Why VECT Ransomware Fails to Decrypt Large Files: A Technical Breakdown

Lead: VECT ransomware contains a cryptographic implementation bug that makes small files decryptable while rendering large files unrecoverable.

Small Files – Unexpected Backdoor

For files smaller than 131,072 bytes (≈128 KB), the ransomware writes two items into the encrypted output:

ChaCha20 encryption key (hard‑coded in the binary)

12‑byte nonce (the random seed for encryption)

Researchers extracted the static key from the malware, read the nonce from the encrypted file, and combined them to recover the original plaintext.

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Large Files – Metadata Loss

When encrypting larger files, VECT splits the input into four 32 KiB blocks and intends to encrypt each block with a distinct nonce.

The implementation generates four nonces but writes only the final nonce to the encrypted file; the first three remain in memory and are never persisted.

Because ChaCha20 requires the correct nonce for each block, the missing nonces make the corresponding ciphertext blocks impossible to decrypt, even for the attacker.

The researchers concluded that the issue is a metadata‑loss bug, not a negotiation tactic; even if victims pay the ransom, the attacker lacks the three missing nonces, leaving the large files undecryptable.

Attack‑Defense Interpretation

Red‑team view: Reusing or mishandling nonces is a critical error. Generating four nonces but persisting only one creates a self‑inflicted trap.

Blue‑team view: Victims should not expect data recovery by paying ransom; the encryption is technically irreversible. Halcyon’s security team warned, “do not assume paying will restore data.”

Key Takeaways

Small files may be recoverable: Files under 128 KB contain both the static key and the nonce, enabling decryption.

Large files are effectively unrecoverable: Missing nonces prevent decryption of any block beyond the last, making ransom payment futile.

Ransomware code quality varies: The VECT bug demonstrates that even malicious software can suffer from fundamental cryptographic implementation flaws.