot.</p>
+
+<p>Active blocks within a keystore are named by UUIDs.  With one
+exception, these are always type-4 (random) UUIDs, generated directly
+from output of the TRNG.  The one exception is the current PIN block,
+which always uses the reserved all-zeros UUID, which cannot possibly
+conflict with a type-4 UUID (by definition).</p>
+
+<p>The core of the keystore mechanism is the <code>ks-&gt;index[]</code> array, which
+contains nothing but a list of block numbers.  This array is divided
+into two parts: the first part is the index of active blocks, which is
+kept sorted (by UUID); the second part is the round-robin free list.
+Everything else in the keystore is indexed by these block numbers,
+which means that the index array is the only data structure which the
+keystore code needs to sort or rotate when adding, removing, or
+updating a block.  Because the block numbers themselves are small
+integers, the index array itself is small enough that shuffling data
+within it using <code>memmove()</code> is a relatively cheap operation, which in
+turn avoids a lot of complexity that would be involved in managing
+more sophisticated data structures.</p>
+
+<p>The keystore code includes both caching of recently used keystore
+blocks (to avoid unnecessary flash reads) and caching of the location
+of the block corresponding to a particular UUID (to avoid unnecessary
+index searches).  Aside from whatever direct performance benefits this
+might bring, this also frees the pkey layer that sits directly on top
+of the keystore code from needing to keep a lot of active state on
+particular keystore objects, which is important given that this whole
+thing sits under an RPC protocol driven by a client program which can
+impose arbitrary delays between any two operations at the pkey layer.</p>
+
+<h2>Key backup</h2>
+
+<p>The key backup mechanism is a straightforward three-step process,
+mediated by a Python script which uses the Python client
+implementation of the RPC mechanism.  Steps:</p>
+
+<ol>
+<li><p>Destination HSM (target of key transfer) generates an RSA keypair,
+exports the public key (the "key encryption key encryption key" or
+"KEKEK").</p></li>
+<li><p>Source HSM (origin of the key transfer) wraps keys to be backed up
+using AES keywrap with key encryption keys (KEKs) generated by the
+TRNG; these key encryption keys are in turn encrypted with RSA
+public key (KEKEK) generated by the receipient HSM.</p></li>
+<li><p>Destination HSM receives wrapped keys, unwraps the KEKs using the
+KEKEK then unwraps the wrapped private keys.</p></li>
+</ol>
+
+<p>Transfer of the wrapped keys between the two HSMs can be by any
+convenient mechanism; for simplicity, <code>cryptech_backup</code> script bundles
+everything up in a text file using JSON and Base64 encoding.</p>
+
+<h2>Multiplexer daemon</h2>
+
+<p>While the C client library can be built to talk directly to the
+Cryptech Alpha board, in most cases it is more convenient to use the
+<code>cryptech_muxd</code> multiplexer daemon, which is now the default.  Client
+code talks to <code>cryptech_muxd</code> via a <code>PF_UNIX</code> socket; <code>cryptech_muxd</code>
+handles interleaving of messages between multiple clients, and also
+manages access to the Alpha's console port.</p>
+
+<p>The multiplexer requires two external Python libraries, Tornado
+(version 4.0 or later) and PySerial (version 3.0 or later).</p>
+
+<p>In the long run, the RPC mechanism will need to be wrapped in some
+kind of secure channel protocol, but we're not there yet.</p>
+
+<h2>API</h2>
+
+<p>Yeah, we ought to document the API, Real Soon Now, perhaps using
+<a href="http://www.doxygen.org/">Doxygen</a>.  For the moment, see the function prototypes in hal.h,
+the Python definitions in cryptech.libhal, and and comments in the
+code.</p>
+}}}
+
+[[RepositoryIndex(format=table,glob=sw/libhal)]]
+
+|| Clone `https://git.cryptech.is/sw/libhal.git` ||