diff --git a/CLAUDE.md b/CLAUDE.md
index 485c997..7c07e14 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -327,6 +327,13 @@ vectors). The runner invokes `require("crypto.<module>").selftest()` per module:
 make test-matrix            # across Lua 5.1–5.4 + LuaJIT
 ```
 
+`crypto.selftest()` fails any module whose selftest leaves the acceleration
+flag changed: a driver that enabled acceleration and then ran it would be left
+unaccelerated, which on Control4 makes SRP unusable. A selftest that toggles the
+flag restores what `openssl_wrapper.enabled()` returned, not the
+`CRYPTO_USE_OPENSSL` default. The runner's lpack pass flips the flag, so every
+run covers both settings.
+
 ## Building
 
 `amalg` produces two single-file distributions from `src/crypto/init.lua`:
diff --git a/run_tests.sh b/run_tests.sh
index fd7554a..88160b5 100755
--- a/run_tests.sh
+++ b/run_tests.sh
@@ -148,10 +148,13 @@ run_selftest "Ed25519"            "ed25519"           "crypto.ed25519"
 run_selftest "OpenSSL gating"     "openssl_wrapper"   "crypto.openssl_wrapper"
 
 # Control4's LuaJIT has string.pack/unpack as lpack, a different dialect: every
-# selftest again with that shape installed before bitn loads.
+# selftest again with that shape installed before bitn loads, and with the
+# acceleration flag flipped so every run covers both settings.
 run_test "All selftests with lpack-shaped string.pack" "lpack" "
     dofile('$script_dir/test/lpack_stub.lua')
-    if not require('crypto').selftest() then
+    local crypto = require('crypto')
+    crypto.use_openssl(not crypto.openssl_wrapper.enabled())
+    if not crypto.selftest() then
         os.exit(1)
     end
   "
diff --git a/src/crypto/bignum.lua b/src/crypto/bignum.lua
index d48a37c..dd98911 100644
--- a/src/crypto/bignum.lua
+++ b/src/crypto/bignum.lua
@@ -177,6 +177,10 @@ end
 --- @param b BigNum Subtrahend (must not exceed a)
 --- @return BigNum difference
 local function sub_raw(a, b)
+  -- The loop reads only #a limbs of b, and with canonical operands a wider b is a larger b.
+  if #b > #a then
+    error("bignum: subtraction would be negative")
+  end
   local r = {}
   local borrow = 0
   local na = #a
@@ -1204,6 +1208,7 @@ function bignum.selftest()
   -- Snapshot the OpenSSL state so the accelerator tests cannot leak.
   local saved_loaded = package.loaded["openssl"]
   local saved_preload = package.preload["openssl"]
+  local saved_use = openssl_wrapper.enabled()
 
   --- Build a stand-in lua-openssl binding. Real lua-openssl is not installed
   --- here, so this validates the *routing* and the bytes-in/hex-out conversion,
@@ -1437,7 +1442,19 @@ function bignum.selftest()
     {
       name = "sub - rejects a negative result",
       test = function()
+        -- Equal widths too: only the final borrow can catch those.
         return pcall(bignum.sub, Y, X) == false
+          and pcall(bignum.sub, from_number(5), from_number(7)) == false
+          and pcall(bignum.sub, Y, bignum.add(Y, bignum.one())) == false
+      end,
+    },
+    {
+      name = "sub - rejects a wider subtrahend whose low limbs do not borrow",
+      test = function()
+        return pcall(bignum.sub, from_number(5), from_hex("1000000")) == false
+          and pcall(bignum.sub, from_hex("ffffff"), from_hex("1000000")) == false
+          and pcall(bignum.sub, X, bignum.add(X, from_hex("1" .. string_rep("0", 66)))) == false
+          and to_hex(bignum.sub(from_hex("1000000"), from_hex("ffffff"))) == "1"
       end,
     },
     {
@@ -1756,10 +1773,10 @@ function bignum.selftest()
     end
   end
 
-  -- Restore the module-load default so later tests in this process are unaffected.
