scripts: imgtool: Add command to dump private keys
This applies a few improvements to a commit previously included in
PR #596:
* Move functions to dump a private key to the private key classes
* Remove language option; always dumps in C format
* Add option to generate a minimal dump. This will remove extra
parameters that are present in keys generated with the `keygen`
command.
For P256 this will remove the public point, which is already
ignored by the parsing function. The resulting key dump shrinks
from 138 to 70 bytes.
For RSA it will remove the DP/DQ/QP parameters which are only
used with CRT enabled, and if not available, can be calculated at
runtime. This reduces the size of a key dump from around 1190
bytes to somewhere close to 800 bytes. A patch to the RSA parsing
routine will be added in another commit.
Signed-off-by: Fabio Utzig <utzig@apache.org>
Signed-off-by: Ioannis Konstantelias <ikonstadel@gmail.com>
diff --git a/scripts/imgtool/keys/rsa.py b/scripts/imgtool/keys/rsa.py
index 0f9a905..85c0342 100644
--- a/scripts/imgtool/keys/rsa.py
+++ b/scripts/imgtool/keys/rsa.py
@@ -42,6 +42,9 @@
encoding=serialization.Encoding.DER,
format=serialization.PublicFormat.PKCS1)
+ def get_private_bytes(self, minimal):
+ self._unsupported('get_private_bytes')
+
def export_private(self, path, passwd=None):
self._unsupported('export_private')
@@ -94,6 +97,49 @@
def _get_public(self):
return self.key.public_key()
+ def _build_minimal_rsa_privkey(self, der):
+ '''
+ Builds a new DER that only includes N/E/D/P/Q RSA parameters;
+ standard DER private bytes provided by OpenSSL also includes
+ CRT params (DP/DQ/QP) which can be removed.
+ '''
+ OFFSET_N = 7 # N is always located at this offset
+ b = bytearray(der)
+ off = OFFSET_N
+ if b[off + 1] != 0x82:
+ raise RSAUsageError("Error parsing N while minimizing")
+ len_N = (b[off + 2] << 8) + b[off + 3] + 4
+ off += len_N
+ if b[off + 1] != 0x03:
+ raise RSAUsageError("Error parsing E while minimizing")
+ len_E = b[off + 2] + 4
+ off += len_E
+ if b[off + 1] != 0x82:
+ raise RSAUsageError("Error parsing D while minimizing")
+ len_D = (b[off + 2] << 8) + b[off + 3] + 4
+ off += len_D
+ if b[off + 1] != 0x81:
+ raise RSAUsageError("Error parsing P while minimizing")
+ len_P = b[off + 2] + 3
+ off += len_P
+ if b[off + 1] != 0x81:
+ raise RSAUsageError("Error parsing Q while minimizing")
+ len_Q = b[off + 2] + 3
+ off += len_Q
+ # adjust DER size for removed elements
+ b[2] = (off - 4) >> 8
+ b[3] = (off - 4) & 0xff
+ return b[:off]
+
+ def get_private_bytes(self, minimal):
+ priv = self.key.private_bytes(
+ encoding=serialization.Encoding.DER,
+ format=serialization.PrivateFormat.TraditionalOpenSSL,
+ encryption_algorithm=serialization.NoEncryption())
+ if minimal:
+ priv = self._build_minimal_rsa_privkey(priv)
+ return priv
+
def export_private(self, path, passwd=None):
"""Write the private key to the given file, protecting it with the
optional password."""