grafana/k6 · error

failed to decode private exponent: %w

Error message

failed to decode private exponent: %w

What it means

When an RSA JWK contains a non-empty d field it is treated as a private key, and d (the private exponent) is decoded with base64.RawURLEncoding. This error means d failed that decoding — padding characters, wrong alphabet, whitespace, or an impossible length. It is raised from crypto.subtle.importKey('jwk', ...) for RSA algorithms.

Source

Thrown at internal/js/modules/k6/webcrypto/jwk.go:358

	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
	}

	dBytes, err := base64URLDecode(jwk.D)
	if err != nil {
		return nil, UnknownCryptoKeyType, 0, fmt.Errorf("failed to decode private exponent: %w", err)
	}
	pBytes, err := base64URLDecode(jwk.P)
	if err != nil {
		return nil, UnknownCryptoKeyType, 0, fmt.Errorf("failed to decode first prime factor: %w", err)
	}
	qBytes, err := base64URLDecode(jwk.Q)
	if err != nil {
		return nil, UnknownCryptoKeyType, 0, fmt.Errorf("failed to decode second prime factor: %w", err)
	}
	dpBytes, err := base64URLDecode(jwk.Dp)
	if err != nil {
		return nil, UnknownCryptoKeyType, 0, fmt.Errorf("failed to decode first exponent: %w", err)
	}
	dqBytes, err := base64URLDecode(jwk.Dq)
	if err != nil {
		return nil, UnknownCryptoKeyType, 0, fmt.Errorf("failed to decode second exponent: %w", err)
	}
	qiBytes, err := base64URLDecode(jwk.Qi)

View on GitHub (pinned to 93accf6570)

Solutions

1. Convert d to unpadded base64url before import
2. Remove whitespace and padding from the value
3. Check the decoded length: for RSA-2048, d is typically 256 bytes
4. If d is unavailable/corrupt but only public operations are needed, drop d (and the other private fields) to import a public key

Example fix

// before
const jwk = { kty: 'RSA', n, e: 'AQAB', d: paddedD, p, q, dp, dq, qi };
// after
const toB64u = (s) => s.replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '').replace(/\s+/g, '');
const jwk = { kty: 'RSA', n, e: 'AQAB', d: toB64u(paddedD), p, q, dp, dq, qi };

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

function isB64uPrivateExponent(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 private exponent')) {
    jwk = { ...jwk, d: toB64u(jwk.d) };
    key = await crypto.subtle.importKey('jwk', jwk, rsaAlg, true, usages);
  } else throw e;
}

When it happens

Trigger: d in standard/padded base64; d containing '+' or '/'; d truncated by secret-store size limits or templating; d with embedded newlines from PEM-style wrapping.

Common situations: Private JWKs sourced from vaults that re-encode or wrap values; manual assembly of private JWKs from PEM dumps; secrets passed through environment variables with mangled characters.

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

AI-assisted analysis of grafana/k6@93accf6570 (2026-08-15). Data as JSON: /api/errors/c53ed86c7b548b91. Report an issue: GitHub.