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ELECTRONIC MAIL SECURITY: PGP, S/MIME 307
Instead of using RSA for key encryption, PGP may use a variant of Diffie–Hellman
(known as ElGamal) that does provide encryption/decryption. In order for the encryption
time to reduce, the combination of conventional and public-key encryption is used in
preference to simply using RSA or ElGamal to encrypt the message directly. In fact,
CAST-128 and other conventional algorithms are substantially faster than RSA or ElGamal.
Since the recipient is able to recover the session key that is bound to the message,
the use of the public-key algorithms solves the session key exchange problem. Finally,
to the extent that the entire scheme is secure, PGP should provide the user with a range
of key size options from 768 to 3072 bits.
Both digital signature and confidentiality services may be applied to the same message.
First, a signature is generated from the message and attached to the message. Then the
message plus signature are encrypted using a symmetric session key. Finally, the session
key is encrypted using public-key encryption and prefixed to the encrypted block.
9.1.2 Authentication via Digital Signature
The digital signature uses a hash code of the message digest algorithm, and a public-key
signature algorithm. Figure 9.2 illustrates the digital signature service provided by PGP.
The sequence is as follows:
• The sender creates a message.
• SHA-1 is used to generate a 160-bit hash code of the message.
• The hash code is encrypted with RSA using the sender’s private key and a digital
signature is produced.
• The binary signature is attached to the message.
• The receiver uses RSA with the sender’s public key to decrypt and recover the
hash code.
• The receiver generates a new hash code for the received message and compares it
with the decrypted hash code. If the two match, the message is accepted as authentic.
The combination of SHA-1 and RSA provides an effective digital signature scheme.
As an alternative, signatures can be generated using DSS/SHA-1. The National Institute
M
KS a
Sender A Receiver B
H Ep Z −1
EKSa [H(M)]
Z Z
M
E KSa [H(M)]
Figure 9.2 PGP authentication computation scheme using compression algorithm.
M
KP a
D p
H
?
=
YES
NO
Authentic
Failed