[kmac] Switch to Trivium-based PRNG implementation

This commit switches the LFSR-based PRNG with an unrolled, Trivium-based
PRNG implementation to avoid brute-forcing attacks on the LFSR states.

The overall PRNG state decreases from 800 bits to 288 bits but due
to the heavy unrolling, the primitive can still generate 800 bits per
cycle as required by the masked SHA3 core.

This resolves lowRISC#20828.

Signed-off-by: Pirmin Vogel <[email protected]>

hw/ip/kmac/data/kmac.hjson

@@ -145,29 +145,29 @@
       desc:    "Width of the hash counter in the entropy"
       local:   "true"
     }
-    { name:    "NumSeedsEntropyLfsr",
+    { name:    "NumSeedsEntropy",
       type:    "int",
-      default: "5",
-      desc:    "Number of words for the LFSR seed used for entropy generation",
+      default: "9",
+      desc:    "Number of words for the PRNG seed used for entropy generation",
       local:   "true"
     }
     { name:      "RndCnstLfsrSeed"
-      desc:      "Compile-time random data for LFSR default seed"
+      desc:      "Compile-time random data for PRNG default seed"
       type:      "kmac_pkg::lfsr_seed_t"
-      randcount: "800"
+      randcount: "288"
       randtype:  "data"
     }
     { name:      "RndCnstLfsrPerm",
-      desc:      "Compile-time random permutation for LFSR output",
+      desc:      "Compile-time random permutation for PRNG output",
       type:      "kmac_pkg::lfsr_perm_t"
       randcount: "800",
       randtype:  "perm",
     }
-    { name:      "RndCnstLfsrFwdPerm",
-      desc:      "Compile-time random permutation for forwarding LFSR state",
-      type:      "kmac_pkg::lfsr_fwd_perm_t"
-      randcount: "32",
-      randtype:  "perm",
+    { name:      "RndCnstBufferLfsrSeed"
+      desc:      "Compile-time random data for PRNG buffer default seed"
+      type:      "kmac_pkg::buffer_lfsr_seed_t"
+      randcount: "800"
+      randtype:  "data"
     }
     { name:      "RndCnstMsgPerm"
       desc:      "Compile-time random permutation for LFSR Message output"
@@ -802,36 +802,30 @@
         }
       ]
     } // R: ENTROPY_REFRESH_THRESHOLD_SHADOWED
-    { multireg: {
-        name: "ENTROPY_SEED"
-        desc: '''Entropy Seed
+    {
+      name: "ENTROPY_SEED"
+      desc: '''Entropy Seed
 
-              Entropy seed registers for the integrated entropy generator.
+            Entropy seed register for the integrated entropy generator.
 
-              If !!CFG_SHADOWED.entropy_mode is set to sw_mode, software first needs to set
-              !!CFG_SHADOWED.entropy_ready and then write the !!ENTROPY_SEED_0 -
-              !!ENTROPY_SEED_4 registers in ascending order. Software writes one 32-bit value
-              to every register which is subsequently loaded into the corresponding 32-bit LFSR
-              chunk of the entropy generator.
+            If !!CFG_SHADOWED.entropy_mode is set to sw_mode, software first needs to set !!CFG_SHADOWED.entropy_ready.
+            Then, software needs to write the !!ENTROPY_SEED register 9 times.
+            Upon each write, the written value is loaded into the corresponding state chunk of the entropy generator.
 
-              After writing all !!ENTROPY_SEED_0 registers, the entropy generator will start
-              its operation. After this point, writing these registers has no longer any
-              effect.
-              '''
-        count: "NumSeedsEntropyLfsr"
-        cname: "KMAC"
-        hwext: "true"
-        hwqe : "true"
-        swaccess: "wo"
-        hwaccess: "hro"
-        fields: [
-          { bits: "31:0"
-            name: "seed"
-            desc: "32-bit chunk of the entropy generator seed"
-          }
-        ]
-      } // R: ENTROPY_SEED
-    } // multireg: ENTROPY_SEED
+            After writing the !!ENTROPY_SEED register 9 times, the entropy generator will start its operation.
+            After this point, writing this register has no longer any effect.
+            '''
+      hwext: "true"
+      hwqe : "true"
+      swaccess: "wo"
+      hwaccess: "hro"
+      fields: [
+        { bits: "31:0"
+          name: "seed"
+          desc: "32-bit chunk of the entropy generator seed"
+        }
+      ]
+    } // R: ENTROPY_SEED
     { multireg: {
         name: "KEY_SHARE0"
         desc: '''KMAC Secret Key