SSH key file formats

File: ssh/file-formats.txt (or file-formats.pdf)

You may work in groups of size 1-3 for this assignment.

Goals

Learn some ways cryptographic keys are stored in practice
Learn a little about ASN.1, DER, PEM, and the PKCS standards documents.

Rubric

1 - author name(s) in comment at top of file-formats.txt 6 - contents and meaning of the private key file 3 - contents and meaning of the public key file 2 - sanity check of the relationships between the private and public key

Background

When we first talk about public-key encryption and key exchange protocols, we talk in abstract mathematical terms. For RSA, for example, we talk about prime integers p and q, least common multiples, modular exponentiation, etc. But if you want to use these mathematical ideas in a practical encryption system, you need to deal with concrete questions like "how are we going to store private and public keys?"

In this assignment, you're going to look at the various standards specifications that are used to store keys for use in the SSH protocol.

Do a little reading first

Start learning about ASN.1 and DER by studying the Example section of the ASN.1 Wikipedia page.
Read this brief description of the Distinguished Encoding Rules (DER)
Skim the Wikipedia page about PEM
Open a tab to (RFC 4716) The Secure Shell (SSH) Public Key File Format
Open a tab to (RFC 8017) PKCS #1: RSA Cryptography Specifications Version 2.2. You'll probably want to scroll down to Appendix A.1.
You might find this intro to ASN.1 and DER from Let's Encrypt handy.

Your job

In your file-formats.txt (or .pdf) file, include the following.

Create an RSA public/private key pair that you will use throughout the rest of this assignment.

ssh-keygen -t rsa -m pem

When you are asked to pick a file name, use "id_rsa_homework" so you can remember to delete these files when you're done with the assignment. (You're going to be posting your observations in a public GitHub repo, so you're not going to ever actually use this key pair for anything.)

Copy the contents of id_rsa_homework and id_rsa_homework.pub into your file-formats.txt file.
Put a heading in your report: "Private Key"
For your private key file (id_rsa_homework), list the items you expect to be contained in the file. (Hint: the Appendix of RFC 8017 should help.)
Use one or more of the following tools (findable by searching the internet for "web asn.1 decoder") to decode your private key file.
Explain briefly the steps you took to decode the private key file.
For each integer in your decoded private key file, tell me:
- What is the meaning/name of the integer? This should correspond to one of the items in your answer to the previous question, articulated as an ASN.1 name from the specification in RFC 8017.
- What is the value of the integer? Write the integer in either decimal or hexadecimal, whichever is more convenient. If hexadecimal, prepend the integer with "0x".
- Which bytes from the decoded base64 data represent this integer? For this, you should specify the starting index (typically known as the "offset") of the integer within the bytes decoded from the base64, and then describe the DER encoding of the integer that you find at that offset. The Lapo Luchini decoder will be particularly helpful for this.
Next, we're moving on to the public key file id_rsa_homework.pub, so put a heading in your report: "Public Key"
Do the same things you did for the private key: write down what you expect to find in id_rsa_homework.pub and why; explain how you decoded the file; for each integer in the file, provide its name, value, and any other relevant details. Unlike with the private key, I'm not telling you exactly where to look in the specifications to figure out this key's file format. (Though you might find this blog post by Leonardo Giordano handy. I'm so nice!)
Put one last heading in your report: "Sanity check"
Demonstrate that the integers you found in these two files work as you expect from an RSA key pair. For example, does e*d mod lambda(n) = 1? There are various relationships that you would expect these numbers to have, so show me that they do in fact have those relationships.