diff --git a/securesystemslib/keys.py b/securesystemslib/keys.py
index 00f98e39d..dc860c682 100755
--- a/securesystemslib/keys.py
+++ b/securesystemslib/keys.py
@@ -70,8 +70,9 @@
 # signatures.
 try:
   import securesystemslib.pyca_crypto_keys
-
+  USE_PYCA = True
 except ImportError: #pragma: no cover
+  USE_PYCA = False
   pass
 
 # Import the PyNaCl library, if available.  It is recommended this library be
@@ -100,8 +101,9 @@
 
 try:
   import securesystemslib.ecdsa_keys
-
+  USE_ECDSA = True
 except ImportError: #pragma: no cover
+  USE_ECDSA = False
   pass
 
 import securesystemslib.exceptions
@@ -200,6 +202,10 @@ def generate_rsa_key(bits=_DEFAULT_RSA_KEY_BITS, scheme='rsassa-pss-sha256'):
     Conforms to 'securesystemslib.formats.RSAKEY_SCHEMA'.
   """
 
+  if not USE_PYCA: # pragma: no cover
+    raise securesystemslib.exceptions.UnsupportedLibraryError(
+      'The cryptography module is required to support RSA keys')
+
   # Does 'bits' have the correct format?  This check will ensure 'bits'
   # conforms to 'securesystemslib.formats.RSAKEYBITS_SCHEMA'.  'bits' must be
   # an integer object, with a minimum value of 2048.  Raise
@@ -287,6 +293,11 @@ def generate_ecdsa_key(scheme='ecdsa-sha2-nistp256'):
     Conforms to 'securesystemslib.formats.ECDSAKEY_SCHEMA'.
   """
 
+  # If the ecdsa module wasn't imported, we can't proceed with this method
+  if not USE_ECDSA: # pragma: no cover
+    raise securesystemslib.exceptions.UnsupportedLibraryError(
+      'The cryptography module is required to support ECDSA keys')
+
   # Does 'scheme' have the correct format?
   # This check will ensure 'scheme' is properly formatted and is a supported
   # ECDSA signature scheme.  Raise 'securesystemslib.exceptions.FormatError' if
@@ -723,6 +734,10 @@ def create_signature(key_dict, data):
   sig = None
 
   if keytype == 'rsa':
+    # If the ecdsa module wasn't imported, we can't handle this type of key
+    if not USE_PYCA: # pragma: no cover
+      raise securesystemslib.exceptions.UnsupportedLibraryError(
+        'The cryptography module is required to support RSA keys')
     if scheme in RSA_SIGNATURE_SCHEMES:
       private = private.replace('\r\n', '\n')
       sig, scheme = securesystemslib.pyca_crypto_keys.create_rsa_signature(
@@ -733,12 +748,20 @@ def create_signature(key_dict, data):
         ' RSA signature scheme specified: ' + repr(scheme))
 
   elif keytype == 'ed25519':
+    if not USE_PYNACL: # pragma: no cover
+      raise securesystemslib.exceptions.UnsupportedLibraryError(
+        'The nacl module is required to support ed25519 signing')
     public = binascii.unhexlify(public.encode('utf-8'))
     private = binascii.unhexlify(private.encode('utf-8'))
     sig, scheme = securesystemslib.ed25519_keys.create_signature(
         public, private, data, scheme)
 
   elif keytype == 'ecdsa-sha2-nistp256':
+    # If the ecdsa module wasn't imported, we can't handle this type of key
+    if not USE_ECDSA: # pragma: no cover
+      raise securesystemslib.exceptions.UnsupportedLibraryError(
+        'The cryptography module is required to support ECDSA keys')
+
     sig, scheme = securesystemslib.ecdsa_keys.create_signature(
         public, private, data, scheme)
 
@@ -864,6 +887,9 @@ def verify_signature(key_dict, signature, data):
 
 
   if keytype == 'rsa':
+    if not USE_PYCA: # pragma: no cover
+      raise securesystemslib.exceptions.UnsupportedLibraryError(
+        'The cryptography module is required to support RSA keys')
     if scheme in RSA_SIGNATURE_SCHEMES:
       valid_signature = securesystemslib.pyca_crypto_keys.verify_rsa_signature(sig,
         scheme, public, data)
@@ -884,6 +910,10 @@ def verify_signature(key_dict, signature, data):
 
   elif keytype == 'ecdsa-sha2-nistp256':
     if scheme == 'ecdsa-sha2-nistp256':
+      if not USE_ECDSA: # pragma: no cover
+        raise securesystemlib.exceptions.UnsupportedLibraryError(
+          'The cryptography module is required to support ECDSA keys')
+
       valid_signature = securesystemslib.ecdsa_keys.verify_signature(public,
         scheme, sig, data)
 
@@ -957,6 +987,10 @@ def import_rsakey_from_private_pem(pem, scheme='rsassa-pss-sha256', password=Non
     Conforms to 'securesystemslib.formats.RSAKEY_SCHEMA'.
   """
 
