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Decryption

Learn how decryption works in the FHEVM SDK.

Overview​

Decryption converts encrypted handles back to plaintext values for authorized users.

Automatic Decryption​

The SDK automatically decrypts in useReadContract:

const { decryptedData } = useReadContract({
  functionName: 'balanceOf',
  args: [userAddress],
  decrypt: true, // Enable auto-decryption
});

// decryptedData contains the plaintext value

Decryption Process​

  1. Handle Detection: SDK detects encrypted handle (bytes32)
  2. Permission Check: Checks for cached decryption signature
  3. Signature Request: If needed, prompts user to sign
  4. Decryption: Decrypts handle using signature
  5. Caching: Caches result for future use

Decryption Signature​

What It Is​

An EIP-712 signature that grants permission to decrypt:

{
  publicKey: "0x...",
  privateKey: "0x...",
  signature: "0x...",
  contractAddresses: ["0x123...", "0x456..."],
  userAddress: "0x789...",
  startTimestamp: 1234567890,
  durationDays: 365
}

Lifecycle​

  1. Generation: User signs once per contract set
  2. Storage: Cached in memory
  3. Reuse: Used for all decryptions in session
  4. Expiration: Valid for specified duration (default: 365 days)

Manual Decryption​

Use useDecryptedValue for specific handles:

const { decryptedValue, isDecrypting } = useDecryptedValue({
  handle: '0x123abc...def',
  contractAddress: '0x456...',
});

Caching​

Cache Behavior​

Decrypted values are cached by:

  • Chain ID
  • User account
  • Contract address
  • Encrypted handle

Configuration​

const config = createConfig({
  cache: {
    enabled: true,
    ttl: 60000, // 1 minute
  },
});

Cache Invalidation​

Cache is cleared when:

  • TTL expires
  • User disconnects
  • Chain changes
  • Manual clear via clearCache()

Performance​

Without Cache​

  • Each decryption: 2-5 seconds
  • Network requests to blockchain
  • User experience degraded

With Cache​

  • Cached reads: less than 1ms
  • No network overhead
  • Smooth user experience

Error Handling​

const { decryptedValue, isError, error } = useDecryptedValue({
  handle,
  contractAddress,
});

if (isError) {
  console.error('Decryption failed:', error);
}

Common errors:

  • SIGNATURE_ERROR: User rejected signature
  • DECRYPT_ERROR: Decryption failed
  • NETWORK_ERROR: Network issues

Security​

What Users Control​

  • Which contracts can decrypt
  • When to grant permission
  • Permission duration

What's Protected​

  • Decryption keys never leave client
  • Signatures only valid for specified contracts
  • Time-limited permissions

Best Practices​

1. Enable Caching​

const config = createConfig({
  cache: { enabled: true, ttl: 60000 },
});

2. Handle Loading States​

if (isDecrypting) return <Spinner />;
if (isError) return <Error />;
return <Display value={decryptedValue} />;

3. Batch Decryptions​

Decrypt multiple values with one signature:

// One signature works for all handles from same contracts
const balance1 = useDecryptedValue({ handle: h1, contractAddress });
const balance2 = useDecryptedValue({ handle: h2, contractAddress });

Notes​

User Permission

Decryption requires user signature. Always handle signature rejection gracefully.

Cache TTL

Set cache TTL based on data update frequency. Financial data: 30-60s, Static data: 5-10 minutes.

Zero Hash

Zero hash (0x000...000) is treated as 0 and cached immediately without decryption.