Cryptography & Encryption 

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Cryptography provides two primary security methods you can use with email: digital signatures and encryption. These are separate processes, but you can digitally sign and encrypt the same email. If you’re planning to take the SY0-501 exam, you should have a good understanding of cryptography and encryption.

For example, can you answer this practice test question?

Q. An organization requested bids for a contract and asked companies to submit their bids via email. After winning the bid, Acme realized it couldn’t meet the requirements of the contract. Acme instead stated that it never submitted the bid. Which of the following would provide proof to the organization that Acme did submit the bid?

A. Digital signature

B. Integrity

C. Repudiation

D. Encryption

More, do you know why the correct answer is correct and the incorrect answers are incorrect? The answer and explanation are available at the end of this post.

Signing Email with Digital Signatures

Digital signatures are similar in concept to handwritten signatures on printed documents that identify individuals, but they provide more security benefits. The digital signature algorithm (DSA) uses an encrypted hash of a message. The hash is encrypted with the sender’s private key. If the recipient of a digitally signed email can decrypt the hash, it provides the following three security benefits:

Authentication. This identifies the sender of the email. Email recipients have assurances the email actually came from who it appears to be coming from. For example, if an executive digitally signs an email, recipients know it came from the executive and not from an attacker impersonating the executive.

Non-repudiation. The sender cannot later deny sending the message. This is sometimes required with online transactions. For example, imagine Homer sends an order to sell stocks using a digitally signed email. If the stocks increase after his sale completes, he can’t deny the transaction.

Integrity. This provides assurances that the message has not been modified or corrupted. Recipients know that the message they received is the same as the sent message.

Digital signatures are much easier to grasp if you understand some other cryptography concepts. These concepts are:

Hashing. Digital signatures start by creating a hash of the message. A hash is simply a number created by executing a hashing algorithm on the message.

Certificates. Digital signatures need certificates, and certificates include the sender’s public key.

Public/private keys. In a digital signature, the sender uses the sender’s private key to encrypt the hash of the message. The recipient uses the sender’s public key to decrypt the hash of the message. The public key is often distributed in an S/MIME.p7s formatted file.

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Certificates

A key element of asymmetric encryption is a certificate. A certificate is a digital document that typically includes the public key and information on the owner of the certificate. Certificate Authorities issue and manage certificates. Certificates are used for a variety of purposes beyond just asymmetric encryption, including authentication and digital signatures.

The figure shows a sample certificate with the public key selected. Users and applications share the certificate file to share the public key. They do not share the private key.

Cryptography & Encryption 

Certificate with public key selected

There is much more information in the certificate than just the public key, but not all of it is visible in the figure. Common elements within a certificate include:

Serial number. The serial number uniquely identifies the certificate. The CA uses this serial number to validate a certificate. If the CA revokes the certificate, it publishes this serial number in a certificate revocation list (CRL).

Issuer. This identifies the CA that issued the certificate.

Validity dates. Certificates include “Valid From” and “Valid To” dates. This ensures a certificate expires at some point.

Subject. This identifies the owner of the certificate. In the figure, it identifies the subject as Google, Inc and indicates this is a wildcard certificate used for all web sites with the google.com root domain name.

Public key. RSA asymmetric encryption uses the public key in combination with the matching private key.

Usage. Some certificates are only for encryption or authentication, whereas other certificates support multiple usages.

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Steganography

Steganography hides data inside other data, or, as some people have said, it hides data in plain sight. The goal is to hide the data in such a way that no one suspects there is a hidden message. It doesn’t actually encrypt the data, so it can’t be classified as either symmetric or asymmetric. However, it can effectively hide information using obfuscation, so it is included with encryption topics. Obfuscation methods attempt to make something unclear or difficult to understand.

There are a variety of steganography tools available that make the process easier. Additionally, these tools attempt to resist detection by forensic methods. For example, Kali Linux includes Steghide and StegoSuite, two tools you can use to embed data into graphic files.

Some common examples of steganography are:

Hide data by manipulating bits. It’s possible to manipulate some bits within an image or sound file to embed a message. One method of embedding data in large files is modifying the least significant bit in some By modifying the least significant bit in some of the individual bytes of a JPEG file, it embeds a message, but the changes are so small that they are difficult to detect. However, if people know the file includes a message, they can easily retrieve it.

Hide data in the white space of a file. Many files have unused space (called white space) at the end of file clusters. Imagine a small 6 KB file stored in two 4 KB clusters. It has an extra 2 KB of unused space and it’s possible to fill this white space with a message. For example, you can embed a message into the white space of a GIF or JPEG file without altering the file

Security professionals use steganalysis techniques to detect steganography, and the most common method is with hashing. If a single bit of a file is modified, the hashing algorithm creates a different hash. By regularly taking the hashes of different files and comparing them with previous hashes, it’s easy to detect when a file has been modified.


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Q. An organization requested bids for a contract and asked companies to submit their bids via email. After winning the bid, Acme realized it couldn’t meet the requirements of the contract. Acme instead stated that it never submitted the bid. Which of the following would provide proof to the organization that Acme did submit the bid?

A. Digital signature

B. Integrity

C. Repudiation

D. Encryption

Answer is A. If Acme submitted the bid via email using a digital signature, it would provide proof that the bid was submitted by Acme. Digital signatures provide verification of who sent a message, non-repudiation preventing them from denying it, and integrity verifying the message wasn’t modified.

Integrity verifies the message wasn’t modified.

Repudiation isn’t a valid security concept.

Encryption protects the confidentiality of data, but it doesn’t verify who sent it or provide non-repudiation.

See Chapter 10 of the CompTIA Security+: Get Certified Get Ahead: SY0-501 Study Guide for more information on cryptography and PKI.

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