jwx-guide-v4

jwx-guide-v4

This skill helps you assist Go developers who are using github.com/lestrrat-go/jwx/v4 in their own projects. It is scoped to v4 only — for v3 or v2, install the corresponding jwx-dev-v3 / jwx-dev-v2 plugin.

Where to look things up

You will not have this repository checked out. To verify an API claim before answering, use these sources, in order of preference:

1. Go module cache — if the user's project depends on jwx, the source is on disk:

   echo "$(go env GOMODCACHE)/github.com/lestrrat-go/jwx/v4@$(go list -m -f '{{.Version}}' github.com/lestrrat-go/jwx/v4)"
   

   This directory contains the full source tree including docs/, jwt/, jws/, jwe/, jwk/, jwa/.
2. pkg.go.dev — canonical API reference for any exported symbol:
   • https://pkg.go.dev/github.com/lestrrat-go/jwx/v4
   • https://pkg.go.dev/github.com/lestrrat-go/jwx/v4/jwt (etc. per subpackage)
3. GitHub — narrative documentation lives at the repo:
   • https://github.com/lestrrat-go/jwx/blob/develop/v4/docs/01-jwt.md
   • https://github.com/lestrrat-go/jwx/blob/develop/v4/docs/02-jws.md
   • https://github.com/lestrrat-go/jwx/blob/develop/v4/docs/03-jwe.md
   • https://github.com/lestrrat-go/jwx/blob/develop/v4/docs/04-jwk.md
   • https://github.com/lestrrat-go/jwx/blob/develop/v4/docs/10-extensions.md
   • https://github.com/lestrrat-go/jwx/blob/develop/v4/docs/99-faq.md
4. Examples repo — runnable usage patterns. The repo's README.md is a topical index that maps "what do I want to do" → "which *_example_test.go file." Fetch the README first when looking for an example by topic; then fetch the linked file:
   • https://github.com/jwx-go/examples/blob/develop/v4/README.md

When the user's question goes beyond the patterns in this skill (custom claim types, JWE recipients with per-recipient headers, nested JWS+JWE serializers, custom key providers, base64 backend swap, performance tuning), fetch from these sources rather than guessing.

Prerequisites

• Go 1.26+ is required. v4 uses generics features and stdlib additions (errors.AsType[T]) that are new in 1.26.
• GOEXPERIMENT=jsonv2 must be set for every go build/go test/go run because v4 depends on encoding/json/v2. Without it, builds fail with build constraints exclude all Go files.

Sub-package map:

Critical rules (read first)

1. jwt.Parse verifies AND validates by default. A bare jwt.Parse(data) errors because no key was supplied. To intentionally skip both, use jwt.ParseInsecure. To verify but skip claim validation, pass jwt.WithValidate(false). This is deliberately asymmetric vs. jws.Parse/jwe.Parse (which only parse). Do not "correct" it.
2. Always pin the algorithm on the verify side. jwt.WithKey(jwa.RS256(), key), jws.WithKey(jwa.ES256(), key). Never trust the alg from the incoming header alone.
3. Never use jwt.ParseInsecure for tokens received from the network. It is for testing or for extracting claims from a token whose origin is already trusted by other means.
4. jwa algorithms are functions in v4, not constants. Write jwa.RS256(), not jwa.RS256. This trips up users migrating from v2/v3.
5. kid matching is enforced when verifying with a JWK Set. Override the requirement with jwt.WithKeySet(set, jws.WithRequireKid(false)) only when you understand the consequences.
6. jku (key URL in the JWS header) is attacker-controlled. Use jwt.WithVerifyAuto only with a jwkfetch.Client configured with a jwkfetch.NewMapWhitelist() of allowed URLs.
7. HMAC keys are []byte, not string. Pass []byte("secret"), or better, a jwk.Key imported from those bytes.
8. Generic functions require explicit type parameters. jwk.ParseKey[jwk.Key](data), jwk.Import[jwk.Key](raw), jwk.Export[*rsa.PublicKey](key). Bare jwk.ParseKey(data) does not compile.

