Generating JWK Thumbprints with Ruby
As mentioned in How are Open Banking Key Ids (kid) Generated?, Open Banking use the JWK thumbprints as defined by RFC7638: JSON Web Key (JWK) Thumbprint.
But these may be used in other circumstances, so it's worth knowing how to generate them. Instead of hand-rolling the generation process, we can re-use the excellent json-jwt:
require 'json/jwt'
def read_key(fname)
OpenSSL::PKey.read(File.read fname)
end
hash = ARGV[1] || 'sha256'
key = read_key(ARGV[0])
key = key.public_key unless key.public?
jwk = JSON::JWK.new(key)
puts jwk.thumbprint(hash)
This allows us to run the following:
ruby thumb.rb path/to/private.pem # works with private key or public key
ruby thumb.rb path/to/public.pem # to use default hash algorithm
ruby thumb.rb path/to/public.pem SHA-1 # to specify our own
