A digital signature
or digital signature scheme
is a type of asymmetric cryptography
used to simulate the security properties of a handwritten signature
on paper. Digital signature schemes consist of at least three algorithms: a key generation algorithm, a signature algorithm, and a verification algorithm. A digital signature mainly provides authentication
of a "message". In theory it can also provide non-repudiation
, meaning that the authenticity of signed messages can be publicly verified, not only by the intended recipient. Messages may be anything, from electronic mail
to a contract
, or even a message sent in a more complicated cryptographic protocol
Digital signatures are often used to implement electronic signatures, a broader term that refers to any electronic data that carries the intent of a signature, but not all electronic signatures use digital signatures. In some countries, including the United States, and in the European Union, electronic signatures have legal significance. However, laws concerning electronic signatures do not always make clear their applicability towards cryptographic digital signatures, leaving their legal importance somewhat unspecified.
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.
In the famous paper "New Directions in Cryptography", Whitfield Diffie and Martin Hellman first described the notion of a digital signature scheme, although they only conjectured that such schemes existed. Soon afterwards, Ronald Rivest, Adi Shamir, and Len Adleman invented the RSA algorithm that could be used for primitive digital signatures. (Note that this just serves as a proof-of-concept, and "plain" RSA signatures are not secure.) The first widely marketed software package to offer digital signature was Lotus Notes 1.0, released in 1989, which used the RSA algorithm.
Basic RSA signatures are computed as follows. To generate RSA signature keys, one simply generates an RSA key pair containing a modulus N that is the product of two large primes, along with integers e and d such that e d = 1 mod φ(N), where φ is the Euler phi-function. The signer's public key consists of N and e, and the signer's secret key contains d.
To sign a message m, the signer computes σ=md mod N. To verify, the receiver checks that σe = m mod N.
As noted earlier, this basic scheme is not very secure. To prevent attacks, one can first apply a cryptographic hash function to the message m and then apply the RSA algorithm described above to the result. This approach can be proven secure in the so-called random oracle model.
Other digital signature schemes were soon developed after RSA, the earliest being Lamport signatures, Merkle signatures (also known as "Merkle trees" or simply "Hash trees"), and Rabin signatures.
In 1984, Shafi Goldwasser, Silvio Micali, and Ronald Rivest became the first to rigorously define the security requirements of digital signature schemes. They described a hierarchy of attack models for signature schemes, and also present the GMR signature scheme, the first that can be proven to prevent even an existential forgery against a chosen message attack.
Most early signature schemes were of a similar type: they involve the use of a trapdoor permutation, such as the RSA function, or in the case of the Rabin signature scheme, computing square modulo composite n. A trapdoor permutation family is a family of permutations, specified by a parameter, that is easy to compute in the forward direction, but is difficult to compute in the reverse direction. However, for every parameter there is a "trapdoor" that enables easy computation of the reverse direction. Trapdoor permutations can be viewed as public-key encryption systems, where the parameter is the public key and the trapdoor is the secret key, and where encrypting corresponds to computing the forward direction of the permutation, while decrypting corresponds to the reverse direction. Trapdoor permutations can also be viewed as digital signature schemes, where computing the reverse direction with the secret key is thought of as signing, and computing the forward direction is done to verify signatures. Because of this correspondence, digital signatures are often described as based on public-key cryptosystems, where signing is equivalent to decryption and verification is equivalent to encryption, but this is not the only way digital signatures are computed.
Used directly, this type of signature scheme is vulnerable to a key-only existential forgery attack. To create a forgery, the attacker picks a random signature σ and uses the verification procedure to determine the message m corresponding to that signature. In practice, however, this type of signature is not used directly, but rather, the message to be signed is first hashed to produce a short digest that is then signed. This forgery attack, then, only produces the hash function output that corresponds to σ, but not a message that leads to that value, which does not lead to an attack. In the random oracle model, this hash-and-decrypt form of signature is existentially unforgeable, even against a chosen-message attack.
There are several reasons to sign such a hash (or message digest) instead of the whole document.
- For efficiency: The signature will be much shorter and thus save time since hashing is generally much faster than signing in practice.
- For compatibility: Messages are typically bit strings, but some signature schemes operate on other domains (such as, in the case of RSA, numbers modulo a composite number N). A hash function can be used to convert an arbitrary input into the proper format.
- For integrity: Without the hash function, the text "to be signed" may have to be split (separated) in blocks small enough for the signature scheme to act on them directly. However, the receiver of the signed blocks is not able to recognize if all the blocks are present and in the appropriate order.
Notions of security
In their foundational paper, Goldwasser, Micali, and Rivest lay out a heirarchy of attack models against digital signatures:
- In a key-only attack, the attacker is only given the public verification key.
- In a known message attack, the attacker is given valid signatures for a variety of messages known by the attacker but not chosen by the attacker.
- In an adaptive chosen message attack, the attacker first learns signatures on arbitrary messages of the attacker's choice.
They also describe a hierarchy of attack results:
- A total break results in the recovery of the signing key.
- A universal forgery attack results in the ability to forge signatures for any message.
- A selective forgery attack results in a signature on a message of the adversary's choice.
- An existential forgery merely results in some valid message/signature pair not already known to the adversary.
The strongest notion of security, therefore, is security against existential forgery under an adaptive chosen message attack.
Benefits of digital signatures
Below are some common reasons for applying a digital signature to communications:
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.
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
(see collision resistance
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.
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.
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.
