CASE STUDY 2: DIGITAL SIGNATURE A digital signature or digital signature scheme is a type of asymmetric cryptography. For messages sent through an insecure channel, a properly implemented digital signature gives the receiver reason to believe the message was sent by the claimed sender. Digital signatures are equivalent to traditional handwritten signatures in many respects; properly implemented digital signatures are more difficult to forge than the handwritten type. Digital signature schemes in the sense used here are cryptographically based, and must be implemented properly to be effective. Digital signatures can also provide nonrepudiation, meaning that the signer cannot successfully claim they did not sign a message, while also claiming their private key remains secret; further, some nonrepudiation schemes offer a time stamp for the digital signature, so that even if the private key is exposed, the signature is valid nonetheless. Digitally signed messages may be anything representable as a bitstring: examples include electronic mail, contracts, or a message sent via some other cryptographic protocol. A digital signature scheme typically consists of three algorithms: A key generation algorithm that selects a private key uniformly at random from a set of possible private keys. The algorithm outputs the private key and a corresponding public key. A signing algorithm which, given a message and a private key, produces a signature. A signature verifying algorithm which given a message, public key and a signature, either accepts or rejects. Two main properties are required. First, a signature generated from a fixed message and fixed private key should verify on that message and the corresponding public key. Secondly, it should be computationally infeasible to generate a valid signature for a party who does not possess the private key. Figure of Signing The Digest How Digital Signatures Work: Digitally Signing Messages Public key cryptography gives a reliable method for digital signing and signature verification based on public/private key pairs. A person can sign a given digital message (file, document, e-mail, and so forth) with his private key. From a technical point of view, the digital signing of a message is performed in two steps: Step 1: Calculate the Message Digest In the first step of the process, a hash-value of the message (often called the message digest) is calculated by applying some cryptographic hashing algorithm (for example, MD2, MD4, MD5, SHA1, or other). The calculated hash-value of a message is a sequence of bits, usually with a fixed length, extracted in some manner from the message. All reliable algorithms for message digest calculation apply such mathematical transformations that when just a single bit from the input message is changed, a completely different digest is obtained. Due to this behavior, these algorithms are very steady in cryptanalytical attacks; in other words, it is almost impossible, from a given hash-value of a given message, to find the message itself. This impossibility for retrieval of the input message is pretty logical if we take into account that a hash-value of a message could have a hundred times smaller size than the input message. Actually, the computing resources needed to find a message by its digest are so huge that, practically, it is unfeasible to do it. It is also interesting to know that, theoretically, it is possible for two entirely different messages to have the same hash-value calculated by some hashing algorithm, but the probability for this to happen is so small that in practice it is ignored. Step 2: Calculate the Digital Signature In the second step of digitally signing a message, the information obtained in the first step hash-value of the message (the message digest) is encrypted with the private key of the person who signs the message and thus an encrypted hash-value, also called digital signature, is obtained. For this purpose, some mathematical cryptographic encrypting algorithm for calculating digital signatures from given message digest is used. The most often used algorithms are RSA (based on the number theory), DSA (based on the theory of the discrete logarithms), and ECDSA (based on the elliptic curves theory). Often, the obtained digital signature is attached to the message in a special format to be verified later if it is necessary. Verifying Digital Signatures Digital signature technology allows the recipient of given signed message to verify its real origin and its integrity. The process of digital signature verification is purposed to ascertain if a given message has been signed by the private key that corresponds to a given public key. The digital signature verification cannot ascertain whether the given message has been signed by a given person. If we need to check whether some person has signed a given message, we need to obtain his real public key in some manner. This is possible either by getting the public key in a secure way (for example, on a floppy disk or CD) or with the help of the Public Key Infrastructure by means of a digital certificate. Without having a secure way to obtain the real public key of given person, we don't have a possibility to check whether the given message is really signed by this person. From a technical point of view, the verification of a digital signature is performed in three steps: Step 1: Calculate the Current Hash-Value In the first step, a hash-value of the signed message is calculated. For this calculation, the same hashing algorithm is used as was used during the signing process. The obtained hash-value is called the current hash-value because it is calculated from the current state of the message. Step 2: Calculate the Original Hash-Value In the second step of the digital signature verification process, the digital signature is decrypted with the same encryption algorithm that was used during the signing process. The decryption is done by the public key that corresponds to the private key used during the signing of the message. As a result, we obtain the original hash-value that was calculated from the original message during the first step of the signing process (the original message digests). Step 3: Compare the Current and the Original Hash-Values In the third step, we compare the current hash-value obtained in the first step with the original hash-value obtained in the second step. If the two values are identical, the verification if successful and proves that the message has been signed with the private key that corresponds to the public key used in the verification process. If the two values differ from onr another, this means that the digital signature is invalid and the verification is unsuccessful. Reasons for Invalid Signatures There are three possible reasons for getting an invalid digital signature: If the digital signature is adulterated (it is not real) and is decrypted with the public key, the obtained original value will not be the original hash-value of the original message but some other value. If the message was changed (adulterated) after its signing, the current hash-value calculated from this adulterated message will differ from the original hash-value because the two different messages correspond to different hash-values. If the public key does not correspond to the private key used for signing, the original hash-value obtained by decrypting the signature with an incorrect key will not be the correct one. Benefits of digital signatures Below are some common reasons for applying a digital signature to communications: Authentication Although messages may often include information about the entity sending a message, that information may not be accurate. Digital signatures can be used to authenticate the source of messages. When ownership of a digital signature secret key is bound to a specific user, a valid signature shows that the message was sent by that user. The importance of high confidence in sender authenticity is especially obvious in a financial context. For example, suppose a bank's branch office sends instructions to the central office requesting a change in the balance of an account. If the central office is not convinced that such a message is truly sent from an authorized source, acting on such a request could be a grave mistake. Integrity In many scenarios, the sender and receiver of a message may have a need for confidence that the message has not been altered during transmission. Although encryption hides the contents of a message, it may be possible to change an encrypted message without understanding it. (Some encryption algorithms, known as nonmalleable ones, prevent this, but others do not.) However, if a message is digitally signed, any change in the message will invalidate the signature. Furthermore, there is no efficient way to modify a message and its signature to produce a new message with a valid signature, because this is still considered to be computationally infeasible by most cryptographic hash functions. Drawbacks of digital signatures Despite their usefulness, digital signatures alone do not solve the following problems: Association of digital signatures and trusted time stamping Digital signature algorithms and protocols do not inherently provide certainty about the date and time at which the underlying document was signed. The signer might have included a time stamp with the signature, or the document itself might have a date mentioned on it. Regardless of the document's contents, a reader cannot be certain the signer did not, for example, backdate the date or time of the signature. Such misuse can be made impracticable by using trusted time stamping in addition to digital signatures. Non-repudiation In a cryptographic context, the word repudiation refers to any act of disclaiming responsibility for a message. A message's recipient may insist the sender attach a signature in order to make later repudiation more difficult, since the recipient can show the signed message to a third party (e.g., a court) to reinforce a claim as to its signatories and integrity. However, loss of control over a user's private key will mean that all digital signatures using that key, and so ostensibly 'from' that user, are suspect. Nonetheless, a user cannot repudiate a signed message without repudiating their signature key. This is aggravated by the fact there is no trusted time stamp, so new documents (after the key compromise) cannot be separated from old ones, further complicating signature key invalidation. A non-repudiation service requires the existence of a public key infrastructure (PKI) which is complex to establish and operate. The Certificate authorities in a PKI usually maintain a public repository of public keys so the associated private key is certified and signatures cannot be repudiated. Expired certificates are normally removed from the repository. It is a matter for the security policy and the responsibility of the authority to keep old certificates for a period of time if non-repudiation of data service is provided. WYSIWYS Technically speaking, a digital signature applies to a string of bits, whereas humans and applications "believe" that they sign the semantic interpretation of those bits. In order to be semantically interpreted the bit string must be transformed into a form that is meaningful for humans and applications, and this is done through a combination of hardware and software based processes on a computer system. The problem is that the semantic interpretation of bits can change as a function of the processes used to transform the bits into semantic content. It is relatively easy to change the interpretation of a digital document by implementing changes on the computer system where the document is being processed. From a semantic perspective this creates uncertainty about what exactly has been signed. WYSIWYS (What You See Is What You Sign) means that the semantic interpretation of a signed message can not be changed. In particular this also means that a message can not contain hidden info that the signer is unaware of, and that can be revealed after the signature has been applied. WYSIWYS is a desirable property of digital signatures that is difficult to guarantee because of the increasing complexity of modern computer systems.
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