tools: Add script for converting public key into device tree include

Allows to create a public key device tree dtsi for inclusion into U-Boot
SPL and proper during first build already. This can be achieved via
CONFIG_DEVICE_TREE_INCLUDES.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

diff --git a/tools/key2dtsi.py b/tools/key2dtsi.py
new file mode 100755 (executable)
index 0000000..1dbb2cc
--- /dev/null
+++ b/tools/key2dtsi.py
@@ -0,0 +1,64 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Public key to dtsi converter.
+#
+# Copyright (c) Siemens AG, 2022
+#
+
+from argparse import ArgumentParser, FileType
+from os.path import basename, splitext
+from Cryptodome.PublicKey import RSA
+from Cryptodome.Util.number import inverse
+
+def int_to_bytestr(n, length=None):
+    if not length:
+        length = (n.bit_length() + 7) // 8
+    byte_array = n.to_bytes(length, 'big')
+    return ' '.join(['{:02x}'.format(byte) for byte in byte_array])
+
+ap = ArgumentParser(description='Public key to dtsi converter')
+
+ap.add_argument('--hash', '-H', default='sha256',
+                help='hash to be used with key (default: sha256)')
+ap.add_argument('--required-conf', '-c', action='store_true',
+                help='mark key required for configuration')
+ap.add_argument('--required-image', '-i', action='store_true',
+                help='mark key required for image')
+ap.add_argument('--spl', '-s', action='store_true',
+                help='mark key for usage in SPL')
+ap.add_argument('key_file', metavar='KEY_FILE', type=FileType('r'),
+                help='key file (formats: X.509, PKCS#1, OpenSSH)')
+ap.add_argument('dtsi_file', metavar='DTSI_FILE', type=FileType('w'),
+                help='dtsi output file')
+
+args = ap.parse_args()
+
+key_name, _ = splitext(basename(args.key_file.name))
+
+key_data = args.key_file.read()
+key = RSA.importKey(key_data)
+
+r_squared = (2**key.size_in_bits())**2 % key.n
+n0_inverse = 2**32 - inverse(key.n, 2**32)
+
+out = args.dtsi_file
+out.write('/ {\n')
+out.write('\tsignature {\n')
+out.write('\t\tkey-{} {{\n'.format(key_name))
+out.write('\t\t\tkey-name-hint = "{}";\n'.format(key_name))
+out.write('\t\t\talgo = "{},rsa{}";\n'.format(args.hash, key.size_in_bits()))
+out.write('\t\t\trsa,num-bits = <{}>;\n'.format(key.size_in_bits()))
+out.write('\t\t\trsa,modulus = [{}];\n'.format(int_to_bytestr(key.n)))
+out.write('\t\t\trsa,exponent = [{}];\n'.format(int_to_bytestr(key.e, 8)))
+out.write('\t\t\trsa,r-squared = [{}];\n'.format(int_to_bytestr(r_squared)))
+out.write('\t\t\trsa,n0-inverse = <0x{:x}>;\n'.format(n0_inverse))
+if args.required_conf:
+    out.write('\t\t\trequired = "conf";\n')
+elif args.required_image:
+    out.write('\t\t\trequired = "image";\n')
+if args.spl:
+    out.write('\t\t\tu-boot,dm-spl;\n')
+out.write('\t\t};\n')
+out.write('\t};\n')
+out.write('};\n')