Does knowing one encrypted character open up a security flaw?

Question

One of the columns in a table in my API's database is an encrypted text formed of two pieces of information concatenated with a given delimiter, lets say a colon. The second piece of information is always, say, 12 characters long.

Does knowing this information open up a security flaw and potentially allow attackers to decrypt the information should they get a hold of it?

The information is encrypted using the php method openssl_encrypt() and with the 'aes-128-gcm' cipher.

Answer 1

What you're talking about is known in the literature as a partially known-plaintext attack (partial-KPA).

The information is encrypted using the php method openssl_encrypt() and with the 'aes-128-gcm' cipher.

AES-GCM combines AES-CTR with GHASH (a polynomial MAC over GF(2^128)).

Answering your question further requires delving a bit deeper into the cryptography weeds, but generally:

• AES is considered a secure block cipher.
• CTR mode turns a block cipher into a stream cipher.
• Stream cipher encryption is conceptually equivalent to One-Time Pads, except the keystream is generated from the key and nonce, and thus can only have a finite (in this case, 128-bit) security level.

One of the columns in a table in my API's database is an encrypted text formed of two pieces of information concatenated with a given delimiter, lets say a colon. The second piece of information is always, say, 12 characters long.

The only thing you can infer from an AES-GCM ciphertext is the length of the plaintext.

In order to be able infer the remainder of the keystream from one byte of ciphertext (and a corresponding known plaintext for that byte), AES would need to be a very insecure block cipher. Since we know (thanks to roughly 21 years of cryptanalysis effort) that AES is a secure block cipher, and AES-GCM is a secure authenticated cipher mode, partial-KPAs are not a concern for this construction.