IMA signing questions

[Ken Dreyer]

Hi folks,

I want to add "intro to IMA signing" instructions to
https://docs.pagure.org/koji/signing/ . I wrote a basic PR at
https://pagure.io/koji/pull-request/3206 but it lacks technical
details.

- How do I generate my own new keypair so I can IMA-sign an RPM?

- Can I use my existing GPG keypair?

- How do I IMA-sign files in an RPM locally (apart from Koji)? (Is it
the --signfiles option from rpmsign(8)?)

- How do I inspect the IMA signatures on an existing RPM?

- When I gpg-sign an RPM with "Key A" and IMA-sign an RPM with "Key
B", does Koji "know" about Key B at all?


- Ken

[Patrick マルタインアンドレアス Uiterwijk]

Hi Ken,

>  
>  I want to add "intro to IMA signing" instructions to
>  https://docs.pagure.org/koji/signing/ . I wrote a basic PR at
>  https://pagure.io/koji/pull-request/3206 but it lacks technical
>  details. 
That'd be cool!

>  
>  - How do I generate my own new keypair so I can IMA-sign an RPM? 
You can generate the key with the standard OpenSSL commands.
For example, an RSA key can be generated like:
openssl genrsa | openssl pkcs8 -topk8 -nocrypt -outform DER -out privatekey.der

(do note that the key will need to be in DER format).

You can then generate a corresponding (self-signed) certificate for validation with:

openssl req -x509 -key privatekey.der -out certificate.pem -days 365 -keyform DER

>  
>  - Can I use my existing GPG keypair? 
Mathematically, yes. Practically, no.
The key format RPM (libimaevm) reads for signing is DER, so you'd have to convert the
actual key bits from the GPG format to DER.

>  
>  - How do I IMA-sign files in an RPM locally (apart from Koji)? (Is it
>  the --signfiles option from rpmsign(8)?) 
Yes, it's the --signfiles option.

>  
>  - How do I inspect the IMA signatures on an existing RPM? 
The signatures are stored in the FILESIGNATURES rpm sighdr (with tag RPMTAG_SIG_BASE + 18,
so 274), as a hex-encoded string array.
I have some code for reading and parsing the signatures at
https://github.com/fedora-iot/rpm-head-signing/blob/main/rpm_head_signing...
.

>  
>  - When I gpg-sign an RPM with "Key A" and IMA-sign an RPM with "Key
>  B", does Koji "know" about Key B at all? 
Koji at this moment does not look at or touch the FILESIGNATURES header.
It copies it into its signature store (because the tag is in the sighdr), and will
re-insert it into the resulting RPM, but it has no clue it's even there.
This also means that the RPMs that are signed with {rpm_key=keyA, ima_key=keyB} and
{rpm_key=keyA, ima_key=keyC} are seen as having the same signature, and thus would result
in the hub rejecting the new signature until the old one gets removed.
It would absolutely be useful to have this information also stored in koji and a part of
the index for the signatures, but that hasn't been done yet.

Patrick

[Simo Sorce]

On Thu, 2022-01-06 at 10:16 +0000, Patrick  マルタインアンドレアス  Uiterwijk
wrote:
>  Hi Ken,
>  
>  > 
>  > I want to add "intro to IMA signing" instructions to
>  > https://docs.pagure.org/koji/signing/ . I wrote a basic PR at
>  > https://pagure.io/koji/pull-request/3206 but it lacks technical
>  > details.
>  
>  That'd be cool!
>  
>  > 
>  > - How do I generate my own new keypair so I can IMA-sign an RPM?
>  
>  You can generate the key with the standard OpenSSL commands.
>  For example, an RSA key can be generated like:
>  openssl genrsa | openssl pkcs8 -topk8 -nocrypt -outform DER -out privatekey.der 

Just a heads-up that genrsa is deprecated and genpkey -algorithm RSA
(or better RSA-PSS) should be used instead (genrsa will simply fail in
FIPS mode for example).

Simo.


>  (do note that the key will need to be in DER format).
>  
>  You can then generate a corresponding (self-signed) certificate for validation with:
>  
>  openssl req -x509 -key privatekey.der -out certificate.pem -days 365 -keyform DER
>  
>  > 
>  > - Can I use my existing GPG keypair?
>  
>  Mathematically, yes. Practically, no.
>  The key format RPM (libimaevm) reads for signing is DER, so you'd have to convert the
> actual key bits from the GPG format to DER.
>  
>  > 
>  > - How do I IMA-sign files in an RPM locally (apart from Koji)? (Is it
>  > the --signfiles option from rpmsign(8)?)
>  
>  Yes, it's the --signfiles option.
>  
>  > 
>  > - How do I inspect the IMA signatures on an existing RPM?
>  
>  The signatures are stored in the FILESIGNATURES rpm sighdr (with tag RPMTAG_SIG_BASE +
> 18, so 274), as a hex-encoded string array.
>  I have some code for reading and parsing the signatures at
> https://github.com/fedora-iot/rpm-head-signing/blob/main/rpm_head_signing...
> .
>  
>  > 
>  > - When I gpg-sign an RPM with "Key A" and IMA-sign an RPM with "Key
>  > B", does Koji "know" about Key B at all?
>  
>  Koji at this moment does not look at or touch the FILESIGNATURES header.
>  It copies it into its signature store (because the tag is in the sighdr), and will
> re-insert it into the resulting RPM, but it has no clue it's even there.
>  This also means that the RPMs that are signed with {rpm_key=keyA, ima_key=keyB} and
> {rpm_key=keyA, ima_key=keyC} are seen as having the same signature, and thus would result
> in the hub rejecting the new signature until the old one gets removed.
>  It would absolutely be useful to have this information also stored in koji and a part of
> the index for the signatures, but that hasn't been done yet.
>  
>  Patrick
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-- 
Simo Sorce
RHEL Crypto Team
Red Hat, Inc

