Guide for building an EdDSA PKI

Guide for building an EdDSA PKI HTT Consulting

Oak Park MI 48237 rgm@labs.htt-consult.com

Fraunhofer SIT

Rheinstrasse 75 Darmstadt 64295 Germany henk.birkholz@sit.fraunhofer.de

Sandelman Software Works

mcr+ietf@sandelman.ca http://www.sandelman.ca/

Security Area wg TBD RFC Request for Comments I-D Internet-Draft PKI EdDSA 802.1AR This memo provides a guide for building a PKI (Public Key Infrastructure) using openSSL. Certificates in this guide can be either ED25519 or ED448 certificates. Along with common End Entity certificates, this guide provides instructions for creating IEEE 802.1AR iDevID Secure Device certificates.

The IETF has adopted the Edwards Elliptic Curve and related algorithms. These algorithms hold out the promise of greater efficiency and better understood security risks. This efficiency could make that critical difference to allow them to be used in some constrained IoT devices. This effort provides the steps, using the openSSL application, to create such a PKI of ED25519 or ED448 certificates . The goal is that any developer or tester can follow these steps, create the basic objects needed and establish the validity of the standard/program design. This guide can even be used to create a production PKI, though additional steps need to be taken. This could be very useful to a small vendor needing to include 802.1AR iDevIDs in their product (Note: EdDSA certificates are not supported in even the forthcoming 802.1AR-2018; this is for future work). This guide was originally developed with openSSL 1.1.1; it is updated using openSSL 3.0.9 on Fedora 38 and creates PEM-based certificates. It closely follows . Current updates follow some lessons learned in developing Previous versions had multiple openssl config files. This version has a single config file to support all the openssl commands used herein.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

This section will contain notations

There are no draft specific definitions at this time

There are two differences between ECDSA and EdDSA certificates that impact the use of openSSL. There are no options with EdDSA, and thus the pkeyopt variable is not used. Likewise there are no hash options. The hashes used by EdDSA are preset and not selectable. As such, none of the hash options should be needed. It should be noted here that ED25519 certificates can be ~100 bytes smaller than corresponding ECDSA certificates not using ECDSA point-compression. This size difference may be critical in some devices and communication technologies. ED448 certificates are similar in size with ECDSA p256 certificates yet with a stronger security claim.

A basic PKI has two levels of hierarchy: Root and Intermediate. The Root level has the greatest risk, and is the least used. It only signs the Intermediate level signing certificate. As such, once the Root level is created and signs the Intermediate level certificate it can be locked up. In fact, the Root level could exist completely on a uSD boot card for an ARM small computer like a RaspberryPi. A copy of this card can be made and securely stored in a different location. The Root level contains the Root certificate private key, a database of all root signed certificates, and the public root certificate. It can also contain the Intermediate level public certificate and a Root level CRL. The Intermediate level contains the Intermediate certificate private key, the public certificate, a database of all its signed certificates, the certificate trust chain, and Intermediate level CRL. It can also contain the End Entity public certificates. The private key file needs to be keep securely. For example as with the Root level, a mSD image for an ARM computer could contain the complete Intermediate level. This image is kept offline. The End Entity CSR is copied to it, signed, and then the signed certificate and updated database are moved to the public image that lacks the private key. For a simple test PKI, all files can be kept on a single system that is managed by the tester. End Entities create a key pair and a Certificate Signing Request (CSR). The private key is stored securely. The CSR is delivered to the Intermediate level which uses the CSR to create the End Entity certificate. This certificate, along with the trust chain back to the root, is then returned to the End Entity. There is more to a PKI, but this suffices for most development and testing needs.

This guide was originally developed on a Fedora 29-beta armv7hl system (Cubieboard2 SoC). It should work on most Linux and similar systems that support openSSL 1.1.1. Current work has been on with openSSL 3.0.9 on Fedora 38. All work was done in a terminal window with extensive "cutting and pasting" from this draft guide into the terminal window. Users of this guide may find different behaviors based on their system.

