Part 1: Olm in detail

Fundamentals

• Key algorithms
• Key types
• Key distribution
• Key verification

Encryption & decryption

Key algorithms

• ed25519
• curve2559
• signed_curve25519

Ed25519

• 32-bit elliptic curve keys
• Bullet Two
• Bullet Three

Key types

• Device fingerprint keys
• Device identity keys
• One time keys
• Fallback keys
• Megolm encryption keys
• Megolm signing keys

Device fingerprint keys

• Ed25519
• The device fingerprint
• Signs all other keys of the device
• Public key shared across the [matrix]

Device identity keys

• Curve25519
• Used to deviate shared secrets
• Signed using the fingerprint keys
• Public key shared across the [matrix]
• Could in future be rotated

One time keys

• Signed Curve25519
• Disposable, single-use keys
• There are looooots of them
• Claimed by other users to establish Olm sessions

Fallback keys

• Signed Curve25519
• Similar to one time keys but not disposable
• Used once one-time keys are consumed
• New fallback keys regenerated once device online

Megolm encryption keys

• Random secret
• Used to derive AES-256 and HMAC-SHA-256 keys
• After each sent message, a hash derives the next key
• Future messages can be decrypted but past ones can't

Megolm signing key

• Ed25519
• Used to sign messages sent via Megolm
• Public key shared in the room along with encryption key