jwt-sign-ES256.sh

#!/bin/bash






echo "DO NOT USE -- incorrect signature format, see comments on gist."
exit 1





# Create and sign a JWT token with ES256 given the path to an ECDSA
# private key and a JSON payload.
# $0 path/to/keypair.der '{"JSON": "payload"}'

# Example keypair creation:
# openssl ecparam -name prime256v1 -genkey -noout -outform DER > example-keypair.der

# A few tips for generating the payload:
# - Pipe raw strings through `jq --raw-input .` to encode them as
#   JSON strings. https://stedolan.github.io/jq/
# - GNU date is great for generating the iat, nbf, and exp time
#   fields: `date --date="15 minutes" +"%s"`

set -eu -o pipefail

keypair_path="$1"
payload="$2"

function base64_urlsafe {
  # Implement own URL-safe Base64 based on standard version. Delete
  # padding, undo wrapping, and swap out chars 62 and 63. Not all
  # versions of `base64` have the `--wrap=0` that GNU coreutils has.
  base64 | tr -d '\r\n=' | tr '+/' '-_'
}

header_enc="$(echo -n '{"typ":"JWT","alg":"ES256"}' | base64_urlsafe)"
payload_enc="$(echo -n "$payload" | base64_urlsafe)"
message="$header_enc.$payload_enc"
# If you're on a Mac, you might have a really old version of openssl
# that doesn't support ECDSA signing this way.
sig="$(echo -n "$message" | openssl dgst -sha256 -sign "$keypair_path" -keyform DER | base64_urlsafe)"

echo -n "$message.$sig"

@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.