Prerequisites:

jq 

You can create a JWT using a library or by encoding the header and payload base64Url and signing it with a secret using a ES256K algorithm. Here is an example of how you can create a JWT using the jq and openssl command line utilities:

Here is an example of how you can use openssl to sign a JWT with the ES256K algorithm:

    Generate an ECDSA key pair using the secp256k1 curve:

openssl ecparam -name secp256k1 -genkey -noout -out private.pem
openssl ec -in private.pem -pubout -out public.pem

    First, create a header object as a JSON object containing the alg (algorithm) and typ (type) fields. For example:

header='{"alg":"ES256K", "issuer": "", "typ":"JWT"}'

    Next, create a payload object as a JSON object containing the claims you want to include in the JWT. For example schema.org :

payload='{"@context": "http://schema.org", "@type": "PlanAction", "identifier": "did:ethr:0xb86913f83A867b5Ef04902419614A6FF67466c12", "name": "Test", "description": "Me"}'

    Encode the header and payload objects as base64Url strings. You can use the jq command line utility to do this:

header_b64=$(echo -n "$header" | jq -c -M . | tr -d '\n')
payload_b64=$(echo -n "$payload" | jq -c -M . | tr -d '\n')

    Concatenate the encoded header, payload, and a secret to create the signing input:

signing_input="$header_b64.$payload_b64"

    Create the signature by signing the signing input with a ES256K algorithm and your secret. You can use the openssl command line utility to do this:

signature=$(echo -n "$signing_input" | openssl dgst -sha256 -sign private.pem)

    Finally, encode the signature as a base64Url string and concatenate it with the signing input to create the JWT:

signature_b64=$(echo -n "$signature" | base64 | tr -d '=' | tr '+' '-' | tr '/' '_')
jwt="$signing_input.$signature_b64"

    This JWT can then be passed in the Authorization header of a HTTP request as a bearer token, for example:

Authorization: Bearer $jwt

    To verify the JWT, you can use the openssl utility with the public key:

openssl dgst -sha256 -verify public.pem -signature <(echo -n "$signature") "$signing_input"

    This will verify the signature and output Verified OK if the signature is valid. If the signature is not valid, it will output an error.

