peass-ng/PEASS-ng · error · ArgumentException

Keys > 32 are not supported

Error message

Keys > 32 are not supported

What it means

Blake2s is a 256-bit hash whose keyed mode (MAC-style) accepts keys of at most 32 bytes. The Blake2sDigest(byte[] key) constructor validates the key length and throws ArgumentException when it exceeds 32 bytes, because the key is copied into the 64-byte internal buffer and the Blake2s parameter block simply cannot encode a longer key.

Solutions

  1. Truncate or derive the key to at most 32 bytes before passing it: use a KDF (e.g. HKDF/PBKDF2) with a 32-byte output, or take key.AsSpan(0, 32).ToArray().
  2. If a longer key must be supported, switch to Blake2bDigest, whose keyed mode accepts up to 64 bytes.
  3. If the byte array is not meant to be a key, pass null or use the keyless constructor and feed the data via BlockUpdate().

Example fix

// before
byte[] longKey = File.ReadAllBytes("secret.pem"); // > 32 bytes
var digest = new Blake2sDigest(longKey);
// after
byte[] key32 = new byte[32];
Array.Copy(longKey, key32, 32); // or derive via HKDF
var digest = new Blake2sDigest(key32);
Defensive patterns

Strategy: validation

Validate before calling

if (key == null || key.Length > 32)
    throw new ArgumentOutOfRangeException(nameof(key), "Blake2s keys must be 1-32 bytes");
var digest = new Blake2sDigest(key);

Type guard

static bool IsValidBlake2sKey(byte[] key) => key != null && key.Length <= 32;

Try / catch

try { var d = new Blake2sDigest(key); }
catch (ArgumentException ex) { /* fall back to keyless hash or derived 32-byte key */ }

Prevention

When it happens

Trigger: Calling new Blake2sDigest(byte[] key) with a key array whose Length > 32 (e.g. a 64-byte HMAC key, an RSA/PEM-derived key blob, or raw user-supplied secret bytes).

Common situations: Developers reuse a long symmetric key or an entire key file (PEM/DER bytes) as the Blake2s key instead of hashing/deriving a 32-byte subkey; porting code from Blake2b (which allows 64-byte keys) to Blake2s; concatenating salt+secret into one key buffer.

Related errors

AI-assisted analysis of peass-ng/PEASS-ng@53fb989abc (2026-09-02). Data as JSON: /api/errors/a0666092ac23a346. Report an issue: GitHub.

Appendix: source

Thrown at winPEAS/winPEASexe/winPEAS/3rdParty/BouncyCastle/crypto/digests/Blake2sDigest.cs:181

            Init();
        }

        /**
         * BLAKE2s for authentication ("Prefix-MAC mode").
         * <p/>
         * After calling the doFinal() method, the key will remain to be used for
         * further computations of this instance. The key can be overwritten using
         * the clearKey() method.
         *
         * @param key a key up to 32 bytes or null
         */
        public Blake2sDigest(byte[] key)
        {
            buffer = new byte[BLOCK_LENGTH_BYTES];
            if (key != null)
            {
                if (key.Length > 32)
                    throw new ArgumentException("Keys > 32 are not supported");

                this.key = new byte[key.Length];
                Array.Copy(key, 0, this.key, 0, key.Length);

                keyLength = key.Length;
                Array.Copy(key, 0, buffer, 0, key.Length);
                bufferPos = BLOCK_LENGTH_BYTES; // zero padding
            }
            digestLength = 32;
            Init();
        }

        /**
         * BLAKE2s with key, required digest length, salt and personalization.
         * <p/>
         * After calling the doFinal() method, the key, the salt and the personal
         * string will remain and might be used for further computations with this
         * instance. The key can be overwritten using the clearKey() method, the

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