Glossary · Cryptography & authentication
Blind index
A blind index is a keyed, deterministic hash of a field (such as an email address) stored in place of the plaintext, so a database can look records up by exact match without holding — or leaking — the value itself.
Encrypting a column normally breaks lookups: two encryptions of the same value differ, so WHERE email = ? stops working. A blind index restores exact-match capability by storing a deterministic derivation of the value — computed with a slow or keyed function — alongside the ciphertext. The database can find the row; nobody browsing the database can read the addresses.
The derivation must resist offline guessing, since email addresses are low-entropy and enumerable. That means a secret key unavailable to the database, an expensive derivation, or both.
Ciphera ID uses a client-side blind index for account lookup: the email address is stretched through PBKDF2 with 1,000,000 iterations in the browser, and only that digest travels to the server as the lookup key. The server can match a returning user without ever storing the address in searchable plaintext.