Additional security precautions
Putting the private key on a smart card
All public key / private key cryptosystems depend entirely on keeping the private key secret. A private key can be stored on a user's computer, and protected by a local password, but this has two disadvantages:
- the user can only sign documents on that particular computer
- the security of the private key depends entirely on the security of the computer
A more secure alternative is to store the private key on a smart card. Many smart cards are designed to be tamper-resistant (although some designs have been broken, notably by Ross Anderson and his students). In a typical digital signature implementation, the hash calculated from the document is sent to the smart card, whose CPU encrypts the hash using the stored private key of the user, and then returns the encrypted hash. Typically, a user must activate his smart card by entering a personal identification number or PIN code (thus providing two-factor authentication). It can be arranged that the private key never leaves the smart card, although this is not always implemented. If the smart card is stolen, the thief will still need the PIN code to generate a digital signature. This reduces the security of the scheme to that of the PIN system, although it still requires an attacker to possess the card. A mitigating factor is that private keys, if generated and stored on smart cards, are usually regarded as difficult to copy, and are assumed to exist in exactly one copy. Thus, the loss of the smart card may be detected by the owner and the corresponding certificate can be immediately revoked. Private keys that are protected by software only may be easier to copy, and such compromises are far more difficult to detect.
Using smart card readers with a separate keyboard
Entering a PIN code to activate the smart card commonly requires a numeric keypad. Some card readers have their own numeric keypad. This is safer than using a card reader integrated into a PC, and then entering the PIN using that computer's keyboard. Readers with a numeric keypad are meant to circumvent the eavesdropping threat where the computer might be running a keystroke logger
, potentially compromising the PIN code. Specialized card readers are also less vulnerable to tampering with their software or hardware and are often EAL3
Other smart card designs
Smart card design is an active field, and there are smart card schemes which are intended to avoid these particular problems, though so far with little security proofs.
Using digital signatures only with trusted applications
One of the main differences between a digital signature and a written signature is that the user does not "see" what he signs. The user application presents a hash code to be encrypted by the digital signing algorithm using the private key. An attacker who gains control of the user's PC can possibly replace the user application with a foreign substitute, in effect replacing the user's own communications with those of the attacker. This could allow a malicious application to trick a user into signing any document by displaying the user's original on-screen, but presenting the attacker's own documents to the signing application.
To protect against this scenario, an authentication system can be set up between the user's application (word processor, email client, etc.) and the signing application. The general idea is to provide some means for both the user app and signing app to verify each other's integrity. For example, the signing application may require all requests to come from digitally-signed binaries.
Some digital signature algorithms
The current state of use — legal and practical
Digital signature schemes share basic prerequisites that— regardless of cryptographic theory or legal provision— they need to have meaning:
- ; Quality algorithms : Some public-key algorithms are known to be insecure, practicable attacks against them having been discovered.
- ; Quality implementations : An implementation of a good algorithm (or protocol) with mistake(s) will not work.
- ; The private key must remain private : if it becomes known to any other party, that party can produce perfect digital signatures of anything whatsoever.
- ; The public key owner must be verifiable : A public key associated with Bob actually came from Bob. This is commonly done using a public key infrastructure and the public keyuser association is attested by the operator of the PKI (called a certificate authority). For 'open' PKIs in which anyone can request such an attestation (universally embodied in a cryptographically protected identity certificate), the possibility of mistaken attestation is non trivial. Commercial PKI operators have suffered several publicly known problems. Such mistakes could lead to falsely signed, and thus wrongly attributed, documents. 'closed' PKI systems are more expensive, but less easily subverted in this way.
- ; Users (and their software) must carry out the signature protocol properly.
Only if all of these conditions are met will a digital signature actually be any evidence of who sent the message, and therefore of their assent to its contents. Legal enactment cannot change this reality of the existing engineering possibilities, though some such have not reflected this actuality.
Legislatures, being importuned by businesses expecting to profit from operating a PKI, or by the technological avant-garde advocating new solutions to old problems, have enacted statutes and/or regulations in many jurisdictions authorizing, endorsing, encouraging, or permitting digital signatures and providing for (or limiting) their legal effect. The first appears to have been in Utah in the United States, followed closely by the states Massachusetts and California. Other countries have also passed statutes or issued regulations in this area as well and the UN has had an active model law project for some time. These enactments (or proposed enactments) vary from place to place, have typically embodied expectations at variance (optimistically or pessimistically) with the state of the underlying cryptographic engineering, and have had the net effect of confusing potential users and specifiers, nearly all of whom are not cryptographically knowledgeable. Adoption of technical standards for digital signatures have lagged behind much of the legislation, delaying a more or less unified engineering position on interoperability, algorithm choice, key lengths, and so on what the engineering is attempting to provide.
- See also: ABA digital signature guidelines
Using separate key pairs for signing and encryption
In several countries, a digital signature has a status somewhat like that of a traditional pen and paper signature. Generally, these provisions mean that what is digitally signed legally binds the signer of the document to the terms therein. For that reason, it is often thought best to use separate key pairs for encrypting and signing. Using the encryption key pair, a person can engage in an encrypted conversation (e.g.
, regarding a real estate transaction), but the encryption does not legally sign every message he sends. Only when both parties come to an agreement do they sign a contract with their signing keys, and only then are they legally bound by the terms of a specific document. After signing, the document can be sent over the encrypted link.
- J. Katz and Y. Lindell, "Introduction to Modern Cryptography" (Chapman & Hall/CRC Press, 2007)
For books in English on electronic signatures, see:
- Stephen Mason, Electronic Signatures in Law (Tottel, second edition, 2007);
- Dennis Campbell, editor, E-Commerce and the Law of Digital Signatures (Oceana Publications, 2005);
- Lorna Brazell, Electronic Signatures Law and Regulation, (Sweet & Maxwell, 2004);
- M. H. M Schellenkens, Electronic Signatures Authentication Technology from a Legal Perspective, (TMC Asser Press, 2004).
For translations of electronic signature cases from Europe, Brazil, China and Colombia into English, see the Digital Evidence and Electronic Signature Law Review