Properly handle strings with internal nul

The strings from NaCl can contain nul characters inside the string
and are not nul terminated. Using ffi.buffer with an explict
size will prevent issues here.

nacl/hash.py

@@ -8,7 +8,7 @@ def sha256(message, binary=False):
     digest = nacl.ffi.new("unsigned char[]", nacl.lib.crypto_hash_sha256_BYTES)
     if not nacl.lib.crypto_hash_sha256(digest, message, len(message)):
         raise CryptoError("Hashing failed")
-    digest = nacl.ffi.string(digest)
+    digest = nacl.ffi.buffer(digest, nacl.lib.crypto_hash_sha256_BYTES)[:]
 
     if binary:
         return digest
@@ -19,7 +19,7 @@ def sha512(message, binary=False):
     digest = nacl.ffi.new("unsigned char[]", nacl.lib.crypto_hash_sha512_BYTES)
     if not nacl.lib.crypto_hash_sha512(digest, message, len(message)):
         raise CryptoError("Hashing failed")
-    digest = nacl.ffi.string(digest)
+    digest = nacl.ffi.buffer(digest, nacl.lib.crypto_hash_sha512_BYTES)[:]
 
     if binary:
         return digest

nacl/random.py

@@ -4,4 +4,4 @@ from . import nacl
 def random(size=32):
     data = nacl.ffi.new("unsigned char[]", size)
     nacl.lib.randombytes(data, size)
-    return nacl.ffi.string(data)
+    return nacl.ffi.buffer(data, size)[:]