+  if not USE_PYCA: # pragma: no cover
+    raise securesystemslib.exceptions.UnsupportedLibraryError(
+      'The cryptography module is required to support RSA keys')
+
   # Does 'pem' have the correct format?
   # This check will ensure 'pem' conforms to
   # 'securesystemslib.formats.PEMRSA_SCHEMA'.
@@ -1051,6 +1085,10 @@ def import_rsakey_from_public_pem(pem, scheme='rsassa-pss-sha256'):
     Conforms to 'securesystemslib.formats.RSAKEY_SCHEMA'.
   """
 
+  if not USE_PYCA: # pragma: no cover
+    raise securesystemslib.exceptions.UnsupportedLibraryError(
+      'The cryptography module is required to support RSA keys')
+
   # Does 'pem' have the correct format?
   # This check will ensure arguments has the appropriate number
   # of objects and object types, and that all dict keys are properly named.
@@ -1127,6 +1165,10 @@ def import_rsakey_from_pem(pem, scheme='rsassa-pss-sha256'):
     Conforms to 'securesystemslib.formats.RSAKEY_SCHEMA'.
   """
 
+  if not USE_PYCA: # pragma: no cover
+    raise securesystemslib.exceptions.UnsupportedLibraryError(
+      'The cryptography module is required to support RSA keys')
+
   # Does 'pem' have the correct format?
   # This check will ensure arguments has the appropriate number
   # of objects and object types, and that all dict keys are properly named.
@@ -1323,6 +1365,10 @@ def encrypt_key(key_object, password):
     'securesystemslib.formats.ENCRYPTEDKEY_SCHEMA'.
   """
 
+  if not USE_PYCA: # pragma: no cover
+    raise securesystemslib.exceptions.UnsupportedLibraryError(
+      'The cryptography module is required to encrypt keys')
+
   # Does 'key_object' have the correct format?
   # This check will ensure 'key_object' has the appropriate number
   # of objects and object types, and that all dict keys are properly named.
@@ -1338,6 +1384,7 @@ def encrypt_key(key_object, password):
 
   # Generate an encrypted string of 'key_object' using AES-256-CTR-Mode, where
   # 'password' is strengthened with PBKDF2-HMAC-SHA256.
+
   encrypted_key = securesystemslib.pyca_crypto_keys.encrypt_key(key_object, password)
 
   return encrypted_key
@@ -1399,6 +1446,10 @@ def decrypt_key(encrypted_key, passphrase):
     RSAKEY_SCHEMA, ED25519KEY_SCHEMA).
   """
 
+  if not USE_PYCA: # pragma: no cover
+    raise securesystemslib.exceptions.UnsupportedLibraryError(
+      'The cryptography module is required to decrypt keys')
+
   # Does 'encrypted_key' have the correct format?
   # This check ensures 'encrypted_key' has the appropriate number
   # of objects and object types, and that all dict keys are properly named.
@@ -1471,6 +1522,10 @@ def create_rsa_encrypted_pem(private_key, passphrase):
     Conforms to 'securesystemslib.formats.PEMRSA_SCHEMA'.
   """
 
+  if not USE_PYCA: # pragma: no cover
+    raise securesystemslib.exceptions.UnsupportedLibraryError(
+      'The cryptography module is required to RSA encryption')
+
   # Does 'private_key' have the correct format?
   # This check will ensure 'private_key' has the appropriate number
   # of objects and object types, and that all dict keys are properly named.
@@ -1701,6 +1756,10 @@ def import_ecdsakey_from_private_pem(pem, scheme='ecdsa-sha2-nistp256', password
     Conforms to 'securesystemslib.formats.ECDSAKEY_SCHEMA'.
   """
 
+  if not USE_ECDSA: # pragma: no cover
+    raise securesystemlib.exceptions.UnsupportedLibraryError(
+      'The cryptography module is required to support ECDSA keys')
+
   # Does 'pem' have the correct format?
   # This check will ensure 'pem' conforms to
   # 'securesystemslib.formats.ECDSARSA_SCHEMA'.
@@ -1800,6 +1859,10 @@ def import_ecdsakey_from_public_pem(pem, scheme='ecdsa-sha2-nistp256'):
     Conforms to 'securesystemslib.formats.ECDSAKEY_SCHEMA'.
   """
 
+  if not USE_ECDSA: # pragma: no cover
+    raise securesystemslib.exceptions.UnsupportedLibraryError(
+      'The cryptography module is required to support ECDSA keys')
+
   # Does 'pem' have the correct format?
   # This check will ensure arguments has the appropriate number
   # of objects and object types, and that all dict keys are properly named.
@@ -1875,6 +1938,10 @@ def import_ecdsakey_from_pem(pem, scheme='ecdsa-sha2-nistp256'):
     Conforms to 'securesystemslib.formats.ECDSAKEY_SCHEMA'.
   """
 
+  if not USE_ECDSA: # pragma: no cover
+    raise securesystemslib.exceptions.UnsupportedLibraryError(
+      'The cryptography module is required to support ECDSA keys')
+
   # Does 'pem' have the correct format?
   # This check will ensure arguments has the appropriate number
   # of objects and object types, and that all dict keys are properly named.