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15

The purpose of a public certificate infrastructure is to create a chain of trust between entities that don't trust each other directly. You pay the certificate authorities because other people trust them to issue certificates to trustworthy entities only. Assuming you trust yourself (or whoever issued the certificate), self-signed certificates are no less ...


13

Revocation is the only method by which a certificate authority may propagate the information that a private key has been compromised. It is, in fact, a damage containment system: in the unfortunate event of a private key being stolen, the revocation system will make sure that nobody trusts the corresponding certificate more than one week or so after the ...


6

Your notation seems to relate to hybrid encryption. The message is encrypted with a symmetric key KAB; that key was probably generated randomly by the sender. For the recipient to know it, it is necessary to send it along with the message, but not as clear text, of course: KAB is encrypted with the recipient's (Bob's) public key. Thus, Alice does the ...


5

To answer your questions: {K_AB}**K_B_E is prepended to the message so that the receiver has the symmetric key to decrypt the message. Because the key K_B_E is used only Bob can decrypt the symmetric key and only he can therefore read the message. The next is the hash {h} which is encrypted/signed by the key K_A_D. I assume that K_A_D is the private key of ...


4

What the page you link to means is that there are known attacks which, when implemented, would allow building collisions with some costs: If the goal is "raw collisions" then the computational effort is equivalent to running 261 times the SHA-1 function. A raw collision is such that the attack produces two messages m and m' which are distinct but hash to ...


3

Only the last line is the actual message sent. Symmetric encryption is generally much faster and potentially more future proof than asymmetric cryptography. It requires far shorter key lengths for security. Since it is the intent to share the entire message with Bob, Alice doesn't have to encrypt the entire message (long) with Bob's public key. Instead, ...


3

"ECDHE" means that the key exchange will use the Diffie-Hellman algorithm (over elliptic curves) with freshly generated DH elements; the first "E" stands for ephemeral. So while DH produces a shared key, it will work with randomly produced values, and nothing in DH will ensure authentication: the client has no way to know whether the DH public key it sees ...


3

SHA1 hash has 160bits. If SHA1 was safe, you would need approximately 2^80 iterations to find a collision. Why 2^80 and not 2^160? Because of birthday paradox: http://en.wikipedia.org/wiki/Birthday_attack The general "collision finding" algorithm works like this: http://en.wikipedia.org/wiki/Cycle_detection#Tortoise_and_hare ...simply, to rabbits, one is ...


3

When Windows verifies a file, it must compute a hash of the file, and match it against the digital signature. In order to compute the hash, Windows does need to read the entire file (see this blog.msdn.com post). Copying a file will naturally take longer than just reading it, as it require write operations, in addition to reads. If Windows did not read the ...


3

You are confused because some people (yeah I am looking at you, Microsoft) have been using the terms inconsistently. A signature algorithm is a cryptographic algorithm such that: The signer owns a public/private key pair. The public key is public, the private key is private; even though both keys are mathematically linked together, it is not feasible to ...


2

None. There's no security benefit. Tools like openssl are designed to work with certificates. This means it's more practical to give openssl a self-signed certificate rather than no cert at all.


2

TL-DR SSL client cert doesn't need KeyUsage but if present it should be digitalSignature except for very-rare-if-ever fixed-*DH. Caveat: You tagged SSL so I assume by "path that requires a certificate" you mean SSL/TLS or something over SSL/TLS (not necessarily HTTP/S). If you mean something more like CMS or S/MIME, or XML-sig, or even PGP, the answer may ...


2

No, this is not the case. A certificate signed by a CA contains only the public key, but for decrypting you need the private key too. This private key is not needed for the CA to sign the key, so they usually don't have it either. But, some CA offer to simplify the process of certificate generation by generating a key pair for the certificate too. In this ...


2

Generally the main use for MitM at the moment is for the attacker to impersonate the website to the victim. The aim being, usually, to get hold of the victim's credentials in order to impersonate them, authenticate correctly to the website and make off with the contents of their bank account, data store, intellectual property etc. This is done, typically, ...


2

The operation performed is similar to server authentication with the following differences. After sending the message ServerKeyExchange, the server indicates to the client that it wishes to authenticate the client via a CertificateRequest message; the client then sends its certificate. The TLS handshake operation is performed normally but after sending the ...