The first step is to create the PKI environment. Modify the variables to suit your needs. The Serial Number length for a public PKI ranges from 8 to 19 bytes. The use of 19 rather than 20 is to accommodate the hex representation of the Serial Number. If it has a one in the high order bit, DER encoding rules will place a 0x00 in front. The DN and SAN fields are examples. Change them to appropriate values. If you leave one blank, it will be left out of the Certificate. "OU" above is an example of an empty DN object.

Create the file, $dir/openssl.cnf from the contents in .

Next are the openssl commands to create the Root certificate keypair, and the Root certificate. Included are commands to view the file contents. Environment variables are used to provide considerable flexibility in the certificate contents. One change is to move from certificate life in days to validity notBefore and notAfter days. This is not supported in the standard self-sign root certificate creation, so a "through-away" self-signed certificate is created first which is used to sign a certificate with the same key and proper validity dates which is then the root certificate. It is not uncommon to get warning messages in "openssl ca" commands on not including some value it expects. For example a warning of not having "default_days", as we are using actual validity dates. These can be ignored. OpenSSL does not allow empty variables in the config file. So in the example below, certextkeyusage is empty. There are a few req_ext sections (e.g. req_ext_bkes, bke, bks, and bk) to cover the more common sets of extensions used. If a different set is needed, add it to the config file. It is not uncommon to get warning messages in "openssl ca" commands on not including some value it expects. For example a warning of not having "default_days", as we are using actual validity dates. These can be ignored. $dir/serial # hex 8 is minimum, 19 is maximum openssl ca -config $dir/openssl.cnf\ -extensions v3_ca -notext \ -in $dir/csr/ca.csr.$format\ -out $dir/certs/ca.cert.$format chmod 444 $dir/certs/ca.cert.$format openssl x509 -inform $format -in $dir/certs/ca.cert.$format\ -text -noout openssl x509 -purpose -inform $format\ -in $dir/certs/ca.cert.$format -inform $format openssl x509 -in $dir/certs/ca.cert.$format\ -out $dir/certs/ca.cert.der -outform der ]]>

The next part is to create the Intermediate PKI environment. Modify the variables to suit your needs. In particular, set the variables for CRL and/or OCSP support. $dir/crlnumber ]]>

The Intermediate level CA now uses the same openssl config file as the root. Just copy it from the main directory into the Intermediate.

Here are the openssl commands to create the Intermediate certificate keypair, Intermediate certificate signed request (CSR), and the Intermediate certificate. Included are commands to view the file contents. $dir/serial # hex 8 is minimum, 19 is maximum # Note 'openssl ca' does not support DER format openssl ca -config $cadir/openssl.cnf\ -extensions v3_intermediate_ca -notext \ -in $dir/csr/intermediate.csr.$format\ -out $dir/certs/intermediate.cert.pem chmod 444 $dir/certs/intermediate.cert.$format openssl verify -CAfile $cadir/certs/ca.cert.$format\ $dir/certs/intermediate.cert.$format openssl x509 -noout -text -in $dir/certs/intermediate.cert.$format openssl x509 -in $dir/certs/intermediate.cert.$format\ -out $dir/certs/intermediate.cert.der -outform der # Create the certificate chain file cat $dir/certs/intermediate.cert.$format\ $cadir/certs/ca.cert.$format > $dir/certs/ca-chain.cert.$format chmod 444 $dir/certs/ca-chain.cert.$format ]]>

Here are the openssl commands to create a Server End Entity certificate keypair, Server certificate signed request (CSR), and the Server certificate. Included are commands to view the file contents. If EE certificates are created at a different time than the Intermediate signing certificate, care needs to be taken that all variables are properly reset. $dir/serial # hex 8 is minimum, 19 is maximum # Note 'openssl ca' does not support DER format openssl ca -config $cadir/openssl.cnf\ -extensions server_cert -notext \ -in $dir/csr/$serverfqdn.csr.$format\ -out $dir/certs/$serverfqdn.cert.$format chmod 444 $dir/certs/$serverfqdn.cert.$format openssl verify -CAfile $dir/certs/ca-chain.cert.$format\ $dir/certs/$serverfqdn.cert.$format openssl x509 -noout -text -in $dir/certs/$serverfqdn.cert.$format openssl x509 -in $dir/certs/$serverfqdn.cert.$format\ -out $dir/certs/$serverfqdn.cert.der -outform der ]]>

