The strength of a RSA public key encryption is based on the difficulty of factoring the product of large prime numbers, and NOT based on the difficulty of finding the discreet logarithm in a finite field. The RSA algorithm can be used for encryption, key exchange, and digital signatures.

The MD5 algorithm, not the Diffie-Hellman algorithm, is a message digest algorithm that takes a message of arbitrary length and generates a 128-bit message digest. On the other hand, the Diffie-Hellman key exchange is a method where subjects exchange secret keys over a non-secure medium without exposing the keys.

Lastly, in a digital signed message, only the message digest is encrypted using the private key of the sender. The entire message is NOT encrypted and the public key is NOT used by the sender to encrypt the data.