Restructure pyspx conditional import

Import third-party pyspx package in securesystemslib.spx_keys
only and require whoever imports securesystemslib.spx_keys to
handle ImportError (or IOError in case of missing C backend).

See securesystemslib.keys for exemplary import handling.

This commit also moves three spx-specific schema definitions from
securesystemslib.formats to securesystemslib.spx_keys, so that
the former does not have to deal with a conditional import of
an optional library.

securesystemslib/formats.py

@@ -79,14 +79,6 @@
 import time
 import six
 
-# Try to import pyspx to get access to lengths of SPX signatures and keys
-try:
-  import pyspx.shake256_192s as pyspx
-
-# pyspx's 'cffi' dependency may raise an 'IOError' exception when importing
-except (ImportError, IOError): # pragma: no cover
-  pass
-
 import securesystemslib.schema as SCHEMA
 import securesystemslib.exceptions
 
@@ -300,15 +292,6 @@
 # An ED25519 raw signature, which must be 64 bytes.
 ED25519SIGNATURE_SCHEMA = SCHEMA.LengthBytes(64)
 
-# Lengths of SPX raw keys and signatures
-try:
-    SPXPUBLIC_SCHEMA = SCHEMA.LengthBytes(pyspx.crypto_sign_PUBLICKEYBYTES)
-    SPXSEED_SCHEMA = SCHEMA.LengthBytes(pyspx.crypto_sign_SECRETKEYBYTES)
-    SPXSIGNATURE_SCHEMA = SCHEMA.LengthBytes(pyspx.crypto_sign_BYTES)
-
-except NameError:  # pragma: no cover
-    pass  # raised when pyspx was not available on import
-
 # An ECDSA signature.
 ECDSASIGNATURE_SCHEMA = SCHEMA.AnyBytes()
 

securesystemslib/keys.py

@@ -102,7 +102,8 @@
 try:
   import securesystemslib.spx_keys
 
-except ImportError: #pragma: no cover
+# pyspx's 'cffi' dependency may raise an 'IOError' exception when importing
+except (ImportError, IOError): #pragma: no cover
   pass
 
 

securesystemslib/spx_keys.py

@@ -43,37 +43,36 @@
 
 # Import the pyspx library, if available. This library is required to use
 # spx signatures.
-#
-# Note: A 'pragma: no cover' comment is intended for test 'coverage'.  Lines
-# or code blocks with this comment should not be flagged as uncovered.
-try:
-  import pyspx.shake256_192s as pyspx
-
-# pyspx's 'cffi' dependency may raise an 'IOError' exception when importing
-except (ImportError, IOError): # pragma: no cover
-  pass
+import pyspx.shake256_192s as pyspx
 
 import securesystemslib.formats
 import securesystemslib.exceptions
+import securesystemslib.schema as SCHEMA
 
 # Supported spx signing schemes: 'spx'.  
 _SUPPORTED_SPX_SIGNING_SCHEMES = ['spx']
 
+# Define lengths of SPX keys and signature bytes
+# NOTE: Define module scope schemas here to avoid conditional imports of
+# optional 'pyspx' package in 'formats' module. ImportError and IOError should
+# be handled by whoever imports this 'spx_keys' module.
+SPX_PUBLIC_BYTES_SCHEMA = SCHEMA.LengthBytes(pyspx.crypto_sign_PUBLICKEYBYTES)
+SPX_PRIVATE_BYTES_SCHEMA = SCHEMA.LengthBytes(pyspx.crypto_sign_SECRETKEYBYTES)
+SPX_SIG_BYTES_SCHEMA = SCHEMA.LengthBytes(pyspx.crypto_sign_BYTES)
 
 def generate_public_and_private():
   """
   <Purpose>
     Generate a pair of spx public and private keys with pyspx.  The public
-    and private keys returned conform to
-    'securesystemslib.formats.SPXPUBLIC_SCHEMA' and
-    'securesystemslib.formats.SPXSEED_SCHEMA', respectively.
+    and private keys returned conform to 'SPX_PUBLIC_BYTES_SCHEMA' and
+    'SPX_PRIVATE_BYTES_SCHEMA', respectively.
 
     An spx seed key is a random 128-byte string. Public keys are 64 bytes.
 
     >>> public, private = generate_public_and_private()
-    >>> securesystemslib.formats.SPXPUBLIC_SCHEMA.matches(public)
+    >>> SPX_PUBLIC_BYTES_SCHEMA.matches(public)
     True
-    >>> securesystemslib.formats.SPXSEED_SCHEMA.matches(private)
+    >>> SPX_PRIVATE_BYTES_SCHEMA.matches(private)
     True
 
   <Arguments>
@@ -91,8 +90,8 @@ def generate_public_and_private():
 
   <Returns>
     A (public, private) tuple that conform to
-    'securesystemslib.formats.SPXPUBLIC_SCHEMA' and
-    'securesystemslib.formats.SPXSEED_SCHEMA', respectively.
+    'SPX_PUBLIC_BYTES_SCHEMA' and
+    'SPX_PRIVATE_BYTES_SCHEMA', respectively.
   """
 
   # Generate spx's seed key by calling os.urandom().  The random bytes
@@ -120,20 +119,20 @@ def create_signature(public_key, private_key, data, scheme):
   <Purpose>
     Return a (signature, scheme) tuple, where the signature scheme is 'spx'
     and is always generated by pyspx.  The signature returned
-    conforms to 'securesystemslib.formats.SPXSIGNATURE_SCHEMA'.
+    conforms to 'SPX_SIG_BYTES_SCHEMA'.
 