+  -- Restore the caller's state, acceleration choice included, so later tests in this process are unaffected.
   package.loaded["openssl"] = saved_loaded
   package.preload["openssl"] = saved_preload
-  openssl_wrapper.use(os.getenv("CRYPTO_USE_OPENSSL") == "1" or os.getenv("CRYPTO_USE_OPENSSL") == "true")
+  openssl_wrapper.use(saved_use)
 
   print(string_format("\nBignum result: %d/%d tests passed\n", passed, #tests))
   return passed == #tests
diff --git a/src/crypto/init.lua b/src/crypto/init.lua
index b7ad931..c3d00b8 100644
--- a/src/crypto/init.lua
+++ b/src/crypto/init.lua
@@ -126,9 +126,15 @@ function crypto.selftest()
   for _, name in ipairs(modules) do
     local mod = crypto[name]
     if type(mod.selftest) == "function" then
+      local accelerated = openssl_wrapper.enabled()
       if mod.selftest() == false then
         ok = false
       end
+      -- A host that enabled acceleration and then ran this must still have it.
+      if openssl_wrapper.enabled() ~= accelerated then
+        print("crypto." .. name .. ".selftest() did not restore the OpenSSL acceleration flag")
+        ok = false
+      end
     end
   end
   return ok
diff --git a/src/crypto/openssl_wrapper.lua b/src/crypto/openssl_wrapper.lua
index a43501f..a3b8b78 100644
--- a/src/crypto/openssl_wrapper.lua
+++ b/src/crypto/openssl_wrapper.lua
@@ -172,6 +172,13 @@ function openssl_wrapper.use(use)
   _openssl_unavailable = false
 end
 
+--- Whether acceleration is requested, as last set by `use`. Snapshot it before toggling the flag temporarily.
+--- @return boolean enabled
+function openssl_wrapper.enabled()
+  -- Truthy like `get`, not `== true`, or restoring this answer could switch acceleration off.
+  return _use_openssl and true or false
+end
+
 --- Parse semantic version string into comparable components
 --- @param version_str string Version string like "0.9.1" or "1.0.0-rc1"
 --- @return number major Major version number
@@ -493,6 +500,16 @@ function openssl_wrapper.selftest()
         return openssl_wrapper.get(OpenSSLFeature.AAD) == nil
       end,
     },
+    {
+      name = "enabled reports what use last set",
+      test = function()
+        openssl_wrapper.use(true)
+        local on = openssl_wrapper.enabled()
+        openssl_wrapper.use(false)
+        local off = openssl_wrapper.enabled()
+        return on == true and off == false
+      end,
+    },
     {
       name = "absent binding falls back gracefully",
       test = function()
diff --git a/src/crypto/random.lua b/src/crypto/random.lua
index 081b3c2..5f0ad0d 100644
--- a/src/crypto/random.lua
+++ b/src/crypto/random.lua
@@ -218,6 +218,7 @@ function random.selftest()
   local saved_path = random._urandom_path
   local saved_preload = package.preload["openssl"]
   local saved_loaded = package.loaded["openssl"]
+  local saved_use = openssl_wrapper.enabled()
 
   --- Force both built-in sources to be unavailable.
   local function starve()
@@ -233,13 +234,13 @@ function random.selftest()
     random._urandom_path = "/nonexistent/crypto-random-selftest"
   end
 
-  --- Restore real detection, including the wrapper's initial opt-in state, so
+  --- Restore real detection, including the caller's acceleration choice, so
   --- running this inside `crypto.selftest()` cannot disturb a later module.
   local function unstarve()
     random.reset()
     package.loaded["openssl"] = saved_loaded
     package.preload["openssl"] = saved_preload
-    openssl_wrapper.use(os.getenv("CRYPTO_USE_OPENSSL") == "1" or os.getenv("CRYPTO_USE_OPENSSL") == "true")
+    openssl_wrapper.use(saved_use)
     random._urandom_path = saved_path
   end