[PATCH] tools: key2dtsi: Write out modulus and r-squared with the correct length

[Hans Gfirtner (Nokia)]

Hi Jan, all,

Your fix seems to work fine, but I only tested with keys of size 256,
please see my "rude" test script attached:
#!/bin/bash


for i in {0..1000} do
{
        openssl genpkey -out privateKey_$i.key -algorithm RSA -pkeyopt rsa_keygen_bits:2048
        openssl rsa -in privateKey_$i.key -pubout > publicKey_$i.pub
        key2dtsi.py publicKey_$i.pub key2dtsi_$i.dtsi
        wc_cmd=`grep r-squared key2dtsi_$i.dtsi |sed -e 's/\s*rsa,r\-squared = \[\(.*\)\]\;/\1/'`

        # size of r-squared is expected to be 256, I print one additional parameter -> 257
        echo "$wc_cmd $i" | wc | awk -e '{if ($2 != 257){ print "key ",$4,": Error value ",$2;exit -1}}'

}
BR
Hans

-----Original Message-----
From: Jan Kiszka <jan.kiszka at siemens.com> 
Sent: Friday, October 31, 2025 10:35 AM
To: U-Boot Mailing List <u-boot at lists.denx.de>; Tom Rini <trini at konsulko.com>; Hans Gfirtner (Nokia) <hans.gfirtner at nokia.com>
Subject: [PATCH] tools: key2dtsi: Write out modulus and r-squared with the correct length


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From: Jan Kiszka <jan.kiszka at siemens.com>

Align the implementation to rsa_add_verify_data() by writing the modulus and r-squared properties with the same length as the key itself. This fixes signature verification issues when one of the parameters has leading zeros.

Reported-by: Hans Gfirtner (Nokia) <hans.gfirtner at nokia.com>
Signed-off-by: Jan Kiszka <jan.kiszka at siemens.com>
---

Hans, please validate. I only had a dry run with your test key.

 tools/key2dtsi.py | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/tools/key2dtsi.py b/tools/key2dtsi.py index 1dbb2cc94bf..320ea930a97 100755
--- a/tools/key2dtsi.py
+++ b/tools/key2dtsi.py
@@ -11,10 +11,8 @@ 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')
+def int_to_bytestr(n, bits):
+    byte_array = n.to_bytes(bits // 8, 'big')
     return ' '.join(['{:02x}'.format(byte) for byte in byte_array])

 ap = ArgumentParser(description='Public key to dtsi converter') @@ -39,7 +37,8 @@ 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
+key_bits = key.size_in_bits()
+r_squared = (2**key_bits)**2 % key.n
 n0_inverse = 2**32 - inverse(key.n, 2**32)

 out = args.dtsi_file
@@ -47,11 +46,13 @@ 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\talgo = "{},rsa{}";\n'.format(args.hash, key_bits)) 
+out.write('\t\t\trsa,num-bits = <{}>;\n'.format(key_bits)) 
+out.write('\t\t\trsa,modulus = [{}];\n'.format(int_to_bytestr(key.n,
+                                                              
+key_bits))) out.write('\t\t\trsa,exponent = 
+[{}];\n'.format(int_to_bytestr(key.e, 64))) out.write('\t\t\trsa,r-squared = [{}];\n'.format(int_to_bytestr(r_squared,
+                                                                
+key_bits)))
 out.write('\t\t\trsa,n0-inverse = <0x{:x}>;\n'.format(n0_inverse))  if args.required_conf:
     out.write('\t\t\trequired = "conf";\n')
--
2.51.0