[sw] Add OTBN app file to run RSA-3072 verification operations.

Signed-off-by: Jade Philipoom <jadep@google.com>

sw/device/silicon_creator/lib/crypto/rsa_3072/rsa_3072_verify.s

diff --git a/sw/device/silicon_creator/lib/crypto/rsa_3072/rsa_3072_verify.s b/sw/device/silicon_creator/lib/crypto/rsa_3072/rsa_3072_verify.s
new file mode 100644
index 0000000..6b0a5e8
--- /dev/null
+++ b/sw/device/silicon_creator/lib/crypto/rsa_3072/rsa_3072_verify.s
@@ -0,0 +1,54 @@
+/* Copyright lowRISC contributors. */
+/* Licensed under the Apache License, Version 2.0, see LICENSE for details. */
+/* SPDX-License-Identifier: Apache-2.0 */
+
+
+.section .text.start
+
+/**
+ * OTBN app to run RSA-3072 verification and constant precomputation.
+ */
+run_rsa_3072:
+  la    x2, mode
+  lw    x2, 0(x2)
+
+  li    x3, 1
+  beq   x2, x3, precomp
+
+  li    x3, 2
+  beq   x2, x3, verify
+
+  /* Mode is neither 1 (= precomp) nor 2 (= verify). Fail. */
+  unimp
+
+.text
+
+/**
+ * Precomputation of Montgomery constants R^2 and m0_inv.
+ *
+ * Expects the modulus (in_mod) to be pre-populated. Results will be stored in
+ * in_rr and in_m0_inv.
+ */
+precomp:
+  /* TODO: not yet implemented */
+  unimp
+
+/**
+ * RSA-3072 signature verification.
+ *
+ * Expects the RSA signature (in_buf), constants (in_rr, in_m0inv) and modulus
+ * (in_mod) to be pre-populated. Recovered message will be stored in out_buf.
+ */
+verify:
+  /* run modular exponentiation */
+  jal      x1, modexp_var_3072_f4
+
+  ecall
+
+.data
+
+/* Operation mode (1 = precomp; 2 = verify) */
+.globl mode
+.balign 4
+mode:
+  .zero 4