A public key is the shareable half of an asymmetric key pair. Anyone may receive it, while the associated private key remains secret. Depending on the primitive, a public key helps others encrypt messages to its owner, derive a shared secret, or verify a signature. section 1.4.1 section 1.4.2
Common uses give the public key different jobs:
- Key exchange: combine a peer’s public key with your private key to produce a shared secret; passive observers cannot reproduce it from public keys alone. section 5.1
- Asymmetric encryption: let anyone encrypt a message to the key owner, who alone should decrypt it with the associated private key. section 6.1
- Digital signatures: combine the public key, signed message, and signature to decide whether the signature is valid. section 1.4.3
- Trust and identity: authenticate that a public key belongs to the intended person or device; publishing the key alone does not establish that association. section 5.1 section 11.3.1
Encrypted message and verified statement
Alice publishes her public key. Anyone can encrypt an email to her, but only Alice’s private key should decrypt it. Alice can also sign a statement with her private key; recipients use her public key, the statement, and the signature to verify it. In Bitcoin, users share public keys to receive funds and sign transactions with private keys to spend them. section 1.4.2 section 12.2.1
Figure 1.7 The final step of a DH key exchange where both participants produce the same shared secret. To do this, Queen Alice combines her private key with Lord Bob’s public key, and Lord Bob combines his private key with Queen Alice’s public key. The shared secret cannot be obtained from solely observing the public keys.
Trace how two participants combine exchanged public keys with their private keys to obtain the same shared secret, without revealing it to an observer. section 1.4.1
