For reference I was able to do what I wanted using python

#!/usr/bin/env python3

# Run with
#   python3 siwa_client_secret.py a.code
#
# requires to install
#   pip3 install cryptography
#   pip3 install pyjwt

import sys
import jwt
import time
import urllib
from urllib.parse import urlencode
from urllib.request import Request, urlopen

authorization_code = sys.argv[1]

bundle_id = "..."
key_id = "..."
team_id = "..."
key = """
-----BEGIN PRIVATE KEY-----
...
-----END PRIVATE KEY-----
"""


issued_at = int(round(time.time()))
expiration = issued_at + 3600

claims = {
    'aud': 'https://appleid.apple.com',
    'sub': bundle_id,
    'iss': team_id,
    'exp': expiration,
    'iat': issued_at,
}


client_secret = jwt.encode(claims, key, algorithm='ES256', headers={'kid': key_id}).decode("utf-8")
print("using secret: %s\n" % client_secret)

url = 'https://appleid.apple.com/auth/token'
payload = {
    "grant_type": "authorization_code",
    "client_secret": client_secret,
    "code": authorization_code,
    "client_id": bundle_id
}
headers = {
    "Content-type": "application/x-www-form-urlencoded"
}

print("calling apple...")

try:
    response = urlopen(Request(url, urlencode(payload).encode()))

    print("\033[1;32;48mcode: %d => %s \033[1;37;0m" % (response.code, response.read.decode("utf8", "ignore")))
except urllib.error.HTTPError as e:
    print("\033[1;31;48mcode: %d => %s \033[1;37;0m" % (e.code, e.read().decode("utf8", 'ignore')))

@bric3, @timmc, I think this is wrong. See this Stack Overflow for details, but effectively openssl is outputting a specific format of signature that isn't actually a valid JWS signature (not even a base64url decoded one). It's wrapping the two integers (R and S) in a DER format that can be read by the ASN1 module within openssl. This SO article goes into detail about that encoding/wrapper.

Hmm, thanks! That's unfortunate.

Luckily, I only ever used this script to generate values for a test suite, or something similar. We did have trouble with it not working on everyone's computer, so I wonder if there was a difference in output for certain openssl versions.

I'll put a giant disclaimer at the top. :-)

I posted my take on the procedure here.

I think you can fix this by making the following switch:

# Current signature construction
sig="$(echo -n "$message" | openssl dgst -sha256 -sign "$keypair_path" -keyform DER | base64_urlsafe)"

# Replacement
sig ="$(echo -n "$message" | openssl dgst -sha256 -sign "$keypair_path" -keyform DER | openssl asn1parse -inform DER | perl -n -e'/INTEGER           :([0-9A-Z]*)$/ && print $1' | xxd -p -r | base64_urlsafe)"

I've just added a couple extra processing steps:

1. To let openssl parse the signature
2. A regex to extract the R and S integers from that output
3. xxd to parse the strings as hex (that's how asn1parse outputs them) and converts them to binary before passing to your base64_urlsafe alg.

I combined all the above with results from multiple stack overflow topics into the following, hopefully it helped someone :)

kid="3UT......."
iss="69a6de78-....."
keyPath="somepath/AuthKey.p8"

iat=$(date +"%s")
exp=$(date --date="15 minutes" +"%s")
header=$(echo -n '{"alg": "ES256", "kid": "'$kid'", "typ": "JWT"}' | openssl base64 -e -A | tr '+/' '-_' | tr -d '=')
payload=$(echo -n '{"iss": "'$iss'", "iat": '$iat', "exp": '$exp', "aud": "appstoreconnect-v1"}' | openssl base64 -e -A | tr '+/' '-_' | tr -d '=')
sig=$(echo -n "$header.$payload" | openssl dgst -sha256 -sign "$keyPath" -keyform PEM | openssl asn1parse -inform DER | perl -n -e'/INTEGER           :([0-9A-Z]*)$/ && print $1' | xxd -p -r | openssl base64 -e -A | tr '+/' '-_' | tr -d '=')
jwt=$(echo -n "$header.$payload.$sig")

curl -X GET \
  -H "Authorization: Bearer $jwt" \
  -H "Content-Type: application/json" \
  https://api.appstoreconnect.apple.com/v1/apps