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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 655
624
SECURITY
CHAP. 9
public-key cryptography only to a relatively small piece of data, the hash. Note
carefully that this method works only if for all
x
E
(
D
(
x
))
=
x
It is not guaranteed a priori that all encryption functions will have this property
since all that we originally asked for was that
D
(
E
(
x
))
=
x
that is,
E
is the encryption function and
D
is the decryption function.
To get the
signature property in addition, the order of application must not matter, that is,
D
and
E
must be commutative functions. Fortunately, the RSA algorithm has this
property.
To use this signature scheme, the receiver must know the sender’s public key.
Some users publish their public key on their Web page.
Others do not because they
may be afraid of an intruder breaking in and secretly altering their key. For them,
an alternative mechanism is needed to distribute public keys. One common meth-
od is for message senders to attach a
certificate
to the message, which contains the
user’s name and public key and is digitally signed by a trusted third party. Once the
user has acquired the public key of the trusted third party, he can accept certificates
from all senders who use this trusted third party to generate their certificates.
A trusted third party that signs certificates is called a
CA
(
Certification Auth-
ority
). However, for a user to verify a certificate signed by a CA, the user needs
the CA’s public key. Where does that come from and how does the user know it is
the real one?
To do this in a general way requires a whole scheme for managing
public keys, called a
PKI
(
Public Key Infrastructure
). For Web browsers, the
problem is solved in an ad hoc way: all browsers come preloaded with the public
keys of about 40 popular CAs.
Above we have described how public-key cryptography can be used for digital
signatures. It is worth mentioning that schemes that do not involve public-key
cryptography also exist.
9.5.5 Trusted Platform Modules
All cryptography requires keys. If the keys are compromised, all the security
based on them is also compromised.
Storing the keys securely is thus essential.
How does one store keys securely on a system that is not secure?
One proposal that the industry has come up with is a chip called the
TPM
(
Trusted Platform Module
), which is a cryptoprocessor with some nonvolatile
storage inside it for keys. The TPM can perform cryptographic operations such as
encrypting blocks of plaintext or decrypting blocks of ciphertext in main memory.
It can also verify digital signatures.
When all these operations are done in spe-
cialized hardware, they become much faster and are likely to be used more widely.
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