Sunday, December 21, 2014

Support for elliptic curves by jarsigner

Summary: Support for cryptography features by jarsigner depends on available Java crypto providers.

Suppose you are defining a PKI profile. You naturally want to use the stronger algorithms with better performance, which (as of year 2014) most likely means elliptic curves. Besides bit strength and performance, you want to be absolutely sure that the curve is supported by your software. If the latter includes jarsigner, you'll be surprised to find that Oracle documentation seems to not mention at all, what elliptic curves does jarsigner support.

Signing a JAR means adding digests of the JAR data to the manifest file, adding digest of the latter to the manifest signature file, and then creating the JAR signature block file. The last step involves two operations:
  1. calculating a digest over the manifest signature file;
  2. signing - meaning, encrypting with the private key - that digest. 
Jarsigner has an option '-sigalg', which is supposed to specify the two algorithms used in these two steps. (There is also '-digestalg' option, but it is not used for the signature block file; it defines the algorithm used in the two initial steps.) Well, this option is irrelevant for the signing step: the curve is in fact defined by the provided private key. So jarsigner will either do the job or choke on the key which comes from an unsupported curve.

A curve may "not work" because it is unknown to jarsigner itself, or to an underlying crypto provider. (The latter case was a reason to a bug 1006776; only three curves actually worked, while many returned a totally unclear error "certificate exception: subject key, Could not create EC public key".)

In a particular setup the support can be tested. For curves, supported by OpenSSL, the test can be done by creating the keypair on each curve and attempting the signing. Create the list of curves with 'openssl ecparam -list_curves', remove manually some extra words openssl puts there, and feed it to the stdin:

# Test, which OpenSSL-supported elliptic curves from the list are supported also by jarsigner.

key_alias="foo"         # Identificator of the key in the keystore
storepass="123456"      # jarsigner requires some

touch $source_data

while read curve; do
        # Generate an ECDSA private key for the selected curve:
        openssl ecparam -name $curve -genkey -out $key

        # Generate the certificate for the key; give some dummy subject:
        openssl req -new -x509 -nodes -key $key -out $cert -subj /CN=foo

        # Wrap key+cert in a PKCS12, so that jarsigner can use it:
        openssl pkcs12 -export -in $cert -inkey $key -passout pass:$storepass -out $pfx -name $key_alias

        # Create a fresh jar and attempt to sign it
        jar cf $jar $source_data
        jarsigner -keystore $pfx -storetype PKCS12 -storepass $storepass $jar $key_alias
        [ $? -eq 0 ] && echo $curve >> $result

rm $source_data $key $cert $pfx $jar

And enjoy the list in supported-curves.txt.

Conclusion: support of elliptic curves by jarsigner depends on jarsigner itself and on the JRE configuration. There is no command-line option to list all supported curves. For a particular system, support of curves supported by OpenSSL can be easily tested.

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