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3ICT1 Security Digital Signatures
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Title: 3ICT1 Security Digital Signatures
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3ICT1 Security- Digital Signatures
Certificates -
- Karl Quinn
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Digital Signatures Certificates
- Overview
- (1) Digital Signatures
- Overview and explanation.
- Secure Digest Function
- Digital Signing.
- (2) Certificates
- X.500 and X.509
- (3) Certificate Infrastructure
- Trusted Third Partys
- Certificate Authorities
- SPKI
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Digital Signatures
- Digital Signatures verify to a 3rd party that a
msg is an unaltered copy of the msg a signer
produces. - Digital signatures are
- (1) Authentic
- Works in a similar manner to hand written
signature. - (2) Unforgeable
- Or at least extremely difficult to forge.
- (3) Non-repudiable, or are they??
- Human can always say it wasnt him/her!
- by use of cryptography.
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Digital Signatures
- Based on an irreversible binding to a msg of a
secret known only to the signer. - Achieved by encrypting a msg, or a digest,
using a key only known to the signer. - The digest is a fixed length value computed via a
secure digest function. - A secure digest function is similar to a checksum
but it is very unlikely that a similar digest
will be produced for two different msg. - The encrypted digest is the signature.
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Digital Signatures
- Digital Signatures can be implement using
- Public Key Cryptography
- i.e. RSA algorithm.
- (2) Secret Keys Cryptography
- Actually, Message Authentication Codes (MACS)
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Digital Signatures
- Public Key Digital Signatures
- Public key cryptography is generally used.
- Originator creates the signature using their
private key. - A recipient can decrypt the signature by using
the originators corresponding public key. - Note that the receiver has to be sure that the
public key really does belong to the originator.
See (public key) Certificates.
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Digital Signatures
- Alice wants to digital sign a document, M, so
that Bob, the recipient, can verify that she is
Ms originator. - Signing
- Alice computes a digest of M. Digest(M).
- Alice encrypts the digest with her private key,
and appends it to M. M, Digest(M)KApriv
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Digital Signatures
- Verifying
- Bob obtains the document, extract M and computes
Digest(M). - Bob decrypts Digest(M)KApriv using Alices
public key, KApub, and compares the result with
his own Digest(M) computation. - If they match then the signatures have been
verified.
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Digital Signatures
- Shared Key Digital Signatures
- Note that the key must be disclosed for
verification. - Also referred to as Message Authentication Codes
(MACs) to reflect their more limited purpose. - Very low cost signing technique
- Alice generates a random key K for signing and
distributes it over secure channels.
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Digital Signatures
- Signing
- Alice concatenates M with the shared secret key
K. - Then she computes the digest of the result h
H(MK). - She now send the signed document, MK M,h
where the digest h is a MAC.
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Digital Signatures
- Verifying
- Bob concatenates the shared secret key K with M.
- Then computes the digest h H(MK)
- The signature is verified if h h.
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Secure Digest Functions
A secure digest function should have the
following properties (1) Given M, it is easy to
compute h. (2) Given h, it is hard to compute
M. (3) Given M, it is hard to find another
message M, such that H(M) H(M). Note that
(1) (2) characterise one-way functions.
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Secure Digest Function Examples
- MD5
- Fifth in a sequence of Message Digest algorithms
developed by Rivest (MD5). - 128-bit digest.
- Efficient Algorithm.
- SHA-7
- Secure Hash Algorithm (SHA).
- 160-bit digest.
- Slower than MD5
- Offers greater security than MD5 against
brute-force and birthday attacks.
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Certificates
- Certificates are documents that contain some
message, M, that is signed by the author. - Consider Alice and Bob the Banker.
- Alice needs to be sure that it is indeed Bob she
is talking too, and conversely Bob needs to
authenticate Alice as being Alice. - Below is Alices bank account certificate
- 1. Certificate Type Account Number
- 2. Name Alice
- 3. Account 123456
- 4. Cert Authority Bobs Bank
- 5. Signature Digest(Field 2 Field 3)KBpriv
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Certificates
- Alices bank account certificate allows her to
certify to a vendor, Carol, that she has a bank
account with Bob the bank. - Carol can accept this cert and charge items to
the bank account number if signature can be
validated. - This is done by finding and using Bobs public
key. - Note If its not Bobs key the Alice can create
KBpriv and KBpub and create a forged certificate
from Bobs bank. - Carol therefore needs a certificate that states
Bobs public key, signed by a trusted authority. - Trusted Authority Banker Federation.
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Certificates
- Fred public key authenticity could also be
questioned. - So the problem becomes recursive
- This recursion can be broken by ensuring Carol
gets Freds public key with a certain degree of
confidence this is a certificate chain. - This can be achieved by personal transfer or via
a trusted third party. - Public Key certificate for Bobs bank.
- 1. Certificate Type Account Number
- 2. Name Alice
- 3. Account 123456
- 4. Cert Authority Bobs Bank
- 5. Signature Digest(Field 2 Field 3)KBpriv
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Certificate Standards
- X.509
- Most widely used standard for certificates.
- Part of the X.500 standard for the construction
of global directories of names and attributes. - X.509 is used in cryptography as a format
definition for free standing certificates. - Public key is bound to a named entity called a
subject. - Binding is in the signature, which is issued by
an Issuer. - X.509 Certificate Format
- Subject Distinguished Name, Public Key
- Issuer Distinguished Name, Signature
- Validity Period Not Before, Not After
- Admin Info Version, Serial
- Extended Info
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Certificate Standards
- X.500 global directory service
- Service that stores collections of bindings
between names and attributes that looks up
entities that match attribute-based
specifications. - I.e. What is the name of the user with Telephone
number 016081335? - So for our purposes what is the public key of the
user with Name X, and Attributes Y,Z.
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Certificates Infrastructure
- Infrastructure
- Certificates need some infrastructure in place to
allow users to verify a given certificate. - This can be done centrally or via a distributed
system. - So how are certificates, and their certificate
chains, verified and disseminated? - (1) Trusted Third Party (TTP)
- (2) Certificate Authority (CA)
- (3) Simple Public Key Infrastructure (SPKI)
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Certificates Infrastructure
- Trusted Third Party (TTP)
- Alice wants to carry out some transaction with
Bob. - Alice wants to be sure that Bob is who he say he
is and Bob vice versa. - Carol, who is trusted by both Alice and Bob,
offers to undertake this authentication process.
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Certificates Infrastructure
- Certificate Authority
- Well known organisation establish themselves to
act as certificate authorities. - Verisign, CREN, etc.
- One can then obtain X.509 public key certificates
from them by submitting satisfactory evidence of
their identity. - Therefore, a 2 step verification process.
- Obtain public key cert of Alice from a CA.
- Validate Alices signature.
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Certificate Infrastructure
- Simple Public Key Infrastructure (SPKI)
- X.509 relies on global uniqueness, which can
cause long chains of certification that must be
validated to someone who is trusted. - SPKI is a scheme for the creation and management
of sets of public certificates. - Chains of certificates can be process using
logical inference to produce derived
certificates. - E.g. Bob believed Alices key is KApub
- Carol trusts Bob on Alices public key
- Therefore, Carol believes that Alices public
- key is KApub.
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Summary
- Summary of main concepts
- (1) Digital Signatures
- Secure Digest Functions.
- Digital Signing.
- (2) Certificates
- X.500 and X.509
- (3) Certificate Infrastructure
- Certificate Authorities
- SPKI
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Reference
- Coukouris, G., Dollimore, J., Kindberg, T.,
Distributed Systems, Concepts and Designs,
Addison-Wesley, Third Edition, 2001.
