Do you know everything there is to know about PGP (Pretty Good Privacy)?
Probably not. The good news is that you don’t need to, especially if you’re planning to take the Security+ exam. However, you should understand some basics. As an example, can you answer this sample Security+ question?
Q. Consider the text in this graphic.
(Click to enlarge.)
Which of the following most closely identifies the purpose of the text in the graphic.
A. Insert a public key within a document
B. Authenticate a document
C. Encrypt a document
D. Hash a document
Can you answer the question? More, do you know why the correct answer is correct and the incorrect answers are incorrect? The answer and explanation is available at the end of this post.
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PGP Goals
Phil Zimmermann created PGP in 1991 with the intention of providing cryptographic privacy using free software (freeware). It has gone through several iterations and owners, but the basic cryptographic goals of PGP remains the same:
- Confidentiality. It includes the ability to encrypt data (primarily emails, but PGP can also encrypt files, folders, and disks). Unauthorized recipients are not able to read the encrypted data.
- Authentication. It authenticates the sender with a digital signature. The digital signature also provides assurances that the email message has not been modified.
PGP emails can be sent in clear text with a digital signature, encrypted without a digital signature, or both encrypted and signed with a digital signature.
PGP Headers
RFC 4880 lists the headers for PGP messages when using the OpenPGP Format. Some of the headers (along with the explanations from RFC 4880) are:
- BEGIN PGP MESSAGE – Used for signed, encrypted, or compressed files.
- BEGIN PGP PUBLIC KEY BLOCK – Used for armoring public keys.
- BEGIN PGP PRIVATE KEY BLOCK – Used for armoring private keys.
- BEGIN PGP SIGNATURE – Used for detached signatures, OpenPGP/MIME signatures, and cleartext signatures.
If you’re not familiar with PGP, you may get overwhelmed with the preceding bullets. However, you can simplify them.
- A message is typically an email message.
- A public key is a key (matched to a private key) used for asymmetric encryption.
- A private key is a key (matched to a public key) used for asymmetric encryption.
- A signature is a digital signature used for authentication, non-repudiation, and integrity.
Note: This assumes you understand the cryptography basics required for Security+. If concepts such as asymmetric encryption and digital signatures are not familiar to you, check out chapter 10 of the CompTIA Security+: Get Certified Get Ahead: SY0-401 Study Guide.
PGP Message
The following graphic shows a sample PGP message.
How do you know it’s a message? It starts with BEGIN PGP MESSAGE.
You can’t read the message because it’s encrypted, but an authorized recipient can decrypt and read it.
PGP Signature
The following graphic shows a sample PGP digital signature.
How do you know it’s a digital signature? It starts with BEGIN PGP SIGNED MESSAGE and the second block starts with BEGIN PGP SIGNATURE.
Notice that in this example, the message isn’t encrypted. It is simply “You can pass.”
The signature isn’t in clear text. However, as with any digital signature, an authorized recipient can use the digital signature to verify the authenticity of the message by authenticating the sender. The sender also provides non-repudiation (preventing the sender from (believably) denying they sent it, and integrity (verifying the message hasn’t been altered).
Remember this
PGP provides confidentiality by encrypting an email message. It provides authenticity with a digital signature. An encrypted email begins with — BEGIN PGP MESSAGE — and includes encrypted data. A PGP email that includes a digital signature with include the header of — BEGIN PGP SIGNED MESSAGE — and —BEGIN PGP SIGNATURE —.
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Q. Consider the text in this graphic.
(Click to enlarge.)
Which of the following most closely identifies the purpose of the text in the graphic.
A. Insert a public key within a document
B. Authenticate a document
C. Encrypt a document
D. Hash a document
Answer is B. The Pretty Good Privacy (PGP) message includes a digital signature, which authenticates the document. You can tell it is a digital signature because it starts with BEGIN PGP SIGNED MESSAGE and the second block starts with BEGIN PGP SIGNATURE.
A digital certificate includes a public key (matched to a private key) for asymmetric encryption. While PGP uses asymmetric encryption, certificates are distributed separately.
The message is simply “You can pass.” It is included in the block that starts with BEGIN PGP SIGNED MESSAGE. It is in clear text (not encrypted).
While a digital signature includes a hash, it does much more so ‘hash a document’ is not the best answer.
