Appendix C: PPK file format

This appendix documents the file format used by PuTTY to store private keys.

In this appendix, binary data structures are described using data type representations such as ‘uint32’, ‘string’ and ‘mpint’ as used in the SSH protocol standards themselves. These are defined authoritatively by RFC 4251 section 5, ‘Data Type Representations Used in the SSH Protocols’.

• C.1 Overview
• C.2 Outer layer
• C.3 Private key encodings
  • C.3.1 RSA
  • C.3.2 DSA
  • C.3.3 NIST elliptic-curve keys
  • C.3.4 EdDSA elliptic-curve keys (Ed25519 and Ed448)
• C.4 Key derivation
• C.5 Older versions of the PPK format
  • C.5.1 Version 2
  • C.5.2 Version 1