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In version 2.1, GnuPG featured primitives that use elliptic curves. For encryption, one option was ECDH, which confused me. I thought ECDH was for key agreement, not public key encryption. How does GnuPG utilize a key agreement scheme for enciphering files/messages?

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https://tools.ietf.org/html/rfc6637#section-8

Elliptic Curve Cryptography (ECC) in OpenPGP

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  1. EC DH Algorithm (ECDH)

    The method is a combination of an ECC Diffie-Hellman method to establish a shared secret, a key derivation method to process the shared secret into a derived key, and a key wrapping method that uses the derived key to protect a session key used to encrypt a message.

    The One-Pass Diffie-Hellman method C(1, 1, ECC CDH) [NIST-SP800-56A] MUST be implemented with the following restrictions: the ECC CDH primitive employed by this method is modified to always assume the cofactor as 1, the KDF specified in Section 7 is used, and the KDF parameters specified below are used.

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    The key wrapping method is described in [RFC3394]. KDF produces a symmetric key that is used as a key-encryption key (KEK) as specified in [RFC3394]. Refer to Section 13 for the details regarding the choice of the KEK algorithm, which SHOULD be one of three AES algorithms. Key wrapping and unwrapping is performed with the default initial value of [RFC3394].

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