diff --git a/RSA.cabal b/RSA.cabal
--- a/RSA.cabal
+++ b/RSA.cabal
@@ -1,6 +1,6 @@
 name:       RSA
 category:   Cryptography, Codec
-version:    2.3.1
+version:    2.4.1
 license:    BSD3
 license-file: LICENSE
 author:     Adam Wick <awick@galois.com>
@@ -25,6 +25,8 @@
                    crypto-api          >= 0.10    && < 0.14,
                    crypto-pubkey-types >= 0.2     && < 0.6,
                    SHA                 >= 1.6.4.1 && < 2.0
+  if impl(ghc < 8.0)
+    build-depends: cipher-aes128       < 0.7.0.4
   exposed-modules: Codec.Crypto.RSA,
                    Codec.Crypto.RSA.Exceptions,
                    Codec.Crypto.RSA.Pure
@@ -41,7 +43,6 @@
                   bytestring                 >  0.8     && < 0.12,
                   crypto-api                 >= 0.10    && < 0.14,
                   crypto-pubkey-types        >= 0.4     && < 0.6,
-                  DRBG                       >= 0.5.2   && < 0.7,
                   QuickCheck                 >= 2.5     && < 3,
                   tagged                     >= 0.2     && < 0.9,
                   test-framework             >= 0.8.0.3 && < 0.10,
diff --git a/Test.hs b/Test.hs
--- a/Test.hs
+++ b/Test.hs
@@ -1,14 +1,12 @@
 import Codec.Crypto.RSA.Pure
 import Control.Monad
 import Data.Binary
-import qualified Data.ByteString as BSS
 import Data.ByteString.Lazy(ByteString)
 import qualified Data.ByteString.Lazy as BS
 import Data.Digest.Pure.SHA
 import System.IO
 import Test.QuickCheck
 import Crypto.Random
-import Crypto.Random.DRBG
 
 import Test.Framework (defaultMain, testGroup)
 import Test.Framework.Providers.QuickCheck2 (testProperty)
@@ -25,7 +23,7 @@
 main = do
   putStr   "Generating testing keys ... "
   hFlush   stdout
-  g :: GenAutoReseed HashDRBG HashDRBG <- newGenIO
+  g :: SystemRandom <- newGenIO
   let (keys, g') = buildRandomKeyPairs g (cycle keySizes) numRandomKeyPairs
   unless (all ((> 5) . public_n . fst) keys) $ fail "Something odd."
   putStrLn "done!"
@@ -37,7 +35,7 @@
     , testGroup "Testing basic helper functions" [
         testProperty "ByteString chunking works"    prop_chunkifyWorks
       , testProperty "Modular exponentiation works" prop_modExpWorks
-      , testProperty "Modular inversion works"      prop_modInvWorks
+      , testProperty "Modular inversion works"      (prop_modInvWorks g')
       ]
     , testGroup "Testing RSA core functions" [
         testProperty "Can roundtrip from Integer to BS and back" prop_i2o2iIdent
@@ -91,19 +89,6 @@
   arbitrary = elements [hashSHA1, hashSHA224,
                        hashSHA256, hashSHA384, hashSHA512]
 
-newtype LargePrime = LP Integer
-  deriving (Show)
-
-instance Arbitrary LargePrime where
-  arbitrary =
-    do seed <- BSS.pack `fmap` replicateM 4096 arbitrary
-       case newGen seed of
-        Left _ -> fail "DRBG initialization error."
-        Right (g :: HashDRBG) ->
-          case largeRandomPrime g 64 of
-            Left _ -> fail "Large prime generation failure."
-            Right (i, _) -> return (LP i)
-
 data KeyPairIdx = KPI Int
  deriving (Show)
 
@@ -149,12 +134,22 @@
   e' = getPositive e
   m' = getPositive m
 
-prop_modInvWorks :: LargePrime -> LargePrime -> Bool
-prop_modInvWorks (LP p) (LP q) = (e * d) `mod` phi == 1
- where 
-  e   = 65537
-  phi = (p - 1) * (q - 1)
-  d   = modular_inverse e phi
+prop_modInvWorks :: CryptoRandomGen g => g -> Word16 -> Bool
+prop_modInvWorks g0 x =
+  let (p, g1) = primeGen (x `mod` 512) g0
+      (q, _)  = primeGen (x `mod` 512) g1
+      e       = 65537
+      phi     = (p - 1) * (q - 1)
+      d       = modular_inverse e phi
+  in (e * d) `mod` phi == 1
+ where
+  primeGen pre g =
+    case randomBS g (fromIntegral pre) of
+      Left e -> error ("Error prefetching bytestring:" ++ show e)
+      Right (_, g') ->
+        case largeRandomPrime g' 64 of
+          Left  _   -> error "Large prime generation failure."
+          Right res -> res
 
 prop_i2o2iIdent :: Positive Integer -> Bool
 prop_i2o2iIdent px =
diff --git a/src/Codec/Crypto/RSA/Pure.hs b/src/Codec/Crypto/RSA/Pure.hs
--- a/src/Codec/Crypto/RSA/Pure.hs
+++ b/src/Codec/Crypto/RSA/Pure.hs
@@ -112,7 +112,7 @@
               return (PrivateKey pub d 0 0 0 0 0)
 
 failOnError :: (Monad m, Show a) => Either a b -> m b
-failOnError (Left e)  = fail (show e)
+failOnError (Left e)  = error (show e)
 failOnError (Right b) = return b
 
 -- ----------------------------------------------------------------------------