OTBN PKA Public Key Accelerator

The OTBN host path is the most complex OpenTitan path. The crypto interconnect applies a CACHE_MODIFIABLE bit fix before the axi_dw_converter, because the VeeR CPU may emit non-modifiable cache attributes for MMIO accesses that would be rejected by the AXI-to-AXI-Lite converter. After data-width conversion and AXI-Lite isolation, sep_crypto_otbn_wrapper bridges the request to TL-UL and drives the standard otbn core configured with Stub = 0 and RegFile = RegFileFF.

OTBN’s instruction and data memories are external SRAM macros; the IMEM and DMEM prim_ram_1p_scr ports are brought out of the wrapper to the sep_crypto top level. The two random-number request ports (edn_rnd, edn_urnd) are served over native-protocol EDN by the shared drbg_axis_edn_adapter instance u_axis_edn_crypto_s3c_scan in sep_crypto, which converts the post-CSRNG DRBG AXI-Stream feed assigned to the crypto sink domain into per-client EDN responses. The adapter forwards the per-beat FIPS tag through a generic AXI-Stream tuser sideband. The CSRNG seed tag and external entropy mux tag are tied high by the integration’s provisional policy. A high tag prevents a FIPS-tag failure for that response but does not prove that the entropy is FIPS compliant. The OTP scramble key handshake is tied to fixed values. Life-cycle escalation and RMA request signals are tied to their inactive state. OTBN occupies alert indices [2:1] in OpenTitan Alert Termination and exports a single intr_done_o interrupt.

The Key Manager delivers a 384-bit OTBN key as twelve words per share, the widest delivery in the subsystem, in the format given in Key Manager Key Delivery. OTBN is the only accelerator that does not combine the shares in hardware. The shares are exposed to the executing program through the KEY_S0_L / KEY_S0_H and KEY_S1_L / KEY_S1_H special registers, each share split into a 256-bit low and a 128-bit high half, and the program is responsible for reconstructing the key as the XOR of the two shares. Each share is qualified independently: reading a share whose key is not valid sets ERR_BITS.KEY_INVALID and stops the program rather than returning key material.

Consult the OpenTitan OTBN documentation for the authoritative core functionality of the IP. The Programmer’s Guide gives the programming guidance for this block. The register map follows below.