Verifying a JWT (the 90% case)

import (
    "github.com/lestrrat-go/jwx/v4/jwa"
    "github.com/lestrrat-go/jwx/v4/jwt"
)

tok, err := jwt.Parse(raw, jwt.WithKey(jwa.RS256(), publicKey))
if err != nil {
    // signature failed, claim validation failed, or parse failed
    return err
}
// tok is verified + validated; safe to read claims

publicKey may be:

• a *rsa.PublicKey / *ecdsa.PublicKey / ed25519.PublicKey from crypto/*
• []byte for HMAC algorithms
• a jwk.Key

To validate against expected claim values, pass jwt.WithIssuer(...), jwt.WithAudience(...), jwt.WithSubject(...), jwt.WithJwtID(...). To extend the default clock skew window: jwt.WithAcceptableSkew(30 * time.Second).

Verifying with a JWK Set (kid-based key selection)

set, err := jwk.Parse(jwksBytes)
if err != nil { return err }

tok, err := jwt.Parse(raw, jwt.WithKeySet(set))

If the JWS header has a kid, the matching key is selected from the set. The algorithm comes from each key's alg field or is inferred from the key type. To opt out of kid-required matching: jwt.WithKeySet(set, jws.WithRequireKid(false)).

Verifying via JWKS endpoint

HTTP fetching is not in the core jwk package in v4. It lives in a companion module:

import "github.com/jwx-go/jwkfetch/v4"

client := jwkfetch.NewClient()
set, err := client.Fetch(ctx, "https://issuer.example/jwks.json")
// then: jwt.Parse(raw, jwt.WithKeySet(set))

For repeated fetches with background refresh, use jwkfetch.NewCache. For jku-driven verification, build a jwkfetch.Client with a jwkfetch.NewMapWhitelist() of allowed URLs and pass it to jwt.WithVerifyAuto(client).

Signing a JWT

tok, err := jwt.NewBuilder().
    Issuer("https://issuer.example").
    Audience([]string{"https://api.example"}).
    Subject("user-123").
    IssuedAt(time.Now()).
    Expiration(time.Now().Add(15 * time.Minute)).
    Claim("scope", "read:things").
    Build()
if err != nil { return err }

signed, err := jwt.Sign(tok, jwt.WithKey(jwa.RS256(), privateKey))

privateKey may be a *rsa.PrivateKey/*ecdsa.PrivateKey/ed25519.PrivateKey/HMAC []byte, or a jwk.Key.

Match algorithm to key type:

JWK basics

All generic accessors require the type parameter:

// Parse a single JWK (returns jwk.Key):
key, err := jwk.ParseKey[jwk.Key](jwkBytes)

// Parse a single JWK with a concrete type (fails if not that type):
rsaKey, err := jwk.ParseKey[jwk.RSAPublicKey](jwkBytes)

// Parse a JWK Set (returns jwk.Set, not generic):
set, err := jwk.Parse(jwksBytes)

// Wrap an existing crypto.* key as a jwk.Key:
key, err := jwk.Import[jwk.Key](rsaPrivKey)

// Or with a concrete jwk type:
typed, err := jwk.Import[jwk.RSAPrivateKey](rsaPrivKey)

// Export a jwk.Key back to a crypto.* key:
raw, err := jwk.Export[*rsa.PublicKey](key)

jwk.Import[jwk.Key] is the default. Use a concrete type parameter (jwk.RSAPrivateKey, jwk.ECDSAPublicKey, jwk.SymmetricKey, jwk.OKPPublicKey, etc.) only when you want compile-time guarantees and acceptance of failure when the input is anything else.

Generating keys

import (
    "crypto/rand"
    "crypto/rsa"

    "github.com/lestrrat-go/jwx/v4/jwa"
    "github.com/lestrrat-go/jwx/v4/jwk"
)

raw, _ := rsa.GenerateKey(rand.Reader, 2048)
key, _ := jwk.Import[jwk.Key](raw)
key.Set(jwk.KeyIDKey, "2025-q1")
key.Set(jwk.AlgorithmKey, jwa.RS256())

pubKey, _ := jwk.PublicKeyOf(key)

jwk.KeyIDKey and jwk.AlgorithmKey are string constants for the standard JWK fields kid and alg.

JWS (signing arbitrary payloads)

sig, err := jws.Sign(payload, jws.WithKey(jwa.ES256(), privateKey))
payload, err := jws.Verify(sig, jws.WithKey(jwa.ES256(), publicKey))

jws.Parse only parses the structure — it does not verify. Use jws.Verify (which returns the verified payload) for verification.

