Cryptography Reference
In-Depth Information
required, either in distributing or in storing this direc-
tory of “public” keys. Anyone wishing to communicate
privately with a user whose key is in the directory only
has to look up the recipient's public key to encrypt a mes-
sage that only the intended receiver can decrypt. The
total number of keys involved is just twice the number
of users, with each user having a key in the public direc-
tory and his own secret key, which he must protect in his
own self-interest. Obviously the public directory must be
authenticated, otherwise
A
could be tricked into commu-
nicating with
C
when he thinks he is communicating with
B
simply by substituting
C
's key for
B
's in
A
's copy of the
directory.
P
ubliC
-K
ey
C
ryPtograPhy
Since they were focused on the key distribution prob-
lem, Diffie and Hellman called their discovery public-key
cryptography. This was the first discussion of two-
key cryptography in the open literature. However,
Admiral Bobby Inman, while director of the U.S. National
Security Agency (NSA) from 1977 to 1981, revealed that
two-key cryptography had been known to the agency
almost a decade earlier, having been discovered by James
Ellis, Clifford Cocks, and Malcolm Williamson at the
British Government Code Headquarters (GCHQ).
In this system, ciphers created with a secret key can
be decrypted by anyone using the corresponding public
key—thereby providing a means to identify the origina-
tor at the expense of completely giving up secrecy. Ciphers
generated using the public key can only be decrypted by
users holding the secret key, not by others holding the
public key—however, the secret-key holder receives no
information concerning the sender. In other words, the
system provides secrecy at the expense of completely
