Skip to content
ecdsa.com

JWK thumbprint · .NET

How to compute a JWK thumbprint (RFC 7638) in C#

In C#, Microsoft.IdentityModel.Tokens computes a JWK thumbprint with JsonWebKey.ComputeJwkThumbprint() — SHA-256 over the RFC 7638 canonical form — and JsonWebKeyConverter turns ECDsa and RSA keys loaded from PEM into a JsonWebKey first. The run reproduces the published RFC 7638 thumbprint and the EC thumbprint every other recipe of this task computes.

Tested with Microsoft.IdentityModel.Tokens 8.23.0 (.NET SDK 10.0.402).

Same recipe in:Node.jsPythonGoOpenSSLWebCryptoJava.NETRustPHPRubyKotlinSwift
app.cs — thumbprints from JWK JSON and from PEM keys (a .NET 10 file-based app)
#:package Microsoft.IdentityModel.Tokens@8.23.0
using System.Security.Cryptography;
using System.Text;
using Microsoft.IdentityModel.Tokens;
 
static string Thumbprint(JsonWebKey jwk) => Base64UrlEncoder.Encode(jwk.ComputeJwkThumbprint()); // SHA-256, RFC 7638
 
// From a JWK: extra members such as kid, use and alg are ignored
foreach (var file in new[] { "ec.jwk.json", "rsa.jwk.json" })
{
    Console.WriteLine($"{file}: {Thumbprint(new JsonWebKey(File.ReadAllText(file)))}");
}
 
// From a PEM public key: convert the key to a JsonWebKey first
using var ecdsa = ECDsa.Create();
ecdsa.ImportFromPem(File.ReadAllText("ec.pub.pem"));
Console.WriteLine($"ec.pub.pem: {Thumbprint(JsonWebKeyConverter.ConvertFromECDsaSecurityKey(new ECDsaSecurityKey(ecdsa)))}");
 
using var rsa = RSA.Create();
rsa.ImportFromPem(File.ReadAllText("rsa.pub.pem"));
Console.WriteLine($"rsa.pub.pem: {Thumbprint(JsonWebKeyConverter.ConvertFromRSASecurityKey(new RsaSecurityKey(rsa)))}");
 
// Ed25519 (OKP): ComputeJwkThumbprint rejects it (IDX10706), so hash the RFC 8037 members directly
var okp = new JsonWebKey(File.ReadAllText("okp.jwk.json"));
byte[] canonical = Encoding.UTF8.GetBytes($$"""{"crv":"{{okp.Crv}}","kty":"{{okp.Kty}}","x":"{{okp.X}}"}""");
Console.WriteLine($"okp.jwk.json: {Base64UrlEncoder.Encode(SHA256.HashData(canonical))}");

How it works

  1. new JsonWebKey(json) parses a JWK; members other than the required ones are kept but ignored by the thumbprint.
  2. ComputeJwkThumbprint() returns the raw SHA-256 digest; Base64UrlEncoder.Encode turns it into the thumbprint string.
  3. For PEM keys, ECDsa.ImportFromPem or RSA.ImportFromPem loads the key, and JsonWebKeyConverter.ConvertFromECDsaSecurityKey or ConvertFromRSASecurityKey builds the JsonWebKey.
  4. CanComputeJwkThumbprint() tells in advance whether a key type is supported; for Ed25519 it is false, so the last lines hash the RFC 8037 members crv, kty, x directly with SHA256.HashData.

Gotchas

  • OKP keys (Ed25519) are not supported: ComputeJwkThumbprint throws IDX10706: Cannot create a JWK thumbprint, 'Kty' must be one of the following: 'EC, RSA, oct, AKP'. The raw string literal above builds their canonical form; it is safe because every member value is base64url or a fixed name.
  • Convert.ToBase64String is standard base64, with +, / and padding; thumbprints need Base64UrlEncoder.Encode.
  • X509Certificate2.Thumbprint is the SHA-1 hash of the whole certificate — a different value with a similar name. A JWK thumbprint covers only the public key.
  • The kid a JsonWebKey carries is whatever the JSON said; IdentityModel does not derive it from the thumbprint unless you set KeyId = Base64UrlEncoder.Encode(jwk.ComputeJwkThumbprint()).

Related recipes