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Cryptography > Email security and secure messaging > Flashcards

Email security and secure messaging Flashcards

1
Q

What does MUA stand for?

A

Message User Agent

A mail client application that grants users access to a mail server in order to create, send, receive email messages

E.g.: Outlook, Gmail

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2
Q

What does MSA stand for?

A

Message Submission Agent

Software agens that receive email messages from a MUA and cooperates with a MTA for delivery of the mail

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3
Q

What does MTA stand for?

A

Mail Transfer Agent

Transfers email messages from one computer to another using the Simple Mail Transfer Protocol

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4
Q

What does SMTP stand for?

A

Simple Mail Transfer Protocol

Communication protocol used for sending and receiving emails over the internet

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5
Q

What does IMAP stand for?

A

Internet Message Access Protocol

Internet standard protocol used by email clients to retrieve email messages from a mail server over a TCP/IP connection

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6
Q

What does POP stand for?

A

Post Office Protocol

Together with IMAP, one of the most common protocols for email retrieval

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7
Q

Describe the Email architecture

A

MUA connects client to mail system

SMTP is used to send mail to MSA

Message handling system (MHS) transfer message from MSA to MS via one or more MTAs

POP or IMAP is used to retrieve mail from message store (MS)

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8
Q

What is webmail?

A

Browser interface to an online email client

SMTP ans POP/IMAP are still used to send and receive email

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9
Q

What security does/may email content require?

A

Confidentiality or Authentication

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10
Q

What are some security threats against email?

A

Availability of email service

Metadata in header information is a source of attacker information

Email content confidentiality/authentication

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11
Q

What is spam?

A

Unsolicited email

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12
Q

What is a common vector for phishing attacks?

A

Spam mail

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13
Q

What are some counter measure against spam/phishing?

A

Email filtering

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14
Q

What types of phishing attacks are harder to filter?

A

Spear phishing: Phishing with more accurate targeting

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15
Q

What type of security is provided between agents in a mail system?

A

Link-to-link basis: Using protocols such as STARTTLS and DKIM

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16
Q

What type of security is provided between clients in a mail system?

A

end-to-end basis: using protocols such as PGP and S/MIME

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17
Q

What is StartTLS?

A

Runs over TLS

Extensions to SMTP, POP and IMAP

Provides link-to-link security

Opportunistic use of TLS security (encryption) - use it if possible

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18
Q

What attack is StartTLS vulnerable to?

A

STRIPTLS attacks: attacker interrupts TLS negotiation and connection falls back to plaintext transmission

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19
Q

What is DKIM?

A

DomainKeys Identified Mail

Allows sending mail domain to sign outgoing mail using RSA signatures

Receiving domain can verify origin of mail

Public verification key of sending domain is retrieved using DNS

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20
Q

What does DKIM help reduce?

A

Email spoofing, and hence reduce spam and phishing

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21
Q

What is email spoofing?

A

A threat involving sending emails with a fake/forged sender address. Can be done by an attacker that changes the metadata of an email.

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22
Q

What does ‘d=’ and ‘s=’ do in the DKIM signature?

A

Specify domain and selector

Example:

d=easychair.org
s=default

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23
Q

Where is the relevant public key for a DKIM signature?

A

In the DNS record for the host defined by the name:
[selector]._domainkey.[domain]

’s=’ : Gives the selector
‘d=’ : Gives the domain

24
Q

What does nslookup do?

A

Command to query internet domain name servers for information about hosts or domains.

25
Q

How is email processing done in PGP (end-to-end security)?

A

Uses hybrid encryption: A new random “session key” is generated for each object (message) and encrypted with the long-term public key of recipient

Signing: RSA or DSA signatures

Compression: ZIP

Coding: Base64

26
Q

Why is base64 used in email processing?

A

Ensure that binary strings can be sent in email body

27
Q

What is PGP used for?

A

End-to-end security between clients

28
Q

How does PGP encryption work?

