In
cryptographic protocols
A security protocol (cryptographic protocol or encryption protocol) is an abstract or concrete protocol that performs a security-related function and applies cryptographic methods, often as sequences of cryptographic primitives. A protocol descri ...
, a key encapsulation mechanism (KEM) is used to secure
symmetric
Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definit ...
key material for transmission using
asymmetric
Asymmetric may refer to:
*Asymmetry in geometry, chemistry, and physics
Computing
* Asymmetric cryptography, in public-key cryptography
*Asymmetric digital subscriber line, Internet connectivity
* Asymmetric multiprocessing, in computer architect ...
(public-key) algorithms. It is commonly used in
hybrid cryptosystems. In practice, public key systems are clumsy to use in transmitting long messages. Instead they are often used to exchange symmetric keys, which are relatively short. The symmetric key is then used to encrypt the longer message.
The traditional approach to sending a symmetric key with public key systems is to first generate a random symmetric key and then encrypt it using the chosen public key algorithm. The recipient then decrypts the public key message to recover the symmetric key. As the symmetric key is generally short, padding is required for full security and proofs of security for padding schemes are often less than complete.
[An OAEP Variant With a Tight Security Proof – Draft 1.0](_blank)
Jakob Jonsson, 2002 KEMs simplify the process by generating a random element in the
finite group underlying the public key system and deriving the symmetric key by hashing that element, eliminating the need for padding.
Example using RSA encryption
Using the same notation employed in the
RSA
RSA may refer to:
Organizations Academia and education
* Rabbinical Seminary of America, a yeshiva in New York City
*Regional Science Association International (formerly the Regional Science Association), a US-based learned society
*Renaissance S ...
system article, say
Alice
Alice may refer to:
* Alice (name), most often a feminine given name, but also used as a surname
Literature
* Alice (''Alice's Adventures in Wonderland''), a character in books by Lewis Carroll
* ''Alice'' series, children's and teen books by ...
has transmitted her public key
to
Bob, while keeping her private key secret, as usual. Bob then wishes to send symmetric key M to Alice. M might be a 128 or 256-bit AES key, for example. Note that the public key
is typically 2048-bits or even longer, thus much larger than typical symmetric keys. If
is small enough that