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CTC-PGP5 - Interoperability

When we embarked on this, we had no documentation of the formats, and have been working largely on our own analysis of samples and the released source.

However there are now a number of documents available from the OpenPGP Working Group in the IETF. These include:-

• PGP Inc.'s initial documentation of PGP Message Exchange Formats
• RFC 1991, PGP Message Exchange Formats
• RFC 2015, MIME Security with Pretty Good Privacy (PGP)
• RFC 2440, OpenPGP specification

We view achieving maximum interoperability with all other PGP-format encryption software very important.

CTC Approach

This out-lines how we intend to handle these changes and the facilities we intend including:-

General

We are far more concerned with inter-site interoperatibility than with individuals being able to use more than one program on the same files on a single machine. In the short term at least, we will be primarily interested in formats routinely transmitted between sites (normally encrypted files, public keys and signatures) than formats that normally stay on the site of origin (conventional encrypted files and secret keys).

We are also far more interested in reading all formats than writing all formats. All PGP-compatible variants in use will read Version 3 (PGP2.6) formats. However in some cases, notably storing new format keys in key-rings, writing packets is also necessary for effective interoperability.

Progress

We now have all the basic operations necessary for inter-operability coded and most have been tested. 3-DES remains a problem.

CTB format

We have implemented:-

• the correct reading of new-style CTB.
• the writing of new-style CTBs for record types greater than 15 only.

Version 4 packets

We have implemented:-

• Reading version 4 public key certificates
• Reading sub-key certificates
• Reading version 4 signature (SKE) packets
• Writing all of the above packets

There are no immediate plans to provide for generating, reading, writing or using DSA/DH secret keys, although this should be tackled eventually.

Operations

We have implemented the following operations:-

• 3-DES encryption/decryption
• CAST5 encryption/decryption
• DSA signature verification
• D-H encryption
• New style (SHA-1 fingerprint based) calculation of key Id.s.