A known-plaintext attack, or KPA, happens when an attacker has both an encrypted message and its original, readable version. By comparing them, the attacker tries to figure out how the encryption works and might use that information to read other messages protected in the same way.

How the attack works

Attackers gather pairs of plaintext and matching ciphertext from sources such as data leaks, intercepted messages, or public records. They look for how parts of the original text match up with the encrypted version. These patterns can help them guess the encryption method or even find the key. Having one pair can help, but more pairs usually make it easier to spot clues.

Common techniques used

When analyzing these pairs, attackers use a few main strategies:

• Frequency analysis: Attackers check how often certain characters or blocks show up and try to match those counts to likely symbols in the original text.
• Pattern matching: They look for repeated phrases, headers, or other predictable parts of messages that match up with repeating patterns in the encrypted text.
• Statistical or automated analysis: Modern tools can quickly run tests and calculations on many pairs to find possible key information faster.
• Brute force checks: When attackers know the original text, it becomes easier to check if a guessed key creates the same encrypted message.

Where KPAs have mattered historically

Some well-known codebreaking efforts used the idea of comparing known and encrypted content. For example, during World War II, analysts used known message formats and repeated content to help break tough ciphers. In other cases, early weak standards and vendor systems were exposed when researchers compared known samples to encrypted messages. These examples show that predictable messages can be a real weakness.

Systems and practices that raise risk

Some choices can make known-plaintext attacks easier:

• Reusing the same key across many messages.
• Using weak or outdated ciphers that do not hide patterns well.
• Encrypting predictable, small, or structured pieces of data, such as fixed headers or repeated forms.
• Omitting integrity checks or using predictable initialization values.
• Compressing data before encryption in a way that leaves obvious patterns.

How to reduce the chance of a successful attack

Good defenses focus on making messages less predictable and keeping keys safe:

• Use modern, vetted algorithms and recommended modes of operation.
• Create strong random values and use a different initialization value for every encryption.
• Change and protect keys regularly, and do not reuse the same key for different sessions.
• Add randomness or salts when needed so that the same original text does not always turn into the same encrypted message.
• Encrypt bigger and less predictable chunks of data instead of small, repetitive pieces.
• Add message authentication or integrity checks to make it harder for attackers to exploit manipulation or padding flaws.
• Keep cryptographic libraries and systems up to date, and test them for known weaknesses.