Invalid JWT signature with ES256

Question

I'm trying to manually create an ES256 JWT token. I've a small script written in python which signs a sha256 hash which uses ecdsa-python. But the signature is invalid on jwt.io.

Steps to reproduce:

Create base64 header + payload:

eyJhbGciOiJFUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWUsImlhdCI6MTUxNjIzOTAyMn0

Create SHA256 hash from the base64 header + payload:

FFC89E33091FFDD3C61798A0A74BF7C2D1A6FD231A6CB519F33952F7696BBE9F

Generate ec_private key:

openssl ec -in ec_private.pem -noout -text

Use the small python program to ecdsa sign the SHA256 hash

from json import dumps
from ellipticcurve.ecdsa import Ecdsa
from ellipticcurve.privateKey import PrivateKey

import base64

def toBase64Url(input):
    return input.replace("+", "-").replace("/", "_").rstrip("=")

# Generate privateKey from PEM string
privateKey = PrivateKey.fromPem("""
    -----BEGIN EC PARAMETERS-----
    BgUrgQQACg==
    -----END EC PARAMETERS-----
    -----BEGIN EC PRIVATE KEY-----
    MHcCAQEEIJfChy9fKFItzqcb8DKBm+2oH0YTZ7N61SQpyABgVZANoAoGCCqGSM49
    AwEHoUQDQgAE1TG2uvIMdfWkteiDWeHNYbQNSW/0uoYcvX4Z7ROUIgYRvgfpsjBa
    Iv70SuYpmBLwl0AuEBoXIVTCclCme6SdEQ==
    -----END EC PRIVATE KEY-----
""")

# Create message from json
message = "FFC89E33091FFDD3C61798A0A74BF7C2D1A6FD231A6CB519F33952F7696BBE9F"

signature = Ecdsa.sign(message, privateKey)

# Generate Signature in base64. This result can be sent to Stark Bank in the request header as the Digital-Signature parameter.
print("Base64: " + signature.toBase64())
print("Base64Url: " +toBase64Url(signature.toBase64()))



# To double check if the message matches the signature, do this:
publicKey = privateKey.publicKey()

print("Hash verification succesfull: " + str(Ecdsa.verify(message, signature, publicKey)))

The output:

Base64: MEQCIFyP4IoZGhzGfDCPX6fVxjtB+nrXDVhOTQwdc5vu8z4eAiBNalfGHqdaO3nCmTqimpAHF+IHzxk8em+OMMHrJkPOhA==

Base64Url: MEQCIFyP4IoZGhzGfDCPX6fVxjtB-nrXDVhOTQwdc5vu8z4eAiBNalfGHqdaO3nCmTqimpAHF-IHzxk8em-OMMHrJkPOhA

Hash verification succesfull: True

Check the signature in jwt.io gives Invalid signature

eyJhbGciOiJFUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWUsImlhdCI6MTUxNjIzOTAyMn0.MEQCIFyP4IoZGhzGfDCPX6fVxjtB-nrXDVhOTQwdc5vu8z4eAiBNalfGHqdaO3nCmTqimpAHF-IHzxk8em-OMMHrJkPOhA

Keys:

Public:

-----BEGIN PUBLIC KEY----- MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE1TG2uvIMdfWkteiDWeHNYbQNSW/0 uoYcvX4Z7ROUIgYRvgfpsjBaIv70SuYpmBLwl0AuEBoXIVTCclCme6SdEQ== -----END PUBLIC KEY-----

Private:

-----BEGIN EC PRIVATE KEY----- MHcCAQEEIJfChy9fKFItzqcb8DKBm+2oH0YTZ7N61SQpyABgVZANoAoGCCqGSM49 AwEHoUQDQgAE1TG2uvIMdfWkteiDWeHNYbQNSW/0uoYcvX4Z7ROUIgYRvgfpsjBa Iv70SuYpmBLwl0AuEBoXIVTCclCme6SdEQ== -----END EC PRIVATE KEY-----

I know that there are many jwt signing python libraries but the use of this is to understand how a jwt token is created.

