grafana/k6 · error

failed to decode modulus: %w

Error message

failed to decode modulus: %w

What it means

During RSA JWK import the modulus n is decoded with base64.RawURLEncoding (base64url, no padding). This error indicates n is not valid unpadded base64url: '=' padding, '+'/'/' alphabet characters, whitespace, or impossible length. It surfaces from crypto.subtle.importKey('jwk', ...) for RSA algorithms.

Source

Thrown at internal/js/modules/k6/webcrypto/jwk.go:337

	// TODO: consider validating the other fields in future
	return nil
}

func importRSAJWK(jsonKeyData []byte) (any, CryptoKeyType, int, error) {
	var jwk rsaJWK
	if err := json.Unmarshal(jsonKeyData, &jwk); err != nil {
		return nil, UnknownCryptoKeyType, 0, fmt.Errorf("failed to parse input as RSA JWK key: %w", err)
	}

	if err := jwk.validate(); err != nil {
		return nil, UnknownCryptoKeyType, 0, fmt.Errorf("invalid RSA JWK key: %w", err)
	}

	// decode the various key components
	nBytes, err := base64URLDecode(jwk.N)
	if err != nil {
		return nil, UnknownCryptoKeyType, 0, fmt.Errorf("failed to decode modulus: %w", err)
	}
	eBytes, err := base64URLDecode(jwk.E)
	if err != nil {
		return nil, UnknownCryptoKeyType, 0, fmt.Errorf("failed to decode exponent: %w", err)
	}

	// convert exponent to an integer
	eInt := new(big.Int).SetBytes(eBytes).Int64()
	pubKey := rsa.PublicKey{
		N: new(big.Int).SetBytes(nBytes),
		E: int(eInt),
	}

	// if the private exponent is missing, return the public key
	if jwk.D == "" {
		return pubKey, PublicCryptoKeyType, pubKey.N.BitLen(), nil
	}

View on GitHub (pinned to 93accf6570)

Solutions

1. Convert n to unpadded base64url (replace '+'->'-', '/'->'_', strip '=')
2. If the value is hex, re-encode the raw bytes as base64url without padding
3. Remove embedded newlines/whitespace
4. Confirm the decoded modulus byte length matches the announced key size (e.g. 256 bytes for RSA-2048)

Example fix

// before
const jwk = { kty: 'RSA', n: stdB64N, e: 'AQAB' };
// after
const toB64u = (s) => s.replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '').replace(/\s+/g, '');
const jwk = { kty: 'RSA', n: toB64u(stdB64N), e: 'AQAB' };

const B64URL = /^[A-Za-z0-9_-]+$/;
const toB64u = (s) => String(s).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '').replace(/\s+/g, '');
const n = toB64u(jwk.n);
if (!B64URL.test(n)) throw new Error('modulus n is not unpadded base64url');
jwk = { ...jwk, n };

function isB64uModulus(s) {
  return typeof s === 'string' && /^[A-Za-z0-9_-]+$/.test(s) && s.length >= 40;
}

try {
  key = await crypto.subtle.importKey('jwk', jwk, rsaAlg, true, usages);
} catch (e) {
  if (e.message.includes('failed to decode modulus')) {
    jwk = { ...jwk, n: toB64u(jwk.n) };
    key = await crypto.subtle.importKey('jwk', jwk, rsaAlg, true, usages);
  } else throw e;
}

When it happens

Trigger: n encoded in standard or padded base64; whitespace or newlines inside the modulus; truncated modulus; hex-encoded modulus (0x... or plain hex) instead of base64url.

Common situations: Moduli copied from OpenSSL output (standard base64 with wrapping); JWKs from systems using padded base64url; values converted from hex representations by hand.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• failed to decode exponent: %w
• failed to decode private exponent: %w
• failed to decode first prime factor: %w
• failed to decode second prime factor: %w
• failed to decode first exponent: %w

AI-assisted analysis of grafana/k6@93accf6570 (2026-08-15). Data as JSON: /api/errors/ff338f3fab93f2cb. Report an issue: GitHub.