Here are the openssl commands to create a Client End Entity certificate keypair, Client certificate signed request (CSR), and the Client certificate. Included are commands to view the file contents. For a Client certificate with "no" subject and only a subjectAltName, set the variable DN to "/". Also the subjectAltName MUST be marked "critical". $dir/serial # hex 8 is minimum, 19 is maximum # Note 'openssl ca' does not support DER format openssl ca -config $dir/openssl.cnf\ -extensions usr_cert -notext \ -in $dir/csr/$clientemail.csr.$format\ -out $dir/certs/$clientemail.cert.$format chmod 444 $dir/certs/$clientemail.cert.$format openssl verify -CAfile $dir/certs/ca-chain.cert.$format\ $dir/certs/$clientemail.cert.$format openssl x509 -noout -text -in $dir/certs/$clientemail.cert.$format openssl x509 -in $dir/certs/$clientemail.cert.$format\ -out $dir/certs/$clientemail.cert.der -outform der ]]>

There is no longer a need for a special 802.1AR Intermediate Certificate CA. The regular Intermediate Certificate CA may be used for 802.1AR iDevID certificates. A special CA may be set up by following the steps outlined in , but into a special intermediate8021AR directory. The difference with 802.1AR device certificates may be in including in the subject the device serial number. These certificates MUST have an afterDate of forever and a specific subjectAltName. Details for these follow.

Here are the openssl commands to create a 802.1AR iDevID certificate keypair, iDevID certificate signed request (CSR), and the iDevID certificate. Included are commands to view the file contents. $dir/serial # hex 8 is minimum, 19 is maximum # Note 'openssl ca' does not support DER format openssl ca -config $dir/openssl.cnf\ -extensions 8021ar_idevid -notext \ -in $dir/csr/$DevID.csr.$format\ -out $dir/certs/$DevID.cert.$format chmod 444 $dir/certs/$DevID.cert.$format openssl verify -CAfile $dir/certs/ca-chain.cert.$format\ $dir/certs/$DevID.cert.$format openssl x509 -noout -text -in $dir/certs/$DevID.cert.$format openssl asn1parse -i -in $dir/certs/$DevID.cert.pem # offset of start of hardwareModuleName and use that in place of 367 openssl asn1parse -i -strparse 367 -in $dir/certs/$DevID.cert.pem openssl x509 -in $dir/certs/$DevID.cert.$format\ -out $dir/certs/$DevID.cert.der -outform der ]]>

This part provides CRL support to an Intermediate CA. In this memo it applies to both Intermediate CAs. Set the crlDistributionPoints as provided via the environment variables.

It is simple to create the CRL. The CRL consists of the certificates flagged with an R (Revoked) in index.txt:

Revoking a certificate is a two step process. First identify the target certificate, examples are listed below. Revoke it then publish a new CRL. Recreate the CRL using .

This part provides OCSP support to an Intermediate CA. In this memo it applies to both Intermediate CAs. Set the authorityInfoAccess as provided via the environment variables.

OCSP needs a signing certificate. This certificate must be signed by the CA that signed the certificate being checked. The steps to create this certificate is the similar to a Server certificate for the CA: $dir/serial # hex 8 is minimum, 19 is maximum # Note 'openssl ca' does not support DER format openssl ca -config $dir/openssl.cnf\ -extensions ocsp -notext \ -in $dir/csr/$ocspurl.csr.$format\ -out $dir/certs/$ocspurl.cert.$format chmod 444 $dir/certs/$ocspurl.cert.$format openssl verify -CAfile $dir/certs/ca-chain.cert.$format\ $dir/certs/$ocspurl.cert.$format openssl x509 -noout -text -in $dir/certs/$ocspurl.cert.$format ]]>

Revoke the certificate as in . The OCSP responder SHOULD detect the flag change in index.txt and, when queried respond appropriately.

