This document describes the pattern for digital signatures using hashing. A digital signature allows a principal to prove that a message was originated from them and verifies that the message has not been altered. It works by hashing the message to a fixed length digest, then encrypting the digest with the sender's private key. When received, the signature can be validated by decrypting with the public key and comparing hashes to ensure the message was not modified. Digital signatures provide message authentication, integrity, and non-repudiation of origin.

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Patterns for Digital Signature
using hashing
Presented by Keiko Hashizume

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Introduction
• A digital signature is a type of public key
cryptography.
• PKC uses a key pair, a private and a public key,
for encryption and decryption operations.
• When a message is encrypted using a key, it
only can be decoded with the other key
(matching key).

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Pattern for Digital Signature with
hashing
• Intent
Digital Signature allows a principal to prove that
a message was originated from it. It also
provides message integrity by indicating
whether a message was altered during
transmission.

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Context
• Participants of electronic transactions that
need to exchange documents or messages
through insecure networks.
• We assume that a principal possesses a key
pair: a private key that is secretly kept by the
principal and a public key that is in an
accessible repository. The generation of these
key pairs and the distribution of public keys
are out of scope of this pattern.

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Problem
• In many applications we need to verify the
origin of a message (message authentication).
How do we verify that a message came from a
particular principal? Messages that travel
through insecure channels can be captured
and intentionally modified by attackers.

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Problem
• The solution for this problem is affected by the
following forces:
– For legal or business reasons we need to be able to
verify who sent a particular message. Otherwise, the
sender may deny having sent it.
– Messages may be altered during transmission, so we
need to verify that the data is in its original form
when it reaches its destination.
– The length of the signed message should not be
significantly larger than the original message;
otherwise we would waste time and bandwidth.

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Solution
• Apply properties of public key cryptographic
algorithms to messages in order to create a
signature that will be unique for each message.
The message first is compressed (hashed) to a
smaller size (digest), and then it is encrypted
using the sender’s private key. When the signed
message arrives at its target, the receiver verifies
the signature using the sender’s public key and
uses the hashed digest to verify that the message
has not been altered.

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• Structure
Solution

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Solution
• Dynamics
Sequence Diagram for Use Case: Sign a message

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Solution
• Dynamics
Sequence Diagram for Use Case: Verify a signature
Signed Message
Message
Signature

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Consequences
• This pattern presents the following advantages:
– A key pair is bound to a principal whose private key is used to
sign the message. If the signature is validated using its public
key, then we know that the sender created and sent the
message. We assume that he keeps his private key secure, and
it has not been compromised.
– Also, when a signature is validated using a principal’s public key,
the sender cannot deny that he created and sent the message.
If a message is signed using another private key that does not
belong to the sender, the validity of the signature fails.
– Any change in the original message will produce another digest
value that will be different from the value obtained after
decrypting the signature using the sender’s public key.
– A message is compressed to a fixed length string using the
digest algorithm before it is signed. As a result, the process of
signing is faster, and the signature is shorter in length.

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Consequences
• The pattern also has some (possible) liabilities:
– This pattern assumes that the owner of the public
key is who he says he is. In other words, both
participants trust in the identity of each other.
Thus, certificates issued by some certification
authority are needed.
– Both the sender and the receiver have to
previously agree what cryptographic algorithm
they support.

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Implementation
• Use Strategy Pattern [Gam94] to select different hashing and
signature algorithm. The most widely used hashing algorithms are
MD5 and SHA1. The two popular digital signature algorithms are
RSA and Digital Signature Algorithm (DSA).
• Digital signatures can be implemented in different applications such
as in email communication, distribution of documents over the
Internet, or web services. For example, one can sign email’s
contents or any other document’s content such as PDF. In both
cases, the signature is appended to the email or document. When
digital signatures are applied in web services, they are also
embedded within XML messages. However, these signatures are
treated as XML elements, and they have additional features such as
signing parts of a message or external resources which can be XML
or any other data type.

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Known Uses
• Digital Signatures have been widely used in different
products.
– Adobe Reader and Acrobat [Ado05] have an extended security
feature that allows users to digitally sing PDF documents.
– CoSign [Arx] digitally signs different types of documents, files,
forms, and other electronic transactions.
– GNuPG [Gnu] digitally signs e-mail messages.
– Java Cryptographic Architecture [Sun] includes APIs for digital
signature.
– Microsoft .Net [Mic07] includes APIs for asymmetric
cryptography such as digital signature.
– XML Signature [W3C08] is one of the foundation web services
security standards that defines the structure and process of
digital signatures in XML messages.

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Related Patterns
• Encryption/Decryption using public key cryptography
[Bra98]
• Generation and Distribution of public keys [Leh02]
• Certificates [Mor06] contain information about its
owner such as public key, name, validity period, and so
forth. Certificates are issued by Certificate Authority
that confirms that the public key contained in the
certificate belongs to the owner of the certificate.
• Strategy Pattern [Gam94] defines how to separate the
implementation of related algorithms from the
selection of one of them.

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• Comments