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1.4.5 Digital signatures
There is one further advantage that asymmetric key algorithms give us.
In the previous example, imagine that Alice uses her private key to encrypt a
message and sends it to Bob. The message that is sent between them is still
scrambled, but it is no longer private, because anyone with the public key can
decrypt it (and we have said that we do not care who has the public key).
So, what can we use this message from Alice for? The answer is: authentication.
Because only Alice has access to the private key that created the message, it
can only have come from her. This is the concept of digital signatures.
Digital signatures are concerned with providing integrity, authentication and
identification, and non-repudiation, whereas the symmetric and asymmetric key
algorithms we have discussed thus far were only concerned with confidentiality.
Hash functions
In order to provide these additional services, we need to introduce a new kind of
cryptographic algorithm: hash functions (also called message digests).
Where you can encrypt and decrypt with symmetric and asymmetric key
algorithms, hash functions only encrypt. This is why they are commonly referred
to as one-way functions. You can never recover the original message from a
hash function.
Also, hash functions are called “functions” because they take an input message
and produce an output. More precisely, they are used to index the original value
or key of a message or a block of data, and then are used later each time the
data associated with the value or key is to be retrieved. A secure hash function
has three main attributes:
1. It takes a message of any size and generates a small, fixed size block of data
from it (called a message digest). Re-executing the hash function on the
same source data will always yield the same resulting digest. This is called
the fingerprint of the message.
2. It is not predictable in operation. That is to say, a small change in the source
message will have an unpredictably large effect on the final digest. Put
another way, even changing a single bit of the message changes half the bits
of the output, if you are using a good hash function.
3. It is, for all intents and purposes, irreversible. In other words, there is no way
to derive the source data, given its digested form.
Chapter 1. Fundamentals of IT security 35