Generate a CSR for an asymmetric key stored in a YubiHSM 2 with OpenSSL and yubihsm-shell

gencsr.sh

#!/bin/bash

# Generate a Certificate Signing Request (CSR) for an asymmetric key stored in a YubiHSM 2

# Usage:
# ./gencsr.sh <id> <cn>
#
# where <id> is the object ID of the asymmetric key,
# and <cn> is the Common Name of the subject DN in the generated CSR.

ID=$1
CN=$2

# note that the subject DN of the generated CSR will consist of a Common Name only
# other fields will typically be overwritten by the CA anyway

if [[ -z "$ID" ]]; then
  echo need an object ID
  echo "usage: $0 <id> <cn>"
  exit
fi

if [[ -z "$CN" ]]; then
  echo need a Common Name
  echo "$usage: 0 <id> <cn>"
  exit
fi


TMP=$(mktemp -d)

echo === Retrieving Public Key
yubihsm-shell -C yhusb:// -p password -a get-public-key -i $ID > pubkey.pem || { echo "ERROR: Cannot find asymmetric key with ID $ID"; exit ; }

# quick & dirty: extract public key bytes from a EC p256 public key
LEN=$(openssl asn1parse -in pubkey.pem | tail -1 | sed -E 's/.*l= +([0-9]+).*/\1/')
pubkey=$(openssl pkey -in pubkey.pem -pubin -outform der | tail -c $(($LEN-1)) | xxd -p | tr -d '\n')

echo === Getting Key Info
ALG=$(yubihsm-shell -C yhusb:// -p password -a get-object-info -i $ID -t asymmetric-key | grep -o 'algorithm: [[:alnum:]]*' | cut -d' ' -f2)
echo === Algorithm $ALG
case "$ALG" in
  rsa2048 | rsa3072 | rsa4096)
    signatureAlgorithm="sha256WithRSAEncryption"
    SIGALG=rsa-pkcs1-sha256
    SIGN=sign-pkcs1v15 ;;
  ecp224 | ecp256 | ecp384 | ecp521 | eck256)
    signatureAlgorithm="ecdsa-with-SHA256"
    SIGALG=ecdsa-sha256
    SIGN=sign-ecdsa ;;
  ed25519*)
    signatureAlgorithm="ED25519"
    SIGALG=ed25519
    SIGN=sign-eddsa ;;
  *)
    echo "unknown algorithm $ALG"
    exit ;;
esac

echo "=== Generate Certificate Signing Request (CSR)"
# CSR template before signing
cat << EOF > $TMP/tbs.asn1
asn1 = SEQUENCE:CertificationRequestInfo

[CertificationRequestInfo]
version = INTEGER:0x00
subject = SEQUENCE:dn
subjectPKInfo = SEQUENCE:PublicKeyInfo
attributes = IMPLICIT:0,SEQUENCE:Attributes

[dn]
dn = SET:rdn

[rdn]
cn = SEQUENCE:cn

[cn]
cn = OBJECT:commonName
val = FORMAT:UTF8,UTF8String:"$CN"

[PublicKeyInfo]
algorithm = SEQUENCE:$ALG
subjectPublicKey = FORMAT:HEX,BITSTRING:$pubkey

[ecp224]
algorithm = OBJECT:id-ecPublicKey
parameters = OBJECT:secp224r1

[ecp256]
algorithm = OBJECT:id-ecPublicKey
parameters = OBJECT:prime256v1

[ecp384]
algorithm = OBJECT:id-ecPublicKey
parameters = OBJECT:secp384r1

[ecp521]
algorithm = OBJECT:id-ecPublicKey
parameters = OBJECT:secp521r1

[eck256]
algorithm = OBJECT:id-ecPublicKey
parameters = OBJECT:secp256k1

[ed25519]
parameters = OBJECT:ED25519

[rsa2048]
parameters = OBJECT:rsaEncryption
parameters = NULL

[rsa3072]
algorithm = OBJECT:rsaEncryption
parameters = NULL

[rsa4096]
algorithm = OBJECT:rsaEncryption
parameters = NULL

[Attributes]
EOF

# The YubiHSM uses different names for PublicKeyInfo algorithm objects than OpenSSL
#
# YubiHSM	OpenSSL
# -------	-------
# ecp224	secp224r1
# ecp256	prime256v1
# ecp384	secp384r1
# ecp521	secp521r1
# eck256	secp256k1
# ecbp256	-
# ecbp384	-
# ecbp512	-
# rsa2048	rsaEncryption
# rsa3072	rsaEncryption
# rsa4096	rsaEncryption

# compile ASN.1
openssl asn1parse -genconf $TMP/tbs.asn1 -out $TMP/tbs.der #>/dev/null

# sign using YubiHSM
echo === Signing $SIGN / $SIGALG
yubihsm-shell -C yhusb:// -p password -a $SIGN -A $SIGALG -i $ID --in $TMP/tbs.der | base64 -d > $TMP/sig.der #2>/dev/null

# double check: verify signature
openssl dgst -sha256 -verify pubkey.pem -signature $TMP/sig.der $TMP/tbs.der
rm $TMP/tbs.der

# Generate CSR, include self-signature
cat << EOF > $TMP/csr.asn1
asn1 = SEQUENCE:csr

[csr]
cri = SEQUENCE:CertificationRequestInfo
signatureAlgorithm = SEQUENCE:signatureAlgorithm
signature = FORMAT:HEX,BITSTRING:$(xxd -p $TMP/sig.der | tr -d '\n')

[signatureAlgorithm]
algorithm = OBJECT:$signatureAlgorithm
parameters = NULL
EOF
rm $TMP/sig.der
cat $TMP/tbs.asn1 | grep -v ^asn1 >> $TMP/csr.asn1
rm  $TMP/tbs.asn1

# compile ASN.1
openssl asn1parse -genconf $TMP/csr.asn1 -out $TMP/csr.der >/dev/null
rm $TMP/csr.asn1

# verify signature
openssl req -in $TMP/csr.der -inform der -verify -out csr.pem && openssl req -in csr.pem
rm $TMP/csr.der
rmdir $TMP