[util] Remove expand_seed routine.

This routine wasn't currently affecting the logic, but would have weakened
certain seeds that happened to have low values. Ideally this should all be
refactored to use byte strings so it's clearer what range the value is sampled
from, but for now just removing the harmful logic should suffice.

Signed-off-by: Jade Philipoom <[email protected]>

util/design/lib/LcStEnc.py

@@ -8,9 +8,8 @@
 from collections import OrderedDict
 
 from Crypto.Hash import cSHAKE128
-from lib.common import (check_int, ecc_encode, expand_seed, get_hd,
-                        hd_histogram, is_valid_codeword, random_or_hexvalue,
-                        scatter_bits)
+from lib.common import (check_int, ecc_encode, get_hd, hd_histogram,
+                        is_valid_codeword, random_or_hexvalue, scatter_bits)
 from topgen import secure_prng as sp
 
 # Seed diversification constant for LcStEnc (this enables to use
@@ -288,7 +287,7 @@ def __init__(self, config):
         log.info('Seed: {0:x}'.format(config['seed']))
         log.info('')
 
-        sp.reseed(expand_seed(LC_SEED_DIVERSIFIER + int(config['seed'])))
+        sp.reseed(LC_SEED_DIVERSIFIER + int(config['seed']))
 
         log.info('Checking SECDED.')
         _validate_secded(config)

util/design/lib/OtpMemImg.py

@@ -224,7 +224,7 @@ def __init__(self, lc_state_config, otp_mmap_config, img_config,
         log.info('')
 
         # Re-initialize with seed to make results reproducible.
-        sp.reseed(common.expand_seed(OTP_IMG_SEED_DIVERSIFIER + int(img_config['seed'])))
+        sp.reseed(OTP_IMG_SEED_DIVERSIFIER + int(img_config['seed']))
 
         if 'partitions' not in img_config:
             raise RuntimeError('Missing partitions key in configuration.')

util/design/lib/OtpMemMap.py

@@ -8,7 +8,7 @@
 import logging as log
 from math import ceil, log2
 
-from lib.common import check_bool, check_int, expand_seed, random_or_hexvalue
+from lib.common import check_bool, check_int, random_or_hexvalue
 from mubi.prim_mubi import is_width_valid, mubi_value_as_int
 from tabulate import tabulate
 from topgen import secure_prng as sp
@@ -405,7 +405,7 @@ def __init__(self, config):
         config["seed"] = check_int(config["seed"])
 
         # Initialize RNG.
-        sp.reseed(expand_seed(OTP_SEED_DIVERSIFIER + int(config['seed'])))
+        sp.reseed(OTP_SEED_DIVERSIFIER + int(config['seed']))
         log.info('Seed: {0:x}'.format(config['seed']))
         log.info('')
 

util/design/lib/common.py

@@ -7,7 +7,7 @@
 import re
 import sys
 import textwrap
-from math import ceil, log, log2
+from math import ceil, log2
 from pathlib import Path
 
 sys.path.append(os.path.join(os.path.dirname(__file__), '../../'))
@@ -116,19 +116,6 @@ def blockify(s, size, limit):
     return (",\n  ".join(s_list))
 
 
-def expand_seed(seed):
-    '''Checks if the input seed is shorter than 256 bits and expands it if
-    it's not.
-    '''
-    new_seed = seed
-    seed_bytes = ceil(log(seed + 1, 256))
-    while new_seed < (1 << 256):
-        new_seed <<= seed_bytes * 8
-        new_seed += seed
-    new_seed %= (1 << 256)
-    return new_seed
-
-
 def get_random_perm_hex_literal(numel):
     '''Compute a random permutation of 'numel' elements and
     return as packed hex literal.'''

util/topgen.py

@@ -19,7 +19,6 @@
 import hjson
 import tlgen
 import version_file
-from design.lib.common import expand_seed
 from ipgen import (IpBlockRenderer, IpConfig, IpDescriptionOnlyRenderer,
                    IpTemplate, TemplateRenderError)
 from mako import exceptions
@@ -1081,7 +1080,8 @@ def main():
     elif "rnd_cnst_seed" not in topcfg:
         log.error('Seed "rnd_cnst_seed" not found in configuration HJSON.')
         exit(1)
-    secure_prng.reseed(expand_seed(topcfg["rnd_cnst_seed"]))
+
+    secure_prng.reseed(topcfg["rnd_cnst_seed"])
 
     # TODO, long term, the levels of dependency should be automatically
     # determined instead of hardcoded.  The following are a few examples:

util/topgen/secure_prng.py

@@ -173,23 +173,25 @@ def reseed(self, seed):
         from previous instantiation.
         """
         if seed == 0:
-            entropy_input = seed.to_bytes(32, 'big')
             log.error("ERROR: PRNG seeded with 0.")
             sys.exit(1)
-        # Check if the provided seed is at least 32 bytes long.
-        elif sys.getsizeof(seed) < sys.getsizeof(1 << 255):
-            # Error out if seed is shorter than 256 bits.
-            log.error("ERROR: Seed shorter than 256 bits.")
-            sys.exit(1)
+
+        if seed.bit_length() < 250:
+            # Warn, but don't fail, because the DV logic always passes in 32-bit
+            # seeds and this can naturally happen about 1% of the time.
+            log.warn('PRNG seed is only {seed.bit_length()} bits long, which is '
+                     'unlikely for a sample from a 256-bit distribution. Please '
+                     'double-check the logic.')
         # Check if the seed is longer than 256 bits. Trim the excess bits and
         # issue a warning if it is.
-        elif seed > (1 << 256):
+        if seed.bit_length() > 256:
             new_seed = seed % (1 << 256)
             log.warning("Seed longer than 256 bits. CTR_DRBG seeded with: " +
                         hex(new_seed))
             entropy_input = new_seed.to_bytes(32, 'big')
         else:
             entropy_input = seed.to_bytes(32, 'big')
+
         self.CTR_DRBG_Instantiate(entropy_input)
         self.returned_bits = []