## Definition

Cryptography is a method of protecting information and communications through the use of codes, so that only those for whom the information is intended can read and process it. The prefix “crypt-” means “hidden” or “vault” — and the suffix “-graphy” stands for “writing.”

In computer science, cryptography refers to secure information and communication techniques derived from mathematical concepts and a set of rule-based calculations called algorithms, to transform messages in ways that are hard to decipher. These deterministic algorithms are used for cryptographic key generation, digital signing, verification to protect data privacy, web browsing on the internet, and confidential communications such as credit card transactions and email.

Cryptography provides for secure communication in the presence of malicious third-parties—known as adversaries. Encryption uses an algorithm and a key to transform an input (i.e., plaintext) into an encrypted output (i.e., ciphertext). A given algorithm will always transform the same plaintext into the same ciphertext if the same key is used.

Algorithms are considered secure if an attacker cannot determine any properties of the plaintext or key, given the ciphertext. An attacker should not be able to determine anything about a key given a large number of plaintext/ciphertext combinations which used the key.

## What is the difference between symmetric and asymmetric cryptography?

With symmetric cryptography, the same key is used for both encryption and decryption. A sender and a recipient must already have a shared key that is known to both. Key distribution is a tricky problem and was the impetus for developing asymmetric cryptography.

With asymmetric crypto, two different keys are used for encryption and decryption. Every user in an asymmetric cryptosystem has both a public key and a private key. The private key is kept secret at all times, but the public key may be freely distributed.

Data encrypted with a public key may only be decrypted with the corresponding private key. So, sending a message to John requires encrypting that message with John’s public key. Only John can decrypt the message, as only John has his private key. Any data encrypted with a private key can only be decrypted with the corresponding public key. Similarly, Jane could digitally sign a message with her private key, and anyone with Jane’s public key could decrypt the signed message and verify that it was in fact Jane who sent it.

Symmetric is generally very fast and ideal for encrypting large amounts of data (e.g., an entire disk partition or database). Asymmetric is much slower and can only encrypt pieces of data that are smaller than the key size (typically 2048 bits or smaller). Thus, asymmetric crypto is generally used to encrypt symmetric encryption keys which are then used to encrypt much larger blocks of data. For digital signatures, asymmetric crypto is generally used to encrypt the hashes of messages rather than entire messages.

A cryptosystem provides for managing cryptographic keys including generation, exchange, storage, use, revocation, and replacement of the keys.

**OBJECTIVES OF CRYPTOGRAPHY**

Modern cryptography concerns itself with the following four objectives:

**Confidentiality**: the information cannot be understood by anyone for whom it was unintended**Integrity:**the information cannot be altered in storage or transit between sender and intended receiver without the alteration being detected**Non-repudiation**: the creator/sender of the information cannot deny at a later stage his or her intentions in the creation or transmission of the information**Authentication**: the sender and receiver can confirm each other’s identity and the origin/destination of the information

## What are the principles?

The most important principle to keep in mind is that you should never attempt to design your own cryptosystem. The world’s most brilliant cryptographers (including Phil Zimmerman and Ron Rivest) routinely create cryptosystems with serious security flaws in them. In order for a cryptosystem to be deemed “secure,” it must face intense scrutiny from the security community. Never rely on security through obscurity, or the fact that attackers may not have knowledge of your system. Remember that malicious insiders and determined attackers will attempt to attack your system.

The only things that should be “secret” when it comes to a secure cryptosystem are the keys themselves. Be sure to take appropriate steps to protect any keys that your systems use. Never store encryption keys in clear text along with the data that they protect. This is akin to locking your front door and placing the key under the doormat. It is the first place an attacker will look. Here are three common methods for protecting keys (from least secure to most secure):

- Store keys in a filesystem and protect them with strong access control lists (ACLs). Remember to adhere to the principal of least privilege.
- Encrypt your data encryption keys (DEKs) with a second key encrypting key (KEK). The KEK should be generated using password-based encryption (PBE). A password known to a minimal number of administrators can be used to generate a key using an algorithm such as bcrypt, scrypt, or PBKDF2 and used to bootstrap the cryptosystem. This removes the need to ever store the key unencrypted anywhere.
- A hardware security module (HSM) is a tamper-resistant hardware appliance that can be used to store keys securely. Code can make API calls to an HSM to provide keys when needed or to perform decryption of data on the HSM itself.

Make sure that you only use algorithms, key strengths, and modes of operation that conform to industry best practices. Advanced encryption standard (AES) (with 128, 192, or 256-bit keys) is the standard for symmetric encryption. RSA and elliptical curve cryptography (ECC) with at least 2048-bit keys are the standard for asymmetric encryption. Be sure to avoid insecure modes of operation such as AES in Electronic Codebook (ECB) mode or RSA with no padding.

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