Update formats to support ecdsa-sha2-nistp384

Note: This commit tries to blend in with the current sslib design.

In future work we should:
- define securesystemslib-wide constants instead of hardcoding
  strings over and over again (see item 3 in secure-systems-lab#183)
- re-think "key type" vs. "signature scheme"

This commit also removes the obsolete REQUIRED_LIBRARIES_SCHEMA,
which only used to be used in the long-gone check_crypto_libraries
tuf function.

Author: lukpueh

securesystemslib/formats.py

@@ -164,9 +164,8 @@
 # http://www.emc.com/emc-plus/rsa-labs/historical/twirl-and-rsa-key-size.htm#table1
 RSAKEYBITS_SCHEMA = SCHEMA.Integer(lo=2048)
 
-# The supported ECDSA signature schemes (ecdsa-sha2-nistp256 is supported by
-# default).
-ECDSA_SCHEME_SCHEMA = SCHEMA.OneOf([SCHEMA.String('ecdsa-sha2-nistp256')])
+# The supported ECDSA signature schemes
+ECDSA_SCHEME_SCHEMA = SCHEMA.RegularExpression(r'ecdsa-sha2-nistp(256|384)')
 
 # A pyca-cryptography signature.
 PYCACRYPTOSIGNATURE_SCHEMA = SCHEMA.AnyBytes()
@@ -201,7 +200,7 @@
 # Supported securesystemslib key types.
 KEYTYPE_SCHEMA = SCHEMA.OneOf(
   [SCHEMA.String('rsa'), SCHEMA.String('ed25519'),
-   SCHEMA.String('ecdsa-sha2-nistp256')])
+   SCHEMA.RegularExpression(r'ecdsa-sha2-nistp(256|384)')])
 
 # A generic securesystemslib key.  All securesystemslib keys should be saved to
 # metadata files in this format.
@@ -254,7 +253,7 @@
 # An ECDSA securesystemslib key.
 ECDSAKEY_SCHEMA = SCHEMA.Object(
   object_name = 'ECDSAKEY_SCHEMA',
-  keytype = SCHEMA.String('ecdsa-sha2-nistp256'),
+  keytype = SCHEMA.RegularExpression(r'ecdsa-sha2-nistp(256|384)'),
   scheme = ECDSA_SCHEME_SCHEMA,
   keyid = KEYID_SCHEMA,
   keyid_hash_algorithms = SCHEMA.Optional(HASHALGORITHMS_SCHEMA),
@@ -272,12 +271,6 @@
 # An ECDSA signature.
 ECDSASIGNATURE_SCHEMA = SCHEMA.AnyBytes()
 
-# Required installation libraries expected by the repository tools and other
-# cryptography modules.
-REQUIRED_LIBRARIES_SCHEMA = SCHEMA.ListOf(SCHEMA.OneOf(
-  [SCHEMA.String('general'), SCHEMA.String('ed25519'), SCHEMA.String('rsa'),
-   SCHEMA.String('ecdsa-sha2-nistp256')]))
-
 # Ed25519 signature schemes.  The vanilla Ed25519 signature scheme is currently
 # supported.
 ED25519_SIG_SCHEMA = SCHEMA.OneOf([SCHEMA.String('ed25519')])