A

Session key encryption: asymmetric

Encryption of messahe text: Symmetric key

Compression applied before encryption

Encryption applied independently of signing (no requirement for authenticated encryption

29
Q

What are PGP signatures?

A

Plaintext message optionally signed with sender’s private key

Can use RSA or DSA signatures

RSA signed messages are hashed with SHA1 or other SHA2 functions

30
Q

What are the requirements of OpenPGP?

A

Support for RSA signatures

Session key enc: ElGamal, recommends RSA

Message enc: 3DES with 3 keys (168 bits in total), recommends AES-128 and CAST5

31
Q

What are OpenPGP PKI used in?

A

PGP email security

32
Q

What does OpenPGP PKI include? (4)

A

ID, public key, validity period, self-signature

33
Q

Who can sign OpenPGP keys?

A

Anyone, no certification authorities

34
Q

Why is PGP difficult to use?

A

Difficult for an average user to understand public key cryptography.

Typical problems:
- Generating new keys securely
- Moving keys between devices
- Renewing keys when expired

35
Q

What are some criticisms of OpenPGP?

A

Outdated cryptographic algorithms still used: SHA1, CAST, Blowfish

No support for SHA3 or auth encryption

A lot of metadata available to an eavesdropper: file length, enc-algorithm used, key identity of recipient

No forward secrecy

No support for streaming mode or random access decryption

36
Q

What is S/MIME

A

Similar security features to PGP

Different format for messages

Not interoperable

Requires X.509 format certificates

37
Q

What is often known as the web of trust?

A

Concept used in PGP: OpenPGP PKI

38
Q

What is the difference between email and messaging?

A

Most instant messages are part of an interactive conversation which extends over many messages and a long time

Proprietary servers are typically used to manage accounts and dedicated applications are used

39
Q

What security is required for secure messaging?

A

Confidentiality. integrity, authentication

Forward secrecy: important for long sessions

Desirable to have post-compromise security (self-healing)

40
Q

How is forward secrecy achieved in secure messaging?

A

Using medium-term public keys stored at the server

41
Q

What happens to an attacker who obtains a long-term key, in a system with post-compromise security?

A

Should be locked out again after communication resumes

42
Q

What is signal?

A

A messaging app, considered the most secure

43
Q

How does the signal protocol work?

A

Server sets up initial auth of user and registers initial public keys

Public keys at the server are used to set up initial communication between users

Key exchange: Elliptic curve DH

Message protection: AES in CBC mode with HMAC

44
Q

In the signal protocol, what is used for key exchange?

A

Elliptic curve DH

45
Q

In the signal protocol, what is used for Message protection?

A

AES in CBC mode with HMAC

46
Q

What is a ratchet?

A

A device which is easy to move forward, but blocked from moving backward

47
Q

What is the continuous key exchange in signal?

A

Signal uses a new unique message key for every message exchanged

48
Q

How does signal use the symmetric ratchet?

A

When successive messages are sent in the same direction, the message key is updated with a symmetric ratchet.

This is done by applying a function such as HMAC

49
Q

What 2 types of ratchets does signal use?

A

Symmetric ratchet

DH ratchet

50
Q

How does signal use the DH ratchet?

A

When a new message is returned on the opposite direction, a new DH ephemeral key is used to compute the new message key.

The new DH ephemeral key is the DH-ratchet

51
Q

How is group messaging implemented securely?

A

DH is the only known good alternative in the multi-party case

Signal uses a simple key distribution for group messaging

52
Q

What does the Signal’s PQXDH protocol protect against?

A

Harvest Now, Decrypt Later attacks

53
Q

What type of security does Signal’s PQXDH protocol provide?

A

Post-quantum forward secrecy

54
Q

What is Signal’s PQXDH protocol?

A

Post-Quantum eXtended Diffie-Hellman key agreement protocol

55
Q

What does Signal’s PQXDH protocol rely on?

A

The hardness of the discrete log problem for mutual authentication

Cryptography (15 decks)

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