OpenSSL provides a simple OCSP service that can be used to test the OCSP certificate and revocation process (Note that this only reads the index.txt to get the certificate status at startup). In a terminal window, set variables dir and ocspurl (examples below), then run the simple OCSP service: In another window, test out a certificate status with: Revoke the certificate, , restart the test Responder again as above, then check the certificate status.

Creating this document was a real education in the state of openSSL, X.509 certificate guidance, and just general level of certificate awareness. Here are a few short notes.

The certificate serial number's role is to provide yet another way to maintain uniqueness of certificates within a PKI as well as a way to index them in a data store. It has taken on other roles, most notably as a defense. The CABForum guideline for a public CA is for the serial number to be a random number at least 8 octets long and no longer than 20 bytes. By default, openssl makes self-signed certificates with 8 octet serial numbers. This guide uses openssl's RAND function to generate the random value and pipe it into the -set_serial option. This number MAY have the first bit as a ONE; the DER encoding rules prepend such numbers with 0x00. Thus the limit of '19' for the variable 'ns'. A private CA need not follow the CABForum rules and can use anything number for the serial number. For example, the root CA (which has no security risks mitigated by using a random value) could use '1' as its serial number. Intermediate and End Entity certificate serial numbers can also be of any value if a strong hash, like SHA256 used here. A value of 4 for ns would provide a sufficient population so that a CA of 10,000 EE certificates will have only a 1.2% probability of a collision. For only 1,000 certificates the probability drops to 0.012%. The following was proposed on the openssl-user list as an alternative to using the RAND function: Keep k bits (k/8 octets) long serial numbers for all your certificates, chose a block cipher operating on blocks of k bits, and operate this block cipher in CTR mode, with a proper secret key and secret starting counter. That way, no collision detection is necessary, you’ll be able to generate 2^(k/2) unique k bits longs serial numbers (in fact, you can generate 2^k unique serial numbers, but after 2^(k/2) you lose some security guarantees). With 3DES, k=64, and with AES, k=128.

There is a bit of inconsistency in how different parts and fields in the config file are used. Environment variables can only be used as values. Some fields can have null values, others cannot. The lack of allowing null fields means a script cannot feed in an environment variable with value null. In such a case, the field has to be removed from the config file. The expectation is each CA within a PKI has its own config file, customized to the certificates supported by that CA.

Older versions of openSSL had limitations in support for subjectAltName (SAN). This is no longer the case. This document sets up the SAN in the config file. Alternatively, the "-addext" option can be used directly in the command line.

In RFC 5280 (sec 4.2.1.6): if the only subject identity in the certificate is in subjectAltName, then Subject MUST be empty and subjectAltName MUST be marked as critical. This can be achieved with the variable DN=/ and subjectAltName (example given):

The long, hard-fought battle with openssl to create a full DER PKI failed. There is no facility to create a DER certificate from a DER CSR. It just is not there in the 'openssl ca' command. Even the 'openssl x509 -req' command cannot do this for a simple certificate. Further, there is no 'hack' for making a certificate chain as there is with PEM. With PEM a simple concatenation of the certificates create a usable certificate chain. For DER, some recommend using PKCS#7, where others point out that this format is poorly support 'in the field', whereas PKCS#12 works for them. Finally, openssl does support converting a PEM certificate to DER: This should also work for the keypair. However, in a highly constrained device it may make more sense to just store the raw keypair in the device's very limited secure storage.

TBD. May be nothing for IANA.

This section is a complete copy of . Changes will be made if anything is found specific to either ECDSA or EdDSA.

