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When can a decryption function detect that the ECC key I use for decryption is incorrect? Is it possible to do that during initialization, or does the complete message have to be decrypted to do that?

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Do you have the public key? This would let you detect that the (private) decryption key is invalid, without even the message. Otherwise: what ECC encryption scheme is used? –  fgrieu Apr 5 '13 at 19:56
    
Typically ECC is not used for encryption at all. It's only used to determine a symmetric key that's used with a symmetric cipher to do the actual encryption. Your question depends on the choice of the symmetric encryption scheme and on the nature of the plaintext. –  CodesInChaos Apr 7 '13 at 12:30
    
I'm programming a bitmessage client, it uses asymetric cryptography only for messages. –  Sibbo Apr 18 '13 at 11:16
    
@Sibbo bitmessage uses ECIES. This means that ECC is used to produce a shared key, which is then used for encryption (AES-CBC) and MAC (HMAC-SHA-256). A wrong key will lead to a MAC verification failure. –  CodesInChaos Mar 5 at 12:30
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