EDIT:

As @Topaco pointed out this library uses curve secp256k1 instead of secp256r1. secp256r1 | prime256v1 | NIST P-256 are all different names chosen by different standards organizations for the same curve (Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)). I changed the library to python-ecdsa and the code to:

from ecdsa import SigningKey, NIST256p
import base64

def toBase64Url(input):
    return input.replace("+", "-").replace("/", "_").rstrip("=")


sk = SigningKey.from_pem("""
    -----BEGIN EC PRIVATE KEY-----
    MHcCAQEEIJfChy9fKFItzqcb8DKBm+2oH0YTZ7N61SQpyABgVZANoAoGCCqGSM49
    AwEHoUQDQgAE1TG2uvIMdfWkteiDWeHNYbQNSW/0uoYcvX4Z7ROUIgYRvgfpsjBa
    Iv70SuYpmBLwl0AuEBoXIVTCclCme6SdEQ==
    -----END EC PRIVATE KEY-----
""")
vk = VerifyingKey.from_pem("""
-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE1TG2uvIMdfWkteiDWeHNYbQNSW/0
uoYcvX4Z7ROUIgYRvgfpsjBaIv70SuYpmBLwl0AuEBoXIVTCclCme6SdEQ==
-----END PUBLIC KEY-----
""")
signature = sk.sign(b"FFC89E33091FFDD3C61798A0A74BF7C2D1A6FD231A6CB519F33952F7696BBE9F")

print(base64.b64encode(signature))
print("Base64: " + base64.b64encode(signature).decode("utf-8"))
print("Base64Url: " + toBase64Url(base64.b64encode(signature).decode("utf-8")))

assert vk.verify(signature, b"FFC89E33091FFDD3C61798A0A74BF7C2D1A6FD231A6CB519F33952F7696BBE9F")
print("Hash verification succesfull: " + str(vk.verify(signature, b"FFC89E33091FFDD3C61798A0A74BF7C2D1A6FD231A6CB519F33952F7696BBE9F")))

The output:

Base64: rMBgC0ismGdd5rd7n1L+LDsQ2UO5+cjBwPNYh+xBZvO6fKoJIfmfyNpxw+kxmyKWlK+55dF5eMH1u327DMJvvA==

Base64Url: rMBgC0ismGdd5rd7n1L-LDsQ2UO5-cjBwPNYh-xBZvO6fKoJIfmfyNpxw-kxmyKWlK-55dF5eMH1u327DMJvvA

Hash verification succesfull: True

But the signature is still invalid.

ES256 is defined for curve P-256 aka secp256r1. The library you are using applies secp256k1, i.e. you need another library that supports secp256r1. — Topaco
@Topaco thank you for pointing that out, I changed libraries and updated my question with the new information — Sam

Topaco Topaco · Accepted Answer · 2021-02-25T15:41:42

The library you are using hashes implicitly, applying SHA1 by default. I.e. for compatibility with ES256 SHA256 must be explicitly specified and the unhashed JWT must be used, e.g.:

from ecdsa import SigningKey, VerifyingKey
import base64
from hashlib import sha256

def toBase64Url(input):
    return input.replace("+", "-").replace("/", "_").rstrip("=")

jwt = b"eyJhbGciOiJFUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWUsImlhdCI6MTUxNjIzOTAyMn0"

sk = SigningKey.from_pem("""
-----BEGIN EC PRIVATE KEY-----
MHcCAQEEIJfChy9fKFItzqcb8DKBm+2oH0YTZ7N61SQpyABgVZANoAoGCCqGSM49
AwEHoUQDQgAE1TG2uvIMdfWkteiDWeHNYbQNSW/0uoYcvX4Z7ROUIgYRvgfpsjBa
Iv70SuYpmBLwl0AuEBoXIVTCclCme6SdEQ==
-----END EC PRIVATE KEY-----
""")
vk = VerifyingKey.from_pem("""
-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE1TG2uvIMdfWkteiDWeHNYbQNSW/0
uoYcvX4Z7ROUIgYRvgfpsjBaIv70SuYpmBLwl0AuEBoXIVTCclCme6SdEQ==
-----END PUBLIC KEY-----
""")

signature = sk.sign(jwt, hashfunc=sha256)

print("Base64: " + base64.b64encode(signature).decode("utf-8"))
print("Base64Url: " + toBase64Url(base64.b64encode(signature).decode("utf-8")))

assert vk.verify(signature, jwt, hashfunc=sha256)
print("Hash verification succesfull: " + str(vk.verify(signature, jwt, hashfunc=sha256)))

A possible output is:

Base64: Mr4/DF87ek66E2GcAc+2H3ulHplCnxygz65h9dkdvm8QsZBbm2N5EjIgyiWsynza9zCGjjnzBUiXYvij9LLikA==
Base64Url: Mr4_DF87ek66E2GcAc-2H3ulHplCnxygz65h9dkdvm8QsZBbm2N5EjIgyiWsynza9zCGjjnzBUiXYvij9LLikA
Hash verification succesfull: True

The resulting signed token

eyJhbGciOiJFUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWUsImlhdCI6MTUxNjIzOTAyMn0.Mr4_DF87ek66E2GcAc-2H3ulHplCnxygz65h9dkdvm8QsZBbm2N5EjIgyiWsynza9zCGjjnzBUiXYvij9LLikA

can then be successfully verified on https://jwt.io/ with the public key used here.

Invalid JWT signature with ES256

1 Answers