[Ken Dreyer]

On Thu, Jan 6, 2022 at 5:17 AM Patrick マルタインアンドレアス Uiterwijk
<puiterwijk(a)redhat.com&gt; wrote:
>  > - How do I generate my own new keypair so I can IMA-sign an
> RPM?
>
>  You can generate the key with the standard OpenSSL commands.
>  For example, an RSA key can be generated like:
>  openssl genrsa | openssl pkcs8 -topk8 -nocrypt -outform DER -out privatekey.der
>
>  (do note that the key will need to be in DER format). 
Thanks for these tips.

rpm-sign complains when I use a DER-formatted key. I switched to a
regular PEM-formatted key file, and that works. Looking at libimaevm's
read_priv_pkey(), it checks for a "pkcs11:" URI, and if it doesn't
find that string prefix, it just calls fopen and PEM_read_PrivateKey.

Reading rpm_head_signing/verify_rpm.py it looks like you're sending a
DER-formatted file to "evmctl ima_verify". I guess that's where the
DER format comes in?

Something else I'm wondering: rpmsign writes those four-byte "keyid"
values to my FILESIGNATURE entries even if I don't have a public cert
at all. How does it do that? I see verify_rpm.py checks the RPM's
keyid values against the final four bytes of a sha1 of a public
certificate, but what if I haven't generated that yet?

Also, on Rawhide, rpmsign fails with an error in EVP_PKEY_sign.
Example with a random SRPM:

rpmsign --addsign --define "_gpg_name security(a)example.com&quot;
--signfiles --fskpath privatekey.pem bash-5.1.8-3.fc36.src.rpm
bash-5.1.8-3.fc36.src.rpm:
hash(sha1): 9958fb4ee30415c75bd992982ac1463c6ff6ce739e00aaf7d7ad992feb0b40f1
sign_hash_v2: signing failed: (invalid digest length) in EVP_PKEY_sign
openssl: error:1C8000A6:Provider routines::invalid digest length
error: sign_hash failed
error: signFile failed

Since this works on CentOS Stream 9, I updated my Rawhide test
environment from ima-evm-utils-1.3.2-4.fc36 to the version in CentOS 9
Stream (ima-evm-utils-1.4-4), then rebuilt rpm-4.17.0-4.fc36 against
the newer libimaevm.so.3.0.0, and then I could use --signfiles in
Rawhide. My builds are at https://fedorapeople.org/~ktdreyer/ima/ .

I think the next step is to get ima-evm-utils 1.4 into Fedora.

- Ken

[Ken Dreyer]

On Mon, Jan 17, 2022 at 4:44 PM Ken Dreyer <ktdreyer(a)ktdreyer.com&gt; wrote:
>  Something else I'm wondering: rpmsign writes those four-byte
> "keyid"
>  values to my FILESIGNATURE entries even if I don't have a public cert
>  at all. How does it do that? I see verify_rpm.py checks the RPM's
>  keyid values against the final four bytes of a sha1 of a public
>  certificate, but what if I haven't generated that yet? 
I understand this part about public and private certs now.
python-cryptography can read the values from the RSA private keyfile
to determine the public key values.

This Python script will find the IMA-style keyid from a RSA private keyfile:

with open('privatekey.pem', 'rb') as f:
    key = serialization.load_pem_private_key(f.read(),
                                             password=None,
                                             backend=default_backend())

public_key = key.public_key()

# The "keyid" is the SHA1 hash of the DER-encoded ASN.1 sequence of the
# modulus and exponent of an RSA public key. (public_key.public_numbers() "n"
# and "e"). This method does it for us:
public_key_id = x509.SubjectKeyIdentifier.from_public_key(public_key)

# The IMA signature's "keyid" is the last four bytes of this SHA1 digest.
ima_keyid = public_key_id.digest[-4:]
print(ima_keyid)


I've been concerned for a while that Koji uses small GPG key ID
values, for the reasons explained at https://evil32.com/

When it comes to IMA signature handling with Koji, I don't want to use
small key ID lengths for that either. Even sha1 is pretty weak now. Is
there any chance of using stronger key IDs for IMA?

- Ken