About
A key is a parameter used in a cipher algorithm that determines:
- and the decryption operation (backward).
It's the only secret parameter that protect the anonymity of the transformation of plaintext into ciphertext (encryption) , and vice versa (decryption).
In designing security systems, it is wise to assume that the details of the cryptographic algorithm are already available to the attacker. This is known as:
- Kerckhoffs' principle only secrecy of the key provides security,
- or, reformulated as Shannon's maxim, the enemy knows the system.
The widely used algorithm are well known. It's easier to replace a key.
An attacker who obtains the key can :
- recover the original message from the encrypted data,
- and issue signatures.
What is the difference with a password?
Cryptographic keys are meant to be randomly generated by an algorithm whereas a password are meant to be created by human beings. See password
Usage
Authentication
Key are used as authentication credential for automated processes
See What is Sender authentication ? (Public Key Authentication based, Certificate-based in Cryptography)
Data Integrity
Signature
Keys may also be used to sign digitally a document. Logically, this is the process underlying authentication because the signature authenticates the sender of the document. See digital signature schemes.
Key exchange
Keys need to be distributed and their is a more secure scheme than to send a letter via postmail. See Cryptography - Key Exchange Algorithm
Management
Creation
Password
Cipher are not using password, they uses keys. Passwords should be then converted into key material with a password-stretching KDF with appropriate work factors.
Passwords were created to be memorized by human beings (low-entropy) where a key were created to be use in automated process by computer. They are too complex and random to be memorized.
Passwords are text whereas cryptographic keys are binary data (even if serialized and deserialized as text via base64) and are generally not meant to input manually.
In a multi-factor authentication context,
- passwords are something you know
- cryptographic keys are something you have.
Keypair
Key pair (public and private) creation
Owner
To be able to tell a key's owner, public keys are often enriched with attributes such as:
- names,
- addresses,
- and similar identifiers.
The packed collection of a public key and its attributes can be digitally signed.
The resulting object is called:
- in the PKI model, a certificate and is signed by a certificate authority (CA).
- In the PGP model, it is still called a “key”, and is signed by various people who personally verified that the attributes match the subject.
Size
Selecting the Size of the Key 2), select the Strength of the crypto.
- For RSA, 2048 bits should currently be sufficient for most purposes.
- For ECDSA, only 256, 384, and 521 bits are supported. (ECDSA offers equivalent security to RSA with smaller key sizes.)
- For Ed25519, the only valid size is 256 bits.
Glossary
Private
A private key is used in the public key cryptography scheme.
Identity
The private keys in a PKI mode that are used for user authentication are called identity keys.
-----BEGIN EC PRIVATE KEY-----
MHcCAQEEIJWbvSW7h50HPwG+bWR3DXgQ6YhOxYbe0ifr1rRUvsUuoAoGCCqGSM49
AwEHoUQDQgAE34yHdT/dZ7hVi1XVCZZQUjMUtNR56CXUCjn9Aa0JEYBmfxvFf0qU
KutYhIiNJgDAJqMgQZI8RnA80wGgrxcPxA==
-----END EC PRIVATE KEY-----
Authorized
Once an SSH server receives a public key from a user and considers the key trustworthy, the server marks the key as authorized in its authorized_keys file. Such keys are called authorized keys. See authorized_keys
Type
The type of key is generally the name of the cipher
- An RSA key is a key used by the Rsa cipher
- EC key (Elliptic Curve Cryptography)
- Octet string, ie symmetric key
- …
Protocol
TLS/SSL is the network protocol that implements the cryptographic concepts:
- authentication during the creation of a connection
- symmetric encryption afterwards
Other secure protocol are a layer above (ie based on) TLS/SSL such as:
- or https