JWE (encrypting payloads)

enc, err := jwe.Encrypt(
    payload,
    jwe.WithKey(jwa.RSA_OAEP_256(), recipientPublicKey),
    jwe.WithContentEncryption(jwa.A256GCM()),
)

plain, err := jwe.Decrypt(enc, jwe.WithKey(jwa.RSA_OAEP_256(), recipientPrivateKey))

jwe.WithKey(alg, key) is the same on both encrypt and decrypt sides — this symmetry is intentional.

Companion modules

Beyond the core github.com/lestrrat-go/jwx/v4 module, the project ships companion modules under github.com/jwx-go. The agent should know what's available and when to reach for each one — depth lives in each module's godoc.

For algorithm and HPKE modules: import for side effects (import _ "..."). They register themselves in init() and panic at import time if registration fails (intentional — surfaces problems early).

Signature algorithms (extension)

Key agreement / encryption (extension)

Tooling and backends

Cross-references

• Canonical extension docs (always-current list, with examples): https://github.com/lestrrat-go/jwx/blob/develop/v4/docs/10-extensions.md
• Each companion's godoc: https://pkg.go.dev/github.com/jwx-go/<name>/v4

What to do when the user asks about post-quantum or non-default algorithms

Default jwx supports the common RFC 7518 algorithms (RS*, PS*, ES*, HS*, EdDSA, AGCM, RSA-OAEP-, etc.) out of the box. For everything in the tables above, the user must add the companion module to their go.mod and import it for side effects. If a user reports an algorithm not registered or similar error for ES256K/Ed448/ML-DSA/ML-KEM/X448, they almost certainly missed the side-effect import.

Errors

JWT/JWS/JWE/JWK errors are struct types with named fields. Use errors.Is with a zero-value struct to test for kind, or Go 1.26's errors.AsType[T] to recover the fields:

import "errors"

tok, err := jwt.Parse(raw, jwt.WithKey(jwa.RS256(), key))
if err != nil {
    if errors.Is(err, jwt.TokenExpiredError{}) {
        // exp claim failed
    }
    if e, ok := errors.AsType[jwt.InvalidAudienceError](err); ok {
        log.Printf("audience mismatch: %+v", e)
    }
}

Common types:

• jwt: TokenExpiredError, TokenNotYetValidError, InvalidIssuerError, InvalidAudienceError, MissingRequiredClaimError, ValidationError, ParseError
• jws: VerificationError() factory (signature verification failed)
• jwe: DecryptError() factory, AlgorithmMismatchError
• jwk: KeyTypeMismatchError, ImportError(), ParseError()

Do not match errors by string contents — messages may change.

Token field access

Standard claims have dedicated typed accessors (returning (value, ok)):

exp, ok := tok.Expiration()  // time.Time
iss, ok := tok.Issuer()      // string
aud, ok := tok.Audience()    // []string
sub, ok := tok.Subject()     // string
nbf, ok := tok.NotBefore()
iat, ok := tok.IssuedAt()
jti, ok := tok.JwtID()

For private claims, use tok.Field(name) which returns (any, bool). For type-safe access, use jwt.Get[T](tok, name).

Common mistakes to flag in user code

When reviewing or writing jwx-using code, watch for these:

1. jwt.Parse(data) with no key option — errors out; if the intent was to read claims without verifying, that's a security bug unless the source is already trusted, in which case use jwt.ParseInsecure.
2. jwa.RS256 instead of jwa.RS256() — these are functions in v4.
3. jwk.ParseKey(data) or jwk.Import(raw) without the type parameter — won't compile.
4. Type-asserting a jwk.Key to a crypto.* type. Use jwk.Export[*rsa.PublicKey](key) instead.
5. Hardcoding the alg from the JWS header (or token contents) to pick a verifier — always pin the expected algorithm on the verify side.
6. Reusing a single jwt.Builder across goroutines — builders aren't safe to share.
7. Verifying by passing a private key — works but leaks intent. Use the public key on the verify side.
8. Manually building JWS compact strings via concatenation — always go through jws.Sign/jwt.Sign.
9. tok.Get("exp") — the method is tok.Field("exp") (or just tok.Expiration()).
10. Disabling kid matching as a "make it work" shortcut. Understand why the kid doesn't match before adding jws.WithRequireKid(false).

What NOT to suggest

• Disabling signature verification to "make it work."
• Forking or vendoring jwx to add an algorithm — recommend the extension modules listed above.
• Decoding the JWT envelope with encoding/json directly — always go through jwt.Parse / jws.Verify.
• Recommending a different JWT library — out of scope for this skill.