2

In many cases, the intermediate certs do mean something. Sometimes it mean the Insurance level (how much the signer will pay you in the event of a fraudulent verification) or it can mean the validation level (only domain verified, or full verified with Company details and Everything). In this case, Comodo isnt even the top signer. Instead its a Company ...


2

It means that rather than trusting a central authority for validating the PKI, you are utilizing your own means to determine that a public key is valid. Pinning allows removal of the trust of a central authority and instead takes that responsibility on directly. The "problem" that it is trying to solve is relying on third party trust, however it requires ...


2

Theory is that everything works automatically. Practice sometimes differs. I suppose that you are talking about S/MIME and X.509 certificates. With S/MIME, when you send an email: The email is encrypted with the public key of the recipient, so you have to know the current recipient's certificate. The email is signed with your private key and the signature ...


2

SSL certificates have an extension field that defines what a certificate is allowed to be used for. When you buy a certificate from VeriSign with your certificate signing request, it typically will not include the extension permission for signing downstream certificates with your certificate; especially for any domain you don't have authority for. Take ...


2

You are missing a very important part of a Digital Certificate here. That part is Digital Signature. Basically, a Digital Certificate consists of 3 parts: A public key. Certificate information. ("Identity" information about user, such as name, user ID, and so on.) One or more digital signatures. A digital signature an encrypted hash of the ...


1

If you want your server to encrypt/sign outgoing traffic, then it needs its own private key. This is normal. What I think you are asking is about the security of having this certificate on a server that is ultimately out of your control. You will simply have to limit the exposure of this certificate and have processes in place to handle the event where you ...


1

Took a while to wrap my head around the bizarre syntax your lecturer is using, but basically the symmetric key is a "session" key generated per message and not reused. Hence the need to include an encrypted copy of it in the final message. I'm assuming H**K_A_D is Alice encrypting the message digest with her private key.


1

One way to do this is open the server up in your browser like so: https://[smtp server]:[port]. Then save the certificate that your browser encounters. For instance, https://smtp.gmail.com:465/ To do this quickly, I used Internet Explorer. To do this in a real browser you will need to override port "protection", as apparently internet users are toddlers to ...


1

It is not uncommon for revocation checking to fail due to the CA revocation service not being available - or it being late in publishing a CRL. So you are probably okay. A revocation check can be either to to CRL file which is published periodically by the CA (e.g. every 8 hours) which has a validity period or using an Online Certificate Status Protocol ...


1

Pipe the output from openssl s_client (which includes the cert in PEM-format) into openssl x509 -noout -text and it gives a human-readable display of the cert fields, including the signing algo (in two places). You don't need the sed -n /BEGIN/,/END/p step; most OpenSSL operations on PEM-format data, including this one, ignore "extra" data outside the PEM ...


1

For the scenario you described you should have the root certificate of the main server sign the child server certificates. The embedded devices would then store the main server's root certificate. In this way any child server could connect to an embedded device, and the embedded device would trust it because the certificate chain is trusted. I would ...


1

I think you're putting two encryption forms together and getting confused. First you need to look at encryption in two ways encrypting data and encrypting a tunnel. Encryption of data is done at the endpoint before the information is send over the wire. This can be done using Symmetric or Asymmetric encryption. Symmetric encryption is an exchange of the ...


1

A man-in-the-middle attack, by definition, involves impersonation. The basic idea is something like this: Alice wants to have a conversation with Bob. Mallory tricks Alice into sending the first message to him instead of Bob (exactly how can vary greatly). Mallory contacts Bob, pretending to be Alice, and passes on Alice's message. Bob replies to ...


1

The answer to your question is: no. MiTM is done to steal information that the user THINKS they are sending to a legitimate site. HOW it accomplishes this usually involves "impersonating" whatever party you're trying to send your data to, and sometimes involves traffic decryption (although most of the time, decryption isn't feasible without some sort of ...


1

It depends. With some ciphers, it is possible to passively eavesdrop the communication once you have the private key. It may be done even for connections wiretapped before you obtained a copy of the private key used. However, for another class of ciphers, those providing forward secrecy (PFS), the client and server generate a new ephemeral key for the ...



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