CODEWITHSUNDEEP
node.jsmongodbsecurityencryption
tsturzl
tsturzl

Reputation: 3137

mongodb: Unique index on encrypted field

I'm encrypting SSNs in mongodb. However, I need to use the SSN as a unique identifier to make sure that a person with that SSN does not insert a duplicate. So basically I want to check for duplicate SSNs before saving. However I'm unsure if I'll be able to do this after encrypting this field with an AES function. If I encrypt and sign 2 strings which are identical with AES, will the output still be identical?

If not, what would be a good alternative? I had thought about hashing the SSN, but an SSN seems to have such little entropy(its 9 numeric digits, some of which are somewhat predictable). If I salt, I lose the ability to assign a unique index on that field, unless I use a static salt which doesn't really do much.

Addition

I would be encrypting at the application level using the node.js crypto core module.

Upvotes: 1

Views: 446

Answers (1)

tsturzl
tsturzl

Reputation: 3137

Using the same symmetric AES key to encipher 2 identical strings will produce an identical output. Therefore you can identify whether or not the encrypted field is unique by comparing it to a value enciphered with the same key.

PoC:

var crypto = require('crypto');

var cipher = crypto.createCipher('aes-256-ctr', "someString");
var cipher2 = crypto.createCipher('aes-256-ctr', "someString");

var crypted = cipher.update("hello world",'utf8','hex');
var crypted2 = cipher2.update("hello world",'utf8','hex');

crypted === crypted2 //true

Upvotes: 1

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