     >>> public, private = generate_public_and_private()
     >>> data = b'The quick brown fox jumps over the lazy dog'
     >>> scheme = 'spx'
     >>> signature, scheme = \
         create_signature(public, private, data, scheme)
-    >>> securesystemslib.formats.SPXSIGNATURE_SCHEMA.matches(signature)
+    >>> SPX_SIG_BYTES_SCHEMA.matches(signature)
     True
     >>> scheme == 'spx'
     True
     >>> signature, scheme = \
         create_signature(public, private, data, scheme)
-    >>> securesystemslib.formats.SPXSIGNATURE_SCHEMA.matches(signature)
+    >>> SPX_SIG_BYTES_SCHEMA.matches(signature)
     True
     >>> scheme == 'spx'
     True
@@ -163,17 +162,9 @@ def create_signature(public_key, private_key, data, scheme):
   <Returns>
     A signature dictionary conformat to 'securesystemslib.format.SIGNATURE_SCHEMA'.  
   """
-
-  # Does 'public_key' have the correct format?
-  # This check will ensure 'public_key' conforms to
-  # 'securesystemslib.formats.SPXPUBLIC_SCHEMA', which must have length 32
-  # bytes.  Raise 'securesystemslib.exceptions.FormatError' if the check fails.
-  securesystemslib.formats.SPXPUBLIC_SCHEMA.check_match(public_key)
-
-  # Is 'private_key' properly formatted?
-  securesystemslib.formats.SPXSEED_SCHEMA.check_match(private_key)
-
-  # Is 'scheme' properly formatted?
+  # Validate arguments
+  SPX_PUBLIC_BYTES_SCHEMA.check_match(public_key)
+  SPX_PRIVATE_BYTES_SCHEMA.check_match(private_key)
   securesystemslib.formats.SPX_SIG_SCHEMA.check_match(scheme)
 
   # Signing the 'data' object requires a seed and public key.
@@ -233,13 +224,13 @@ def verify_signature(public_key, scheme, signature, data):
 
   <Arguments>
     public_key:
-      The public key is a simple byte string of length spx.crypto_sign_PUBLICKEYBYTES
+      The public key is a simple byte string of length SPX_PUBLIC_BYTES_SCHEMA.
 
     scheme:
       'spx' signature scheme
 
     signature:
-      The signature is a simple byte string of length spx.crypto_sign_BYTES
+      The signature is a simple byte string of length SPX_SIG_BYTES_SCHEMA.
 
     data:
       Data object used by securesystemslib.spx_keys.create_signature() to
@@ -259,18 +250,11 @@ def verify_signature(public_key, scheme, signature, data):
   <Returns>
     Boolean.  True if the signature is valid, False otherwise.
   """
-
-  # Does 'public_key' have the correct format?
-  # This check will ensure 'public_key' conforms to
-  # 'securesystemslib.formats.SPXPUBLIC_SCHEMA', bytes.  
-  # Raise 'securesystemslib.exceptions.FormatError' if the check fails.
-  securesystemslib.formats.SPXPUBLIC_SCHEMA.check_match(public_key)
-
-  # Is 'scheme' properly formatted?
+  # Validate arguments
+  SPX_PUBLIC_BYTES_SCHEMA.check_match(public_key)
+  SPX_SIG_BYTES_SCHEMA.check_match(signature)
   securesystemslib.formats.SPX_SIG_SCHEMA.check_match(scheme)
 
-  # Is 'signature' properly formatted?
-  securesystemslib.formats.SPXSIGNATURE_SCHEMA.check_match(signature)
 
   # Verify 'signature'.  Before returning the Boolean result, ensure 'spx'
   # was used as the signature scheme.  Raise

tests/test_spx_keys.py

@@ -48,8 +48,8 @@ def test_generate_public_and_private(self):
     pub, priv = securesystemslib.spx_keys.generate_public_and_private()
 
     # Check format of 'pub' and 'priv'.
-    self.assertEqual(True, securesystemslib.formats.SPXPUBLIC_SCHEMA.matches(pub))
-    self.assertEqual(True, securesystemslib.formats.SPXSEED_SCHEMA.matches(priv))
+    self.assertEqual(True, securesystemslib.spx_keys.SPX_PUBLIC_BYTES_SCHEMA.matches(pub))
+    self.assertEqual(True, securesystemslib.spx_keys.SPX_PRIVATE_BYTES_SCHEMA.matches(priv))
 
 
 
@@ -63,7 +63,7 @@ def test_create_signature(self):
 
     # Verify format of returned values.
     self.assertEqual(True,
-        securesystemslib.formats.SPXSIGNATURE_SCHEMA.matches(signature))
+        securesystemslib.spx_keys.SPX_SIG_BYTES_SCHEMA.matches(signature))
 
     self.assertEqual(True, securesystemslib.formats.SPX_SIG_SCHEMA.matches(scheme))
     self.assertEqual('spx', scheme)