OpenSSH Key Structure Guide

# OpenSSH Key Structure Guide Source: [https://sshref.dev/](https://sshref.dev/) **Chunk 3\.0\.0 to 3\.0\.1:**These blocks are not present in unencrypted keys \(see the[encrypted key structure](https://sshref.dev/#bkdn_rsa_priv_v1_crypt_struct)for what these look like\)\.**3\.0**reflects this, as it’s always going to be`00000000`\(0\)\. **Chunk 4\.0:**This is technically currently unused; upstream hardcodes to 1 \(left zero\-padded 0x01\)\. **Chunk 4\.0\.0\.1\.0, 4\.0\.1\.4\.0:**This is almost always`65537`for a couple reasons\. It’s the["Fermat Prime"](https://en.wikipedia.org/wiki/Fermat_number)*F4*\. **Chunk 4\.0\.0\.1\.0, 4\.0\.0\.2\.0, 4\.0\.1\.3\.0, 4\.0\.1\.4\.0:**Note that the ordering of`e`/`n`in**4\.0\.0**is changed to`n`/`e`in**4\.0\.1**\. **Chunk 4\.0\.1\.6\.0:**The CRT coefficient, or[Chinese Remainder Theorem](https://en.wikipedia.org/wiki/Chinese_remainder_theorem), is a helper used during decryption and signing\. It’s commonly referred to as`qnlv`or*inverse coefficient*\. The value for this chunk is the result of`q^\(\-1\) % p`\. **Chunk 4\.0\.1\.10:**The padding used aligns the private key \(**4\.0\.1\.0**to**4\.0\.1\.9\.0**\) to the cipher blocksize\. For plaintext keys, a blocksize of 8 is used\.