Creating certificates takes a lot of random numbers. A good source of random numbers is critical. Studies have found excessive amount of certificates, all with the same keys due to bad randomness on the generating systems. The amount of entropy available for these random numbers can be tested. On Fedora/Centos use: If the value is low (below 1000) check your system's randomness source. Is rng-tools installed? Consider adding an entropy collection service like haveged from issihosts.com/haveged.

During the certificate creation, particularly during keypair generation, the files are vulnerable to theft. This can be mitigate using umask. Before using openssl, set umask: Afterwards, restore it with:

This work is possible because of the availability of openSSL 1.1.1. As in , the openssl-user mailing list, with its many supportive experts, was of immense help in the nuance differences between ECDSA and EdDSA.

References Detection of Widespread Weak Keys in Network Devices University of California, San Diego

The University of Michigan

The following is the openssl.cnf file contents . #countryName = Country Name (2 letter code) #stateOrProvinceName = State or Province Name #localityName = Locality Name #0.organizationName = Organization Name #organizationalUnitName = Organizational Unit Name commonName = Common Name [ req_ext ] [ req_ext_bkes ] basicConstraints = $ENV::basicConstraints keyUsage = $ENV::certkeyusage extendedKeyUsage = $ENV::certextkeyusage subjectAltName = $ENV::subjectAltName [ req_ext_bke ] basicConstraints = $ENV::basicConstraints keyUsage = $ENV::certkeyusage extendedKeyUsage = $ENV::certextkeyusage [ req_ext_bks ] basicConstraints = $ENV::basicConstraints keyUsage = $ENV::certkeyusage subjectAltName = $ENV::subjectAltName [ req_ext_bk ] basicConstraints = $ENV::basicConstraints keyUsage = $ENV::certkeyusage [ req_ext_8021AR ] basicConstraints = $ENV::basicConstraints keyUsage = $ENV::certkeyusage subjectAltName = $ENV::subjectAltName [ hmodname ] hwType = OID:$ENV::hwType hwSerialNum = FORMAT:HEX,OCT:$ENV::hwSerialNum [ v3_ca ] # Extensions for a typical CA (`man x509v3_config`). subjectKeyIdentifier = hash authorityKeyIdentifier = keyid:always basicConstraints = $ENV::basicConstraints keyUsage = $ENV::certkeyusage [ v3_intermediate_ca ] # Extensions for a typical intermediate CA (`man x509v3_config`). subjectKeyIdentifier = hash authorityKeyIdentifier = keyid:always # basicConstraints = $ENV::basicConstraints # keyUsage = $ENV::certkeyusage # subjectAltName = $ENV::subjectAltName [ usr_cert ] # Extensions for client certificates (`man x509v3_config`). subjectKeyIdentifier = hash authorityKeyIdentifier = keyid:always # uncomment the following if the ENV variables set # crlDistributionPoints = $ENV::crlDP # authorityInfoAccess = $ENV::ocspIAI [ usr_req ] # Extensions for client certificates (`man x509v3_config`). subjectAltName = critical, $ENV::subjectAltName [ server_cert ] # Extensions for server certificates (`man x509v3_config`). subjectKeyIdentifier = hash authorityKeyIdentifier = keyid:always # uncomment the following if the ENV variables set # crlDistributionPoints = $ENV::crlDP # authorityInfoAccess = $ENV::ocspIAI [ 8021ar_idevid ] # Extensions for IEEE 802.1AR iDevID # certificates (`man x509v3_config`). subjectKeyIdentifier = hash authorityKeyIdentifier = keyid:always # uncomment the following if the ENV variables set # crlDistributionPoints = $ENV::crlDP # authorityInfoAccess = $ENV::ocspIAI [ crl_ext ] # Extension for CRLs (`man x509v3_config`). authorityKeyIdentifier=keyid:always [ ocsp ] # Extension for OCSP signing certificates (`man ocsp`). subjectKeyIdentifier = hash authorityKeyIdentifier = keyid:always ]]>