90 lines
2.6 KiB
Go
90 lines
2.6 KiB
Go
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// Package zitadel talks to the ZITADEL management API with the controller's own
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// machine key.
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//
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// The controller needs a credential to rotate credentials. It signs a short
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// assertion with its private key, exchanges it for an access token, and reuses
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// that token until shortly before it expires.
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package zitadel
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import (
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"crypto"
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"crypto/rand"
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"crypto/rsa"
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"crypto/sha256"
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"crypto/x509"
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"encoding/base64"
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"encoding/json"
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"encoding/pem"
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"fmt"
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"time"
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)
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// assertionLifetime is deliberately short. The assertion proves one request.
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const assertionLifetime = 60 * time.Second
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// newAssertionClaims builds the claim set for a machine-key assertion.
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//
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// ZITADEL expects `iss` to name the caller, `sub` to name the user it acts as,
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// and `aud` to equal the issuer of the instance.
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func newAssertionClaims(clientID, userID, issuer string, now time.Time) map[string]any {
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if userID == "" {
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userID = clientID
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}
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return map[string]any{
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"iss": clientID,
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"sub": userID,
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"aud": issuer,
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"iat": now.Unix(),
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"exp": now.Add(assertionLifetime).Unix(),
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}
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}
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func base64URL(b []byte) string {
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return base64.RawURLEncoding.EncodeToString(b)
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}
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// SignAssertion produces a signed RS256 JWT for the jwt-bearer grant.
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//
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// The header carries `kid`, so ZITADEL selects the matching registered key. That
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// is what makes overlapping keys possible: two keys can be valid at once and the
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// assertion says which one it used.
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func SignAssertion(privateKeyPEM []byte, keyID, clientID, userID, issuer string, now time.Time) (string, error) {
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block, _ := pem.Decode(privateKeyPEM)
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if block == nil {
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return "", fmt.Errorf("private key is not PEM encoded")
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}
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key, err := x509.ParsePKCS8PrivateKey(block.Bytes)
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if err != nil {
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// ZITADEL exports PKCS#1 keys as well.
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if rsaKey, rsaErr := x509.ParsePKCS1PrivateKey(block.Bytes); rsaErr == nil {
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key = rsaKey
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} else {
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return "", fmt.Errorf("private key does not parse: %w", err)
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}
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}
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rsaKey, ok := key.(*rsa.PrivateKey)
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if !ok {
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return "", fmt.Errorf("private key is not RSA")
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}
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header := map[string]any{"alg": "RS256", "typ": "JWT", "kid": keyID}
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headerJSON, err := json.Marshal(header)
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if err != nil {
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return "", err
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}
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claimsJSON, err := json.Marshal(newAssertionClaims(clientID, userID, issuer, now))
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if err != nil {
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return "", err
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}
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signingInput := base64URL(headerJSON) + "." + base64URL(claimsJSON)
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digest := sha256.Sum256([]byte(signingInput))
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signature, err := rsa.SignPKCS1v15(rand.Reader, rsaKey, crypto.SHA256, digest[:])
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if err != nil {
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return "", fmt.Errorf("cannot sign the assertion: %w", err)
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}
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return signingInput + "." + base64URL(signature), nil
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}
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