When the sender creates the digital signature, PGP GPG
first scrunches the message to an 128 bit cryptographic
checksum, using MD5 SHA-1. Cryptographic checksum algorithms
are constructed so that they are
difficult to reverse. That is, given a given cryptographic
checksum, it is difficult to find a message that will
map to it. RSA DSA encryption is applied to the
cryptographic checksum using the user's secret key
yielding the digital signature. The digital
signature is combined with the original message
to create a signed message. Or if the PGP GPG
is specified, the digital signature
is kept in a seperate file, called a detached
To check a digital signature, PGP GPG applies RSA DSA decryption using the public key to the signature. It applies MD5 SHA-1 to the message to yield a cryptographic checksum. Since public key decryption is the inverse of secret key encryption, thses two values should be equal. If so, the digital signature checks.
Now let us consider a PGP GPG digital signature from the point of view of a would be forger. How can we create digital signature and a message, that will pass the above test?